Scientific Investigations Report 2010−5215

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Colorado Department of Public Health and Environment

Use of Diverse Geochemical Data Sets to Determine Sources 

and Sinks of Nitrate and Methane in Groundwater, 

Garfield County, Colorado, 2009
COLLAGE (LEFT TO RIGHT)

Horses near a domestic well, Garfield County, Colorado. Photograph by Peter McMahon, 

U.S. Geological Survey.

Livestock grazing in a pasture, Garfield County, Colorado. Photograph by Jim Collins, 

U.S. Geological Survey.

Drilling operation for a natural gas well, Garfield County, Colorado. Photograph by 

Jim Collins, U.S. Geological Survey.

FRONT COVER

Top

U.S. Geological Survey hydrologist measuring dissolved-oxygen concentrations in 

water from a domestic well, Garfield County, Colorado. Photograph by Peter McMahon, 

U.S. Geological Survey.

Bottom

U.S. Geological Survey hydrologists preparing to sample a domestic well, Garfield 

County, Colorado. Photograph by Jim Collins, U.S. Geological Survey.
Use of Diverse Geochemical Data Sets 

to Determine Sources and Sinks of 

Nitrate and Methane in Groundwater, 

Garfield County, Colorado, 2009

By P.B. McMahon, J.C. Thomas, and A.G. Hunt

Prepared in cooperation with the Colorado Department of Public Health and Environment

Scientific Investigations Report 2010–5215

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: 2011

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Suggested citation:

McMahon, P.B., Thomas, J.C., and Hunt, A.G., 2011, Use of diverse geochemical data sets to determine sources 

and sinks of nitrate and methane in groundwater, Garfield County, Colorado, 2009: U.S. Geological Survey Scientific 

Investigations Report 2010–5215, 40 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Study Methods ...............................................................................................................................................2

Sample Collection.................................................................................................................................2

Sample Analysis....................................................................................................................................5

Groundwater Age................................................................................................................................19

Geochemical Data Sets ..............................................................................................................................21

Distributions of Oxygen, Nitrate, and Methane in Groundwater................................................21

Major-Ion Chemistry...........................................................................................................................21

Groundwater Mixing....................................................................................................................................25

Stable Isotopes of Water...................................................................................................................26

Apparent Groundwater Ages............................................................................................................27

Chloride/Bromide Mass Ratios.........................................................................................................28

Summary of Mixing Results ..............................................................................................................30

Sources and Sinks of Nitrate in Groundwater........................................................................................30

Sources and Sinks of Methane in Groundwater....................................................................................30

Hydrocarbon Molecular and Isotopic Compositions....................................................................30

Helium Concentrations and Isotopes ..............................................................................................32

Methane Transport.............................................................................................................................36

Summary........................................................................................................................................................37

Acknowledgments.......................................................................................................................................37

References Cited..........................................................................................................................................38

Figures

1. Map showing location of the study area ..................................................................................3

2. Map showing location of the domestic wells sampled for this study..................................4

3. Graph showing concentrations of chlorofluorocarbons and sulfur hexafluoride 

in North American air, and concentrations of tritium in precipitation at Salt Lake 

City, Utah, decayed to 2010 .......................................................................................................20

4–6. Maps showing:

 4. Concentrations of dissolved oxygen in water from domestic wells sampled 

for this study.........................................................................................................................22

 5. Concentrations of nitrate in water from domestic wells sampled for this study.......... 23

 6. Concentrations of methane in water from domestic wells sampled for this study.........24

7. Diagram showing major-ion chemistry of water from domestic wells sampled for 

this study, and major-ion chemistry of produced water from the Mesaverde Group ........25

8. Diagram showing major-ion chemistry of water from previously sampled domestic 

wells in the study area, and major-ion chemistry of produced water from the 

Mesaverde Group........................................................................................................................26
iv

9–14. Graphs showing:

 9. Stable isotopic compositions of water from domesitic wells (A) sampled for 

this study and (B) sampled during previous studies......................................................27

 10. Concentrations of tritium and chlorofluorocarbon-113 in water from domestic 

wells sampled for this study ..............................................................................................28

 11. Apparent ages of the young fraction of water from domestic wells sampled 

for this study.........................................................................................................................28

 12. Chloride/bromide mass ratios in water from domestic wells sampled for 

this study and during previous studies ............................................................................29

 13. Measured concentrations of (A) nitrate and (B) methane in water from 

domestic wells sampled for this study, and (C) concentrations of nitrate in 

the samples at the time they were recharged, in relation to concentrations 

of dissolved oxygen.............................................................................................................31

 14. Molecular and isotopic composition of short-chain hydrocarbons in water 

from domestic wells (A) sampled for this study and (B) sampled during 

previous studies...................................................................................................................32

15. Map showing concentrations of helium-4 in water from domestic wells sampled 

for this study ................................................................................................................................33

16. Graphs showing (A) Methane concentrations in relation to helium-4 concentrations 

in water from domestic wells sampled for this study and (B) measured helium-3/

helium-4 ratios corrected for excess air in relation to the relative amount of 

helium-4 derived from air-water equilibrium with respect to total helium-4 in the 

sample, corrected for excess air................................................................................................34

17. Graph showing concentrations of helium-4 in water from domestic wells sampled for 

this study in relation to depth below the water table to the midpoint of the well screen...... 35

Tables

1. Chemical data for water collected from domestic wells in Garfield County, Colorado ...........6

2. Modeled helium-4 ages, in years, of water from well grflds9, Garfield County, 

Colorado. ......................................................................................................................................36

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Volume

liter (L) 0.2642 gallon (gal)

cubic centimeter (cm3) 0.06102 cubic inch (in3) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
v

Abbreviations and Acronyms

‰ per mil or parts per thousand

14C carbon-14

3

H tritium

4

He helium-4

4

Hea helium-4 in water derived from excess air

4

Hes total helium-4 in a water sample 

4

Heeq helium-4 in water derived from air-water equilibrium

4

Hesol rate at which helium-4 enters solution

4Heterr terrigenic helium-4

cm3 STP/g cubic centimeters at standard temperature and pressure, 

per gram of water

cm3 STP/g∙yr cubic centimeters at standard temperature and pressure, 

per gram of water, per year

cm3 STP He/cm2∙yr cubic centimeters of helium at standard temperature and pressure, 

per square centimeter, per year

CFC chlorofluorocarbon

CH4 methane

CO2 carbon dioxide

GMWL global meteoric water line

J helium flux

MCL Maximum Contaminant Level

mg/L as N milligrams per liter as nitrogen

mm/yr millimeters per year

n porosity

N2 nitrogen gas

NO3 nitrate

O2 dissolved oxygen

ppm parts per million

ρs rock bulk density

ρw density of water

RA helium-3/helium-4 ratio in air

SF6 sulfur hexafluoride

TU tritium units

USGS U.S. Geological Survey

VPDB Vienna Peedee Belemnite

VSMOW Vienna Standard Mean Ocean Water

Use of Diverse Geochemical Data Sets to Determine 

Sources and Sinks of Nitrate and Methane in Groundwater, 

Garfield County, Colorado, 2009

By P.B. McMahon, J.C. Thomas, and A.G. Hunt

was derived from deeper zones in the Wasatch Formation. 

The primary methane sink in the aquifer appeared to be methane oxidation on the basis of dissolved-oxygen and methane 

concentrations and methane isotopic data.

The diverse data sets used in this study enhance previous water-quality assessments by providing new and more 

complete insights into the sources and sinks of nitrate and 

methane in groundwater. Field measurements of dissolved 

oxygen in groundwater were useful indicators of the Wasatch 

Formation’s vulnerability to nitrate and methane contamination 

or enrichment. Results from this study also provide new evidence for the movement of water, ions, and gases into the shallow Wasatch Formation from sources such as the Mesaverde 

Group and deeper Wasatch Formation.

Introduction

Nitrate, methane, and a few other chemical constituents 

were detected in relatively high concentrations in water from 

domestic wells screened in shallow zones of the Wasatch 

Formation, Garfield County, Colorado during previous waterquality assessments (URS Corporation, 2006; Papadopulos & 

Associates, Inc., 2008). In 2009, the U.S. Geological Survey, 

in cooperation with the Colorado Department of Public Health 

and Environment, analyzed samples collected from 26 domestic wells for a diverse set of geochemical tracers for the purpose of determining sources and sinks of nitrate and methane

in groundwater from the Wasatch Formation. Interpretations 

regarding possible sources and sinks of those constituents are 

important because they could help water managers understand 

why and where groundwater is vulnerable to certain types of 

contamination. Interpretations based on a relatively limited set 

of chemical indicators or tracers could have large uncertainties 

if the sources and sinks of chemicals in groundwater are complex, or if the chemical tracers on which interpretations are 

based have been altered by subsurface processes. Other environmental tracers that could be helpful in these circumstances 

include reduction/oxidation (redox) constituents (Chapelle 

Abstract

Previous water-quality assessments reported elevated 

concentrations of nitrate and methane in water from domestic 

wells screened in shallow zones of the Wasatch Formation, 

Garfield County, Colorado. In 2009, the U.S. Geological 

Survey, in cooperation with the Colorado Department of 

Public Health and Environment, analyzed samples collected 

from 26 domestic wells for a diverse set of geochemical tracers for the purpose of determining sources and sinks of nitrate 

and methane in groundwater from the Wasatch Formation.

Nitrate concentrations ranged from less than 0.04 to 

6.74 milligrams per liter as nitrogen (mg/L as N) and were 

significantly lower in water samples with dissolved-oxygen 

concentrations less than 0.5 mg/L than in samples with 

dissolved-oxygen concentrations greater than or equal to 

0.5 mg/L. Chloride/bromide mass ratios and tracers of groundwater age (tritium, chlorofluorocarbons, and sulfur hexafluoride) 

indicate that septic-system effluent or animal waste was a source 

of nitrate in some young groundwater (less than 50 years), 

although other sources such as fertilizer also may have contributed nitrate to the groundwater. Nitrate and nitrogen gas (N2

) 

concentrations indicate that denitrification was the primary sink 

for nitrate in anoxic groundwater, removing 99 percent of the 

original nitrate content in some samples that had nitrate concentrations greater than 10 mg/L as N at the time of recharge.

