Preliminary assessment of sources of nitrogen in groundwater at a biosolids-application area near Deer Trail

Yager, Tracy J.B., McMahon, Peter B.- pmcmahon@usgs.gov · 2012 · Report

Concentrations of dissolved nitrite plus nitrate increased fairly steadily in samples from four shallow groundwater monitoring wells after biosolids

Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.

Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado, 2005 U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Report 2012-5056 Prepared in cooperation with the Metro Wastewater Reclamation District Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado, 2005

Preliminary Assessment of Sources of Nitrogen in Groundwater at a BiosolidsApplication Area near Deer Trail, Colorado, 2005 By Tracy J.B. Yager and Peter B. McMahon Prepared in cooperation with the Metro Wastewater Reclamation District Scientific Investigations Report 2012-5056 U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012

This and other USGS information products are available at http://store.usgs.gov/ U.S. Geological Survey Box 25286, Denver Federal Center Denver, CO 80225 To learn about the USGS and its information products visit http://www.usgs.gov/ 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Yager, T.J.B., and McMahon, P.B., 2012, Preliminary assessment of sources of nitrogen in groundwater at a biosolidsapplication area near Deer Trail, Colorado, 2005: U.S. Geological Survey Scientific Investigations Report 2012-5056, 30 p.

Contents Abstract 1 Introduction 1 Purpose and Scope 3 Description of Study Area 3 Approach 6 Nitrogen Sources in the Study Area 7 Biosolids 8 Animal Manure 11 Inorganic Fertilizer 13 Atmospheric Deposition 14 Geologic Materials 14 Source Contributions 15 Groundwater Characteristics 15 Preliminary Assessment of Sources of Nitrogen in the Groundwater 21 Nitrogen Isotopic Composition 21 Major-Ion Concentrations 22 Chloride/Bromide Mass Ratios 24 Water-Extractable Nitrate in Soil 25 Summary 25 Acknowledgments 27 References Cited 27 Figures

1.  Map showing location of the nitrogen source study near Deer Trail, Colorado 2

2.  Map showing locations of U.S. Geological Survey sampling sites related to the nitrogen source study near Deer Trail, Colorado 4

3.  Graph showing concentrations of dissolved nitrite plus nitrate in water from selected monitoring wells as a function of time 5

4.  Graph showing nitrogen isotopic composition of selected solid source materials and of nitrate in groundwater from this study and total nitrogen content of the solids 8

5.  Metro Wastewater Reclamation District biosolids-application areas near Deer Trail, Colorado 9

6.  Graphs showing concentrations of water-extractable chloride and nitrate in the upper 1.5 meters of the soil column at five sites with variable land-use histories, δ15N[NO3] values for selected samples, and table of chemical inventories of total mass of chloride and nitrate stored in the soil column 18

7.  Graph showing measured values of δ15N[NO3] and δ18O[NO3] in water from the monitoring wells 21

8.  Graphs showing concentrations of nitrate and selected major ions in water from well D6 with solid lines for linear regressions 23

9.  Graph showing chloride/bromide mass ratios in water from selected monitoring wells near Deer Trail as a function of time and the ratios measured in domestic sewage, leachate from biosolids, animal waste, and atmospheric deposition 24

Tables

1.  Nitrogen data for biosolids and manure materials related to the Metro Wastewater Reclamation District properties near Deer Trail, Colorado 10

2.  Chemical data from analyses of leachates prepared from biosolids materials and associated quality-control samples 12

3.  Mineralogy and nitrogen concentration of geologic materials collected near Deer Trail, Colorado 16

4.  Summary of nitrogen content, annual input, load, and isotopic composition for nitrogen sources in the study area 19

5.  Chemical data for groundwater from selected monitoring wells sampled in 2005 20 Conversion Factors Multiply By To obtain Length centimeter (cm) inch (in) kilometer (km) mile (mi) meter (m) foot (ft) micron (µm) inch (in) Area square kilometer (km2) square mile (mi2) Volume cubic centimeter (cm3) cubic inch (in3) liter (L) gallon (gal) Mass gram (g) ounce, avoirdupois (oz) kilogram (kg) pound avoirdupois (lb) Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25°C). Concentrations of chemical constituents in liquids are given either in milligrams per liter (mg/L) or micrograms per liter (µg/L). Concentrations of chemical constituents in solid materials are given either in milligrams per kilogram (mg/kg) or micrograms per gram (µg/g). Isotopic composition data are provided in delta (δ) notation in units of per mil (parts per thousand or 0/00).

Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado, 2005 By Tracy J.B. Yager and Peter B. McMahon Abstract Concentrations of dissolved nitrite plus nitrate increased fairly steadily in samples from four shallow groundwater monitoring wells after biosolids applications to nonirrigated farmland began in 1993. The U.S. Geological Survey began a preliminary assessment of sources of nitrogen in shallow groundwater at part of the biosolids-application area near Deer Trail, Colorado, in 2005 in cooperation with the Metro Wastewater Reclamation District. Possible nitrogen sources in the area include biosolids, animal manure, inorganic fertil­ izer, atmospheric deposition, and geologic materials (bedrock and soil). Biosolids from the Metro Wastewater Reclamation District plant in Denver and biosolids, cow manure, geologic materials (bedrock and soil), and groundwater from the study area were sampled to measure nitrogen content and nitrogen isotopic compositions of nitrate or total nitrogen. Biosolids also were leached, and the leachates were analyzed for nitro­ gen content and other concentrations. Geologic materials from the study area also were sampled to determine mineralogy. Estimates of nitrogen contributed from inorganic fertilizer and atmospheric deposition were calculated from other published reports. The nitrogen information from the study indicates that each of the sources contain sufficient nitrogen to potentially affect groundwater nitrate concentrations. Natural processes can transform the nitrogen in any of the sources to nitrate in the groundwater. Load calculations indicate that animal manure, inorganic fertilizer, or atmospheric deposition could have contributed the largest nitrogen load to the study area in the 13 years before biosolids applications began, but biosolids likely contributed the largest nitrogen load to the study area in the 13 years after biosolids applications began. Various approaches provided insights into sources of nitrate in the groundwater samples from 2005. The isotopic data indicate that, of the source materials considered, biosolids and (or) animal manure were the most likely sources of nitrate in the wells at the time of sampling (2005), and that inorganic fertilizer, atmospheric deposition, and geologic materials were not substantial sources of nitrate in the wells in 2005. The large total nitrogen content of the biosolids and animal-manure samples and biosolids leachates also indicates that the bio­ solids and animal manure had potential to leach nitrogen and produce large dissolved nitrate concentrations in groundwater. The available data, however, could not be used to distinguish between biosolids or manure as the dominant source of nitrate in the groundwater because the nitrogen isotopic composi­ tion of the two materials is similar. Major-ion data also could not be used to distinguish between biosolids or manure as the dominant source of nitrate in the groundwater because the major-ion composition (as well as the isotopic composition) of the two materials is similar. Without additional data, chloride/ bromide mass ratios do not necessarily support or refute the hypothesis that biosolids and (or) animal manure were the primary sources of nitrate in water from the study-area wells in 2005. Concentrations of water-extractable nitrate in the soil indicate that biosolids could be an important source of nitrate in the groundwater recharge. Nitrogen inventories in the soil beneath biosolids-application areas and the nitrogen-input estimates for the study area both support the comparisons of isotopic composition, which indicate that some type of human waste (such as biosolids) and (or) animal manure was the source of nitrate in groundwater sampled from the wells in 2005. The nitrogen-load estimates considered with the nitro­ gen isotopic data and the soil-nitrogen inventories indicate that biosolids applications likely are a major source of nitrogen to the shallow groundwater at these monitoring wells. Introduction The Metro Wastewater Reclamation District (Metro District) applied biosolids from their wastewater treatment process in Denver, Colo., to their agricultural land near Deer Trail, Colo. (fig. 1). The wastewater treatment processes that produced the biosolids included an industrial-waste pretreatment program, trash and grit screening, primary clari­ fication, aeration, secondary clarification, solids concentra­ tion, solids blending, anaerobic digestion, and centrifugation (http://www.metrowastewater.com/, accessed August 9, 2011).

2    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado EL PASO COUNTY LINCOLN COUNTY ARAPAHOE COUNTY ADAMS COUNTY DENVER COUNTY WELD COUNTY LARIMER COUNTY MORGAN COUNTY LOGAN CO WASHINGTON COUNTY Denver Colorado Springs Longmont Castle Rock Deer Trail Limon Greeley Fort Morgan Loveland ELBERT COUNTY DOUGLAS COUNTY JEFFERSON COUNTY BOULDER COUNTY TELLER COUNTY Area of figure 2 105° 40° 39° 104° 40 MILES 40 KILOMETERS South Platte River South Platte River Kiowa Muddy Beaver Big Sandy Creek Badger Creek Creek Creek Creek ROCKY MOUNTAINS Byers South Platte R Denver Study area C O L O R A D O Gun n is o n Ri ve r o o r ado

R er River Rocky Mountain National Park Ar k a n sas Metro Wastewater Reclamation District property in 1999 Thompson Big River Figure 1.  Location of the nitrogen source study near Deer Trail, Colorado.

Introduction    3 Biosolids applications to the area have been permitted by the State of Colorado (Colorado Department of Public Health and Environment, 2003) and documented by the wastewater treatment plant. The biosolids were applied intermittently as a fertilizer and soil amendment at agronomic loading rates to the Metro District properties beginning in late 1993. Groundwater-quality monitoring by the U.S. Geologi­ cal Survey (USGS) at the Metro District properties near Deer Trail (fig. 2) indicated that concentrations of dissolved nitrate in water from some shallow wells increased after monitoring began in 1993 (Yager and others, 2004, 2009; unpub. data on file at U.S. Geological Survey Colorado Water Science Center, 2011). The increase in nitrate concentration in samples from one well in particular, well D6, was of interest to the Metro District because (1) the well was located near a property boundary (fig. 2), (2) measured nitrate concentrations were largest at this location, and (3) nitrate concentration increased from about 5 milligrams per liter as nitrogen (mg-N/L) in 1993 before biosolids were applied to about 40 mg-N/L in 2007 (fig. 3). Nitrate concentrations also increased in water from three other shallow monitoring wells in the drainage area upgradient from well D6, although to a lesser extent. The gradual increase in nitrate concentration in water from well D6 during a period of intermittent biosolids applications led to concerns that biosolids were causing the nitrate increase. How­ ever, nitrate concentrations were at least 5 mg-N/L in some of these monitoring wells before biosolids applications to the area began, which indicates that biosolids applications were not the only source of nitrogen to groundwater in the area. The cycling of nitrogen between organic nitrogen, ammonia, oxidized forms such as nitrate, and other nitrogen forms is well documented (Russell-Hunter, 1970; Freeze and Cherry, 1979; Odum, 1989; Hem, 1992), so nitrate in the groundwater could have been derived from sources that contained nitrogen in some form other than nitrate. Biosolids, animal manure, inorganic fertilizer, atmo­ spheric deposition, and geologic materials (bedrock and soil) are possible nitrogen sources in the area. Because of the multiple possible sources of nitrogen at the Metro District properties, the USGS began a preliminary assessment of sources of nitrogen in shallow groundwater at part of the biosolids-application area in 2005 in cooperation with the Metro District. The Metro District properties near Deer Trail and the surrounding land (fig. 2) constitute the study area. Biosolids from the Metro District plant in Denver (fig. 1) and biosolids, cow manure, geologic materials (bedrock and soil), and groundwater from the study area (fig. 2) were sampled to measure nitrogen content and nitrogen isotopic compositions of nitrate or total nitrogen. Biosolids also were leached, and the leachates were analyzed for nitrogen content and other constituent concentrations. Geologic materials from the study area (fig. 2) also were sampled to determine mineralogy. Estimates of nitrogen contributed from inorganic fertilizer and atmospheric deposition were calculated from other published reports. Purpose and Scope This report presents a preliminary assessment of sources of nitrogen in shallow groundwater at part of a biosolidsapplication area near Deer Trail, Colo., in 2005. Results from mineralogical, chemical, and isotopic analyses of possible source materials are evaluated with respect to groundwa­ ter characteristics. The preliminary assessment of nitrogen sources in groundwater pertains to conditions in 2005, although data from 1993 through 2008 were used to support the assessment. This report includes the data collected from the study area that were used to evaluate sources of groundwa­ ter nitrate. This report does not include all the data collected by the USGS from the study area or the Metro District plant in 2005; those data are included in Yager and others (2009) and Crock and others (2008). Description of Study Area The study area (described in more detail in Yager and Arnold, 2003, and Yager and others, 2004) is located on the eastern plains of Colorado about 16 kilometers (km) east of the town of Deer Trail (fig. 1). The study area is on the eastern margin of the Denver Basin, a bowl-shaped sequence of sedi­ mentary rocks that was formed in an ocean or near-ocean envi­ ronment. The geology of the study area consists of interbedded shale, siltstone, and sandstone, which is overlain in some areas by clay, windblown silt and sand, or alluvial sand and gravel (Sharps, 1980; Major and others, 1983; Robson and Banta, 1995). The primary water-supply aquifer is the Laramie-Fox Hills aquifer, which is a bedrock aquifer that ranges from 0 to about 61 meters (m) thick in the study area and is the lower­ most aquifer in the Denver Basin aquifer sequence (Robson and others, 1981; Robson and Banta, 1995; Yager and Arnold, 2003). Multiple alluvial aquifers are present in the study area. These aquifers are associated with the surficial drainage network and contain water of variable quality, are of limited extent, and generally yield little water (Stevens and others, 2003; Yager and Arnold, 2003). The study area is within the South Platte River drainage basin; all streams in this area drain northward to the South Platte River (fig. 1; Yager and Arnold, 2003). Short segments of some of the streams are intermittent, but in general, the streams are ephemeral and flow only after storms. No surface water flows from the Metro District prop­ erties except after storms. Most ponds in the area have been created by detention structures. Soils in the area generally are sandy or loamy on flood plains and stream terraces, clayey to loamy on gently sloping to rolling uplands, and sandy and shaley on steeper uplands (Larsen and others, 1966; Larsen and Brown, 1971). Land use in the study area was historically rangeland or cropland and pasture (U.S. Geological Survey, 1980). Some petroleum exploration was done in the area (Drew and others, 1979), but no oil or gas production took place on the Metro

4    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Figure 2.  Locations of U.S. Geological Survey sampling sites related to the nitrogen source study near Deer Trail, Colorado. Creek South-1 DC-300 D6 4 MILES 4 KILOMETERS EXPLANATION Metro Wastewater Reclamation District property (METROGRO Farm, 2003 boundaries) USGS alluvial-aquifer monitoring well and identifier USGS soil-sampling location and identifier for nitrogen source study USGS weathered-biosolids sampling location and identifier for nitrogen source study Soil-sampling area for the USGS Expanded Monitoring Program near Deer Trail

ELBERT COUNTY Muddy Creek Rattlesnake Rattlesnake Creek 39°30' COTTONWOOD South-2 D6 D5 D10 LINCOLN COUNTY Beaver Beaver Creek Creek Middlemist South-1 ADAMS COUNTY ARAPAHOE COUNTY WASHINGTON COUNTY Badger Creek Creek Creek 103°52'30" 103°42' VALLEY Route 34 Muddy Creek 39°37'30" 39°44'45" D32 Cottonwood To Deer Trail South property Central property North property Base from U.S. Geological Survey 1:50,000 quadrangles, Elbert County and Arapahoe County Denver Study area C O L O R A D O S. P a t t e R DC-300 D6-2 D6-1 USGS manure-sampling location for nitrogen source study USGS core- or rock-sampling location and identifier for nitrogen source study DTX11 DTX8 DTX10 D6A D9A D17A D25A D31 DTX2WB DTX2A DTX3 DTX3 D29A

Introduction    5 District properties during 1993 through 2005 (N. Crews, Metro Wastewater Reclamation District, written commun., April 13, 2011). Land in the study area was used as range­ land or cropland during 1993 through 2005. Cattle and sheep were the primary domesticated animals grazing the area, and wildlife also were present. No concentrated animal-feeding operations were located on or near the Metro District proper­ ties during 1993 through 2005; no evidence of large confined or unconfined animal feeding or dairy operations has been produced for the period before 1993. Wheat was the primary crop; farmland was not irrigated. Herbicides and other chemi­ cals were applied to the study area during 1993 through 2005 for farming purposes. Pesticides and fertilizers also may have been applied to the Metro District properties historically, but little information is available about these historical applica­ tions. Inorganic fertilizer has not been used in the area since 1993 (N. Crews, Metro Wastewater Reclamation District, oral commun., April 13, 2011); no documentation of type or quan­ tity of inorganic fertilizer use in the area has been produced for the period before 1993. Biosolids were applied as a fertilizer and soil amendment only on the Metro District properties. Monitoring well D6 is located on the east side of the Metro District Central property (fig. 2). This area is drained by Cottonwood Creek, and the contributing surficial drain­ age area for the D6 location is about 2.8 square kilometers (km2) (http://water.usgs.gov/osw/streamstats/colorado.html, accessed March 6, 2012). About 80 percent of this surficial drainage area is Metro District property. The topography of this drainage area is steep to gently rolling hills. Well D6 was located in a lowland depositional area at the confluence of ephemeral Cottonwood Creek and an unnamed tributary. Groundwater in well D6 likely was a mixture of water from 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 EXPLANATION Well D6 Well D5 Well D10 Well D32 Concentration of dissolved nitrite plus nitrate, in milligrams per liter as nitrogen Date Colorado Human Health Standard Samples collected for this study Figure 3.  Concentrations of dissolved nitrite plus nitrate in water from selected monitoring wells as a function of time using data from this study (in blue column), Stevens and others (2003), Yager and others (2004), and U.S. Geological Survey (unpub. data on file at U.S. Geological Survey Colorado Water Science Center, 2011).

