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

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Denver, CO 80225

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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.
iii

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
iv

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) 0.3937 inch (in)

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft)

micron (μm) 0.000039 inch (in)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic centimeter (cm3

) 0.06102 cubic inch (in3)

liter (L) 0.2642 gallon (gal)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 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 fertilizer, 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 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 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 biosolids 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 composition 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 nitrogen 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 clarification, aeration, secondary clarification, solids concentration, 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

36

24

71

70

70

25

25

Area of figure 2

105°

40°

39°

104°

0

0

10

10 20

20 30 40 MILES

30 40 KILOMETERS

South

Platte

River

South

Platte

River

Kiowa

Muddy

Beaver

Big

Sandy

Creek

Badger

Creek

Creek

Creek

Creek

76

ROCKY MOUNTAINS

Byers

South

Platte R Denver

Study

area

C O L O R A D O 

Gunnison River

Colorado River

River

Rocky Mountain National Park

Arkansas

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. Geological 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. 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 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, atmospheric 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 groundwater 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 groundwater 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 sedimentary rocks that was formed in an ocean or near-ocean environment. 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 lowermost 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 properties 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

3 4 MILES

4 KILOMETERS

2

2 3

1

1

0

0

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

36

103°52'30" 103°42'

VALLEY

Route 34

Muddy

Creek

39°37'30"

39°44'45"

D32

Cottonwood

70

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. Pl a tte 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 rangeland 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 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; farmland was not irrigated. Herbicides and other chemicals 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 applications. 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 quantity 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 drainage 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 

1/1/19931/1/19961/1/19991/1/20021/1/20051/1/20081/1/2011

0

10

20

30

40

50

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 Characteristics 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 biosolids 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 containers of fresh sample material into a large, shallow tray, substantially 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 Protection 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 analyzed 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 September 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 concentration 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 volume 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 Laboratory 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 collected 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 concentration 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 nitrogen 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 barrels 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 isotopic 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 Laboratory (http://isotopes.usgs.gov/) in Reston, Va., using procedures 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 monitoring 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 temperature 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 superscripts are the approximate atomic masses of the isotopes.

15

14

15 sample

15

14

standard

N

N

δ N 1 1,000 N

N

    =  − ×    

(1)

18

16

18 sample

18

16

standard

O

O

δ O 1 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 Standard 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 microbial 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, measured 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 environment can be farm fertilizer, although animal manure and 

atmospheric deposition also are nitrogen sources in agricultural environments (Ruddy and others, 2006). Farm fertilizers 

can include biosolids, livestock manure, and inorganic fertilizers. 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 materials 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. 

0

2

4

6

8

10

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

8

18

19

30

31

6

8 9 10 11

14

23

26

35

2

8 9 10 11 12 7

2 1 6

11

14 13 18

6

7

18

19

30

31

2

35 36 31 32 33 34 35 36 31

26 25 30 29 28 27 26 25

23

14 13

11 12

5 4 3

7 10 11

18 16 15 14 13

24

29 2526

32 36

5 12

2019

6

36

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 346 DC 350

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 

326

DC 

327

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

8 9

17

9 10

34

33 34 35

29 28

20 21

17 16

27

22

15

2728

2221 23

5 4 3

32 33 34

2 1

12 7 8 9 10

13 18 17 16 15

24 19 20 21 22

6 5 4

25 30 29 28 27

31 32 33 34

1 6

7

5 4 3

12

23 24

26 25

35 36

11 12

14 13

1

DC 329

DC 332 DC 328 DC 330

3 4 MILES

4 KILOMETERS

2

2 3

1

1

0

0

EXPLANATION

Metro Wastewater Reclamation District 

 property at METROGRO Farm (2003 

 boundaries) showing Destination Code (DC, used

 to track biosolids applications) and section 

DC 351

28

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, combustion 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)

2002 2002 440 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 

2004

2005 -- -- -- 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 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 2005 -- -- -- 3,650

Biosolids, fresh, wet 

(from a centrifuge at the wastewater treatment plant)