Methane concentrations ranged from less than 0.0005 to 

32.5 mg/L and were significantly higher in water samples with 

dissolved-oxygen concentrations less than 0.5 mg/L than in 

samples with dissolved-oxygen concentrations greater than or 

equal to 0.5 mg/L. High methane concentrations (greater than 

1 mg/L) in some samples were biogenic in origin and appeared 

to be derived from a relatively deep source on the basis of 

helium concentrations and isotopic data. One such sample 

had water-isotopic and major-ion compositions similar to that 

of produced water from the underlying Mesaverde Group, 

which was the primary natural-gas producing interval in the 

study area. Methane in the Mesaverde Group was largely thermogenic in origin so biogenic methane in the sample probably 
2 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

and others, 1995; McMahon and Chapelle, 2008), stable isotopes of water (Böhlke and others, 2007; Landon and others, 

2008), tracers of groundwater age (Manning and others, 2005; 

Rupert and Plummer, 2009), and dissolved noble gases (Zhou 

and others, 2005; Sherwood Lollar and Ballentine, 2009). 

Each of those tracers measures different processes and when 

used in combination, the diverse data sets can sometimes 

provide new and more complete insights into the sources and 

sinks of chemical constituents in groundwater.

Purpose and Scope

The purpose of this report was to determine sources and 

sinks of nitrate and methane in groundwater from the Wasatch 

Formation in Garfield County, Colorado, by use of diverse 

data sets that included concentrations of major ions, nutrients, 

redox constituents, and noble gases, hydrocarbon molecular 

and isotopic compositions, water isotopic compositions, and 

tracers of groundwater age. The study area is located between 

Silt and Rifle, Colorado (fig. 1). Twenty-six domestic wells 

were sampled for this broad suite of chemical and isotopic 

tracers (fig. 2). Ten wells were located north of the Colorado 

River and sampled in April 2009. Sixteen wells were located 

south of the river and sampled in April and October 2009.

Description of Study Area

The study area is located between Silt and Rifle, 

Colorado (fig. 1). The mean annual air temperature in Rifle, 

Colorado, was 8.7°C for the period 1910 to 2007, and mean 

annual precipitation was 29.4 cm (High Plains Regional 

Climate Center, 2010). The elevation of sampling locations 

ranged from 1,672 to 2,155 m. Mean annual precipitation and 

air temperatures at the highest-elevation sites are likely to be 

wetter and colder than sampling sites at lower elevations.

The primary geologic units of interest in the study were 

the Williams Fork and Wasatch Formations. The Williams 

Fork Formation, part of the Mesaverde Group, consists of 

mostly fluvial sedimentary rock of Late Cretaceous age 

(Johnson and Rice, 1990). Most natural gas wells in the 

study area were completed in the Williams Fork Formation 

at depths of about 1,830 to 3,050 m below ground surface 

(URS Corporation, 2006), but deeper sandstones in the 

Mesaverde Group and pre-Cretaceous sedimentary rock 

below the Mesaverde Group also produce gas (Johnson and 

Rice, 1990). The Wasatch Formation is the primary bedrock 

unit at land surface in the study area. The Wasatch Formation 

consists of mostly fluvial sedimentary rock of Tertiary age 

(Paleocene and Eocene) (Donnell, 1969). Most domestic wells 

in the study area were screened in the Wasatch Formation 

at depths generally less than 185 m below land surface. 

The thickness of the Wasatch Formation ranges from about 

365 m near the southern end of the Divide Creek anticline 

to more than 1,710 m along the west side of the study area 

(URS Corporation, 2006). The Divide Creek anticline, located 

in the southeastern part of the study area, plunges into the 

central part of the study area (Hoak and Klawitter, 1977; 

Grout and Verbeek, 1992; URS Corporation, 2006). Structural 

features (for example, faults and fractures) associated with the 

anticline may be important pathways for water or gas movement, or both, from deeper to shallower zones in the area 

(URS Corporation, 2006).

Static water levels in the Wasatch Formation reported 

in drillers’ logs ranged from about 0 to 105 m below land 

surface, with a mean of 22 m (URS Corporation, 2006). The 

primary sources of recharge to the Wasatch Formation were 

precipitation in upland areas and irrigation at lower elevations. 

In general, groundwater flow in the study area is probably 

topography controlled, moving from upland areas to local 

streams and springs. The Colorado River is interpreted as 

the regional discharge area for groundwater in the Wasatch 

Formation (URS Corporation, 2006).

More detailed descriptions of the geology and hydrology, 

as well as the natural gas production history of the study area, 

can be found in Johnson and Rice (1990), Shroba and Scott 

(1997, 2001), URS Corporation (2006), and Papadopulos & 

Associates, Inc. (2008).

Study Methods

Water samples were collected from 26 domestic wells in 

April and October 2009. Wells were selected to provide relatively broad spatial coverage of the aquifer north and south of 

the Colorado River. Sixteen wells were located south of the 

river and 10 wells were located north of the river (fig. 2). 

Several criteria had to be met before a well was selected for 

sampling. The criteria included (1) well owner permission 

to sample, (2) availability of information on well depth, top 

and bottom of screened interval, and depth to water at time of 

drilling, (3) availability of information showing geology of the 

screened interval, and (4) availability of a sampling point on 

the well upstream from any pressure tank or treatment device.

Sample Collection

Water samples were collected from the well discharge 

after readings of specific conductance, pH, temperature, 

turbidity, and dissolved-oxygen concentration had become 

stable (as defined by Koterba and others, 1995). Three general 

groups of samples were collected: (1) inorganic ions, (2) stable 

isotopes and dissolved gases, and (3) environmental tracers 

of groundwater age. Processing chambers were used during 

the collection and preservation of samples to reduce airborne 

contamination in the samples.
Study Methods 3

Inorganic ions.—Samples for the analysis of alkalinity, 

major ions, and nutrients were filtered (0.45 micron) in the 

field and stored in precleaned plastic bottles. Cation samples 

were acidified in the field to pH less than 2 using nitric acid. 

Nutrient samples were kept chilled on ice until delivered to 

the laboratory.

Stable isotopes and dissolved gases.—Water-isotope samples (δ18O[H2

O] and δ2H[H2O]) were unfiltered and collected 

in precleaned glass bottles with polyseal caps that were secured 

with tape. Samples for analysis of the major gases methane, 

nitrogen (N2), argon, and carbon dioxide were unfiltered and 

collected in precleaned glass bottles that were filled and capped 

with thick rubber stoppers under water to exclude headspace 

(atmospheric contamination). Major-gas samples were collected 

in duplicate at each site. Samples for analysis of the hydrocarbon gases methane, ethane, and propane and the isotopic composition of methane (δ13C[CH4

] and δ2H[CH4]) were unfiltered 

and collected in precleaned plastic bottles that were filled and 

capped under water. The samples were preserved in the field 

with a bactericide and kept chilled on ice until delivered to the 

laboratory. In April, unfiltered noble-gas samples were collected 

in precleaned glass bottles that were filled and capped with thick 

rubber stoppers under water to exclude headspace. These samples were analyzed for helium and neon and were used to identify helium-enriched waters that could not be age dated using 

the tritium/helium-3 method (Ludin and others, 1998). Wells 

with the greatest helium enrichment were resampled in October 

for analysis of the full suite of noble gases (helium, neon, argon, 

krypton, xenon). The October samples were unfiltered and collected in copper tubes that were sealed on both ends with pinch 

clamps. The copper tubes were connected in line with the pumpdischarge line to prevent atmospheric contamination and slight 

backpressure was applied to the tubes while filling to help keep 

the noble gases in solution.

Environmental tracers of groundwater age.—Tritium

samples were unfiltered and collected in precleaned plastic 

bottles with polyseal caps that were secured with tape. Samples 

for analysis of chlorofluorocarbon-11, chlorofluorocarbon-12, 

and chlorofluorocarbon-113 were unfiltered and collected in 

precleaned glass bottles that were filled and capped under 

water to exclude headspace. The caps were secured with tape 

and the bottles were kept chilled on ice until delivered to the 

Figure 1. Location of the study area.

Colorado River

70

Glenwood 

Springs

Rifle Silt

107°30'

108°30' 109°00' 108°00'

40°00'

39°30'

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Transverse Mercator projection, Zone 13

Study area

C O L O R A D O

0 10 20 30 KILOMETERS

0 10 20 30 MILES

GARFIELD COUNTY
4 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Figure 2. Location of the domestic wells sampled for this study.

grfldn1

grfldn7

grflds9

grflds8

grflds7

grflds5 grflds4

grflds6

grflds11

grflds10

grflds27

grflds1

grflds2

grflds23

grflds22

grflds20

grflds21

grflds24

grfldn2 grfldn5

grfldn4

grfldn10

grfldn6

grfldn8

grfldn3 grfldn9

Rifle

Silt

Divide Creek anticline

Rifle-Grand hogback syncline

Unnamed syncline

Government Creek

Ma mm Creek

Col orado River

Divide Creek

107°50'

39°35'

39°30'

39°25'

107°45' 107°40'

EXPLANATION

Geologic unit

Quaternary deposits

Tertiary units

Cretaceous units

Well

0 5 KILOMETERS

0 5 MILES

Fault (approximate)

Axis and direction of plunge 

 of anticline or syncline

Direction of dip of limb 

 of anticline

Direction of dip of limb 

 of syncline

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Tansverse Mercator projection, Zone 13

Geology from Tweto (1979)

Geologic structure modified from URS Corporation (2006)

and Hoak and Klawitter (1997)
Study Methods 5

laboratory. Sulfur hexafluoride samples were collected and 

preserved in the same way as chlorofluorocarbon samples. 

Samples for tritium/helium-3 dating were collected in copper 

tubes as described above.

Sample Analysis

Inorganic ions.—Alkalinity was measured in the field by 

incremental titration using 0.16 normal sulfuric acid. Major 

ions and nutrients were measured by standard methods of 

the U.S. Geological Survey (USGS) National Water Quality 

Laboratory in Lakewood, Colorado (Fishman and Friedman, 

1989), with a reproducibility of about ±1 percent.

Stable isotopes and dissolved gases.—The water-isotope 

samples were analyzed at the USGS Stable Isotope Laboratory 

in Reston, Virginia, by hydrogen- and carbon dioxide-gas 

equilibration and mass spectrometry (U.S. Geological Survey, 

2008a). Stable-isotope results are reported using the standard 

delta (δ) notation, in per mil (‰, parts per thousand). For 

example, the stable oxygen-isotopic composition of a water 

sample (δ18O[H2O]sample) is equal to

δ18O[H2O]sample = ((18O/16O)sample/(18O/16O)ref – 1)∙1000 (1)

where 18O/16O is the ratio of the oxygen-18 to oxygen-16 isotopes 

in the sample and a reference (ref) material.