6    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado alluvium in paleochannels and water from the unconfined part of the Laramie-Fox Hills aquifer (Yager and Arnold, 2003). Completion information for this well and other monitoring wells in the study is provided in the Groundwater Characteris­ tics section of the report. Biosolids were applied intermittently in the surficial drainage area that contained well D6, but this drainage area is not known to contain any current or former pit toilets or septic tanks. Approach Samples of fresh biosolids were collected from the Metro District wastewater treatment plant in Denver (fig. 1), and samples of weathered biosolids were collected from the study area near Deer Trail (fig. 2). The study area near Deer Trail was the same as that used by the USGS Expanded Monitoring Program near Deer Trail (Yager and others, 2009). Fresh bio­ solids were collected into large jars from an active centrifuge at the Metro District plant by Metro District personnel then were refrigerated and provided to the USGS. A fresh biosolids sample obtained in September 2004 was dried and archived at the laboratories of the USGS in Denver prior to grinding and analysis. The drying process included compositing all contain­ ers of fresh sample material into a large, shallow tray, sub­ stantially stirring the sample to homogenize, and leaving the sample in forced-air conditions (typically under a heat lamp) until dry before splitting the sample in a stainless-steel soil splitter. The aliquot of dried biosolids for analysis was ground to less than 150 microns (μm) and analyzed for nitrogen by methods 350.1, 351.2, and 353.2 (U.S. Environmental Protec­ tion Agency, 1993) or by elemental analyzer (Révész and Qi, 2006). In contrast, fresh biosolids samples obtained in 2010 were analyzed for nitrogen compounds in chilled, hydrated, unground materials within one month of collection and ana­ lyzed for nitrogen by methods 350.1, 351.2, and 353.2 (U.S. Environmental Protection Agency, 1993). Weathered biosolids were sampled by the USGS from site DTX2WB (fig. 2) in 2002 and from site DC-300 (fig. 2) in 2005 by hand picking biosolids aggregates from the land surface using laboratorycleaned spatulas and tweezers. The biosolids sampled at site DC-300 in late May 2005 and the fresh biosolids from Sep­ tember 2004 were produced at the same wastewater treatment plant at approximately the same time. The dried, ground 2004 fresh biosolids sample and corresponding weathered biosolids sample (from site DC-300) were analyzed for nitrogen concen­ tration and the isotopic composition of total nitrogen at the USGS Stable Isotope Laboratory using methods described by Révész and Qi (2006). To better understand how biosolids could be a source of inorganic constituents in groundwater, leachate samples were analyzed. The leachates were prepared from three types of biosolids materials using procedures that were based on the methods described by Hageman (2007). Fresh, wet biosolids for leachates were subsampled from the wet material bottled at the Metro District the day previous to leaching before any drying or grinding of the sample and then refrigerated. Frozen, partly thawed biosolids for leachates also were subsampled from fresh, wet biosolids from the Metro District before any drying or grinding of the sample; they were then frozen for 2 months, then partially thawed. Laboratory-dried biosolids for leachates were subsampled from the air-dried, ground aliquot of Metro District biosolids that had been prepared for analysis. The target mass of biosolids material to leach with 1 liter (L) of solution (Hageman, 2007) was 50 grams (g). The leaching solution was laboratory-grade deionized water (known as inorganic blank water). For wet samples, the mass of biosolids and the volume of deionized water were adjusted for water content of the material so that the final leachate vol­ ume was consistent among the samples; water content ranged from about 75 to 85 percent by weight (%). After mixing, the leachate was filtered through a 0.45-μm nitro-cellulose filter. Leachate samples were analyzed for major ions, nutrients, and trace elements by the USGS National Water Quality Labo­ ratory according to methods described by Fishman (1993). Leachate concentrations are reported as mass (milligram or microgram) per 1 L of total leachate solution (mg/L or μg/L). Samples of fresh and weathered cow manure were col­ lected from the study area in 2005. The manure samples were obtained from the land surface at South-2, a part of the study area that had not received biosolids (fig. 2). Both the fresh and weathered manure samples were collected with laboratorycleaned utensils, then were air-dried and ground (<150 μm) at the laboratories of the USGS in Denver prior to analysis. These samples were analyzed for their total nitrogen concen­ tration and the isotopic composition of total nitrogen at the USGS Stable Isotope Laboratory using methods described by Révész and Qi (2006). Information about inorganic fertilizer and atmospheric deposition is available in the literature (for example, Zublena and others, 1991; Kendall and Doctor, 2004; Ruddy and others, 2006). Site-specific information for these sources were not collected as part of this study but were estimated from literature information. Samples of geologic materials that were used in the nitro­ gen source assessment included bedrock and soil. Nitrogen in soil is from the bedrock minerals or from other sources of nitrogen that are applied or transported to the soil. Samples of rock were collected at various times from the study area (fig. 2). A few samples were collected from the soil surface or from surficial outcrops. Most samples were collected from cores obtained from split-spoon core bar­ rels in hollow-stem augers during the drilling of exploratory boreholes or monitoring wells. Rock samples were analyzed for mineralogy by using X-ray diffraction (Cullity and Stock, 2001) with and without the Rietveld Method (Young, 1995). Most of the rock samples that were analyzed for nitrogen concentration were ground prior to analysis and submitted to a commercial or research laboratory through a contract with the USGS. A single rock sample (shale) from the study area was analyzed for total nitrogen concentration and the isoto­ pic composition of total nitrogen at the USGS Stable Isotope

Nitrogen Sources in the Study Area    7 Laboratory using methods described by Révész and Qi (2006). The sample was from archived core of unweathered shale that underlies the site at DTX11 (fig. 2) at about 11.6 m below land surface. The core was obtained from a borehole in 2000 when a monitoring well was installed at this location. In 2005, samples of soil from five sites in the study area (fig. 2) were collected, dried, and ground. Soil samples were collected at approximately every 0.15-m depth in the top 1.5 m of the soil column. Samples from the upper 0.3 m were analyzed for the total nitrogen concentration and the isotopic composition of total nitrogen at the USGS Stable Isotope Lab­ oratory (http://isotopes.usgs.gov/) in Reston, Va., using proce­ dures described by Révész and Qi (2006). The remaining soil samples were analyzed for concentrations of water-extractable nitrate and chloride, as described below. Soil-sampling sites D6-1, D6-2, and DC-300 (fig. 2) were located in or adjacent to cultivated fields to which biosolids had been applied. Grazing also may have occurred at sites D6-1 and D6-2. Site South-1 (fig. 2) was located in a cultivated field that was not grazed during 1995 through 2005 and to which biosolids had not been applied; this site was in one of the soil-sampling fields used by the USGS Expanded Monitoring Program near Deer Trail (Yager and others, 2004). Site South-2 was located in a pasture to which biosolids had not been applied (fig. 2). Nitrate and chloride concentrations in the soil samples were determined by first extracting the anions from the oven-dried soil using deionized water and then quantifying the concentrations of nitrate and chloride in the extracts by ion chromatography (McMahon and others, 2003). The measured concentrations were corrected for dilution by the added deionized water and then normalized to the dry weight of the soil used in the extraction. Concentrations are reported in micrograms as nitrogen per gram of dry sediment (μg-N/g) for nitrate or micrograms per gram of dry sediment (μg/g) for chloride. Water samples were collected from four USGS monitor­ ing wells (D5, D6, D10, D32) (fig. 2) during May through July 2005. Detailed information about these monitoring wells is provided by Yager and Arnold (2003). The sampling protocols are described in detail elsewhere (Stevens and others, 2003; McMahon and others, 2004) and are only briefly summarized here. Approximately three casing volumes of water were purged from the wells prior to sampling using a submersible pump. Values of pH, specific conductance, and water tempera­ ture and concentrations of dissolved oxygen were monitored in the field using a flow-through chamber during purging. Samples for the laboratory analysis of dissolved nutrients; dissolved nitrogen gas (N2), argon, and methane; nitrogen and oxygen isotopes of nitrate (δ15N[NO3] and δ 18O[NO3]); and nitrogen isotopes of N2 (δ 15N[N2]) were collected after the field properties stabilized. Nutrient samples were analyzed at the USGS National Water Quality Laboratory in Denver using procedures described by Fishman (1993). Analyses of the dissolved gases—N2, argon, and methane—were done at the USGS Chlorofluorocarbon Laboratory (http://water.usgs.gov/ lab/dissolved-gas/) in Reston, Va., using procedures described by Busenberg and others (1993). Analyses of δ15N[NO3], δ18O[NO3], and δ15N[N2] were done at the USGS Stable Isotope Laboratory (http://isotopes.usgs.gov/) in Reston, Va., using procedures described by Böhlke and Denver (1995), Casciotti and others (2002), and Böhlke and others (2003). The delta "δ" notation, in per mil (parts per thousand or ‰), is defined by equations 1 and 2, where 15N, 14N, 18O, and 16O are stable isotopes of nitrogen (N) and oxygen (O) and the super­ scripts are the approximate atomic masses of the isotopes.

sample standard N N δ N 1,000 N N    

− ×        

(1)

sample standard O O δ O 1,000 O O    

− ×        

(2) Values of δ15N[NO3] and δ18O[NO3] were measured using the denitrifier method (Casciotti and others, 2002) and reported relative to atmospheric N2 (AIR) and Vienna Stan­ dard Mean Ocean Water (VSMOW), respectively (Böhlke and others, 2003). The 2-sigma precisions of the δ15N[NO3] and δ18O[NO3] isotopic results are ±0.5 per mil and ±1.0 per mil, respectively. Values of δ15N[N2] are reported relative to AIR with a 2-sigma precision of ±0.2 per mil. Values of δ15N[NO3] in groundwater can be altered from their original values at the time of recharge by natural micro­ bial processes in the aquifer that reduce nitrate to N2 gas, potentially reducing the usefulness of nitrogen isotopes for understanding nitrate sources. This process is referred to as denitrification and generally occurs when concentrations of dissolved oxygen in groundwater are less than about 2 mg/L (Böhlke and others, 2002). Thus, if nitrogen isotopes are to be used to assess sources of nitrate in groundwater, it is important to correct for denitrification effects. When necessary, mea­ sured values of δ15N[NO3] in the water samples were corrected for denitrification effects produced in the aquifer using the approaches described by Vogel and others (1981), Böhlke (2002), and McMahon and Böhlke (2006). Nitrogen Sources in the Study Area Information about nitrogen sources is important to the understanding of variability of nitrogen concentrations in groundwater over time and space (Ruddy and others, 2006). One of the largest sources of nitrogen in an agricultural envi­ ronment can be farm fertilizer, although animal manure and atmospheric deposition also are nitrogen sources in agricul­ tural environments (Ruddy and others, 2006). Farm fertilizers can include biosolids, livestock manure, and inorganic fertil­ izers. Holloway and Dahlgren (2002) reported that rocks can

8    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado be a natural source of nitrogen to groundwater and streams. In addition to biosolids, other forms of human sewage such as pit toilets or septic tanks can be present in rural environments and contribute nitrogen. Because no known current or former pit toilets or septic tanks are located in the vicinity of well D6 and because the source-assessment tools used for this study do not distinguish between different types of human sewage, these potential nitrogen sources are not discussed further. Total nitrogen content and isotopic composition of total nitrogen in samples of biosolids, animal manure, and geologic materi­ als from the study area are shown in figure 4. Total nitrogen content measured for the sources in 2005 ranged from 0.070% in shale to 0.518% in fresh biosolids. The nitrogen isotopic values measured for the sources in 2005 ranged from about +2.8 per mil in shale to +7.3 per mil in weathered biosolids. Biosolids Biosolids have been applied to parts of the study area (fig. 5) intermittently since 1993. Fresh biosolids from the Metro District were measured by the USGS to contain as much as 71,000 milligrams per kilogram as nitrogen (mg-N/kg) in 2010 (table 1), which is 7.10%. The nitrogen in fresh biosolids is primarily in the reduced forms, organic nitrogen and ammonia (table 1) or ammonium. These forms of nitrogen are transformed to nitrate in the presence of oxygen Figure 4.  Nitrogen isotopic composition of selected solid source materials and of nitrate in groundwater (corrected for denitrification effects produced in the aquifer, if necessary; see table 5) from this study and total nitrogen content of the solids. EXPLANATION Nitrogen isotopic composition ( ) of total nitrogen in solid source materials Nitrogen isotopic composition ( ) of nitrate in groundwater (corrected for denitrification effects) (0.365% N) Total nitrogen content of solid measured in 2005 sample, in percent by weight well D10 well D32 δ15 , in per mil relative to atmospheric N2 (AIR)

N Shale Fresh cow manure Weathered cow manure Fresh biosolids Weathered biosolids well D6 Soil n=2 The number of samples analyzed for that material—The composition of groundwater

was based on analysis of a single sample from each well. n=1 n=1 n=2 n=1 n=1 n=2 (no biosolids) δ15N δ15N (0.070% N) (0.090 to 0.110% N) (0.186% N) (0.159% N) (0.518% N) (0.365 to 0.374% N)