October 

2010

October 

2010

15,000 71,000 <47 --

Biosolids, fresh, wet, replicate 

(from a centrifuge at the wastewater treatment plant)

October 

2010

October 

2010

15,000 57,000 <47 --

Biosolids, fresh, wet 

(from a centrifuge at the wastewater treatment plant)

November 

2010

November 

2010

9,000 71,000 <40 --

Biosolids, fresh, wet, replicate 

(from a centrifuge at the wastewater treatment plant)

November 

2010

November 

2010

8,000 63,000 6.0 --

Biosolids, fresh, wet 

(from a centrifuge at the wastewater treatment plant)

December 

2010

December 

2010

15,000 68,000 <44 --

Biosolids, fresh, wet, replicate 

(from a centrifuge at the wastewater treatment plant)

December 

2010

December 

2010

15,000 70,000 <44 --

Cow manure, fresh 

(collected from the land surface at site South-2)

June 2005 2005 -- -- -- 1,860

Cow manure, field weathered 

(collected from the land surface at site South-2)

June 2005 2005 -- -- -- 1,590

1

Analyses 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 keeping 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 draining 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 nitrogen 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), chloride, 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 potential 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 Minnesota, 2002; Sullivan, 2008). These forms of nitrogen are transformed to nitrate in the presence of oxygen through ammonification (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 others (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 pronghorn 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 centimeter 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 21.6 99.7 115 213 <1.6