The δ2H[H2O] and δ18O[H2O] values were referenced to 

the VSMOW scale (Vienna Standard Mean Ocean Water) 

(Coplen, 1988), with reproducibility of about ±1‰ and 

±0.1‰, respectively. Major gases were measured at the 

USGS Chlorofluorocarbon Laboratory in Reston, Virginia 

(U.S. Geological Survey, 2008b), by gas chromatography 

with reproducibility of about ±2 to 4 percent. Major gases 

were measured in duplicate and average values were reported 

in table 1. Hydrocarbon molecular and isotopic compositions were measured at the Isotech Laboratory, Champaign, 

Illinois, by gas chromatography and mass spectrometry, 

respectively. Concentrations of methane, ethane, and propane 

were reported in mole percent with a reproducibility of about 

±2 percent. The δ13C[CH4] values were reported relative to 

Vienna Peedee Belemnite (VPDB) with a reproducibility 

of ±0.3‰. The δ2H[CH4] values were reported relative to 

VSMOW with a reproducibility of ±2‰. Noble-gas samples 

(helium and neon) collected in April were analyzed at the 

USGS Chlorofluorocarbon Laboratory in Reston, Virginia 

(U.S. Geological Survey, 2008b), using gas chromatography, 

with a reproducibility of about ±10 to 20 percent. Noble-gas 

samples collected in October were analyzed at the USGS 

Noble Gas Laboratory in Denver, Colorado (Hunt and others, 

2010), using mass spectrometry. Reproducibilities for helium, 

neon, argon, krypton, and xenon were about ±1, 3, 2, 3, and 

3 percent, respectively.

Environmental tracers of groundwater age.—Tritium 

was analyzed at the USGS Tritium Laboratory in Menlo Park, 

California, using electrolytic enrichment and gas counting 

(Thatcher and others, 1977). Reproducibility of the tritium 

analyses generally was better than ±5 percent. Chlorofluorocarbons and sulfur hexafluoride were analyzed at the 

USGS Chlorofluorocarbon Laboratory in Reston, Virginia, 

using gas chromatography (Plummer and Busenberg, 2000; 

Busenberg and Plummer, 2000). The reproducibility of chlorofluorocarbon and sulfur hexafluoride measurements was 

about ±3 percent. Samples for tritium/helium-3 dating were 

analyzed at the Lamont-Doherty Earth Observatory Noble 

Gas Laboratory in Palisades, New York (Schlosser and others, 

1989; Ludin and others, 1998). These samples were analyzed for helium-4, neon, and helium-3/helium-4 ratios using 

mass spectrometry, with reproducibilities of about ±1, 3, and 

1 percent, respectively.

Determination of recharge temperature, excess air, 

excess nitrogen gas, and initial nitrate concentration in 

recharge.—Recharge temperature refers to the temperature of 

water at the time it recharged the aquifer and excess air refers 

to air bubbles that get trapped in groundwater at the time of 

recharge (Andrews and Lee, 1979; Heaton and Vogel, 1981). 

Recharge temperatures and concentrations of excess air were 

calculated using measured concentrations of nitrogen gas and 

argon (Herzberg and Mazor, 1979; Heaton and Vogel, 1981; 

Böhlke and others, 2002). In the calculations, recharge elevation was assumed to be the elevation of the water table in the 

sampled well and salinity was assumed to equal the concentration of dissolved solids in the sample (table 1). Generally, this 

method for determining recharge temperature and excess air 

is suitable for samples containing more than 2 mg/L dissolved 

oxygen because they are not likely to contain excess nitrogen gas from denitrification. In other words, the only source 

of nitrogen gas in those samples would be the atmosphere. 

Excess nitrogen gas from denitrification is more likely to 

occur in groundwater containing less than 2 mg/L dissolved 

oxygen because denitrification is a microbial process that 

occurs in the absence of dissolved oxygen. McMahon and 

Chapelle (2008) proposed a dissolved-oxygen concentration threshold of 0.5 mg/L for the onset of denitrification in 

groundwater, but because of mixing between oxic and anoxic 

waters in some well screens, the effects of denitrification 

sometimes can be observed in samples containing more than 

0.5 mg/L dissolved oxygen (Böhlke and others, 2002, 2007; 

McMahon and others, 2004). In this study, for samples with 

dissolved-oxygen concentrations less than or equal to 2 mg/L, 

it was assumed that recharge temperatures and concentrations 

of excess air were equal to the median recharge temperature 

and excess-air concentration in samples with dissolved-oxygen 

concentrations greater than 2 mg/L (7.9°C and 1.9 cubic centimeters at standard temperature and pressure, per liter of water 

(cm3 STP/L), respectively).
6 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Land surface 

elevation

(m above NGVD 29)

Well depth

(m below

land surface)

Depth to top of

screened interval

(m below

land surface)

Depth to bottom of 

screened interval

(m below

land surface)

Screen

length

(m)

Formation/

aquifer name

grfldn1 393528107453601 1,730 30.5 6.1 30.5 24.4 Wasatch

grfldn2 393237107450901 1,711 70.1 61.0 70.1 9.1 Wasatch

grfldn3 393318107373601 1,750 30.5 12.2 24.4 12.2 Wasatch

grfldn4 393310107422101 1,672 24.4 12.2 18.3 6.1 Wasatch

grfldn5 393407107430901 1,718 37.5 18.3 30.5 12.2 Wasatch

grfldn6 393456107404501 1,814 61.0 24.4 61.0 36.6 Wasatch

grfldn7 393314107455801 1,708 33.5 28.0 33.5 5.5 Wasatch

grfldn8 393337107394601 1,719 36.6 36.6 44.2 7.6 Wasatch

grfldn9 393311107402201 1,693 21.9 9.8 15.8 6.1 Wasatch

grfldn10 393415107415201 1,742 45.1 21.3 42.1 20.7 Wasatch

grfldn10-rep 393415107415201 1,742 45.1 21.3 42.1 20.7 Wasatch

grflds1 392631107411401 1,903 36.6 24.4 36.6 12.2 Wasatch

grflds2 392712107440101 1,951 30.2 23.8 30.2 6.4 Wasatch

grflds4 392922107375001 1,811 150.9 120.4 150.9 30.5 Wasatch

grflds4 392922107375001 1,811 150.9 120.4 150.9 30.5 Wasatch

grflds5 392928107392901 1,775 43.6 18.3 43.6 25.3 Wasatch

grflds6 392921107382601 1,779 61.0 54.9 61.0 6.1 Wasatch

grflds7 393009107375401 1,821 42.7 27.4 42.7 15.2 Wasatch

grflds8 393101107391001 1,708 45.7 29.0 45.7 16.8 Wasatch

grflds9 393101107393201 1,722 137.2 120.4 134.1 13.7 Wasatch

grflds9 393101107393201 1,722 137.2 120.4 134.1 13.7 Wasatch

grflds10 392524107370301 1,934 45.7 24.4 45.7 21.3 Wasatch

grflds11 392800107382101 1,868 52.7 18.3 30.5 12.2 Wasatch

grflds20 393051107490601 1,724 48.8 36.6 48.8 12.2 Wasatch/

colluvium

grflds21 393031107490201 1,756 73.2 57.9 72.2 14.3 Colluvium

grflds22 392750107465801 2,155 50.3 30.5 48.8 18.3 Wasatch

grflds22-rep 392750107465801 2,155 50.3 30.5 48.8 18.3 Wasatch

grflds23 392853107462101 2,045 54.9 42.7 54.9 12.2 Wasatch/

colluvium

grflds24 392930107454601 1,971 61.0 54.9 61.0 6.1 Wasatch

grflds27 392421107392701 1,993 53.3 44.2 53.3 9.1 Wasatch

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 7

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Collection

date

Depth

to water

(m below

land surface)

Source

of water

level data

Depth below

water to

screen midpoint 

(m)

Specific

conductance

(μS/cm at 25°C)

pH

Water

temperature

(°C)

grfldn1 393528107453601 4/26/2009 11.9 driller log 6.4 2,765 7.75 10.6

grfldn2 393237107450901 4/24/2009 53.3 driller log 12.2 1,983 7.10 11.5

grfldn3 393318107373601 4/20/2009 5.2 driller log 13.1 1,895 7.02 10.8

grfldn4 393310107422101 4/21/2009 5.5 driller log 9.8 1,672 7.20 10.4

grfldn5 393407107430901 4/24/2009 7.6 driller log 16.8 1,150 7.30 11.8

grfldn6 393456107404501 4/22/2009 18.3 driller log 24.4 813 7.55 13.3

grfldn7 393314107455801 4/26/2009 29.0 driller log 1.8 2,866 6.85 12.4

grfldn8 393337107394601 4/24/2009 30.5 driller log 9.9 1,580 7.24 12.7

grfldn9 393311107402201 4/22/2009 6.4 driller log 6.4 1,415 7.33 10.6

grfldn10 393415107415201 4/22/2009 6.1 driller log 25.6 1,510 8.20 10.5

grfldn10-rep 393415107415201 4/22/2009 6.1 driller log 25.6 1,510 8.20 10.5

grflds1 392631107411401 4/21/2009 19.8 driller log 10.7 1,310 7.20 10.8

grflds2 392712107440101 4/25/2009 21.3 driller log 5.6 1,190 7.25 11.9

grflds4 392922107375001 4/26/2009 13.1 driller log 122.5 1,834 7.45 12.0

grflds4 392922107375001 10/26/2009 13.1 driller log 122.5 1,814 7.75 13.6

grflds5 392928107392901 4/27/2009 6.4 driller log 24.5 1,232 7.51 11.1

grflds6 392921107382601 4/23/2009 48.8 driller log 9.1 2,460 7.90 9.1

grflds7 393009107375401 4/23/2009 21.3 driller log 13.7 2,130 7.10 12.0

grflds8 393101107391001 4/25/2009 19.5 driller log 17.8 6,622 7.55 10.1

grflds9 393101107393201 4/25/2009 25.3 driller log 102.0 3,530 8.36 15.3

grflds9 393101107393201 10/28/2009 25.3 driller log 102.0 5,320 8.10 16.0

grflds10 392524107370301 4/21/2009 13.7 driller log 21.3 1,640 7.20 11.7

grflds11 392800107382101 4/23/2009 3.0 driller log 21.3 1,430 8.30 11.0

grflds20 393051107490601 10/27/2009 35.1 driller log 7.6 962 7.97 12.6

grflds21 393031107490201 10/27/2009 56.4 driller log 8.7 719 8.10 13.1

grflds22 392750107465801 10/27/2009 25.3 driller log 14.3 592 7.55 10.4

grflds22-rep 392750107465801 10/27/2009 25.3 driller log 14.3 592 7.55 10.4

grflds23 392853107462101 10/28/2009 35.1 driller log 13.7 711 7.64 11.4

grflds24 392930107454601 10/28/2009 21.0 driller log 36.9 585 8.21 12.0

grflds27 392421107392701 10/26/2009 6.4 driller log 42.4 1,440 7.90 11.0

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
8 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Turbidity