Nitrogen Sources in the Study Area    9 U.S. HIGHWAY 36 T. 6 S. T. 5 S. T. 6 S. T. 5 S. T. 7 S. T. 7 S. T. 6 S. T. 6 S. T. 5 S. T. 4 S. R. 59 W. R. 58 W. R. 58 W. R. 57 W. R. 58 W. R. 57 W. R. 57 W. R. 56 W. R. 57 W. R. 56 W. R. 57 W. R. 56 W. R. 58 W. R. 57 W. T. 5 S. T. 4 S. ARAPAHOE COUNTY ARAPAHOE COUNTY ROAD 34 JOLLY ROAD ELBERT COUNTY RD 166 DC 341 DC 342 DC 343 DC 344 DC 333 DC 334 DC 345 DC 347 DC 348 DC 349 DC 350 DC 346 DC 351 DC 352 DC 375 DC 374 DC 359 DC 360 DC 363 DC 364 DC 304 DC 305 DC 306 DC 307 DC 314 DC 315 DC 316 DC 317 DC DC DC 324 DC 325 DC 322 DC 323 DC 399 DC 397 DC 398 DC 400 DC 401 DC 402 DC 403 DC 416 DC 417 DC 414 DC 415 DC 428 DC 429 DC 430 DC 431 DC 432 DC 433 DC 434 DC 435 DC 436 DC 437 DC 438 DC 439 DC 452 DC 453 DC 450 DC 451 DC 448 DC 449 DC 446 DC 447 DC 444 DC 445 DC 442 DC 443 DC 440 DC 441 DC 454 DC 455 DC 456 DC 457 DC 458 DC 459 DC 460 DC 461 DC 462 DC 463 DC 464 DC 465 DC 466 DC 467 DC 468 DC 469 DC 470 DC 471 DC 472 DC 473 DC 474 DC 475 DC 476 DC 477 DC 478 DC 479 DC 480 DC 481 DC 482 DC 483 DC 493 DC 494 DC 484 DC 485 DC 486 DC 487 DC 488 DC 489 DC 490 DC 491 DC 492 DC 404 DC 405 DC 406 DC 407 DC 408 DC 409 DC 412 DC 413 DC 426 DC 427 DC 424 DC 425 DC 422 DC 423 DC 420 DC 421 DC 418 DC 419 DC 411 DC 318 DC 319 DC 320 DC 321 DC 312 DC 313 DC 308 DC 309 DC 310 DC 311 DC 302 DC 303 DC 300 DC 301 DC 365 DC 366 DC 361 DC 362 DC 376 DC 377 DC 355 DC 356 DC 357 DC 358 DC 353 DC 354 DC 340 DC 329 DC 332 DC 328 DC 330 4 MILES 4 KILOMETERS EXPLANATION Metro Wastewater Reclamation District property at METROGRO Farm (2003 boundaries) showing Destination Code (DC, used to track biosolids applications) and section DC 351 DC 352 Property outside the METROGRO Farm as of 2003. These areas usually do not receive biosolids applications. Sections with Destination Code (DC, used to track biosolids applications) formerly were owned by the Metro Wastewater Reclamation District and received biosolids applications. Roads (RD, County road) Modified from Metro Wastewater Reclamation District, written commun., 2003. Figure 5.  Metro Wastewater Reclamation District biosolids-application areas near Deer Trail, Colorado.

10    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Table 1.  Nitrogen data for biosolids and manure materials related to the Metro Wastewater Reclamation District properties near Deer Trail, Colorado. [All data reported as dry weight; mg-N/kg, milligram per kilogram (parts per million) as nitrogen; DSAC, distillation with semi-automated colorimetry, method 350.1 (U.S. Environmental Protection Agency, 1993); KSAC, Kjeldahl with semi-automated colorimetry, method 351.2 (U.S. Environmental Protection Agency, 1993); LSAC, deionized water leaching procedure with semi-automated colorimetry, method 353.2 (U.S. Environmental Protection Agency, 1993); CEA, com­ bustion by elemental analyzer (Révész and Qi, 2006); --, not determined; less than] Sample description (sites are shown in fig. 2) Sample date Analysis date Ammonia,1 soluble (mg-N/kg, by DSAC) Organic nitrogen plus ammonia,1 soluble (mg-N/kg, by KSAC) Nitrite plus nitrate, soluble (mg-N/kg, by LSAC) Nitrogen, total (mg/kg, by CEA) Biosolids, field weathered (applied to the land surface at site DTX2WB several months before sample was collected) 4,200 1,100 Biosolids, fresh (from a centrifuge at the wastewater treatment plant, then dried in a laboratory and archived prior to analysis) September 5,180 Biosolids, field weathered (applied to the land surface at site DC-300 between 9/30/04 and 10/4/04, about 8 months before sample was collected) May 2005 3,740 Biosolids, field weathered, replicate (applied to the land surface at site DC-300 about 8 months before sample was collected) May 2005 3,650 Biosolids, fresh, wet (from a centrifuge at the wastewater treatment plant) October October 15,000 71,000 <47 Biosolids, fresh, wet, replicate (from a centrifuge at the wastewater treatment plant) October October 15,000 57,000 <47 Biosolids, fresh, wet (from a centrifuge at the wastewater treatment plant) November November 9,000 71,000 <40 Biosolids, fresh, wet, replicate (from a centrifuge at the wastewater treatment plant) November November 8,000 63,000 Biosolids, fresh, wet (from a centrifuge at the wastewater treatment plant) December December 15,000 68,000 <44 Biosolids, fresh, wet, replicate (from a centrifuge at the wastewater treatment plant) December December 15,000 70,000 <44 Cow manure, fresh (collected from the land surface at site South-2) June 2005 1,860 Cow manure, field weathered (collected from the land surface at site South-2) June 2005 1,590 1Analyses for "ammonia" quantify ammonia plus ammonium.

Nitrogen Sources in the Study Area    11 through ammonification (mineralization) and nitrification (Barbarick and Ippolito, 2000; Gilmour and others, 2003; Kadlec and Wallace, 2009). The substantial nitrogen content measured by the USGS in Metro District biosolids (table 1) confirms that biosolids have fertilizer potential and also indicates that biosolids are one potential source of nitrogen in groundwater of the study area. Biosolids applications are regulated for content, application method, and record keep­ ing through the Colorado Department of Public Health and Environment (2003). The regulation specifies how to calculate nitrogen content for biosolids applications. Using the approach specified by the regulation, the nitrogen applied to all the Metro District properties in the study area during 1993-2008 was calculated from Metro District application records to be 4,930 kilograms per square kilometer as nitrogen (kg-N/km2) per biosolids application (median from 614 applications). Biosolids applications are documented by the Metro District by half-section (Destination Codes or DCs), not by watershed (fig. 5), and multiple watersheds are included in the DCs that have drainage area contributing to the D6 location (DC 301 in section 15 and DCs 310 and 311 in section 22). About 70-80 percent of the approximately 2.8-km2 area drain­ ing to the D6 location received biosolids applications. Most of the biosolids-application area likely to directly affect the D6 location is contained within areas DC 310 and DC 311 (fig. 5). Biosolids were applied to areas DC 310 and DC 311 during 1994, 1997, 2000, 2001, and 2003; no biosolids were applied to these areas during 1993, 1995, 1996, 1998, 1999, 2002, or 2004 through 2008. Applications to areas DC 310 and DC 311 during the application years ranged from 103 to 7,695 kg-N/km2 (N. Stefonick, Metro Wastewater Reclamation District, written commun., October 2009). Various values for nitrogen isotopic composition are provided for biosolids. Buck (2003) reported a nitrogen isotopic composition for biosolids of +9.6 per mil. Kendall and Doctor (2004) reported a large range of isotopic nitro­ gen values for human waste (about -5 to +25 per mil for δ15N[NO3] with a corresponding δ18O[NO3] range of about -7 to +14 per mil). The fresh biosolids sample from the Metro District had a higher nitrogen content (0.518%) and a lower isotopic nitrogen value (+6.6 per mil) than the corresponding weathered biosolids sample (0.365 to 0.374% and +7.3 per mil) from this study area (fig. 4). Thus, weathering can affect the nitrogen content and isotopic composition of biosolids. It was observed by the USGS that biosolids weathered longer than about 9 months at this site disaggregated so completely that pure biosolids could no longer be sampled from the land surface, although the biosolids-derived material may continue to weather and change chemically and isotopically in the soil after this stage. Concentrations of nitrogen (including nitrate), chlo­ ride, and other inorganic constituents in leachate from fresh biosolids are listed in table 2. Concentrations of ammonia plus organic nitrogen in leachate from biosolids materials ranged from 249 to 650 mg-N/L of leachate, whereas concentrations of nitrate were less than 0.1 mg-N/L. These data and those in table 1 indicate that most of the nitrogen in fresh biosolids was in reduced forms such as organic nitrogen and ammonia. The data in tables 1 and 2 also indicate that nitrogen was lost from the fresh biosolids during the drying process, which could reflect the loss of volatile forms of reduced nitrogen such as ammonia. This result is similar to field results that were reported by Robinson and Polglase (2000) documenting ammonia volatilization following biosolids application. The leachate data (table 2) also indicate the chemical heterogeneity of the biosolids materials, which likely results from distinct clay masses that are present with organic-rich material in the final biosolids product. Animal Manure Manure from domestic livestock and wildlife is a poten­ tial source of nitrate in the groundwater at the Metro District properties. Nitrogen in manure is primarily in the reduced forms, organic nitrogen and ammonium (University of Minne­ sota, 2002; Sullivan, 2008). These forms of nitrogen are trans­ formed to nitrate in the presence of oxygen through ammoni­ fication (mineralization) and nitrification (Hartz and others, 2000; Gilmour and others, 2003; Sullivan, 2008; Kadlec and Wallace, 2009). Manure freshly deposited on the land surface can contain less nitrogen (less than 2,000 mg-N/kg or 0.2% according to fig. 4; Gormly and Spalding, 1979; Lysyk and others, 1985) than fresh biosolids (fig. 4; table 1). However, manure can be concentrated in a single location whereas a small volume of biosolids is distributed over a large area during the Metro District application process. The estimated annual nitrogen input from livestock manure in Arapahoe County was about 160 kg-N/km2 from unconfined or confined operations (median of the annual values from 1982, 1987, and 1992 from Ruddy and others, 2006). The estimated annual nitrogen input from livestock manure in Elbert County was about 480 kg-N/km2 from unconfined operations and 58 kg-N/km2 from confined operations (median of the annual values from 1982, 1987, and 1992 from Ruddy and others, 2006). No data for site-specific annual nitrogen input from livestock or wildlife manure are available for the study area. No confined livestock operations were observed at or near the Metro District properties by the USGS during 1993 through 2005. The nearest animal-feeding operation on record with the State of Colorado (S. Scott, Colorado Department of Public Health and Environment, written commun., August 16, 2011) is about 24 km northwest of well D6, which is much farther than the ammonia concentration and deposition distances from feedlots that were documented by McGinn and others (2003) but less than those documented by Occhipinti and oth­ ers (2008). Cattle occasionally were observed on the Metro District properties by the USGS during 1993 through 2005, but not in the D6 drainage area. Wildlife was observed in the study area by the USGS, particularly herds of deer and prong­ horn in addition to various birds. No identifiable, subsurface deposits of livestock or wildlife manure were observed in

12    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Table 2.  Chemical data from analyses of leachates prepared from biosolids materials and associated quality-control samples.— Continued [All leachates were prepared 12/18/2007 from Metro Wastewater Reclamation District biosolids according to procedures that were based on the methods of Hageman (2007). Analyses were done by the U.S. Geological Survey National Water Quality Laboratory on filtered leachates according to methods described by Fishman (1993). Oct., October; Dec., December; mg/L, micrograms per liter; ICP-MS, inductively coupled plasma-mass spectroscopy; DA, discrete analyzer; mg-N/L, milligrams per liter as nitrogen; less than; cICP-MS, collision-cell inductively coupled plasma-mass spectroscopy; IC, ion chromatography; mg/L, milligrams per liter; E, estimated by laboratory; +, plus; N, nitrogen; mg-P/L, milligrams per liter as phosphorus; std., standard; µS/cm, microsiemens per centi­ meter at 25 degrees Celsius; --, not applicable; oC, degrees Celsius; IBW, inorganic blank water] Analyte, analytical method Units Concentration or characteristic in leachate from indicated biosolids material1 Laboratory dried (from Oct. 2007) Frozen, partly thawed, nondried (from Oct. 2007) Fresh, wet (from Dec. 2007) Replicate of fresh, wet (from Dec. 2007) None (leachate equipment blank) Aluminum, ICP-MS µg/L Antimony, ICP-MS µg/L Arsenic, cICP-MS µg/L Barium, ICP-MS µg/L Beryllium, ICP-MS µg/L Boron, ICP µg/L Bromide, IC mg/L E0.07 Cadmium, ICP-MS µg/L Calcium, ICP mg/L E0.04 Chloride, IC mg/L Cobalt, cICP-MS µg/L Copper, cICP-MS µg/L

Chromium, cICP-MS µg/L Fluoride, IC mg/L Iron, ICP µg/L 1,220

Lead, ICP-MS µg/L Magnesium, ICP mg/L Manganese, ICP-MS µg/L Molybdenum, ICP-MS µg/L Nickel, cICP-MS µg/L Nitrogen, ammonia,2 colorimetry, DA, salicylate-hypochlorite mg-N/L Nitrogen, nitrite + nitrate (NO2+NO3), colorimetry, continuous flow, cadmiumreduction mg-N/L E0.024 Nitrogen, nitrite, colorimetry, DA mg-N/L E0.073 E0.055 Nitrogen, ammonia2 + organic, alkaline persulfate mg-N/L E0.03 Orthophosphate, colorimetry, DA, phosphomolybdate mg-P/L pH, electrode std. units Potassium, ICP mg/L Selenium, cICP-MS µg/L Silica, ICP mg/L Silver, ICP-MS µg/L Sodium, ICP mg/L Specific conductance, Wheatstone bridge µS/cm 2,280 1,400 1,340 Strontium, ICP µg/L Table 2.  Chemical data from analyses of leachates prepared from biosolids materials and associated quality-control samples. [All leachates were prepared 12/18/2007 from Metro Wastewater Reclamation District biosolids according to procedures that were based on the methods of Hageman (2007). Analyses were done by the U.S. Geological Survey National Water Quality Laboratory on filtered leachates according to methods described by Fishman (1993). Oct., October; Dec., December; µg/L, micrograms per liter; ICP-MS, inductively coupled plasma-mass spectroscopy; DA, discrete analyzer; mg-N/L, milligrams per liter as nitrogen; less than; cICP-MS, collision-cell inductively coupled plasma-mass spectroscopy; IC, ion chromatography; mg/L, milligrams per liter; E, estimated by laboratory; +, plus; N, nitrogen; mg-P/L, milligrams per liter as phosphorus; std., standard; µS/cm, microsiemens per centi­ meter at 25 degrees Celsius; --, not applicable; °C, degrees Celsius; IBW, inorganic blank water]