Antimony, ICP-MS μg/L 2.1 1.6 2.4 1.7 <0.1

Arsenic, cICP-MS μg/L 7.0 9.0 6.0 5.4 <0.06

Barium, ICP-MS μg/L 2.1 2.4 6.5 2.2 <0.4

Beryllium, ICP-MS μg/L <0.008 <0.008 <0.008 <0.008 <0.008

Boron, ICP μg/L 73.5 55.3 51.5 43.5 <1.2

Bromide, IC mg/L 0.07 E0.07 0.08 0.08 <0.02

Cadmium, ICP-MS μg/L 0.28 0.13 0.19 0.11 <0.04

Calcium, ICP mg/L 9.54 7.67 4.89 4.49 E0.04

Chloride, IC mg/L 48.4 61.0 59.6 58.0 <0.12

Cobalt, cICP-MS μg/L 5.86 8.43 5.13 4.57 <0.02

Copper, cICP-MS μg/L 24.6 3.16 2.0 2.2 <1

Chromium, cICP-MS μg/L 0.6 1.7 0.8 0.9 <0.1

Fluoride, IC mg/L 0.37 1.18 1.21 1.12 <0.12

Iron, ICP μg/L 288 1,220 322 373 <8

Lead, ICP-MS μg/L 0.65 0.45 0.84 0.38 <0.08

Magnesium, ICP mg/L 2.92 1.77 1.71 1.3 <0.02

Manganese, ICP-MS μg/L 32.5 25.6 8.2 7.8 <0.2

Molybdenum, ICP-MS μg/L 85.1 40.8 61.5 58.0 <0.2

Nickel, cICP-MS μg/L 77.1 74.9 27.9 28.4 <0.2

Nitrogen, ammonia,2

 colorimetry, DA, 

salicylate-hypochlorite

mg-N/L 110 323 224 258 0.086

Nitrogen, nitrite + nitrate (NO2

+NO3), 

colorimetry, continuous flow, cadmiumreduction

mg-N/L E0.024 0.041 0.044 0.042 <0.04

Nitrogen, nitrite, colorimetry, DA mg-N/L <0.100 E0.073 E0.055 <0.100 <0.002

Nitrogen, ammonia2

 + organic, alkaline 

persulfate

mg-N/L 249 519 650 379 E0.03

Orthophosphate, colorimetry, DA, 

phosphomolybdate

mg-P/L 1.97 10.4 15.2 13.3 <0.006

pH, electrode std. units 6.9 8.4 8.6 8.7 5.6

Potassium, ICP mg/L 15.9 27.5 25.8 23 <0.02

Selenium, cICP-MS μg/L 16.0 18.1 8.1 8.4 <0.04

Silica, ICP mg/L 3.31 2.49 6.29 5.57 <0.02

Silver, ICP-MS μg/L 0.34 0.11 <0.1 <0.1 <0.1

Sodium, ICP mg/L 11.3 18.0 15.4 13.9 <0.12

Specific conductance, Wheatstone bridge μS/cm 575 2,280 1,400 1,340 2

Strontium, ICP μg/L 31.2 23.9 16.9 16.2 <0.4

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 centimeter 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 livestock 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 weathered 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 nitrogen 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 centimeter 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 192 20 38.1 49.6 <0.18

Uranium, ICP-MS μg/L 0.41 74.3 37.1 33.2 <0.02

Tungsten, cICP-MS μg/L 7.0 18.2 14.1 12.4 <0.06

Zinc, cICP-MS μg/L 38.6 27.1 12.8 14.5 <1.8

Chloride:Bromide none 691 871 745 725 --

Sample temperature at time of preparation, 

thermistor of pH meter

°C 25 25 25 25 25

Mass of biosolids material leached grams 50.01 216.32 216.54 216.20 0

Volume of IBW added liter 1.00 0.82 0.82 0.82 1.00

Estimated water content of solid material percent 7 78.5 78.5 78.5 100 

(all water, 

no solid)

1

All biosolids samples were collected from the centrifuges at the wastewater treatment plant (1- to 2-day composites) in Denver, Colorado.

2

Analyses 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 composition 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 Spalding (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 inorganic 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 atmospheric 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 elevation area about 161 km northwest of the study area) as ranging 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 ranging 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). Nitrogen 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 concentrations 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 application 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 concentrations at the other cultivated sites indicate that land applications 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 concentrations 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 nitrogen 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, atmospheric 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. Geological 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); o

C, 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]

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 -- -- 850 <1 2,344 ± 34 

(n=3)

--

DTX8 42.2–42.4 3/2/1999 Hard, black, unweathered shale with silt -- -- 750 <1 2,101 ± 65 

(n=3)

--

DTX8 48.9–49.1 3/2/1999 Gray silt (weathered siltstone and shale) with shale -- -- 370 <1 869 ± 49 (n=2) --

DTX8 49.8–50.0 3/2/1999 Gray silt (weathered siltstone and shale) with shale -- -- 360 <1 874 ± 6 (n=2) --

DTX10 25.0–25.2 2/24/1999 Shale, clay (weathered shale), and silt (weathered siltstone) -- -- 500 <1 798 ± 51 (n=2) --

DTX10 26.1–26.3 2/24/1999 Gray silt (weathered siltstone and shale) with shale -- -- 450 <1 1,031 ± 37 

(n=2)

--

DTX10 31.2–31.4 2/24/1999 Black shale with silt (weathered shale) and fossil fragments -- -- 400 <1 1,086 ± 17 

(n=3)

--

DTX10 109.9–110.8 2/24/1999 Hard black clay (weathered shale) with shale and silt (weathered siltstone) -- -- 470 1 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 ~1.2 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 8.2 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 ~10.7 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, (possible kaolinite, clinochlore), gypsum, pyrite, dolomite-- -- -- -- --

DTX11 ~11.6 1/18/20003 Hard, black, unweathered shale -- -- -- -- -- 700

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 identified as gypsum

<2 94 <1 -- --

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

<2 51 5.2 -- --

D9A 17.1 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 leaching procedure with semi-automated colorimetry, method 353.2 (U.S. Environmental Protection Agency, 1993); o

C, 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); o

C, 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]

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 mineral identified as jarosite surrounded by minerals quartz and gypsum

-- -- -- -- --

D9A 17.7 2/7/2002 Consolidated, friable, heterogeneous, weathered, grey, black, yellow and red loamy siltstone, sandstone, and shale with vein of clear/white crystalline mineralQuartz (major), gypsum, orthoclase, montmorillonite (minor), with trace of albite, jarosite, muscovite; Clear/white mineral identified as gypsum with a trace of quartz sand-- -- -- -- --