(NTU)

Dissolved

oxygen

(mg/L)

Redox state

Alkalinity

(mg/L as 

CaCO3)

Dissolved solids, 

residue on evaporation 

at 180°C 

(mg/L)

Calcium,

dissolved

(mg/L)

grfldn1 393528107453601 0.6 0.7 oxygen reducing 475 1,954 53.2

grfldn2 393237107450901 -- 2.4 oxygen reducing 481 1,614 157

grfldn3 393318107373601 -- 0.5 oxygen reducing 604 1,518 151

grfldn4 393310107422101 0.2 5.5 oxygen reducing 456 1,102 87.0

grfldn5 393407107430901 0.4 8.6 oxygen reducing 364 773 94.5

grfldn6 393456107404501 0.9 7.7 oxygen reducing 257 558 51.3

grfldn7 393314107455801 0.2 0.7 oxygen reducing 352 2,760 384

grfldn8 393337107394601 -- 4.2 oxygen reducing 460 1,086 114

grfldn9 393311107402201 0.2 5.5 oxygen reducing 394 1,003 95.5

grfldn10 393415107415201 2.5 3.7 oxygen reducing 374 1,001 11.4

grfldn10-rep 393415107415201 2.5 3.7 -- 374 1,006 10.9

grflds1 392631107411401 1.6 2.0 oxygen reducing 645 863 83.5

grflds2 392712107440101 -- 1.5 oxygen reducing 488 782 90.0

grflds4 392922107375001 2.2 1.0 mixed oxygenmanganese reducing594 1,170 16.6

grflds4 392922107375001 -- 2.9 -- -- -- --

grflds5 392928107392901 0.2 8.6 oxygen reducing 505 796 52.4

grflds6 392921107382601 2.0 0.2 suboxic 402 1,667 21.1

grflds7 393009107375401 0.2 0.2 denitrifying 551 1,491 87.4

grflds8 393101107391001 0.2 0.2 manganese reducing 249 5,280 258

grflds9 393101107393201 0.6 0.2 suboxic 113 1,960 29.4

grflds9 393101107393201 -- 0.6 -- -- -- --

grflds10 392524107370301 0.2 2.8 oxygen reducing 445 1,184 133

grflds11 392800107382101 0.3 0.1 suboxic 354 922 8.33

grflds20 393051107490601 0.2 8.3 oxygen reducing 334 630 31.6

grflds21 393031107490201 0.1 8.2 oxygen reducing 348 461 22.8

grflds22 392750107465801 0.1 6.8 oxygen reducing 291 373 70.2

grflds22-rep 392750107465801 0.1 6.8 -- 291 378 70.1

grflds23 392853107462101 0.2 3.5 oxygen reducing 296 434 58.1

grflds24 392930107454601 0.2 0.8 oxygen reducing 303 372 26.9

grflds27 392421107392701 0.5 0.1 suboxic 580 943 11.2

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 9

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Magnesium,

dissolved

(mg/L)

Sodium,

dissolved

(mg/L)

Potassium,

dissolved

(mg/L)

Sulfate,

dissolved

(mg/L)

Chloride,

dissolved

(mg/L)

Fluoride,

dissolved

(mg/L)

Bromide,

dissolved

(mg/L)

grfldn1 393528107453601 19.7 567 4.28 860 82.6 1.27 0.10

grfldn2 393237107450901 111 166 6.42 713 14.2 0.327 0.120

grfldn3 393318107373601 153 100 2.98 603 4.42 0.680 0.06

grfldn4 393310107422101 83.4 165 3.46 244 152 0.974 0.12

grfldn5 393407107430901 73.3 65.8 2.01 248 31.5 0.512 0.07

grfldn6 393456107404501 53.0 45.4 5.25 187 9.38 0.686 0.05

grfldn7 393314107455801 159 174 4.47 1,590 13.4 0.350 0.08

grfldn8 393337107394601 67.9 157 2.90 396 27.6 0.534 0.12

grfldn9 393311107402201 62.2 159 3.17 354 39.9 0.741 0.14

grfldn10 393415107415201 3.56 343 1.84 357 23.3 1.01 0.06

grfldn10-rep 393415107415201 3.47 348 1.81 358 23.6 1.02 0.066

grflds1 392631107411401 53.2 150 3.17 106 5.71 0.353 0.05

grflds2 392712107440101 59.9 105 3.40 180 9.59 0.378 0.09

grflds4 392922107375001 4.65 395 1.53 176 126 4.07 0.90

grflds4 392922107375001 -- -- -- -- -- -- --

grflds5 392928107392901 51.6 164 1.64 140 24.4 1.96 0.19

grflds6 392921107382601 4.87 538 1.70 748 70.9 2.52 0.79

grflds7 393009107375401 41.3 367 2.72 568 43.3 0.784 0.49

grflds8 393101107391001 62.1 1,340 5.05 2,880 391 1.30 1.63

grflds9 393101107393201 1.29 682 1.61 15.7 1,120 3.76 4.75

grflds9 393101107393201 -- -- -- -- -- -- --

grflds10 392524107370301 114 75.5 8.99 362 108 0.929 0.24

grflds11 392800107382101 0.248 305 0.548 302 43.4 2.37 0.36

grflds20 393051107490601 59.0 101 11.7 167 15.1 0.451 0.210

grflds21 393031107490201 25.5 111 6.19 42.0 3.68 0.650 0.049

grflds22 392750107465801 29.9 20.5 3.03 19.1 13.5 0.280 0.152

grflds22-rep 392750107465801 29.6 20.4 2.99 19.1 13.5 0.274 0.154

grflds23 392853107462101 48.0 27.3 3.52 21.8 41.8 0.290 0.281

grflds24 392930107454601 30.8 67.3 2.61 21.4 7.65 0.382 0.109

grflds27 392421107392701 0.755 352 0.989 55.7 128 1.63 0.903

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
10 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Chloride/

bromide 

mass ratio

Silica,

dissolved

(mg/L)

Nitrite,

dissolved

(mg/L as N)

Nitrite+Nitrate, 

dissolved

(mg/L as N)

Ammonia,

dissolved

(mg/L as N)

Orthophosphate,

dissolved

(mg/L as P)

grfldn1 393528107453601 826 5.51 <0.002 4.03 <0.02 0.008

grfldn2 393237107450901 118 17.9 <0.002 1.51 <0.02 0.010

grfldn3 393318107373601 74 15.7 0.002 0.18 <0.02 0.014

grfldn4 393310107422101 1269 18.6 <0.002 2.24 <0.02 0.014

grfldn5 393407107430901 450 23.6 <0.002 1.61 <0.02 0.013

grfldn6 393456107404501 188 23.2 <0.002 0.62 <0.02 0.017

grfldn7 393314107455801 168 14.6 <0.002 0.08 0.025 0.009

grfldn8 393337107394601 230 14.8 <0.002 1.14 <0.02 0.011

grfldn9 393311107402201 285 15.8 <0.002 0.79 <0.02 0.013

grfldn10 393415107415201 388 6.97 <0.002 0.67 <0.02 E0.00441

grfldn10-rep 393415107415201 360 7.01 <0.002 0.67 <0.02 E0.00473

grflds1 392631107411401 114 29.7 0.013 6.74 <0.02 0.021

grflds2 392712107440101 107 32.7 <0.002 3.03 0.033 0.026

grflds4 392922107375001 140 9.17 <0.002 <0.04 0.087 0.011

grflds4 392922107375001 -- -- -- -- --

grflds5 392928107392901 128 36.2 <0.002 0.25 <0.02 0.023

grflds6 392921107382601 90 7.18 <0.002 0.17 E0.01358 E0.00652

grflds7 393009107375401 88 8.99 <0.002 0.94 <0.02 E0.00626

grflds8 393101107391001 240 7.23 E 0.001 0.40 0.068 E0.00516

grflds9 393101107393201 236 8.32 <0.002 <0.04 0.171 E0.00428

grflds9 393101107393201 -- -- -- -- --

grflds10 392524107370301 449 32.1 <0.002 1.20 <0.02 0.023

grflds11 392800107382101 121 9.22 0.013 0.21 0.028 0.009

grflds20 393051107490601 72 31.5 <0.002 3.26 <0.02 0.039

grflds21 393031107490201 75 30.6 <0.002 1.54 <0.02 0.056

grflds22 392750107465801 89 32.2 <0.002 0.72 <0.02 0.033

grflds22-rep 392750107465801 88 32.2 <0.002 0.72 <0.02 0.033

grflds23 392853107462101 149 35.0 <0.002 3.37 <0.02 0.040

grflds24 392930107454601 70 26.3 <0.002 0.11 0.09 0.069

grflds27 392421107392701 142 7.19 0.012 0.16 0.07 E0.0068

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 11

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Iron,

dissolved

(μg/L)

Manganese,

dissolved

(μg/L)

Carbon

dioxide

(mg/L)

Nitrogen

gas (N2)

(mg/L)

Methane

(mg/L)

Methane

(mol %)

Ethane

(mol %)

Propane

(mol %)

grfldn1 393528107453601 <8 0.44 21.57 20.51 <0.0005 -- -- --

grfldn2 393237107450901 11 E0.236 72.91 19.43 <0.0005 -- -- --

grfldn3 393318107373601 31 2.12 109.4 19.63 <0.0005 -- -- --

grfldn4 393310107422101 4.4 E0.182 51.14 17.23 <0.0005 -- -- --

grfldn5 393407107430901 8.4 0.82 35.56 19.96 <0.0005 -- -- --

grfldn6 393456107404501 13 0.59 14.85 16.04 <0.0005 -- -- --

grfldn7 393314107455801 E4.6 10.1 93.76 20.65 0.001 -- -- --

grfldn8 393337107394601 20 0.88 53.09 16.17 <0.0005 -- -- --

grfldn9 393311107402201 13 2.12 33.80 17.12 <0.0005 -- -- --

grfldn10 393415107415201 5.1 1.62 5.675 23.14 <0.0005 -- -- --

grfldn10-rep 393415107415201 8.9 1.60 -- -- -- -- --

grflds1 392631107411401 16 35.0 90.25 20.81 <0.0005 -- -- --

grflds2 392712107440101 5.6 <0.2 55.21 20.07 <0.0005 -- -- --

grflds4 392922107375001 92 73.5 52.24 9.197 27.6 -- -- --

grflds4 392922107375001 -- -- 23.49 18.05 0.072 6.59 0.028 0.001

grflds5 392928107392901 9.2 <0.2 41.69 15.78 <0.0005 -- -- --

grflds6 392921107382601 39 12.4 4.351 34.68 0.004 -- -- --

grflds7 393009107375401 <8 1.70 46.46 23.77 <0.0005 -- -- --

grflds8 393101107391001 23 220 13.90 28.21 0.005 -- -- --

grflds9 393101107393201 E 7.5 16.7 1.485 5.554 32.5 -- -- --

grflds9 393101107393201 -- -- 0.535 5.378 29.5 81.0 0.091 <0.0001

grflds10 392524107370301 9.1 2.12 58.00 20.10 0.002 -- -- --

grflds11 392800107382101 E3.0 8.79 3.547 28.42 0.197 -- -- --

grflds20 393051107490601 7.6 0.35 10.440 14.21 <0.0005 -- -- --

grflds21 393031107490201 <6.0 <0.20 9.125 14.92 <0.0005 -- -- --

grflds22 392750107465801 <6.0 <0.20 4.010 16.16 <0.0005 -- -- --

grflds22-rep 392750107465801 <6.0 <0.20 3.69 16.15 <0.0005 -- -- --

grflds23 392853107462101 10 1.67 18.025 19.19 <0.0005 -- -- --

grflds24 392930107454601 6.1 10.5 5.310 18.13 0.001 -- -- --

grflds27 392421107392701 20 14.9 12.710 27.68 2.87 5.22 0.001 <0.0001

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
12 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Methane/