Nitrogen Sources in the Study Area    13 the cores obtained from the drilling of exploratory boreholes or monitoring wells in the study area. No fresh or weathered concentrated manure-deposition areas were observed on the surface of the Metro District properties by the USGS during 1993 through 2005. Therefore, the estimated annual nitrogen input that was calculated from Ruddy and others (2006) was assumed to apply to the surficial drainage area that contains well D6. Various values for nitrogen isotopic composition are provided for animal manure. Gormly and Spalding (1979) reported a range of nitrogen isotopic composition for live­ stock manure of +4.8 to +44 per mil. Kreitler and Jones (1975) reported a nitrogen isotopic composition of +14 per mil for barnyard soils with definite animal-waste contribution. Kendall and Doctor (2004) reported a large range of isotopic composition values for animal manure (about -5 to +25 per mil for δ15N[NO3] with a corresponding δ18O[NO3] range of about -7 to +14 per mil). The fresh cow-manure sample from the study area had a higher nitrogen content (0.186%) and a higher δ15N[NO3] (+5.2 per mil) than the corresponding weath­ ered cow-manure sample (0.159% and +4.8 per mil) from this study area (fig. 4). Thus, weathering can affect the nitrogen content and isotopic composition of animal manure. Inorganic Fertilizer Inorganic fertilizer used in farming is a possible source of nitrate in the groundwater of the study area. Parts of the Metro District properties near Deer Trail were farmed before biosolids applications began in 1993. The nitrogen content of inorganic fertilizers can range from about 5 to about 80% (Zublena and others, 1991), which is as much as 800,000 mg-N/kg. The forms of nitrogen in inorganic fertilizer vary by product but commonly include ammonium or nitrate (Zublena and others, 1991). If the nitrogen is not already in nitrate form, the nitrogen can be transformed to nitrate in the presence of oxygen through nitrification. Estimated median annual nitro­ gen input from fertilizer use on farmland during 1987 through 1992 is calculated from Ruddy and others (2006) as about 190 kg-N/km2 for Arapahoe County and about 120 kg-N/km2 for Elbert County. No inorganic fertilizers reportedly were used at the Metro District properties near Deer Trail during 1993 through 2005 (Metro Wastewater Reclamation District, written commun., 2007), but this type of fertilizer could have been used on the properties before the Metro District acquired the properties. Table 2.  Chemical data from analyses of leachates prepared from biosolids materials and associated quality-control samples.— Continued [All leachates were prepared 12/18/2007 from Metro Wastewater Reclamation District biosolids according to procedures that were based on the methods of Hageman (2007). Analyses were done by the U.S. Geological Survey National Water Quality Laboratory on filtered leachates according to methods described by Fishman (1993). Oct., October; Dec., December; mg/L, micrograms per liter; ICP-MS, inductively coupled plasma-mass spectroscopy; DA, discrete analyzer; mg-N/L, milligrams per liter as nitrogen; less than; cICP-MS, collision-cell inductively coupled plasma-mass spectroscopy; IC, ion chromatography; mg/L, milligrams per liter; E, estimated by laboratory; +, plus; N, nitrogen; mg-P/L, milligrams per liter as phosphorus; std., standard; µS/cm, microsiemens per centi­ meter at 25 degrees Celsius; --, not applicable; oC, degrees Celsius; IBW, inorganic blank water] Analyte, analytical method Units Concentration or characteristic in leachate from indicated biosolids material1 Laboratory dried (from Oct. 2007) Frozen, partly thawed, nondried (from Oct. 2007) Fresh, wet (from Dec. 2007) Replicate of fresh, wet (from Dec. 2007) None (leachate equipment blank) Sulfate, IC mg/L Uranium, ICP-MS µg/L Tungsten, cICP-MS µg/L Zinc, cICP-MS µg/L Chloride:Bromide none Sample temperature at time of preparation, thermistor of pH meter °C Mass of biosolids material leached grams Volume of IBW added liter Estimated water content of solid material percent (all water, no solid) 1All biosolids samples were collected from the centrifuges at the wastewater treatment plant (1- to 2-day composites) in Denver, Colorado. 2Analyses for "ammonia" quantify ammonia plus ammonium.

14    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Various values for nitrogen isotopic composition are provided for inorganic fertilizer. The nitrogen isotopic com­ position of inorganic fertilizers can range from less than +1 to +5.6 per mil (δ15N[NO3]) but generally is characterized by low isotopic values (Vitoria and others, 2004). Gormly and Spald­ ing (1979) reported a nitrogen isotopic composition of -1.1 ± 1.0 per mil for inorganic fertilizers. Kendall and Doctor (2004) reported a range of about -3 to +3 per mil for δ15N[NO3] with a corresponding δ18O[NO3] ranging from about -7 to about +23 per mil in inorganic fertilizer. Kreitler (1979) reported a nitrogen isotopic composition of -38 to +1.9 per mil for inor­ ganic fertilizer. Atmospheric Deposition Atmospheric nitrogen is a possible source of nitrate in groundwater at the Metro District properties. Natural sources of nitrogen in arid environments include precipitation, eolian deposition of nitrate salts, and biological assimilation of atmospheric nitrogen by biota (Walvoord and others, 2003). The nitrogen content of atmospheric deposition in eastern Colorado can be about 0.7 mg-N/L in precipitation (National Atmospheric Deposition Program, http://nadp.sws.uiuc.edu/, accessed April 28, 2011), but the nitrogen content of atmo­ spheric deposition varies. Estimated median annual nitrogen input from atmospheric deposition during 1985 through 2001 is calculated from Ruddy and others (2006) to be about 280 kg-N/km2 for Arapahoe County and about 250 kg-N/km2 for Elbert County based on median values. The nitrogen input from precipitation can be small compared to other sources in agricultural environments. Various values for nitrogen isotopic composition are provided for precipitation. The nitrogen isotopic composition of atmospheric deposition generally is characterized by low isotopic values. Billy and others (2010) reported δ15N[NO3] for precipitation in France as -7 to -2 per mil with a mean of -4 per mil. Nanus and others (2008) reported δ15N[NO3] for precipitation in Rocky Mountain National Park (a higher ele­ vation area about 161 km northwest of the study area) as rang­ ing from -3.6 to -2.1 per mil with a corresponding δ18O[NO3] for precipitation ranging from +71.2 to +74.8 per mil. Kendall and Doctor (2004) reported δ18O[NO3] for precipitation rang­ ing from about +17 per mil to about +67 per mil. Geologic Materials Another potential source of nitrate in groundwater at the Metro District properties is nitrogen naturally present in geologic materials (bedrock and soil) of the study area. The forms of nitrogen in bedrock of the study area are most likely organic nitrogen in recalcitrant organic matter and ammonium in silicate minerals (Holloway and Dahlgren, 2002). Nitro­ gen in rocks can result in increased nitrogen in terrestrial and aquatic systems when bedrock weathers (Holloway and Dahlgren, 2002). Nitrogen can be substantial in coal (Holloway and Dahlgren, 2002; Schweinfurth, 2003), and coal is found in the Laramie Formation west of the Metro District properties (Kirkham and Ladwig, 1979). Only trace amounts of coal were detected in or above the geologic materials that contain groundwater at well D6 as determined by exploratory drilling at sites D6A, D9A, and D25A (fig. 2) in 2002 (table 3). However, geologic materials (both bedrock and soil) of the study area contained nitrogen. Samples of weathered and unweathered shale, siltstone, sandstone, loess, and near-surface soil from the study area contained nitrogen (table 3), and nitrogen content was largest in the shale samples (as much as 2,344 mg-N/kg). Soil that received biosolids applications (sites D6-1 and DC-300) contained as much as 1,700 mg-N/kg nitrogen (table 3). Soil that did not receive biosolids applications (sites South-1 and South-2) contained as much as 1,100 mg-N/kg nitrogen (table 3). Soil nitrogen is from surficial applications (biosolids, animal manure, and inorganic fertilizer), plant materials, atmospheric deposition, and bedrock. A mineralogical analysis did not identify any of the nitrogen-bearing minerals listed by Holloway and Dahlgren (2002, table 1) in the geologic materials from the vicinity of the Metro District properties. However, nitrogen concentrations of shale samples from sites DTX3 and DTX8 (fig. 2, table 3) were substantially larger than the range reported by McMahon and others (1999) for Pierre Shale and larger than many other rock concentrations reported by Holloway and Dahlgren (2002). This study did not assess annual nitrogen input from bedrock in the study area, but Holloway and others (2001) reported cumulative nitrogen release from rocks in laboratory experiments as 3.6×10-12 moles per square centimeter (mol/cm2) (0.0005 kg-N/km2) for slate and 4.2×10-11 mol/cm2 (0.0059 kg-N/km2) for greenstone. Holloway and others (2001) reported cumulative nitrogen release from soils derived from nitrogen-bearing bedrock in laboratory experiments at similar or slightly smaller levels (0.0002 to 0.0004 kg-N/km2). Various values for nitrogen isotopic composition are provided for geologic materials. Kendall and Doctor (2004) reported δ15N[NO3] for soil as ranging from about +2 per mil to +7 per mil with a corresponding δ18O[NO3] for soil ranging from about -7 per mil to about +13 per mil. The isotopic composition of total nitrogen in geologic materials from the study area was +2.8 per mil in shale and +4.3 per mil to +4.7 per mil in soil. The nitrogen isotopic composition of the geologic materials from the study area was lower than that measured for biosolids or animal manure (fig. 4). Concentrations of water-extractable nitrate and chloride in the soil samples are shown in figure 6. The largest concen­ trations of nitrate occurred in the upper 0.6 m at sites D6-1 and DC-300 where concentrations exceeded 75 μg/g. Those two sites also had the largest inventories of nitrate stored in the upper 1.5 m of the soil column. Both of those sites received biosolids applications and were cultivated. Some grazing probably occurred at both sites as well. Sites South-1 and South-2 were cultivated and grazed, respectively, but received no biosolids applications. The largest nitrate concentrations

Groundwater Characteristics    15 at those sites were less than 35 μg/g. Site D6-2 received biosolids applications, but it had by far the smallest nitrate concentrations and largest chloride concentrations of any site. The presence of large chloride concentrations in the soil zone in semiarid regions has been interpreted to mean that little to no water movement has occurred through the soil (Scanlon and Goldsmith, 1997; Walvoord and others, 2003; McMahon and others, 2006) because otherwise the chloride would be flushed from the soil. This interpretation assumes no applica­ tion of chemical amendments, such as potassium chloride, to a cultivated field. Although historical chemical-application data for the site are lacking, the relatively small chloride concentra­ tions at the other cultivated sites indicate that land applica­ tions of potassium chloride probably did not occur (fig. 6). A general lack of water movement through the soil at site D6-2 is supported by the small concentrations of nitrate in the soil zone at that site (fig. 6). In contrast, all of the other sites had larger nitrate concentrations and smaller chloride concentra­ tions than site D6-2, consistent with the interpretation that water movement through the soil did occur at those other sites. Source Contributions The nitrogen information from the study indicates that each of the sources contain sufficient nitrogen to potentially affect groundwater nitrate concentrations. Natural processes can transform the nitrogen in any of the sources to nitrate in the groundwater. Nitrogen information for each source in the study area is summarized in table 4 and was used to estimate nitrogen loads. The nitrogen load from biosolids applications in areas DC 310 and DC 311 during 1993 through 2008 ranged from 75 to 9,310 kilograms of nitrogen (kg-N) per application. The annual nitrogen load to this area ranged from 0 during non-application years to 16,482 kg-N in 1997. The total nitro­ gen load (summation from all applications to areas DC 310 and DC 311) was about 48,300 kg-N for 1993 through 2005 (table 4). The actual load of biosolids-applied nitrogen to the surficial drainage area contributing to the D6 location cannot be calculated from available application records. However, estimates calculated from Metro District application records indicate that the load of biosolids-applied nitrogen to the drainage area contributing to the D6 location did not exceed 72,440 kg-N for the period 1993 through 2005 or 74,770 kg-N for the period 1993 through 2008. Estimated annual nitrogen input was used to estimate the nitrogen load to the contributing surficial drainage area of well D6 from animal manure, inorganic fertilizer, atmo­ spheric deposition, and geologic materials (bedrock and soil) (table 4). Because biosolids applications began in the study area in 1993, the nitrogen load was calculated separately for the 13 years prior to biosolids application (1980 to 1993) and for the 13 years after biosolids applications began (1993 through 2005). Estimates calculated from Ruddy and others (2006) indicate that the nitrogen load from animal manure likely did not exceed 17,000 kg-N for the period 1993 through 2005. The nitrogen load from inorganic fertilizer is estimated as 0 kg-N for the period 1993 through 2005 because only biosolids were used as the fertilizer. Estimates calculated from Ruddy and others (2006) and the National Atmospheric Deposition Program (http://nadp.sws.uiuc.edu/) indicate that the nitrogen load from atmospheric deposition likely did not exceed 11,000 kg-N for the period 1993 through 2005. Using nitrogen-release rates from Holloway and others (2001), the nitrogen load from bedrock is estimated as 0.20 kg-N or less and the nitrogen load from soil is estimated as 0.01 kg-N or less for the period 1993 through 2005. Thus, calculated estimates from Holloway and others (2001) indicate that the nitrogen load from all geologic materials likely did not exceed 0.21 kg-N for the period 1993 through 2005, which is much less than the estimated nitrogen load from biosolids, animal manure, and atmospheric deposition for the same period. The load estimates (table 4) indicate that animal manure, inorganic fertilizer, or atmospheric deposition could have contributed the largest nitrogen load to the study area in the 13 years before biosolids applications began, but biosolids likely contributed the largest nitrogen load to the study area in the 13 years after biosolids applications began. Groundwater Characteristics Groundwater information for the study area indicates that nitrate concentration varied both temporally and spatially. In 1993, before any biosolids applications began, dissolved nitrite plus nitrate concentrations were about 5 mg-N/L or larger in groundwater samples from monitoring wells D5, D6, and D9 (well D9 was at the D9A location shown in fig. 2); concentrations were about 2.4 mg-N/L in groundwater samples from monitoring well D10 in 1993 (unpub. data on file at U.S. Geological Survey Colorado Water Science Center, 2011). Concentrations of dissolved nitrite plus nitrate increased fairly steadily in samples from well D6 after biosolids applications began; concentrations of dissolved nitrite plus nitrate also increased similarly at monitoring well D32 and to a lesser extent at wells D5 and D10 (fig. 3). Wells D5 and D6 were completed at similar depths (table 5); well D10 was completed about 3 m shallower, and well D32 was completed about 3 m deeper (table 5). Wells D5, D10, and D32 were upgradient topographically as well as hydrogeologically from well D6 and located in the same drainage basin as well D6, so they are of the most interest in understanding well D6 nitrogen sources. Concentrations of dissolved nitrite plus nitrate were much lower at 24 other monitoring wells during the same period, but spiked as high as 25-31 mg-N/L in late 1995 and 1998 at well D9 and in late 1996 at well D25 (well D25 was at the D25A location shown in fig. 2) (unpub. data on file at U.S. Geo­ logical Survey Colorado Water Science Center, 2011). This information indicates that (1) groundwater in the study area had a nitrogen source before biosolids applications started, and (2) concentration patterns for dissolved nitrite plus nitrate in groundwater were diverse and highly variable throughout the study area during 1993-2005.