D9A 18.1 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

<2 52 <1 -- --

D9A 18.29 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, muscovite, jarosite-- -- -- -- --

D9A 18.35 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 sandstoneClear 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 sandstoneQuartz and gypsum (major), jarosite and montmorillonite clay (minor), with trace of microcline, 

albite, jarosite, muscovite; Clear crystals identified as gypsum

-- -- -- -- --

D9A 18.44 2/7/2002 Consolidated, friable, weathered, grey clayey shale with yellow, clear, and orange minerals; unconsolidated weathered, yellow, orange, grey, and brown siltstone and sandstoneQuartz and gypsum (major), jarosite and montmorillonite clay (minor), with trace of microcline, 

albite, muscovite; Yellow minerals identified as natrojarosite, jarosite; clear minerals identified as quartz, gypsum; orange minerals identified as natrojarosite, jarosite, amorphous iron 

hydroxides

-- -- -- -- --

D17A ~32.0 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 51.5 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 7.0 2/5/2002 Semi-consolidated to unconsolidated orange-brown sand or weathered sandstone Quartz (major), microcline and albite (minor), with trace of muscovite, montmorillonite, possible 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 -- -- -- -- -- 700

South-1 0.2–0.3 5/31/2005 Soil on lowland terrace that has received cultivation but no biosolids applications -- -- -- -- -- 900

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

1

Analyzed in 2005 by X-ray diffraction (Cullity and Stock, 2001) with and without Rietveld Method (Young, 1995).

2

Analyses for “ammonia” quantify ammonia plus ammonium. 

3

Chemical analysis done in 2005.

4

Chemical 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 increasing nitrate concentrations in the well (fig. 3). Nitrate concentration 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 apparent 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.

1.5

1.2

0.9

0.6

0.3

0

0 100 200 300 400 500

0 30 60 90 120

 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

0 30 60 90 120 0 30 60 90 120

0 30 60 90 120 0 30 60 90 120

0 100 200 300 400 500 0 100 200 300 400 500

0 100 200 300 400 500 0 100 200 300 400 500

 Chloride concentration, in micrograms per gram 

 Depth below land surface, in meters 

1.5

1.2

0.9

0.6

0.3

0

1.5

1.2

0.9

0.6

0.3

0

1.5

1.2

0.9

0.6

0.3

0

1.5

1.2

0.9

0.6

0.3

0

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 <560

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 1

3,650–71,000 24,930 0 248,300 1+6.6 to +7.3; 3

+9.6 

--

--

Animal manure 1

1,860 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 8

800,000 4120–190 44,070–6,340 0 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

4

250–280; 

10220–270

4

6,480–7,680 106,930

4

9,410–10,920; 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 140.01 140.01 1

+4.3 to +4.7; 

7

+2 to +7 

7

-7 to +13 

1

Data from this study.

2

Calculated 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.

3

From Buck (2003).

4

Calculated 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.

5

From Gormly and Spalding (1979).

6

7

From Kendall and Doctor (2004).

8

From Zublena and others (1991).

9

From 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 denitrification 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 0.76 1.84 1.74 -0.46 0.27