(ethane+propane)

Helium-4

(10–8 cm3 STP/g)

Neon

(10–7 cm3 STP/g)

Argon

(10–4 cm3 STP/g)

Krypton

(10–8 cm3 STP/g)

grfldn1 393528107453601 -- 8.77 2.19 3.623 --

grfldn2 393237107450901 -- 5.56 2.31 3.665 --

grfldn3 393318107373601 -- 9.56 2.35 3.700 --

grfldn4 393310107422101 -- 191 -- 3.420 --

grfldn5 393407107430901 -- 6.03 2.49 3.833 --

grfldn6 393456107404501 -- 4.85 2.03 3.049 --

grfldn7 393314107455801 -- 5.69 2.31 3.478 --

grfldn8 393337107394601 -- 8.09 2.02 3.282 --

grfldn9 393311107402201 -- 4.65 1.93 3.515 --

grfldn10 393415107415201 -- 7.21 2.97 4.076 --

grfldn10-rep 393415107415201 -- -- -- -- --

grflds1 392631107411401 -- 5.76 2.41 3.802 --

grflds2 392712107440101 -- 5.87 2.40 3.705 --

grflds4 392922107375001 -- 1,100 -- 2.128 --

grflds4 392922107375001 226 729 1.68 3.432 8.18

grflds5 392928107392901 -- 31.3 2.33 3.330 --

grflds6 392921107382601 -- 145 -- 3.710 --

grflds7 393009107375401 -- 61.9 2.80 4.056 --

grflds8 393101107391001 -- 1,960 -- 3.599 --

grflds9 393101107393201 -- 4,900 -- 0.488 --

grflds9 393101107393201 888 4,250 0.206 0.433 1.32

grflds10 392524107370301 -- 6.30 2.44 3.703 --

grflds11 392800107382101 -- 104 -- 4.238 --

grflds20 393051107490601 -- -- -- 3.164 --

grflds21 393031107490201 -- 149 1.57 3.136 7.33

grflds22 392750107465801 -- -- -- 3.254 --

grflds22-rep 392750107465801 -- -- -- 3.248 --

grflds23 392853107462101 -- -- -- 3.513 --

grflds24 392930107454601 -- -- -- 3.340 --

grflds27 392421107392701 3,729 -- -- 4.014 --

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 13

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Xenon

(10–8 cm3 STP/g) R/Ra

Recharge

temperature

(°C)

Excess air

(cm3 STP/L)

Excess

nitrogen gas 

(mg/L as N)

Nitrate

concentration at 

the time of recharge 

(mg/L as N)

grfldn1 393528107453601 -- 0.67 7.9 4.0 1.4 5.4

grfldn2 393237107450901 -- 1.02 7.9 4.2 <1 1.51

grfldn3 393318107373601 -- 0.64 7.4 4.3 <1 0.18

grfldn4 393310107422101 -- 9.0 2.2 <1 2.24

grfldn5 393407107430901 -- 0.99 5.6 3.8 <1 1.61

grfldn6 393456107404501 -- 0.98 14.8 3.0 <1 0.62

grfldn7 393314107455801 -- 1.15 7.9 2.4 3.1 3.2

grfldn8 393337107394601 -- 0.63 9.5 1.4 <1 1.14

grfldn9 393311107402201 -- 0.95 6.5 1.3 <1 0.79

grfldn10 393415107415201 -- 1.07 6.6 7.6 <1 0.67

grfldn10-rep 393415107415201 -- -- -- -- -- --

grflds1 392631107411401 -- 1.15 7.0 5.6 <1 6.74

grflds2 392712107440101 -- 1.15 7.2 5.1 <1 3.03

grflds4 392922107375001 -- -- -- -- --

grflds4 392922107375001 1.13 0.017 7.9 2.2 0.8 --

grflds5 392928107392901 -- -- 7.4 0.4 <1 0.25

grflds6 392921107382601 -- -- 8.8 1.9 14.0 14.2

grflds7 393009107375401 -- -- 7.8 8.8 <1 0.94

grflds8 393101107391001 -- -- 7.9 4.0 9.2 9.6

grflds9 393101107393201 -- -- -- -- -- --

grflds9 393101107393201 0.112 0.009 -- -- -- --

grflds10 392524107370301 -- 1.11 7.4 5.2 <1 1.20

grflds11 392800107382101 -- -- 7.9 11.0 2.6 2.8

grflds20 393051107490601 -- -- 8.0 –1.1 <1 3.26

grflds21 393031107490201 1.04 0.174 10.1 0.4 <1 1.54

grflds22 392750107465801 -- -- 8.2 1.7 <1 0.72

grflds22-rep 392750107465801 -- -- -- -- -- --

grflds23 392853107462101 -- -- 7.9 4.0 <1 3.37

grflds24 392930107454601 -- -- 8.2 1.9 1.5 1.6

grflds27 392421107392701 -- -- 7.9 9.2 3.9 4.1

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
14 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Denitrification 

reaction progress

Sulfur hexafluoride, 

mean concentration

in solution

(fmol/L)

Sulfur hexafluoride,

calculated atmospheric

concentration

(pptv)

Chlorofluorocarbon-11,

mean concentration

in solution

(pg/kg)

grfldn1 393528107453601 0.25 -- -- --

grfldn2 393237107450901 0.00 -- -- --

grfldn3 393318107373601 0.00 -- -- --

grfldn4 393310107422101 0.00 6.92 16.14 456

grfldn5 393407107430901 0.00 -- -- --

grfldn6 393456107404501 0.00 -- -- --

grfldn7 393314107455801 0.98 -- -- --

grfldn8 393337107394601 0.00 -- -- --

grfldn9 393311107402201 0.00 -- -- 538

grfldn10 393415107415201 0.00 -- -- --

grfldn10-rep 393415107415201 -- -- -- --

grflds1 392631107411401 0.00 -- -- 21

grflds2 392712107440101 0.00 -- -- 30

grflds4 392922107375001 -- -- -- --

grflds4 392922107375001 -- -- -- --

grflds5 392928107392901 0.00 2.35 6.38 554

grflds6 392921107382601 0.99 0.29 0.69 30

grflds7 393009107375401 0.00 -- -- 31

grflds8 393101107391001 0.96 0.28 0.53 20

grflds9 393101107393201 -- -- -- --

grflds9 393101107393201 -- -- -- --

grflds10 392524107370301 0.00 -- -- 374

grflds11 392800107382101 0.92 0.76 0.92 16

grflds20 393051107490601 0.00 0.95 2.77 200

grflds21 393031107490201 0.00 0.48 1.46 189

grflds22 392750107465801 0.00 0.90 2.23 45

grflds22-rep 392750107465801 -- -- -- --

grflds23 392853107462101 0.00 0.63 1.23 76

grflds24 392930107454601 0.93 0.07 0.16 19

grflds27 392421107392701 0.96 1.26 1.70 17

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 15

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Chlorofluorocarbon-11,

calculated atmospheric

concentration

(pptv)

Chlorofluorocarbon-12,

mean concentration

in solution

(pg/kg)

Chlorofluorocarbon-12,

calculated atmospheric

concentration

(pptv)

Chlorofluorocarbon-113,

mean concentration

in solution

(pg/kg)

grfldn1 393528107453601 -- -- -- --

grfldn2 393237107450901 -- -- -- --

grfldn3 393318107373601 -- -- -- --

grfldn4 393310107422101 186 272 478 62

grfldn5 393407107430901 -- -- -- --

grfldn6 393456107404501 -- -- -- --

grfldn7 393314107455801 -- -- -- --

grfldn8 393337107394601 -- -- -- --

grfldn9 393311107402201 191 352 554 74

grfldn10 393415107415201 -- -- -- --

grfldn10-rep 393415107415201 -- -- -- --

grflds1 392631107411401 7.8 219 347 2.66

grflds2 392712107440101 11 84 135 5

grflds4 392922107375001 -- -- -- --

grflds4 392922107375001 -- -- -- --

grflds5 392928107392901 208 325 541 68

grflds6 392921107382601 12 28 50 10

grflds7 393009107375401 12 104 166 3.6

grflds8 393101107391001 8 68 115 4

grflds9 393101107393201 -- -- -- --

grflds9 393101107393201 -- -- -- --

grflds10 392524107370301 142 258 420 56

grflds11 392800107382101 6.3 46 73 1.8

grflds20 393051107490601 77 222 379 43

grflds21 393031107490201 82 222 423 28

grflds22 392750107465801 18 104 186 5.6

grflds22-rep 392750107465801 -- -- -- --

grflds23 392853107462101 34 72 126 14

grflds24 392930107454601 7.4 6 10 3

grflds27 392421107392701 7 30 49 5

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
16 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier

Chlorofluorocarbon-113,

calculated atmospheric

concentration

(pptv)

Apparent age

of young fraction 

(years)