16    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Table 3.  Mineralogy and nitrogen concentration of geologic materials collected near Deer Trail, Colorado.—Continued [Samples were obtained from borehole cores when monitoring wells were installed (DTX3, DTX8, DTX10, DTX11, D6A, D25A, and D31), from exploratory coreholes (D9A, D17A, D6-1, DC-300, South-1, and South-2), from outcrops (D29A), and from recently tilled fields (DTX2A). mg-N/kg, milligram per kilogram (parts per million) as nitrogen; DSAC, distillation with semi-automated colorimetry, method 350.1 (U.S. Environmental Protection Agency, 1993); +, plus; KSAC, Kjeldahl with semi-automated colorimetry, method 351.2 (U.S. Environmental Protection Agency, 1993); LSAC, deionized water leaching procedure with semi-automated colorimetry, method 353.2 (U.S. Environmental Protection Agency, 1993); oC, degrees Celsius; ±, plus or minus; std. dev., standard deviation; mg/kg, milligram per kilogram; CEA, combustion by elemental analyzer (Révész and Qi, 2006); --, not determined; less than; n, num­ ber of samples analyzed; cm, centimeter; approximately; major, minerals that compose greater than 25 percent by weight; minor, minerals that compose between 5 and 25 percent by weight; trace, minerals that compose less than 5 percent by weight] Site (fig. 2) Sample depth, meters below land surface Sample date Sample description Mineralogy1 Ammonia,2 soluble (mg-N/kg, by DSAC) Organic nitrogen + ammonia,2 soluble (mg-N/kg, by KSAC) Nitrate + nitrite, soluble (mg-N/kg, by LSAC) Nitrogen, total (mg/kg, by pyrolysis at 1,050°C [± std. dev.]) Nitrogen, total (mg/kg, by CEA) DTX3 4.3-4.5 2/12/1999 Black unweathered shale

2,344 ± 34 (n=3) DTX8 42.2-42.4 3/2/1999 Hard, black, unweathered shale with silt

2,101 ± 65 (n=3) DTX8 48.9-49.1 3/2/1999 Gray silt (weathered siltstone and shale) with shale

869 ± 49 (n=2) DTX8 49.8-50.0 3/2/1999 Gray silt (weathered siltstone and shale) with shale

874 ± 6 (n=2) DTX10 25.0-25.2 2/24/1999 Shale, clay (weathered shale), and silt (weathered siltstone)

798 ± 51 (n=2) DTX10 26.1-26.3 2/24/1999 Gray silt (weathered siltstone and shale) with shale

1,031 ± 37 (n=2) DTX10 31.2-31.4 2/24/1999 Black shale with silt (weathered shale) and fossil fragments

1,086 ± 17 (n=3) DTX10 109.9-110.8 2/24/1999 Hard black clay (weathered shale) with shale and silt (weathered siltstone) 1,358 ± 6 (n=2) DTX2A 0 (surface) 7/6/2000 Tabular, clear to white crystal (9 cm × 5 cm × 2 cm) Gypsum with trace of quartz sand DTX11 1/18/2000 Dense, semi-consolidated, grey, silty clay (loess) with white crystalline rootlet deposits Quartz (major), microcline, (minor), with trace of albite, gypsum, muscovite, (possible kaolinite or chlorite, clinochlore), dolomite; White crystalline deposits identified as gypsum DTX11 1/18/2000 Unconsolidated pink, brown, black, white, and gray silt and sand with arkosic fragments Quartz, microcline (major), albite (minor), with trace of gypsum, muscovite DTX11 1/18/2000 Consolidated fragments of grey sandstone and darker siltstone in unconsolidated, homogeneous, grey, very fine sand and silt Quartz (major), albite, microcline (minor), with trace of montmorillonite clay, muscovite, (pos­ sible kaolinite, clinochlore), gypsum, pyrite, dolomite DTX11 1/18/20003 Hard, black, unweathered shale D6A 0.6-0.8 2/6/2002 Semi-consolidated, brown clay balls in dark grey-brown loam (loess) Quartz (major), albite, orthoclase (minor), with trace of montmorillonite, muscovite, calcite, kaolinite, magnesiohornblende D6A 3.0-3.2 2/6/2002 Unconsolidated, light brown loam with black flecks that possibly are coal (loess) Quartz (major), gypsum (minor), with trace of albite, montmorillonite, muscovite, orthoclase, kaolinite (sample has smectite, probably montmorillonite) D6A

19-19.5 2/6/20024 Unconsolidated, dense, brown, clayey silt (loess) with white megascopic crystals lining a vertical planar void Quartz (major), gypsum (minor), with trace of albite, montmorillonite, muscovite, orthoclase, and possible kaolinite (sample has smectite, probably montmorillonite); White crystals identi­ fied as gypsum

D6A 8.7-9.1 (below screen) 2/6/2002 Consolidated, dark grey to black shale Quartz (major), gypsum (minor), with trace of orthoclase, albite, montmorillonite, muscovite, and possible kaolinite (sample has smectite, probably montmorillonite) D9A 8.7-10.2 2/7/20024 Black amorphous-mineral vein (0.3-0.6 cm thick) in semiconsolidated brown silty loess Quartz (major), albite (minor), with trace of orthoclase, montmorillonite, muscovite; Black mineral is amorphous, consistent with coal

D9A 2/7/2002 Consolidated, friable, orange sandstone with unconsolidated, homogeneous, orange loamy sand (weathered sandstone) Quartz (major), albite (minor), with trace of orthoclase, montmorillonite, muscovite; Black mineral is amorphous, consistent with coal Table 3.  Mineralogy and nitrogen concentration of geologic materials collected near Deer Trail, Colorado. [Samples were obtained from borehole cores when monitoring wells were installed (DTX3, DTX8, DTX10, DTX11, D6A, D25A, and D31), from exploratory coreholes (D9A, D17A, D6-1, DC-300, South-1, and South-2), from outcrops (D29A), and from recently tilled fields (DTX2A). mg-N/kg, milligram per kilogram (parts per million) as nitrogen; DSAC, distillation with semi-automated colorimetry, method 350.1 (U.S. Environmental Protection Agency, 1993); +, plus; KSAC, Kjeldahl with semi-automated colorimetry, method 351.2 (U.S. Environmental Protection Agency, 1993); LSAC, deionized water leach­ ing procedure with semi-automated colorimetry, method 353.2 (U.S. Environmental Protection Agency, 1993); oC, degrees Celsius; ±, plus or minus; std. dev., standard deviation; mg/kg, milligram per kilogram; CEA, combustion by elemental analyzer (Révész and Qi, 2006); --, not determined; less than; n, number of samples analyzed; cm, centimeter; approximately; major, minerals that compose greater than 25 percent by weight; minor, minerals that compose between 5 and 25 percent by weight; trace, minerals that compose less than 5 percent by weight]

Groundwater Characteristics    17 Table 3.  Mineralogy and nitrogen concentration of geologic materials collected near Deer Trail, Colorado.—Continued [Samples were obtained from borehole cores when monitoring wells were installed (DTX3, DTX8, DTX10, DTX11, D6A, D25A, and D31), from exploratory coreholes (D9A, D17A, D6-1, DC-300, South-1, and South-2), from outcrops (D29A), and from recently tilled fields (DTX2A). mg-N/kg, milligram per kilogram (parts per million) as nitrogen; DSAC, distillation with semi-automated colorimetry, method 350.1 (U.S. Environmental Protection Agency, 1993); +, plus; KSAC, Kjeldahl with semi-automated colorimetry, method 351.2 (U.S. Environmental Protection Agency, 1993); LSAC, deionized water leaching procedure with semi-automated colorimetry, method 353.2 (U.S. Environmental Protection Agency, 1993); oC, degrees Celsius; ±, plus or minus; std. dev., standard deviation; mg/kg, milligram per kilogram; CEA, combustion by elemental analyzer (Révész and Qi, 2006); --, not determined; less than; n, num­ ber of samples analyzed; cm, centimeter; approximately; major, minerals that compose greater than 25 percent by weight; minor, minerals that compose between 5 and 25 percent by weight; trace, minerals that compose less than 5 percent by weight] Site (fig. 2) Sample depth, meters below land surface Sample date Sample description Mineralogy1 Ammonia,2 soluble (mg-N/kg, by DSAC) Organic nitrogen + ammonia,2 soluble (mg-N/kg, by KSAC) Nitrate + nitrite, soluble (mg-N/kg, by LSAC) Nitrogen, total (mg/kg, by pyrolysis at 1,050°C [± std. dev.]) Nitrogen, total (mg/kg, by CEA) D9A 17.4-17.5 2/7/2002 Consolidated, friable, weathered, yellow sandstone and gray shale with heterogeneous gray, black, and yellow loamy sand and shale Quartz (major), with trace of orthoclase, montmorillonite, albite, gypsum, jarosite, muscovite, possible kaolinite (sample has amorphous smectite, probably montmorillonite); Yellow min­ eral identified as jarosite surrounded by minerals quartz and gypsum D9A 2/7/2002 Consolidated, friable, heterogeneous, weathered, grey, black, yellow and red loamy siltstone, sand­ stone, and shale with vein of clear/white crystalline mineral Quartz (major), gypsum, orthoclase, montmorillonite (minor), with trace of albite, jarosite, mus­ covite; Clear/white mineral identified as gypsum with a trace of quartz sand D9A 2/7/20024 Consolidated, weathered, friable red-orange siltstone and sandstone with concretions, just above grey shale Quartz (major), gypsum (minor), with trace of albite, montmorillonite, orthoclase, muscovite, jarosite

D9A 2/7/2002 Consolidated, friable, weathered, orange and brown sandstone with orange, black, and yellow shale Quartz (major), gypsum and microcline (minor), with trace of albite, montmorillonite, musco­ vite, jarosite D9A 2/7/2002 Clear crystals in consolidated, friable, weathered, grey shale with yellow and orange minerals; semiconsolidated weathered, yellow, orange, grey, and brown siltstone, shale, and sandstone Clear crystals identified as gypsum D9A 18.38-18.41 2/7/2002 Clear crystals in consolidated, friable, weathered grey shale with yellow and orange minerals; semiconsolidated weathered, yellow, orange, grey, and brown siltstone, shale, and sandstone Quartz and gypsum (major), jarosite and montmorillonite clay (minor), with trace of microcline, albite, jarosite, muscovite; Clear crystals identified as gypsum D9A

2/7/2002 Consolidated, friable, weathered, grey clayey shale with yellow, clear, and orange minerals; unconsoli­ dated weathered, yellow, orange, grey, and brown siltstone and sandstone Quartz and gypsum (major), jarosite and montmorillonite clay (minor), with trace of microcline, albite, muscovite; Yellow minerals identified as natrojarosite, jarosite; clear minerals identi­ fied as quartz, gypsum; orange minerals identified as natrojarosite, jarosite, amorphous iron hydroxides D17A 10/22/1997 Consolidated, dense, dark grey shale and siltstone Quartz (major), microcline, albite, montmorillonite (minor), with trace of gypsum, muscovite, (possible kaolinite or chlorite, clinochlore), dolomite D17A 10/22/1997 Consolidated, dense, dark grey shale and siltstone Quartz (major), dolomite (minor), with trace of microcline, albite, muscovite, montmorillonite, gypsum, possible kaolinite or chlorite and clinochlore D25A 2/5/2002 Semi-consolidated to unconsolidated orange-brown sand or weathered sandstone Quartz (major), microcline and albite (minor), with trace of muscovite, montmorillonite, pos­ sible kaolinite or chlorite, clinochlore D29A 0 (surface) 3/6/1997 Consolidated, hard, light brown sandstone on top of field along road Quartz and calcite (major), albite (minor), with a trace of microcline, muscovite D31 1.4-2.7 5/4/1995 Unconsolidated, hard, dry balls of brown clay and clay-loam (loess) with 0.2- to 0.3-cm pods of anhedral white crystals; light brown silty loam to clay loam (loess) White crystals identified as gypsum surrounded with a trace of quartz D6-1 0.2-0.3 5/31/2005 Soil in valley area that has received biosolids applications, cultivation, and possibly grazing 1,000 D6-1 0.4-0.6 5/31/2005 Soil in valley area that has received biosolids applications, cultivation, and possibly grazing 1,700 DC-300 0.2-0.3 5/31/2005 Soil on south-facing hillslope that has received biosolids applications and cultivation South-1 0.2-0.3 5/31/2005 Soil on lowland terrace that has received cultivation but no biosolids applications South-2 0.1-0.2 5/31/2005 Soil on upland terrace that has had grazing but has not received biosolids applications or cultivation 1,100 1Analyzed in 2005 by X-ray diffraction (Cullity and Stock, 2001) with and without Rietveld Method (Young, 1995). 2Analyses for "ammonia" quantify ammonia plus ammonium. 3Chemical analysis done in 2005. 4Chemical analysis done in 2002; mineralogic analysis done in 2005.

18    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Chemical and isotopic data collected from wells D5, D6, D10, and D32 in 2005 for this study are listed in table 5. Although nitrite concentrations were not measured in the 2005 samples, other measurements from the site in 2005 and from historical data (Stevens and others, 2003, table 14) indicate that nitrite probably represents less than 1% of the dissolved nitrite plus nitrate concentration in groundwater; therefore, dissolved nitrite plus nitrate in groundwater is referred to as nitrate in the remainder of this report. Nitrate concentrations for the 2005 samples ranged from a low of 4.91 mg-N/L in well D10 to a high of an estimated 31.5 mg-N/L in well D6. Nitrate concentration in the sample collected from well D5 in 2005 (6.40 mg-N/L) is not consistent with the apparent long-term trend of increasing nitrate concentrations in the well (fig. 3); the 2005 concentration was substantially less than that of a subsequent sample collected in late 2007 (fig. 3; Yager and others, 2011, table 10). Nitrate concentrations in the samples collected from well D6 in 2005 and in late 2007 are consistent with the apparent long-term trend of increas­ ing nitrate concentrations in the well (fig. 3). Nitrate con­ centration in the sample collected from well D10 in 2005 is consistent with the apparent long-term trend of irregularly increasing nitrate concentrations in the well (fig. 3); however, the 2005 concentration was substantially less than that of a subsequent sample collected in late 2007 (fig. 3; Yager and others, 2011, table 10). Nitrate concentration in the sample collected from well D32 in 2005 is consistent with the appar­ ent long-term trend of increasing nitrate concentrations in the well observed for 1995 through 1998 (fig. 3); however, the 2005 concentration was substantially greater than that of a subsequent sample collected in late 2007 (fig. 3; Yager and others, 2011, table 10). Figure 6.  Concentrations of water-extractable chloride and nitrate in the upper 1.5 meters of the soil column at five sites with variable land-use histories, δ15N[NO3] values for selected samples, and table of chemical inventories of total mass of chloride and nitrate stored in the soil column.