Dissolved oxygen, mg/L 2.6 -- 3.0 4.1 --

Field pH, standard units 7.2 7.0 7.0 7.2 6.9

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 0.20 E1.7 -- 0.30 0.18

Ammonia, dissolved, mg-N/L <0.040 0.067 -- <0 .040 E0.029

Nitrite plus nitrate, dissolved, mg-N/L 6.40 E31.5 2

E27 4.91 21.9

N2

, mg/L degassed degassed 16.434 14.690 18.085

Ar, mg/L degassed degassed 0.5212 0.5487 0.6350

CH4

, mg/L degassed degassed <0.001 <0.001 <0.001

Recharge temperature, °C nd nd E15 E12 E8

Excess air, cc STP/L nd nd 1.0 0.7 2.5

Excess N2

, mg-N/L nd nd 2.0 <0.5 <0.5

Measured δ15N[NO3

–], ‰ relative to AIR 15.5 -- 10.1 8.5 9.6

Measured δ18O[NO3

–], ‰ relative to VSMOW 6.79 -- 0.62 –0.94 –0.39

Measured δ15N[N2

], ‰ relative to AIR degassed degassed –0.29 0.84 0.44

Corrected3

 nitrate, mg-N/L nd nd E29 4.91 21.9

Corrected3

 δ15N[NO3–], ‰ relative to AIR nd nd E8.9 8.5 9.6

1

Last 11 digits, only. First four digits are “3936” for each of these sites. 

2

Value is nitrate as nitrogen analyzed by ion chomatography (estimated because of chloride interference). 

3

Corrected 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 information. 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 biosolids) (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 

8 10 12 14 16 -2

0

2

4

6

8

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 explanation 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 animal 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 preferential 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 dissolved 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 materials 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 differentiating 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 concentrations 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 significant 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 studies (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 magnesium 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 concentrations 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

1/1/19931/1/19961/1/19991/1/20021/1/20051/1/20081/1/2011

0

10

20

30

40

50

Concentration of dissolved nitrite plus nitrate,

in milligrams per liter as nitrogen

300

350

400

450

p = 0.02

r

2

 = -0.14

 Concentration of dissolved chloride,

in milligrams per liter

350

400

450

500

p = 0.03

r

2

 = 0.12

Concentration of dissolved calcium,

in milligrams per liter

2,000

2,200

2,400

2,600

2,800

p < 0.01

r

2

 = 0.49

 Concentration of dissolved magnesium,

in milligrams per liter

1,800

2,000

2,200

2,400

p < 0.01

r

2

 = 0.74

Concentration of dissolved sodium,

in milligrams per liter

12,500

13,000

13,500

14,000

14,500

p < 0.01

r

2

 = 0.58

 Concentration of dissolved sulfate,

in milligrams per liter

3.0

3.5

4.0

4.5

5.0

p < 0.01

r

2

 = 0.20

Concentration of dissolved bromide,

in milligrams per liter

Date

10

12

14

16

18

p < 0.01

r

2

 = 0.46

Concentration of dissolved potassium,

in milligrams per liter

Date

1/1/19931/1/19961/1/19991/1/20021/1/20051/1/20081/1/2011

1/1/19931/1/19961/1/19991/1/20021/1/20051/1/20081/1/2011

1/1/19931/1/1996

1/1/19991/1/2002

1/1/20051/1/20081/1/2011

1/1/19931/1/19961/1/19991/1/2002

1/1/20051/1/2008

1/1/2011

1/1/19931/1/1996

1/1/19991/1/20021/1/20051/1/20081/1/2011

1/1/19931/1/1996

1/1/1999

1/1/20021/1/2005

1/1/2008

1/1/2011

1/1/1993

1/1/19961/1/1999

1/1/20021/1/2005

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 others, 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 differentiating 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

0

200

400

600

800

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 during 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. Concentrations 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 cultivated 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-extractable 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 evaluate 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 composition (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. Measurements 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 composition (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/bromide 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 shallow groundwater in the vicinity of well D6 (table 4). The isotopic 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 characterization 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 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. Nitrate concentrations also increased in water from three other shallow monitoring 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 groundwater in the area.

Biosolids, animal manure, inorganic fertilizer, atmospheric 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 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 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 quantity 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 weathered 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 composition 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 composition 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 biosolids applications contained as much as 1,700 mg-N/kg nitrogen. 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. Calculated estimates 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 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.

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 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 biosolids 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 composition 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 shallow groundwater at these monitoring wells. To reduce uncertainty in identifying the dominant source of nitrate in groundwater 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.

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Publishing support provided by: 

Denver and Lafayette Publishing Service Centers

For more information concerning this publication, contact:

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Box 25046, Mail Stop 415

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Or visit the Colorado Water Science Center Web site at: 

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