Percent

young 

water

Tracers used to determine age

grfldn1 393528107453601 -- 8 100 tritium/helium-3

grfldn2 393237107450901 -- 3 100 tritium/helium-3

grfldn3 393318107373601 -- 6 100 tritium/helium-3

grfldn4 393310107422101 59 20 100 tritium, chlorofluorocarbon-12 and -113

grfldn5 393407107430901 -- <1 -- tritium/helium-3

grfldn6 393456107404501 -- <1 -- tritium/helium-3

grfldn7 393314107455801 -- 12 -- tritium/helium-3

grfldn8 393337107394601 -- 3 100 tritium/helium-3

grfldn9 393311107402201 62 21 100 tritium, chlorofluorocarbon-113

grfldn10 393415107415201 -- 5 -- tritium/helium-3

grfldn10-rep 393415107415201 -- -- --

grflds1 392631107411401 2.2 30 100 tritium, chlorofluorocarbon-12

grflds2 392712107440101 4.3 39 100 chlorofluorocarbon-12 and -113

grflds4 392922107375001 -- -- -- --

grflds4 392922107375001 -- -- -- --

grflds5 392928107392901 60 21 100 tritium, chlorofluorocarbon-113

grflds6 392921107382601 10 31 100 tritium and sulfur hexafluoride

grflds7 393009107375401 3.1 37 100 tritium, chlorofluorocarbon-12

grflds8 393101107391001 3.6 27 50 tritium and sulfur hexafluoride

grflds9 393101107393201 -- >50 0 tritium

grflds9 393101107393201 -- -- -- --

grflds10 392524107370301 49 23 100 tritium, chlorofluorocarbon-12 and -113

grflds11 392800107382101 1.5 28 100 tritium and sulfur hexafluoride

grflds20 393051107490601 39 25 100 tritium, chlorofluorocarbon-12 and -113

grflds21 393031107490201 28 28 100 tritium, chlorofluorocarbon-113

grflds22 392750107465801 5.4 25 13 tritium, chlorofluorocarbon-113

grflds22-rep 392750107465801 -- -- --

grflds23 392853107462101 12 25 30 chlorofluorocarbon-12 and -113

grflds24 392930107454601 3 >50 0 tritium

grflds27 392421107392701 4 23 100 tritium and sulfur hexafluoride

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 17

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier Model used to determine age Age comment

grfldn1 393528107453601 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn2 393237107450901 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn3 393318107373601 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn4 393310107422101 piston-flow model

grfldn5 393407107430901 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn6 393456107404501 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn7 393314107455801 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn8 393337107394601 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn9 393311107402201 piston-flow model

grfldn10 393415107415201 piston-flow model minimum age, tritiogenic helium-3 degassing likely

grfldn10-rep 393415107415201

grflds1 392631107411401 exponential mixing model

grflds2 392712107440101 piston-flow model

grflds4 392922107375001 -- high methane, gas stripped

grflds4 392922107375001 -- high methane, gas stripped

grflds5 392928107392901 piston-flow model

grflds6 392921107382601 piston-flow model

grflds7 393009107375401 piston-flow model low dissolved oxygen, chlorofluorocarbon-12 could be degraded

grflds8 393101107391001 binary-mixing model

grflds9 393101107393201 --

grflds9 393101107393201 --

grflds10 392524107370301 piston-flow model

grflds11 392800107382101 piston-flow model

grflds20 393051107490601 piston-flow model

grflds21 393031107490201 piston-flow model

grflds22 392750107465801 binary-mixing model

grflds22-rep 392750107465801

grflds23 392853107462101 binary-mixing model

grflds24 392930107454601 --

grflds27 392421107392701 piston-flow model

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
18 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 1. Chemical data for water collected from domestic wells in Garfield County, Colorado.—Continued

[NGVD29, North American Vertical Datum of 1929; m, meters; °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; NTU, nephelometric turbidity units; <, less than; --, no data; E, estimated1; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; 

‰, per mil; VSMOW, Vienna Standard Mean Ocean Water; TU, tritium units; VPDB, Vienna PeeDee Belemnite; mol %, mole percent; cm3 STP/g, cubic 

centimeters at standard temperature and pressure, per gram of water; R/Ra, helium-3/helium-4 ratio in the sample divided by the helium-3/helium-4 ratio in air; 

cm3 STP/L, cubic centimeters at standard temperature and pressure, per liter of water; g, gram; fmol/L, femtomole per liter; pptv, parts per trillion by volume; 

pg/kg, picogram per kilogram]

Site name Site identifier δ2

H, water

(‰, VSMOW)

δ18O, water

(‰, VSMOW)

Tritium

(TU)

δ2H, methane

(‰, VSMOW)

δ13C, methane

(‰, VPDB)

grfldn1 393528107453601 –115.7 –15.18 5.5 -- --

grfldn2 393237107450901 –118.03 –15.44 5.4 -- --

grfldn3 393318107373601 –116.12 –15.05 5.6 -- --

grfldn4 393310107422101 –121.75 –16.25 6.6 -- --

grfldn5 393407107430901 –119.57 –15.7 5.5 -- --

grfldn6 393456107404501 –121.58 –16.02 5.0 -- --

grfldn7 393314107455801 –120.63 –15.75 5.5 -- --

grfldn8 393337107394601 –119.64 –15.74 6.0 -- --

grfldn9 393311107402201 –120.56 –15.68 5.9 -- --

grfldn10 393415107415201 –120.8 –15.93 6.0 -- --

grfldn10-rep 393415107415201 -- -- -- -- --

grflds1 392631107411401 –113.32 –15.22 11.5 -- --

grflds2 392712107440101 –119.07 –15.99 13.1 -- --

grflds4 392922107375001 –100.25 –13.7 6.2 -- --

grflds4 392922107375001 -- -- -- –152.1 –41.76

grflds5 392928107392901 –113.4 –15.15 8.2 -- --

grflds6 392921107382601 –121.23 –15.48 8.6 -- --

grflds7 393009107375401 –112.51 –15.02 8.3 -- --

grflds8 393101107391001 –115.03 –14.98 2.1 -- --

grflds9 393101107393201 –58.28 –8.9 <0.3 -- --

grflds9 393101107393201 -- -- -- –192.4 –65.75

grflds10 392524107370301 –114.46 –15.33 7.4 -- --

grflds11 392800107382101 –120.63 –15.77 10.5 -- --

grflds20 393051107490601 –119.35 –16.02 7.1 -- --

grflds21 393031107490201 –117.99 –15.94 9.5 -- --

grflds22 392750107465801 –123.35 –16.29 0.9 -- --

grflds22-rep 392750107465801 -- -- -- -- --

grflds23 392853107462101 –122.96 –16.17 0.3 -- --

grflds24 392930107454601 –124.82 –16.25 <0.5 -- --

grflds27 392421107392701 –116.84 –15.59 6.3 –146.3 –50.37

1

Estimated (E) concentrations are those concentrations that are greater than or equal to the long-term method detection limit but less than the laboratory 

reporting level or lowest calibration standard, whichever is greater (Childress and others, 1999).
Study Methods 19

For samples with dissolved-oxygen concentrations less 

than or equal to 2 mg/L, excess nitrogen gas from denitrification was calculated according to equation 2:

Excess N2 = N2(measured)

– N2(equilibrium) – N2(excess air) (2)

where

Excess N2 is the concentration of excess nitrogen gas 

from denitrification in the sample,

N2(measured) is the measured concentration of nitrogen 

gas in the sample,

N2(equilibrium) is the concentration of nitrogen gas from 

equilibration with the atmosphere in the 

sample (calculated for each sample on 

the basis of its recharge temperature and 

elevation, and salinity),

and

N2(excess air) is the concentration of nitrogen gas from 

excess air in the sample.

The initial nitrate concentration (Initial nitrate) in the 

sample at the time it recharged the aquifer was calculated 

according to equation 3:

Initial nitrate = Measured nitrate + Excess N2 (3)

Measured nitrate is the measured concentration of nitrate 

in the sample. If the sample was not affected by denitrification 

then Excess N2 equals zero, and Initial nitrate equals Measured 

nitrate. An estimate of the extent to which nitrate was removed 

from groundwater by denitrification was calculated according 

to equation 4:

Denitrification reaction progress = 1

– (Measured nitrate/Initial nitrate) (4)

Denitrification reaction progress would equal zero if no 

nitrate was removed by denitrification, and it would equal one 

if all the nitrate was removed by denitrification.

Groundwater Age

Groundwater ages calculated from environmental tracers 

like tritium, chlorofluorocarbons, sulfur hexafluoride, and 

tritium/helium-3 are referred to as “apparent ages” because 

they are based on simplifying assumptions regarding transport 

processes that may affect tracer concentrations in groundwater 

(Plummer and Busenberg, 2000; Rupert and Plummer, 2009). 

The most straightforward assumption is that transport processes (for example, mixing or dispersion) have no effect on 

the concentration of tracers as they move in an aquifer. Thus 

the tracer concentration in water at the sampling point (such as 

a well) is the same as the concentration at the time the water 

recharged the aquifer. This model is referred to as the “pistonflow model.” The piston-flow model appears to be a reasonable representation of groundwater age in some instances, such 

as in shallow, short-screened wells. In other instances, mixing 

or dispersion affect measured tracer concentrations, particularly for wells with long-screened intervals. The simplest 

model describing the effect of mixing on tracer concentrations 

is the binary-mixing model, which assumes the water is a 

mixture of old (pretracer or pre 1950s) water and young water 

(containing tracer or post 1950s). More complicated models 

of mixing assume the sampled water represents a distribution of ages. These models, called lumped-parameter models, yield a mean age of the water sample. Perhaps the most 

common lumped-parameter model is the exponential model 

(Maloszewski and Zuber, 1982, 1996; Cook and Böhlke, 

2000). The exponential model could describe the mean age of 

water discharged from a long-screened well in an unconfined 

aquifer receiving distributed recharge, for example.

The following sections discuss in more specific terms 

the application of tritium, chlorofluorocarbons, sulfur hexafluoride, and tritium/helium-3 tracers to groundwater dating. 

These tracers were used to determine apparent ages of young 

(generally, post 1950s) groundwater and the dilution of young 

groundwater with old (pre 1950s) groundwater.

Tritium.—Tritium is a radioactive isotope of hydrogen with a half-life of 12.32 years (Lucas and Unterweger, 

2000). Small concentrations of tritium are produced naturally 

by interactions between the atmosphere and cosmic rays. It 

is an excellent tracer of water movement because it is part 

of the water molecule. In general, tritium in groundwater 

originates from precipitation. Because tritium is radioactive, 

its concentration in groundwater decreases over time because 

of radioactive decay. Before the onset of atmospheric testing 

of nuclear weapons in 1953 (prebomb), the tritium content 

of precipitation in the central United States probably ranged 

from about 3 to 8 tritium units (TU) (Kaufman and Libby, 

1954; Thatcher, 1962). As a result of radioactive decay, 

groundwater derived from precipitation that fell before 1953 

would have contained less than 0.5 TU tritium in 2010. The 

tritium content of precipitation increased substantially after 

the onset of atmospheric nuclear weapons testing with the 

addition of bomb tritium but has slowly decreased from its 

peak in the early 1960s (fig. 3). Even with the variability in 

tritium content of precipitation over time, groundwater totally 

derived from precipitation that fell since 1953 (postbomb) 

contained more than 0.5 TU in 2010.