Chloride Nitrate Nitrate concentration, in micrograms per gram as nitrogen

D6-1 (valley) -Biosolids and cultivation -Possible grazing D6-2 (upland) -Biosolids and cultivation -Possible grazing

DC-300 -Biosolids and cultivation -Possible grazing

South-2 -Grazing

Explanation

South-1 -Cultivation

Chloride concentration, in micrograms per gram Depth below land surface, in meters δ15N[NO3] 6.8‰ δ15N[NO3] 5.6‰ δ15N[NO3] 8.2‰ ‰ per mil δ15N[NO3] Isotopic concentration of nitrate-nitrogen (upland) (terrace) (upland) kg/km2 kilograms per square kilometer Chemical inventory for 0- to 1.5-meter depth Site (fig. 2) Chloride (kg/km2) Nitrate as nitrogen (kg/km2) D6-1 71,061 50,326 D6-2 419,753 DC-300 21,072 61,198 South-1 89,331 33,065 South-2 5,268 21,184 less than

Groundwater Characteristics 19 Table 4. Summary of nitrogen content, annual input, load, and isotopic composition for nitrogen sources in the study area. [Values from this study are from measurements; values from other studies are a summary of literature data, are estimates, or may be from other measurements of materials from other locations. mg, milligram; N, nitrogen; kg, kilogram; km2, square kilometer; O, oxygen; --, no information; L, liter; CEA, combustion by elemental analyzer (Révész and Qi, 2006)] Source Nitrogen content (mg-N/kg) Estimated annual input (kg-N/km2) Estimated N load from source during 1980 to 1993 (kg-N) Estimated N load from source during 1993 through 2005 (kg-N) N-isotopic composition (per mil) O-isotopic composition of nitrate (per mil) Biosolids 13,650-71,000 24,930 248,300 1+6.6 to +7.3; 3+9.6 Animal manure 11,860 4160-480 45,520-16,190 47,170-16,980 1+4.8 to +5.2; 5+4.8 to +44; 6+14 7-7 to +14 Inorganic fertilizer As much as 8800,000 4120-190 44,070-6,340 5-1.1 ± 1.0; 7-3 to +3; 9-38 to +1.9 7-7 to +23 Atmospheric deposition About 0.7 mg-N/L in precipitation10 4250-280; 10220-270 46,480-7,680 106,930 49,410-10,920; 108,190 11-3.6 to -2.1; 12-7 to -2 7+17 to +67; 11+71.2 to +74.8 Bedrock 1,058 (median of 8 samples analyzed by pyrolysis; table 3)1 130.0005-0.0059 130.02-0.20 130.02-0.20 1+2.8 Soil derived from nitrogen-bearing bedrock As much as 1,100 (no biosolids applied; table 3)1 140.0002-0.0004 1+4.3 to +4.7; 7+2 to +7 7-7 to +13 1Data from this study. 2Calculated from application records provided by the Metro Wastewater Reclamation District for 1993 through 2008. Estimated annual input is the median for all applications to all Metro Wastewater Reclamation District property in the study area during 1993 through 2008, although biosolids were applied to the same well D6 for 1993 through 2005. 3From Buck (2003). 4Calculated from county-level estimates reported by Ruddy and others (2006) for Arapahoe and Elbert Counties by assuming that the nitrogen reported for each county was evenly distributed throughout the county. Estimated annual input was determined by dividing the median of the annual nitrogen input for all applicable reported years for the county by the area of the county. Estimated nitrogen load was determined by multiplying the estimated annual input for the ganic fertilizer are for farmed areas. 5From Gormly and Spalding (1979). 7From Kendall and Doctor (2004). 8From Zublena and others (1991). 9From Kreitler (1979). 10Calculated from National Atmospheric Deposition Program (http://nadp.sws.uiuc.edu/, accessed April 28, 2011); median of depositional totals from annual data for 1983-2005, nitrate plus ammonia data, Bent County site and Weld County site in Colorado. 11Values in precipitation from Nanus and others (2008) for National Atmospheric Deposition Program sites in Rocky Mountain National Park, a higher-elevation area about 100 miles northwest of the study area. 12From Billy and others (2010). 13Calculated from Holloway and others (2001) for slate and greenstone based on steady-state values from laboratory experiments. 14Calculated from Holloway and others (2001) for a sandy California BC horizon soil and a saprolite derived from slate and greenstone based on steady-state values from laboratory experiments.

20    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Measured δ15N[NO3] values in water samples from the wells ranged from +8.5 to +15.5 per mil for samples collected in 2005 (table 5). The lowest δ15N[NO3] value was measured in the sample from well D10 (table 5). The highest δ15N[NO3] value was measured in the sample from well D5 (table 5). Denitrification probably affected the nitrogen isotopic composition of nitrate in water from wells D5 and D6 more than it did nitrate in water from wells D10 and D32. The samples from wells D5 and D6 plot farther along isotopefractionation trend lines characteristic of denitrification (Böttcher and others, 1990; Granger and others, 2004) than the samples from D10 and D32 (fig. 7). Analysis of the dissolvedgas data for wells D6, D10, and D32 also indicates that water from wells D10 and D32 contained little or no excess N2 from denitrification, whereas water from well D6 apparently contained a small amount of excess N2, about 2 mg-N/L (table 5). The nitrogen isotopic composition of nitrate in water from well D6 was corrected for denitrification effects using the chemical and isotopic data (Böhlke and others, 2002), resulting in a shift in the measured value from +10.1 to +8.9 per mil (table 5). The dissolved-gas sample from well D5 apparently was degassed (probably during sampling), so the measured nitrogen isotopic composition of nitrate in water from that well could not be corrected for denitrifica­ tion effects. Thus, isotopic data from well D5 is not discussed further. Table 5.  Chemical data for groundwater from selected monitoring wells sampled in 2005. [NWIS, U.S. Geological Survey National Water Information System database; m, meters; mg/L, milligrams per liter; --, not measured; µS/cm, microsiemens per centimeter; °C, degrees Celsius; mg-N/L, milligrams per liter as nitrogen; E, estimated by laboratory; less than; N2, nitrogen gas; Ar, argon; CH4, methane; nd, not determined; cc STP/L, cubic centimeters at standard temperature and pressure per liter; δ15N[NO3 -], the delta N-15 isotope composition of the nitrate nitrogen; ‰, per mil or parts per thousand; AIR, atmospheric N2; δ18O[NO3 -], the delta O-18 isotope composition of the nitrate oxygen; VSMOW, Vienna Standard Mean Ocean Water; δ15N[N2], the delta N-15 isotope composition of the nitrogen gas]

Well name D5 D6 D6 D10 D32 NWIS station number1 19103515500 33103512300 33103512300 09103514600 09103513900 Screen interval, m below land surface 4.9 to 7.9 4.0 to 7.0 2.3 to 5.3 2.3 to 5.3 8.5 to 11.6 Sample date 5/31/2005 6/1/2005 7/13/2005 5/31/2005 6/1/2005 Depth to top of screen, m below water level Dissolved oxygen, mg/L Field pH, standard units Specific conductance, μS/cm at 25°C 4,140 17,600 17,000 4,400 3,600 Ammonia plus organic nitrogen, dissolved, mg-N/L as N E1.7 Ammonia, dissolved, mg-N/L E0.029 Nitrite plus nitrate, dissolved, mg-N/L E31.5 2E27 N2, mg/L degassed degassed Ar, mg/L degassed degassed CH4, mg/L degassed degassed Recharge temperature, °C nd nd E15 E12 E8 Excess air, cc STP/L nd nd Excess N2, mg-N/L nd nd Measured δ15N[NO3-], ‰ relative to AIR Measured δ18O[NO3-], ‰ relative to VSMOW -0.94 -0.39 Measured δ15N[N2], ‰ relative to AIR degassed degassed -0.29 Corrected3 nitrate, mg-N/L nd nd E29 Corrected3 δ15N[NO3-], ‰ relative to AIR nd nd E8.9 1Last 11 digits, only. First four digits are "3936" for each of these sites. 2Value is nitrate as nitrogen analyzed by ion chomatography (estimated because of chloride interference). 3Corrected for denitrification effects in the aquifer; these corrected values are graphed in figure 4.

Preliminary Assessment of Sources of Nitrogen in the Groundwater    21 Preliminary Assessment of Sources of Nitrogen in the Groundwater Various approaches provided insights into sources of nitrogen in the groundwater samples from 2005. Nitrogen isotopic composition of source materials was compared to that of the groundwater samples. Major-ion concentrations of the groundwater samples were evaluated for source informa­ tion. Chloride/bromide mass ratios of source materials were evaluated for source information. Characteristics of waterextractable nitrate in soil also were evaluated for source information. Nitrogen Isotopic Composition Nitrate-nitrogen isotopic values for water samples from wells D6, D10, and D32 (fig. 4, table 5) were not similar to the isotopic values commonly observed for nitrate derived from inorganic fertilizers, atmospheric deposition, bedrock, or soil (fig. 4, table 4). Nitrate-nitrogen isotopic values for the groundwater samples were similar to the isotopic values commonly observed for nitrate derived from organic waste materials, such as animal manure and sewage (including bio­ solids) (fig. 4, table 4). Among the source materials considered in this study, however, weathered biosolids had the closest nitrogen isotopic values to those for nitrate in water from the 1:1 slope 1:2 slope D5 D6 D10 D32 Range of fractionation trends reported for denitrification δ15N[NO3], in per mil relative to atmospheric N2 (AIR) δ18O[NO3], in per mil relative to Vienna Standard Mean Ocean Water (VSMOW) EXPLANATION δ15N[NO3] Isotopic composition of

nitrate-nitrogen δ18O[NO3] Isotopic composition of

nitrate-oxygen Figure 7.  Measured values of δ15N[NO3] and δ18O[NO3] in water from the monitoring wells.

22    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado wells (fig. 4). The isotopic values for total nitrogen in the three biosolids samples analyzed for this study were somewhat smaller than the isotopic values for nitrate in the water from wells D6, D10, and D32. However, a nitrogen isotopic value of +9.6 per mil was reported for biosolids from Oregon (Buck, 2003), which is similar to the values for nitrate in water from the selected Deer Trail area wells. The weathered biosolids had larger nitrogen isotopic values than the fresh biosolids (fig. 4), which probably reflects the preferential loss of 14N during volatilization of ammonia, or other volatile nitrogen species, from the biosolids as they dried and weathered. As discussed previously, a substantial amount of nitrogen was lost from fresh biosolids when they were dried in the laboratory (tables 1 and 2). A similar loss of nitrogen can be observed in the nitrogen data from fresh and field-weathered biosolids (table 1). Thus, one possible expla­ nation for the difference in isotopic values between nitrate in water samples from the wells and total nitrogen in biosolids, assuming biosolids were the source of that nitrate, could be that nitrate in water from the wells was derived from biosolids that were more highly weathered than those measured in this study. The isotopic values for total nitrogen in samples of ani­ mal manure, shale, and soil from this study were smaller than the isotopic values for total nitrogen in biosolids and for nitrate in water from the wells (fig. 4). Typically, the pref­ erential loss of 14N from animal manure as it weathers results in larger isotopic values for the nitrogen that still remains in the animal manure, as was observed with the biosolids samples, but the weathered animal-manure sample from this study actually had a smaller nitrogen isotopic value than the fresh animal-manure sample. The reason for this is unclear, but it could simply indicate that the fresh animal manure from which the weathered sample was derived had a smaller initial nitrogen isotopic value to begin with than the fresh animalmanure sample collected for this study. Nonetheless, nitrate derived from animal manure commonly has nitrogen isotopic values in the range of the values for nitrate in water samples from the wells, or higher (Gormly and Spalding, 1979). In contrast, nitrate derived from shale and soil commonly has nitrogen isotopic values smaller than what was measured in nitrate in water samples from the wells (McMahon and others, 1999; Kendall and Doctor, 2004; McMahon and Böhlke, 2006). The isotopic data indicate that, of the source materials considered, biosolids and (or) animal manure were the most likely sources of nitrate in the wells at the time of sampling (2005) and that inorganic fertilizer, atmospheric deposition, and geologic materials were not substantial sources of nitrate in the wells in 2005. The large total nitrogen content of the biosolids and animal-manure samples (table 1) and biosolids leachates (table 2) also indicates that the biosolids and animal manure had potential to leach nitrogen and produce large dis­ solved nitrate concentrations in groundwater. The available data, however, cannot distinguish between biosolids or manure as the dominant source of nitrate in water from well D6 because the nitrogen isotopic composition of the two materi­ als is similar. Additional isotopic measurements of biosolids and groundwater nitrate could indicate whether those values change with time. Major-Ion Concentrations Major-ion concentrations sometimes are helpful in dif­ ferentiating sources. Concentrations of several major ions in water from well D6 increased with time along with the nitrate concentrations. Concentrations of sodium, potassium, calcium, magnesium, sulfate, and bromide exhibited significant positive correlations with time. The correlations of sodium concentra­ tions with time and sulfate concentrations with time were the strongest among the time correlations examined here (fig. 8). In figure 8, p values less than 0.05 indicate statistically sig­ nificant correlations at the 95-percent confidence level, and the correlation coefficient (r2; 0 to 1 for positive correlations) indicates the strength of the correlation (Helsel and Hirsch, 1992). Increases in concentrations of major ions in shallow groundwater under cropland have been reported in other stud­ ies (Böhlke, 2002; McMahon and others, 2007) and have been attributed to a combination of processes. Soil amendments or fertilizers such as calcium sulfate (gypsum), calcium magne­ sium carbonate (dolomite), and potassium chloride are applied in some cropland settings to improve soil properties, and dissolution of those chemicals in the soil zone can increase concentrations of major ions in recharge. It is unlikely that gypsum and dolomite were used at the Deer Trail site because the soils already contain those minerals (table 3). As discussed previously, it appears that potassium chloride also was not used to any substantial degree at the site. In fact, chloride con­ centrations in well D6 appear to have decreased slightly over time (fig. 8). Nitrogen in fresh biosolids and manure occurs primarily in reduced forms (table 1), but upon exposure to air those reduced forms of nitrogen can be oxidized to nitrate, thereby producing protons (acidity) that may react with and dissolve minerals in the soil:

Nh4 + + 2O2 → No3 - + 2H+ + H2O (3)

CaMg(CO3)2 + 2H+ → Ca2+ + Mg2+ + 2HCO3 - (4) The positive correlations of nitrate concentrations with time and major-ion concentrations with time for samples from well D6 are consistent with the interpretation based on isotope data that nitrate in water from well D6 was derived from biosolids, animal manure, or a combination of the two source materials. The major-ion data, however, could not be used to distinguish between biosolids or manure as the dominant source of nitrate in water from well D6 because the major-ion composition (as well as the isotopic composition) of the two materials is similar.

Preliminary Assessment of Sources of Nitrogen in the Groundwater    23 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 Concentration of dissolved nitrite plus nitrate, in milligrams per liter as nitrogen p 0.02 r2 -0.14 Concentration of dissolved chloride, in milligrams per liter p 0.03 r2 0.12 Concentration of dissolved calcium, in milligrams per liter 2,000 2,200 2,400 2,600 2,800 r2 0.49 Concentration of dissolved magnesium, in milligrams per liter 1,800 2,000 2,200 2,400 r2 0.74 Concentration of dissolved sodium, in milligrams per liter 12,500 13,000 13,500 14,000 14,500 r2 0.58 Concentration of dissolved sulfate, in milligrams per liter r2 0.20 Concentration of dissolved bromide, in milligrams per liter Date r2 0.46 Concentration of dissolved potassium, in milligrams per liter Date 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 Nitrate Potassium Calcium Magnesium Sodium Sulfate Bromide Chloride Figure 8.  Concentrations of nitrate and selected major ions in water from well D6 with solid lines for linear regressions (regression statistics are shown in each plot).

24    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Chloride/Bromide Mass Ratios Chloride/bromide mass ratios can be used to assess sources of chloride and groundwater contamination (Davis and others, 1998). Chloride/bromide mass ratios in water from the wells near Deer Trail were substantially smaller than the ratios measured in leachate from fresh biosolids and in some domestic sewage effluent and animal manure (Davis and others, 1998; Panno and others, 2006; Mullaney and oth­ ers, 2009) but were similar to chloride/bromide mass ratios reported by Hudak (2003) and Katz and others (2011) for some animal wastes and atmospheric deposition (fig. 9). These data imply that atmospheric deposition and animal manure may be sources of the nitrate in groundwater from well D6 and that biosolids or other sewage materials cannot have been an important nitrate source. However, a large range of chloride/ bromide mass ratios are reported in the literature for animal wastes (Katz and others, 2011), and chloride/bromide ratio data for animal waste at the Deer Trail site are not available, so chloride/bromide mass ratios are not definitive in differ­ entiating animal-waste sources of chloride or nitrate for this site. It is also possible that the large baseline concentrations of chloride and bromide in the groundwater from this site (fig. 8) overwhelmed any contribution of those anions from biosolids or other sewage, thus reducing the sensitivity of the ratio method for detecting contributions from those sources at this site. It is unknown whether the chloride/bromide ratio of biosolids changes as biosolids weather in the field. This is important to know because biosolids were not applied on a regular basis in the vicinity of well D6, which means that the chloride/bromide ratio of weathered biosolids could be more relevant than the ratio for fresh biosolids in characterizing biosolids-affected recharge. A reduction in chloride/bromide ratios in biosolids during weathering would bring these ratios in closer agreement with the ratios measured in water from the wells. 1/1/1993 1/1/1996 1/1/1999 1/1/2002 1/1/2005 1/1/2008 1/1/2011 1,000 1,200 EXPLANATION Well D6 Well D5 Well D10 Well D32 Chloride/bromide mass ratio Date Leachate from fresh and weathered biosolids, Metro Wastewater Reclamation District treatment plant (data from table 2) 1,400 1,600 Hog and horse waste (Panno and others, 2006) Atmospheric deposition for inland United States (Katz and others, 2011) Domestic sewage (Davis and others, 1998) Leachates from cow and horse manure (Hudak, 2003) Sewage and animal waste (Mullaney and others, 2009) Figure 9.  Chloride/bromide mass ratios in water from selected monitoring wells near Deer Trail as a function of time and the ratios measured in domestic sewage, leachate from biosolids, animal waste, and atmospheric deposition.