Chlorofluorocarbons.—Chlorofluorocarbons are stable 

synthetic organic compounds that were used as refrigerants 

and in other industrial applications beginning in the 1930s 

(Plummer and Busenberg, 2000). Concentrations of chlorofluorocarbons in the atmosphere rose sharply in the 1960s 

and peaked in the early 1990s (Plummer and Busenberg, 

2000; Plummer and others, 2006) (fig. 3). The use of 
20 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

chlorofluorocarbons to determine groundwater age depends 

on relating their concentrations in groundwater to atmospheric 

concentrations in the recharge area at the time of recharge.

The concentration of chlorofluorocarbons in the atmosphere at the time a sample recharged the aquifer can be back 

calculated using the measured chlorofluorocarbon concentrations in the sample; the recharge temperature and excess-air 

concentration determined from the nitrogen-gas and argon 

concentrations, as discussed in the Sample Analysis section; salinity; recharge elevation; and Henry’s Law. The year 

of recharge is determined by referring the back-calculated 

atmospheric concentration to a curve of the atmosphericconcentration history and picking the year corresponding to 

the back-calculated concentration (fig. 3). Apparent age simply 

is the sample date minus the recharge date. Since the early to 

mid 1990s, atmospheric concentrations of chlorofluorocarbons 

have begun to level off or decline (fig. 3), greatly limiting 

their use as age tracers for water recharged during that period. 

Details of the chlorofluorocarbon-dating method, including a 

discussion of subsurface processes that could produce erroneous apparent ages, can be found in Plummer and Busenberg 

(2000) and Plummer and others (2006). One such process 

relevant to the current study is chlorofluorocarbon biodegradation in anoxic groundwater. Biodegradation would reduce 

chlorofluorocarbon concentrations in groundwater and result 

in the calculated ages being biased old.

Sulfur Hexafluoride.—Sulfur hexafluoride is a stable 

gas that mainly has been used as an electrical insulator in 

high-voltage switches and transformers. It was produced 

in significant quantities beginning in the 1950s (Busenberg 

and Plummer, 2000). Release of sulfur hexafluoride to the 

atmosphere rose sharply in the 1980s and is ongoing (fig. 3). 

Dating groundwater with sulfur hexafluoride essentially follows the same principals as chlorofluorocarbon dating. Two 

advantages of sulfur hexafluoride over chlorofluorocarbons 

are that sulfur hexafluoride does not degrade under anoxic 

conditions and it can be used as a tracer of groundwater age 

for water recharged since the 1990s because of its continued 

increase in the atmosphere during that period. Details of the 

sulfur-hexafluoride dating method, including a discussion of 

subsurface processes that could produce erroneous apparent 

ages, can be found in Busenberg and Plummer (2000).

Tritium/Helium-3.—Helium-3 is produced from the 

radioactive decay of tritium. As discussed previously, tritium 

in groundwater recharged since the early 1950s is dominated 

by bomb tritium. Helium-3 derived from the decay of bomb 

tritium is referred to as “tritiogenic helium-3”. Given that 

the half-life of tritium is well known (12.32 years), if the 

amount of bomb tritium and tritiogenic helium-3 in a sample 

can be measured then the apparent age of that sample can 

be calculated using a form of the radioactive-decay equation 

(Solomon and Cook, 2000). Whereas the tritium concentration 

in a sample can be measured directly, the tritiogenic helium-3 

concentration in a sample must be calculated from mass 

balances on helium-3 and helium-4 concentrations, and the 

helium-3/helium-4 ratios of the predominant helium sources in 

the system. Typically, these helium sources include the atmosphere (helium-3/helium-4 = 1.384 × 10–6; Clarke and others, 

1976) and the decay of uranium and thorium in rocks and 

minerals in the earth’s crust (helium-3/helium-4 = 2 × 10–8; 

Mamyrin and Tolstikhin, 1984). In some instances, mantle 

helium also is important (helium-3/helium-4 = ~1 × 10–5; 

Ozima and Podosek, 1983). Helium mass balance is necessary 

because the vast majority (usually greater than 99.9 percent) 

of the measured helium in a groundwater sample is helium-4. 

Moreover, the helium-4 content of groundwater typically 

increases over time because of helium-4 production in the sediment from uranium and thorium decay. For tritium/helium-3 

dating in this study, it was assumed that the earth’s crust was 

the only subsurface source of helium (in other words, no 

mantle helium), with a helium-3/helium-4 ratio of 2 × 10–8. 

The basis for this assumption is discussed in more detail in 

the Helium Concentrations and Isotopes section of this report. 

Details of the tritium/helium-3 dating method, including a 

discussion of subsurface processes that could produce erroneous apparent ages, can be found in Schlosser and others 

(1988, 1989), Solomon and Cook (2000), Böhlke and others 

(2007), and McMahon and others (2010). One such process 

relevant to the current study is incomplete tritiogenic helium-3 

confinement in the aquifer because of low recharge rates. 

Tritiogenic helium-3 could diffuse across the water table and 

exit the system if recharge rates were too low to trap it below 

the water table. Incomplete tritiogenic helium-3 confinement 

would result in the calculated ages being biased young.

Figure 3. Concentrations of chlorofluorocarbons and sulfur 

hexafluoride in North American air, and concentrations of tritium 

in precipitation at Salt Lake City, Utah, decayed to 2010.

600

500

400

300

200

100

0

1940 1950 1960 1970 1980 1990 2000 2010

0

1

2

3

4

5

6

7

8

Concentration of SF6 in air, in pptv

Year

Concentration of CFC’s in air, in pptv, and concentration 

of 3H in precipitation (decayed to 2010), in TU

CFC-12

SF6

CFC-11

CFC-113

3

H

EXPLANATION

CFC, chlorofluorocarbon

SF6

, sulfur hexafluoride

pptv, parts per trillion by volume

TU, tritium units

3

H, tritium (Manning and others, 2005)
Geochemical Data Sets 21

Geochemical Data Sets

This section describes and compares the spatial distributions of oxygen, nitrate, methane, and geochemical water 

types (based on major-ion data) in the Wasatch Formation.

Distributions of Oxygen, Nitrate, and Methane 

in Groundwater

Concentrations of dissolved oxygen in water from the 

sampled wells ranged from 0.1 to 8.6 mg/L (table 1), with the 

lowest concentrations occurring in a north-south trending zone 

located south of Silt (fig. 4). All the samples collected north of the 

Colorado River and 9 of 16 samples collected south of the river 

were considered to be oxygen reducing, or oxic (dissolved oxygen concentration greater than or equal to 0.5 mg/L), on the basis 

of the redox classification scheme of McMahon and Chapelle 

(2008) (table 1). Of the remaining 7 samples collected south of 

the river, 4 were classified as suboxic, 1 was manganese reducing, 

1 was a mixture of oxygen and manganese reducing, and 1 was 

denitrifying (table 1).

Concentrations of nitrite in most of the samples were less 

than the reporting level of 0.002 mg/L as N (table 1); therefore, 

concentrations of nitrite plus nitrate consisted almost entirely of 

nitrate and will be referred to as such for the remainder of this 

report. Concentrations of nitrate ranged from less than 0.04 to 

6.74 mg/L as N (table 1). Nitrate concentrations in water samples 

with dissolved-oxygen concentrations less than 0.5 mg/L were 

significantly (p = 0.03) lower than nitrate concentrations in water 

samples with dissolved-oxygen concentrations greater than or 

equal to 0.5 mg/L on the basis of the nonparametric Wilcoxon 

rank sum test (Helsel and Hirsch, 1992). Some of the lowest 

nitrate concentrations occurred in the north-south trending zone 

of low dissolved-oxygen concentrations located south of Silt 

(fig. 5). In contrast, that zone hosted the highest methane concentrations (fig. 6). Overall, methane concentrations ranged from 

less than 0.0005 to 32.5 mg/L (table 1) and were significantly 

(p = 0.007) higher in water samples with dissolved-oxygen 

concentrations less than 0.5 mg/L than in samples with dissolvedoxygen concentrations greater than or equal to 0.5 mg/L. This 

pattern of low nitrate and high methane concentrations in groundwater containing little or no dissolved oxygen can be explained 

by the general principals of redox chemistry. Nitrate, presumably 

from anthropogenic or natural sources at the land surface, is likely 

to be degraded by denitrifying bacteria in anoxic groundwater 

(Chapelle and others, 1995; McMahon and Chapelle, 2008). In 

contrast, the potential for local biogenic methane production is 

greater in anoxic groundwater than in oxic groundwater. Methane 

produced externally by biogenic or thermogenic processes, and 

subsequently transported to the aquifer by natural or anthropogenic processes, also would be more likely to persist in anoxic 

groundwater than in oxic groundwater because of the possibility 

of methane degradation (oxidation) by methanotrophic bacteria 

in oxic groundwater (Barker and Fritz, 1981; Whiticar, 1999). 

Anoxic methane oxidation also may occur in aquifers, but generally that process is more common in marine sediments (Smith and 

others, 1991; Zhang and others, 1998; Whiticar, 1999; Grossman 

and others, 2002; Van Stempvoort and others, 2005). The processes of denitrification and methane oxidation are discussed in 

more detail in the Sources and Sinks of Nitrate in Groundwater 

and the Sources and Sinks of Methane in Groundwater sections 

of this report.

In general, the data in figures 4–6 indicate that field measurements of dissolved oxygen in groundwater could be useful 

indicators of the Wasatch Formation’s vulnerability to nitrate 

and methane contamination, or enrichment in the case of 

naturally derived constituents. Oxic parts of the aquifer would 

be relatively more vulnerable to nitrate contamination and 

less vulnerable to methane contamination. Anoxic parts of the 

aquifer would be relatively less vulnerable to nitrate contamination and more vulnerable to methane contamination.