Summary    25 A reduction in chloride/bromide ratios of biosolids dur­ ing weathering would mean that chloride is preferentially lost during that process. There is some evidence from the literature that chloride is preferentially lost relative to bromide from organic matter as weathering proceeds. Shotyk (1997) reported that the percentage of bromide retained in Scottish peat bogs was 50 to 60 times greater than that of chloride. Similarly, Biester and others (2004) reported that 7.5% to 50% of the bromine in Chilean peat bogs was retained compared with 0.2% to 2% of the chlorine. The organic matter in biosolids may be similar to that of peat bogs with respect to retention of bromide. Although the chloride/bromide ratio of weathered animal manure in the study area is not known, it is possible that the organic matter in animal manure could retain more bromide than chloride as was the case for the peat bogs. Thus, without additional data, the chloride/bromide mass ratios do not necessarily support or refute the hypothesis that biosolids and (or) animal manure were the primary sources of nitrate in water from the study-area wells in 2005. Water-Extractable Nitrate in Soil Characteristics of water-extractable nitrate in soil can provide information about likely nitrogen sources. Concen­ trations of water-extractable nitrate in the soil indicate that biosolids could be an important source of nitrate in recharge at well D6. As discussed previously, the soil samples with the largest concentrations of water-extractable nitrate were from the two sites where biosolids had been applied (D6-1 and DC-300) (fig. 6). Site D6-2 was not included in this group, even though biosolids were applied there, because it appears that very little water movement through the soil zone occurred at that site (fig. 6). Sites D6-1 and DC-300 also were culti­ vated and may have been grazed at times, but they still had much larger soil nitrate inventories than the two sites that were only cultivated (South-1) or grazed (South-2) (fig. 6). Values of δ15N[NO3] for the largest concentrations of water-extract­ able nitrate at sites D6-1 and DC-300 were +6.8 and +8.2 per mil, respectively (fig. 6), which are similar to the values for fresh and weathered biosolids and approach the values for nitrate in water from wells D6, D10, and D32 (fig. 4). The concentrations of water-extractable nitrate were used to evalu­ ate nitrogen inventories in the soil (fig. 6), which can provide additional information about likely nitrogen sources. The nitrogen inventories in the soil beneath biosolids-application areas (fig. 6) and the nitrogen-input estimates for the study area (table 4) both support the comparisons of isotopic compo­ sition (fig. 4), which indicate that some type of human waste (such as biosolids) and (or) animal manure was the source of nitrate in groundwater sampled from well D6 in 2005. Mea­ surements of water-extractable nitrate in soil profiles at more sites would help to determine how representative the data in figure 6 are with respect to the larger study area. Of the various methods used to evaluate nitrate sources in the study area, isotopic composition is better established and better constrained by end-member data than other methods like chloride/bromide mass ratios. Therefore, isotopic compo­ sition (which indicates that inorganic fertilizer, atmospheric deposition, and geologic materials were not the source of nitrate at well D6) was given more weight than chloride/bro­ mide mass ratios (which indicate that atmospheric deposition or animal manure was the source of nitrate at well D6). The nitrogen-load estimates considered with the nitrogen isotopic data and the soil-nitrogen inventories indicate that biosolids applications likely are a major source of nitrogen to the shal­ low groundwater in the vicinity of well D6 (table 4). The iso­ topic and other data collected for this study appear to be useful for identifying the likely sources of nitrate in groundwater from well D6 (biosolids and (or) animal manure), but the data only represent a single point in time relative to the 15-year apparent trend of increasing nitrate concentrations (fig. 3). To reduce uncertainty in identifying the dominant source of nitrate in groundwater from well D6, additional characteriza­ tion of the nitrogen isotopic composition and chloride/bromide ratios of fresh and weathered biosolids and manure, as well as additional soil-nitrogen inventories, would be useful. Summary Biosolids from municipal wastewater treatment in Denver, Colo., have been applied intermittently as a fertilizer and soil amendment to some nonirrigated farmland near Deer Trail, Colo., since 1993. Biosolids applications to the area have been permitted by the State of Colorado and documented by the wastewater treatment plant. Concentrations of dissolved nitrate have increased over time in some of the shallow monitoring wells at the biosolidsapplication area. The increase in nitrate concentration in samples from one well in particular, well D6, was of interest to the Metro Wastewater Reclamation District (Metro District) because (1) the well was located near a property boundary, (2) measured nitrate concentrations were largest at this loca­ tion, and (3) nitrate concentration increased from about 5 mil­ ligrams per liter as nitrogen (mg-N/L) in 1993 before biosolids were applied to about 40 mg-N/L in 2007. Nitrate concentra­ tions also increased in water from three other shallow monitor­ ing wells in the drainage area upgradient from D6, although to a lesser extent. The gradual increase in nitrate concentration in water from well D6 during a period of intermittent biosolids applications led to concerns that biosolids were causing the nitrate increase. However, nitrate concentrations were at least 5 mg-N/L in some of these monitoring wells before biosolids applications to the area began, which indicates that biosolids applications were not the only source of nitrogen to ground­ water in the area. Biosolids, animal manure, inorganic fertilizer, atmo­ spheric deposition, and geologic materials (bedrock and soil) are possible nitrogen sources in the area. Because of the multiple possible sources of nitrogen at the Metro District

26    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado properties, the USGS began a preliminary assessment of sources of nitrogen in shallow groundwater at part of the biosolids-application area in 2005 in cooperation with the Metro District. The Metro District properties near Deer Trail and the surrounding land constitute the study area. Biosolids from the Metro District plant in Denver and biosolids, cow manure, geologic materials (bedrock and soil), and ground­ water from the study area were sampled to measure nitrogen content and nitrogen isotopic compositions of nitrate or total nitrogen. Biosolids also were leached, and the leachates were analyzed for nitrogen content and other concentrations. Geologic materials from the study area also were sampled to determine mineralogy. Estimates of nitrogen contributed from inorganic fertilizer and atmospheric deposition were calculated from other published reports. The geology of the study area consists of interbedded shale, siltstone, and sandstone, which is overlain in some areas by clay, windblown silt and sand, or alluvial sand and gravel. The primary water-supply aquifer is the Laramie-Fox Hills aquifer, and multiple alluvial aquifers are associated with the surficial drainage network in the study area. Land use in the study area was rangeland or cropland and pasture. Cattle and sheep were the primary domesticated animals grazing the area, and wildlife also were present. No concentrated animal-feeding operations were located on or near the Metro District properties during 1993 through 2005; no evidence of large confined or unconfined animal feeding or dairy operations has been produced for the period before 1993. Wheat was the primary crop. Inorganic fertilizer has not been used in the area since 1993; no documentation of type or quan­ tity of inorganic fertilizer use in the area has been produced for the period before 1993. Biosolids were applied intermittently in the surficial drainage area that contained well D6, but this drainage area is not known to contain any current or former pit toilets or septic tanks. Fresh biosolids from the Metro District contained as much as 71,000 milligrams per kilogram as nitrogen (mg-N/kg), primarily in the reduced forms, organic nitrogen and ammonia or ammonium. A fresh biosolids sample from the Metro District had a higher nitrogen content and a lower nitrogen isotopic composition than the corresponding weath­ ered biosolids sample from this study area. The leachate data indicate that nitrogen was lost from the fresh biosolids during the drying process, which could reflect the loss of volatile forms of reduced nitrogen such as ammonia. Manure freshly deposited on the land surface can contain less nitrogen (less than 2,000 mg-N/kg) than fresh biosolids, however manure can be concentrated in a single location whereas a small volume of biosolids is distributed over a large area. The fresh cow-manure sample from the study area had a higher nitrogen content and a higher nitrogen isotopic compo­ sition than the corresponding weathered cow-manure sample from this study area. Inorganic fertilizers can contain as much as 800,000 mg-N/kg. No inorganic fertilizers reportedly were used at the Metro District properties near Deer Trail during 1993 through 2005, but this type of fertilizer could have been used on the properties before the Metro District acquired the properties. The nitrogen isotopic composition of inorganic fertilizers generally is characterized by low isotopic values. Atmospheric deposition in eastern Colorado can contain about 0.7 mg-N/L in precipitation, but the nitrogen content of atmospheric deposition varies. The nitrogen isotopic composi­ tion of atmospheric deposition generally is characterized by low isotopic values. Geologic materials (both bedrock and soil) of the study area contained nitrogen. Nitrogen content was largest in shale samples (as much as 2,344 mg-N/kg). Soil that received bio­ solids applications contained as much as 1,700 mg-N/kg nitro­ gen. Soil that did not receive biosolids applications contained as much as 1,100 mg-N/kg nitrogen. The nitrogen isotopic composition of the geologic materials from the study area was lower than that measured for biosolids or animal manure. The nitrogen information from the study indicates that each of the sources contain sufficient nitrogen to potentially affect groundwater nitrate concentrations. Natural processes can transform the nitrogen in any of the sources to nitrate in the groundwater. Nitrogen information for each source was used to estimate nitrogen loads to the surficial drainage area contributing to the well D6 location. Estimates calculated from Metro District application records indicate that the load of biosolids-applied nitrogen did not exceed 72,440 kilograms as nitrogen (kg-N) for the period 1993 through 2005. Calculated estimates indicate that the nitrogen load from animal manure to the drainage area contributing to the D6 location likely did not exceed 17,000 kg-N for the period 1993 through 2005. The load of inorganic-fertilizer nitrogen to the drainage area contributing to the D6 location is estimated as 0 kg-N for the period 1993 through 2005. Calculated estimates indicate that the load of atmospheric-deposition nitrogen likely did not exceed 11,000 kg-N for the period 1993 through 2005. Calcu­ lated estimates indicate that the nitrogen load from all geo­ logic materials likely did not exceed 0.21 kg-N for the period 1993 through 2005, which is much less than the estimated nitrogen load from biosolids, animal manure, and atmospheric deposition for the same period. The load estimates indicate that animal manure, inorganic fertilizer, or atmospheric depo­ sition could have contributed the largest nitrogen load to the study area in the 13 years before biosolids applications began, but biosolids likely contributed the largest nitrogen load to the study area in the 13 years after biosolids applications began. Concentrations of dissolved nitrite plus nitrate increased fairly steadily in samples from well D6 after biosolids applica­ tions began. Concentrations of dissolved nitrite plus nitrate also increased at three other shallow monitoring wells in the vicinity. Water-quality data indicate that (1) groundwater in the study area had a nitrogen source before biosolids applications started, and (2) concentration patterns for dissolved nitrite plus nitrate in groundwater were diverse and highly variable throughout the study area during 1993-2005. Various approaches provided insights into sources of nitrate in the groundwater samples from 2005. The isotopic

References Cited    27 data indicate that, of the source materials considered, biosolids and animal manure were the most likely sources of nitrate in the wells at the time of sampling (2005) and that inorganic fertilizer, atmospheric deposition, and geologic materials were not substantial sources of nitrate in the wells in 2005. The large total nitrogen content of the biosolids and animal-manure samples and biosolids leachates also indicates that the bio­ solids and animal manure had potential to leach nitrogen and produce large dissolved nitrate concentrations in groundwater. The available data, however, could not be used to distinguish between biosolids or manure as the dominant source of nitrate in water from well D6 because the nitrogen isotopic composi­ tion of the two materials is similar. Major-ion data also could not be used to distinguish between biosolids or manure as the dominant source of nitrate in water from well D6 because the major-ion composition (as well as the isotopic composition) of the two materials is similar. Without additional data, chloride/ bromide mass ratios do not necessarily support or refute the hypothesis that biosolids and (or) animal manure were the primary sources of nitrate in water from the study-area wells in 2005. Concentrations of water-extractable nitrate in the soil indicate that biosolids could be an important source of nitrate in recharge at well D6. Nitrogen inventories in the soil beneath biosolids-application areas and the nitrogen-input estimates for the study area both support the comparisons of isotopic composition, which indicate that some type of human waste (such as biosolids) and (or) animal manure was the source of nitrate in groundwater sampled from well D6 in 2005. The nitrogen-load estimates considered with the nitrogen isotopic data and the soil-nitrogen inventories indicate that biosolids applications likely are a major source of nitrogen to the shal­ low groundwater at these monitoring wells. To reduce uncer­ tainty in identifying the dominant source of nitrate in ground­ water from well D6, additional characterization of the nitrogen isotopic composition and chloride/bromide ratios of fresh and weathered biosolids and manure, as well as additional soilnitrogen inventories, would be useful. Acknowledgments The authors thank the various property owners for access to collect samples, JoAnn M. Holloway for analyses of geologic materials for total nitrogen using the pyrolysis method, and Philip L. Hageman for assistance in preparation of leachates. References Cited Barbarick, K.A., and Ippolito, J.A., 2000, Nitrogen fertilizer equivalency of sewage biosolids applied to drlyand winter wheat: Journal of Environmental Quality, v. 29, p. 1345- Biester, H., Keppler, F., Putschew, A., Martinez-Cortizas, A., and Petri, M., 2004, Halogen retention, organohalogens, and the role of organic matter decomposition on halogen enrich­ ment in two Chilean peat bogs: Environmental Science and Technology, v. 38, p. 1984-1991. Billy, Claire; Billen, Gilles; Sebilo, Mathieu; Birgand, Francois; and Tournebize, Julien, 2010, Nitrogen isotopic composition of leached nitrate and soil organic matter as an indicator of denitrification in a sloping drained agricultural plot and adjacent uncultivated riparian buffer strips: Soil Biology and Biochemistry, v. 42, p. 108-117. Böhlke, J.K., 2002, Groundwater recharge and agricultural contamination: Hydrogeology Journal, v. 10, p. 153-179 [Erratum, 2002, Hydrogeology Journal, v. 10, p. 438-439]. Böhlke, J.K., and Denver, J.M., 1995, Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic Coastal Plain, Maryland: Water Resources Research, v. 31, p. 2319-2339. Böhlke, J.K., Mroczkowski, S.J., and Coplen, T.B., 2003, Oxygen isotopes in nitrate—New reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibration: Rapid Communications in Mass Spectrom­ etry, v. 17, p. 1835-1846. Böhlke, J.K., Wanty, R., Tuttle, M., Delin, G., and Landon, M., 2002, Denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer, Minnesota: Water Resources Research, v. 38, no. 7, p. 1105-1131. Böttcher, J., Strebel, O., Voerkelius, S., and Schmidt, H.-L., 1990, Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer: Journal of Hydrology, v. 114, p. 413-424. Buck, Jeremy, 2003, A preliminary investigation of nutrients and isotopic nitrogen in Oregon Chub habitat adjacent to Oakridge sewage treatment plant: Portland, Oreg., U.S. Fish and Wildlife Service, accessed October 13, 2011, at http://www.fws.gov/oregonfwo/Contaminants/FieldStudies/ OregonChub/SewageSludgeAndOregonChubReport.pdf. Busenberg, Eurybiades, Weeks, E.P., Plummer, L.N., and Bartholomy, R.C., 1993, Age dating ground water by use of chlorofluorocarbons (CCl3F and CCl2F2) and distribu­ tion of chlorofluorocarbons in the unsaturated zone, Snake River Plain aquifer, Idaho National Engineering Labora­ tory, Idaho, USA: U.S. Geological Survey Water-Resources Investigations Report 93-4054, 47 p. Casciotti, K.L., Sigman, D.M., Galanter Hastings, M., Böhlke, J.K., and Hilkert, A., 2002, Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method: Analytical Chemistry, v. 74, p. 4905-4912.

28    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado Colorado Department of Public Health and Environment, 2003, Biosolids regulation: Colorado Department of Public Health and Environment, 5CCR 1002-64 [variously paged]. [Revised on April 14, 2003, and subsequently.] Crock, J.G., Smith, D.B., Yager, T.J.B., Brown, Z.A., and Adams, M.G., 2008, Analytical results for municipal biosolids samples from a monitoring program near Deer Trail, Colorado (USA), 1999 through 2006: U.S. Geological Survey Open-File Report 2008-1172, 67 p. (Also available at http://pubs.usgs.gov/of/2008/1172/.) Cullity, B.D., and Stock, S.R., 2001, Elements of X-ray dif­ fraction (3d ed.): Reading, Mass., Addison-Wesley, 664 p. Davis, S.N., Whittemore, D.O., and Fabryka-Martin, June, 1998, Uses of chloride/bromide ratios in studies of potable water: Ground Water, v. 36, no. 2, p. 338-350. Drew, L.J., Schuenemeyer, J.H., and Bawiec, W.J., 1979, Petroleum exhaustion maps of the Cretaceous "D-J" sandstone stratigraphic interval of the Denver Basin: U.S. Geological Survey Miscellaneous Investigations Series Map I-1138, scale 1:200,000, sheet 2. Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory— Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey OpenFile Report 93-125, 217 p. Freeze, R.A., and Cherry, J.A., 1979, Groundwater: New Jer­ sey, Prentice-Hall Inc., 604 p. Gilmour, J.T., Cogger, C.G., Jacobs, L.W., Evanylo, G.K., and Sullivan, D.M., 2003, Decomposition and plant-available nitrogen in biosolids—Laboratory studies, field studies, and computer simulation: Journal of Environmental Quality, v. 32, p. 1498-1507. Gormly, J.R., and Spalding, R.F., 1979, Sources and concen­ trations of nitrate-nitrogen in ground water of the central Platte region, Nebraska: Ground Water, v. 17, 291-301. Granger, Julie, Sigman, D.M., Needoba, J.A., and Harrison, P.J., 2004, Coupled nitrogen and oxygen isotope fraction­ ation of nitrate during assimilation by cultures of marine phytoplankton: Limnology and Oceanography, v. 49, p. 1763-1773. Hageman, P.L., 2007, U.S. Geological Survey field leach test for assessing water reactivity and leaching potential of mine-wastes, soils, and other geologic and environmental materials: U.S. Geological Survey Techniques and Methods, book 5, chap. D3, 14 p. (Also available at http://pubs.usgs. gov/tm/2007/05D03/.) Hartz, T.K., Mitchell, J.P., and Giannini, C., 2000, Nitrogen and carbon mineralization dynamics of manures and com­ posts: HortScience, v. 35, p. 209-212. Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in water resources: New York, Elsevier, 522 p. Hem, J.D., 1992, Study and interpretation of the chemical characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p. Holloway, J.M., and Dahlgren, R.A., 2002, Nitrogen in rock— Occurrences and biogeochemical implications: Global Biogeochemical Cycles, v. 16, no. 4, p. 1118-1135. Holloway, J.M., Dahlgren, R.A., and Casey, W.H., 2001, Nitrogen release from rock and soil under simulated field conditions: Chemical Geology, v. 174, p. 403-414. Hudak, P.F., 2003, Chloride/bromide ratios in leachate derived from farm-animal waste: Environmental Pollution, v. 121, p. 23-25. Kadlec, R.H., and Wallace, Scott, 2009, Treatment wetlands (2d ed.): Boca Raton, Fla., CRC Press, 1016 p. Katz, B.G., Eberts, S.M., and Kauffman, L.J., 2011, Using Cl/Br ratios and other indicators to assess potential impacts on groundwater quality from septic systems—A review and examples from principal aquifers in the United States: Journal of Hydrology, v. 397, p. 151-166. Kendall, Carol, and Doctor, D.H., 2004, Stable isotope appli­ cations in hydrologic studies, in Drever, J.I., ed., Surface and ground water, weathering, and soils: Treatise on Geo­ chemistry, v. 5, p. 319-364. Kirkham, R.M., and Ladwig, L.R., 1979, Coal resources of the Denver and Cheyenne Basins, Colorado: Colorado Geologi­ cal Survey Resource Series 5, 49 p. Kreitler, C.W., 1979, Nitrogen-isotope ratio studies of soils and groundwater nitrate from alluvial fan aquifers in Texas: Journal of Hydrology, v. 42, p. 147-170. Kreitler, C.W., and Jones, D.C., 1975, Natural soil nitrate— The cause of the nitrate contamination of ground water in Runnels County, Texas: Ground Water, v. 13, p. 53-61. Larsen, L.S., Baber, T.G., Wesswick, E.L., McCoy, D.E., and Harman, J.B., 1966, Soil survey of Elbert County, Colorado: U.S. Department of Agriculture Soil Conservation Service Soil Survey series, 79 p. Larsen, L.S., and Brown, J.B., 1971, Soil survey of Arapahoe County, Colorado: U.S. Department of Agriculture Soil Conservation Service Soil Survey series, 78 p. Lysyk, T.J., Easton, E.R., and Evenson, P.D., 1985, Seasonal changes in nitrogen and moisture content of cattle manure in cool-season pastures: Journal of Range Management, v. 38, p. 251-254. Major, T.J., Robson, S.G., Romero, J.C., and Zawistowski, Stanley, 1983, Hydrogeologic data from parts of the Denver Basin, Colorado: U.S. Geological Survey Open-File Report 83-274, 425 p.

References Cited    29 McGinn, S.M., Janzen, H.H., and Coates, T., 2003, Atmo­ spheric ammonia, volatile fatty acids, and other odorants near beef feedlots: Journal of Environmental Quality, v. 32, p. 1173-1182. McMahon, P.B., and Böhlke, J.K., 2006, Regional patterns in the isotopic composition of natural and anthropogenic nitrate in groundwater, High Plains, USA: Environmental Science and Technology, v. 40, p. 2965-2970. McMahon, P.B., Böhlke, J.K., and Bruce, B.W., 1999, Deni­ trification in marine shales in northeastern Colorado, USA: Water Resources Research, v. 35, p. 1629-1642. McMahon, P.B., Böhlke, J.K., and Christenson, S.C., 2004, Geochemistry, radiocarbon ages, and paleorecharge condi­ tions along a transect in the central High Plains aquifer, southwestern Kansas, USA: Applied Geochemistry, v. 19, p. 1655-1686. McMahon, P.B., Dennehy, K.F., Bruce, B.W., Böhlke, J.K., Michel, R.L., Gurdak, J.J., and Hurlbut, D.B., 2006, Stor­ age and transit time of chemicals in thick unsaturated zones under rangeland and irrigated cropland, High Plains, United States: Water Resources Research, v. 42, W03413, doi:10.1029/2005WR004417. McMahon, P.B., Dennehy, K.F., Bruce, B.W., Gurdak, J.J., and Qi, S.L., 2007, Water quality assessment of the High Plains aquifer, 1999-2004: U.S. Geological Survey Professional Paper 1749, 136 p. McMahon, P.B., Dennehy, K.F., Michel, R.L., Sophocleous, M.A., Ellett, K.M., and Hurlbut, D.B., 2003, Water move­ ment through thick unsaturated zones overlying the central High Plains aquifer, southwestern Kansas, 2000-2001: U.S. Geological Survey Water-Resources Investigations Report 03-4171, 30 p. Mullaney, J.R., Lorenz, D.L., Arntson, A.D., 2009, Chloride in groundwater and surface water in areas underlain by the glacial aquifer system, northern United States: U.S. Geo­ logical Survey Scientific Investigations Report 2009-5086, 41 p. Nanus, Leora, Williams, M.W., Campbell, D.H., Elliott, E.M., and Kendall, Carol, 2008, Evaluating regional patterns of nitrate sources to watersheds in national parks of the Rocky Mountains using nitrate isotopes: Environmental Science and Technology, v. 42, no. 17, p. 6487-6493. Occhipinti, Chris, Aneja, V.P., Showers, William, and Niyogi, Dev, 2008, Back-trajectory analysis and source-receptor relationships—Particulate matter and nitrogen isotopic com­ position in rainwater: Journal of the Air and Waste Manage­ ment Association, v. 58, p. 1215-1222. Odum, E.P., 1989, Ecology and our endangered life-support systems: Sunderland, Mass., Sinauer Associates, Inc. Pub­ lishers, 283 p. Panno, S.V., Hackley, K.C., Hwang, H.H., Greenberg, S.E., Krapac, I.G., Landsberger, S., and O'Kelly, D.J., 2006, Characterization and identification of Na-Cl sources in ground water: Ground Water, v. 44, no. 2, p. 176-187. Révész, K.M., and Qi, Haiping, 2006, Determination of the δ(15N/14N) of total nitrogen in solids—RSIL Lab Code 2893: U.S. Geological Survey Techniques and Methods, book 10, chap. C11, 31 p. Robinson, M.B., and Polglase, P.J., 2000, Volatilization of nitrogen from dewatered biosolids: Journal of Environmen­ tal Quality, v. 29, p. 1351-1355. Robson, S.G., and Banta, E.R., 1995, Groundwater atlas of the United States—Arizona, Colorado, New Mexico, Utah: U.S. Geological Survey Hydrologic Investigations Atlas 730-C, p. C20-C22. Robson, S.G., Wacinski, Andrew, Zawistowski, Stanley, and Romero, J.C., 1981, Geologic structure, hydrology, and water quality of the Laramie-Fox Hills aquifer in the Den­ ver Basin, Colorado: U.S. Geological Survey Hydrologic Investigations Atlas 650, scale 1:500,000, sheet 1. Ruddy, B.C., Lorenz, D.L., and Mueller, D.K., 2006, Countylevel estimates of nutrient inputs to the land surface of the conterminous United States, 1982-2001: U.S. Geological Survey Scientific Investigations Report 2006-5012, 17 p. (Also available at http://pubs.usgs.gov/sir/2006/5012/pdf/ sir2006_5012.pdf.) Russell-Hunter, W.D., 1970, Aquatic productivity: New York, The Macmillan Company, 306 p. Scanlon, B.R., and Goldsmith, R.S., 1997, Field study of spa­ tial variability in unsaturated flow beneath and adjacent to playas: Water Resources Research, v. 33, p. 2239-2252. Schweinfurth, S.P., 2003, Coal—A complex natural resource: U.S. Geological Survey Circular 1143, 37 p. Sharps, J.A., 1980, Geologic map of the Limon 1 × 2 quadran­ gle, Colorado and Kansas: U.S. Geological Survey Miscel­ laneous Investigations Series Map I-1250, scale 1:250,000, sheet 1. Shotyk, William, 1997, Atmospheric deposition and mass bal­ ance of major and tracer elements in two oceanic peat bog profiles, northern Scotland and the Shetland Islands: Chemi­ cal Geology, v. 138, p. 55-72. Stevens, M.R., Yager, T.J.B., Smith, D.B., and Crock, J.G., 2003, Biosolids, soils, ground-water, and streambedsediment data for a biosolids-application area near Deer Trail, Colorado, 1999: U.S. Geological Survey Open-File Report 02-51, 118 p. Sullivan, D.M., 2008, Estimating plant-available nitrogen from manure: Oregon State University Extension publica­ tion EM 8954-E, 21 p., accessed October 13, 2011, at http:// extension.oregonstate.edu/catalog/pdf/em/em8954-e.pdf.

30    Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado University of Minnesota, 2002, Soil scientist—What is manure: University of Minnesota Extension publication WW-07401 [unpaged], accessed October 13, 2011, at http://www.extension.umn.edu/distribution/cropsystems/ components/7401_02.html. U.S. Environmental Protection Agency, 1993 [and subse­ quent revisions], Methods for the determination of inor­ ganic substances in environmental samples: U.S. Environ­ mental Protection Agency EPA-600/R-93-100 [variously paged]. U.S. Geological Survey, 1980, Land use and land cover, 1975, Limon, Colorado; Kansas: U.S. Geological Survey Land Use Series L-191, scale 1:250,000, 1 sheet. Vitoria, Laura; Otero, Neus; Soler, Albert; and Canals, Angels, 2004, Fertilizer characterization—Isotopic data (N, S, O, C, and Sr): Environmental Science and Technology, v. 38, p. 3254-3262. Vogel, J.C., Talma, A.S., and Heaton, T.H.E., 1981, Gaseous nitrogen as evidence for denitrification in groundwater: Journal of Hydrology, v. 50, p. 191-200. Walvoord, M.A., Phillips, F.M., Stonestrom, D.A., Evans, R.D., Hartsough, P.C., Newman, B.D., and Striegl, R.G., 2003, A reservoir of nitrate beneath desert soils: Science, v. 302, p. 1021-1024. Yager, T.J.B., and Arnold, L.R., 2003, Hydrogeology of a bio­ solids-application site near Deer Trail, Colorado, 1993-99: U.S. Geological Survey Water-Resources Investigations Report 03-4209, 90 p. Yager, T.J.B., Smith, D.B., and Crock, J.G., 2004, Effects of surface applications of biosolids on soil, crops, ground water, and streambed sediment near Deer Trail, Colorado, 1999-2003: U.S. Geological Survey Scientific Investiga­ tions Report 2004-5289, 93 p. Yager, T.J.B., Smith, D.B., and Crock, J.G., 2009, Biosolids, crop, and ground-water data for a biosolids-application area near Deer Trail, Colorado, 2004-2006: Denver, U.S. Geo­ logical Survey Data Series 379, 57 p. Yager, T.J.B., Smith, D.B., and Crock, J.G., 2011, Biosolids, crop, and groundwater data for a biosolids-application area near Deer Trail, Colorado, 2007 and 2008: U.S. Geological Survey Data Series 589, 53 p. Young, R.A., ed., 1995, The Rietveld method: Oxford Univer­ sity Press, 298 p. Zublena, J.P., Baird, J.V., and Lilly, J.P., 1991, Nutrient content of fertilizer and organic materials: North Caro­ lina Cooperative Extension Service Soil Facts Publication AG-439-18, 11 p., accessed March 22, 2011, at http://www. soil.ncsu.edu/publications/Soilfacts/AG-439-18/. (Revised 1997.) Publishing support provided by: Denver and Lafayette Publishing Service Centers For more information concerning this publication, contact: Director, USGS Colorado Water Science Center Box 25046, Mail Stop 415 Denver, CO 80225 (303) 236-4882 Or visit the Colorado Water Science Center Web site at: http://co.water.usgs.gov/

Yager and McMahon—Preliminary Assessment of Sources of Nitrogen in Groundwater at a Biosolids-Application Area near Deer Trail, Colorado—SIR 2012-5056