Major-Ion Chemistry

Water samples containing less than 1 mg/L methane and 

collected north of the Colorado River generally were mixedcation-sulfate-bicarbonate waters, whereas low-methane 

samples collected south of the river mostly were mixedcation-bicarbonate-sulfate waters (fig. 7). Three wells located 

south of the river (grflds4, grflds9, and grflds27) produced 

water with more than 1 mg/L methane, and they were sodiumbicarbonate to sodium-chloride waters (fig. 7). Of those 

samples, the one from grflds9 appeared to be closest in majorion composition to produced water from the Mesaverde Group 

(fig. 7). Well grflds9 was screened from 120 to 134 m below 

land surface, which is considerably deeper than all the other 

wells except grflds4 (screened from 120 to 151 m below land 

surface) (table 1). Samples from two wells located north of 

the river (grfldn1 and grfldn10) and three wells located south 

of the river (grflds6, grflds8, and grflds11) also were relatively 

enriched in sodium but did not have methane concentrations 

greater than 1 mg/L (fig. 7).

The major-ion chemistry of water collected for this 

study showed similar overall patterns to the chemistry of 

water collected from domestic wells during previous studies (fig. 8). The data in figure 8 were retrieved from the 

U.S. Geological Survey Piceance Basin common data repository (U.S. Geological Survey, 2010b). Many of the wells in 

the data repository were sampled on more than one date so 

only the most recent sample with complete data was used in 

figure 8. This same approach was used for other figures in the 

report in which data from the common data repository were 

displayed. Many of the previously collected samples containing less than 1 mg/L methane were mixed-cation-bicarbonatesulfate waters, but several low-methane waters also were relatively enriched in sodium. Most of the high-methane waters 

were relatively enriched in sodium as well.
22 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Figure 4. Concentrations of dissolved oxygen in water from domestic wells sampled for this study.

grfldn1

grfldn2

grfldn3

grfldn4

grfldn5

grfldn6

grfldn7 grfldn8

grfldn9

grfldn10

grflds1

grflds2

grflds4 grflds5

grflds6

grflds7

grflds8

grflds9

grflds10

grflds11

grflds20

grflds21

grflds22

grflds23

grflds24

grflds27

Rifle

Silt

Divide Creek anticline

Rifle-Grand hogback syncline

Unnamed syncline

Government

Creek

Ma mm Creek

Col orado River

Divide Creek

107°50' 107°45' 107°40'

39°35'

39°30'

39°25'

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Transverse Mercator projection, Zone 13.

Geology from Tweto (1979)

Geologic structure modified from URS Corporation (2006)

and Hoak and Klawitter (1997)

EXPLANATION

Geologic unit

Quaternary deposits

Tertiary units

Cretaceous units

Fault (approximate)

Dissolved oxygen, 

 milligrams per liter

Less than 0.5

0.5 to less than 2.0

Greater than or equal 

 to 2.0

0 5 KILOMETERS

0 5 MILES

Axis and direction of plunge 

 of anticline or syncline

Direction of dip of limb 

 of anticline

Direction of dip of limb 

 of syncline
Geochemical Data Sets 23

Figure 5. Concentrations of nitrate in water from domestic wells sampled for this study.

grfldn1

grfldn2

grfldn3

grfldn4

grfldn5

grfldn6

grfldn7 grfldn8

grfldn9

grfldn10

grflds1

grflds2

grflds4 grflds5

grflds6

grflds7

grflds8

grflds9

grflds10

grflds11

grflds20

grflds21

grflds22

grflds23

grflds24

grflds27

Rifle

Silt

Divide Creek anticline

Rifle-Grand hogback syncline

Unnamed syncline

Government Creek

Ma mm Creek

Col orado River

Divide Creek

107°50' 107°45' 107°40'

39°35'

39°30'

39°25'

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Transverse Mercator projection, Zone 13.

Geology from Tweto (1979)

Geologic structure modified from URS Corporation (2006)

and Hoak and Klawitter (1997)

EXPLANATION

Geologic unit

Quaternary deposits

Tertiary units

Cretaceous units

Fault (approximate)

Nitrate, milligrams per 

 liter as nitrogen

less than 1

1 to less than 5

greater or equal to 5

0 5 KILOMETERS

0 5 MILES

Axis and direction of plunge

 of anticline or syncline

Direction of dip of limb 

 of anticline

Direction of dip of limb 

 of syncline
24 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Figure 6. Concentrations of methane in water from domestic wells sampled for this study.

grfldn1

grfldn2

grfldn3

grfldn4

grfldn5

grfldn6

grfldn7 grfldn8

grfldn9

grfldn10

grflds1

grflds2

grflds4 grflds5

grflds6

grflds7

grflds8

grflds9

grflds10

grflds11

grflds20

grflds21

grflds22

grflds23

grflds24

grflds27

Rifle

Silt

Divide Creek anticline

Rifle-Grand hogback syncline

Unnamed syncline

Government Creek

Ma mm Creek

Col orado River

Divide Creek

107°50' 107°45' 107°40'

39°35'

39°30'

39°25'

EXPLANATION

Geologic unit

Quaternary deposits

Tertiary units

Cretaceous units

Fault (approximate)

Methane, milligrams 

 per liter

Less than 0.0005

0.0005 to less than 1.0

Greater or equal to 1.0

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Transverse Mercator projection, Zone 13

Geology from Tweto (1979)

Geologic structure modified from URS Corporation (2006)

and Hoak and Klawitter (1997)

Axis and direction of plunge

 of anticline or syncline

Direction of dip of limb 

 of anticline

Direction of dip of limb 

 of syncline

0 5 KILOMETERS

0 5 MILES
Groundwater Mixing 25

Domestic wells

North of Colorado River,

Methane less than 1 milligram per liter 

Mixed cation-SO4

-HCO3

South of Colorado River,

Methane less than 1 milligram per liter 

Mixed cation-HCO3

-SO4

South of Colorado River,

Methane greater than or equal to 1 milligram per liter 

Na-HCO3

 to Na-Cl

Produced water, Mesaverde Group, Garfield County, Colorado 

 (U.S. Geological Survey, 2010a)

EXPLANATION

Calcium (Ca) + magnesium (Mg)

Sulfate (SO4

)

Chloride (Cl) Calcium (Ca)

Magnesium (Mg)

Sulfate (SO4

) + chloride (Cl)

Bicarbonate (HCO3

) + carbonate (CO3

)

Sodium (Na) + potassium (K)

PERCENT MILLIEQUIVALENTS

grflds8

grflds6

grflds11

grflds9

grflds4

grflds27

grfldn10

grfldn1

20

20

20

40

40

40

60

60

60

80

80

80

80

80

60

60

40

40

20

20

20

20

20

40

40

40

60

60

60

80

80

80

Groundwater Mixing

Mixing groundwater from different sources could have 

important implications for the quality of groundwater in 

the Wasatch Formation. For example, when young, nitratecontaminated groundwater is mixed with old, nitrate-free 

groundwater or when young, dilute groundwater is mixed with 

old, saline groundwater or old, methane-enriched groundwater 

the quality of the resulting mixture could be better or worse 

than the end-member waters. Sorting out mixing processes 

that could involve water, ions, and gases requires multiple 

tracers because those constituents may not all be derived from 

the same sources. This section of the report examines mixing 

processes in the Wasatch Formation using stable isotopes of 

water (δ2H[H2O] and δ18O[H2O]), groundwater apparent ages, 

and chloride/bromide mass ratios.

Figure 7. Major-ion chemistry of water from domestic wells sampled for this study, and major-ion chemistry 

of produced water from the Mesaverde Group.
26 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Stable Isotopes of Water

δ2H [H2O] and δ18O[H2O] values for samples collected for 

this study plotted near the Global Meteoric Water Line (Craig, 

1961), indicating the water was derived from precipitation 

(fig. 9A). The samples plotted between the isotopic values for 

snow from Grand Mesa (located about 60 km southwest of the 

study area) and water from the Mesaverde Group. All but two of 

the samples (from grflds4, grflds9) plotted near the snow values, 

although shifted toward slightly more positive values, indicating 

they probably were derived from lower elevation precipitation 

than what was represented by the Grand Mesa snow samples. 

The Grand Mesa snow samples were collected at an elevation of 

3,230 m, whereas the land-surface elevations of the sampled wells 

ranged from 1,672 to 2,155 m (table 1). The samples also could 

have contained small fractions of water from the Mesaverde 

Group, but that is difficult to determine from the isotopic values 

alone. The samples from grflds4 and grflds9 appeared to contain 

much larger fractions of water from the Mesaverde Group than 

the other samples (fig. 9A). All the water from grflds9 could have 

been from the Mesaverde Group on the basis of the δ2H[H2O] and 

δ18O[H2O] values and major-ion data (fig. 7).

A similar pattern in the water isotopic values was 

observed for samples collected from domestic wells during previous studies (fig. 9B). All but two of those samples 

(LLOY2WW, SCHW1WW) consisted mostly of snowmelt 

Figure 8. Major-ion chemistry of water from previously sampled domestic wells in the study area, and 

major-ion chemistry of produced water from the Mesaverde Group.

Domestic wells (U.S. Geological Survey, 2010b)

South of Colorado River, methane 

 less than 1 milligram per liter 

South of Colorado River, methane greater 

 than or equal to 1 milligram per liter 

Produced water, Mesaverde Group, Garfield County, Colorado 

 (U.S. Geological Survey, 2010a)

EXPLANATION

Magnesium (Mg)

Sodium (Na) + potassium (K)

Calcium (Ca)

PERCENT MILLIEQUIVALENTS

Bicarbonate (HCO3

) + carbonate (CO3

)

Chloride (Cl)

Sulfate (SO4

)

Sulfate (SO4

) + chloride (Cl)

Calcium (Ca) + magnesium (Mg)

20

20

40

40

60

60

80

80

20

20

40

40

60

60

80

80

80

80

60

60

40

40

20

20

80

60

40

20

20

40

60

80

LLOY2WW

DIET1WW

LANG13WW
Groundwater Mixing 27

recharge. LLOY2WW and SCHW1WW apparently con- entirely of young groundwater plotted along the piston-flow line, 

tained relatively large fractions of water from the Mesaverde whereas samples that were mixtures of old and young groundwaGroup. Previously collected major-ion data for LLOY2WW ter plotted below the piston-flow line. For the example shown by 

also indicated a similar composition to that of water from the the dashed line in figure 10, the percentages refer to the fracMesaverde Group (fig. 8). All four wells in figure 9 that con- tion of young groundwater in a mixture of old water and water 

tained relatively large fractions of water from the Mesaverde recharged in 1985. Some of the data points that plotted below the 

Group were located south of Silt in the zone that had ground- piston-flow line were not necessarily mixtures of young and old 

water with low concentrations of dissolved oxygen and nitrate water, but rather had apparently undergone some chlorofluorocarbon-113 loss, possibly because of biodegradation under anoxic and high concentrations of methane (figs. 4–6). conditions (Plummer and others, 2006).

The relatively old age of water from grflds9, as indicated by Apparent Groundwater Ages its tritium concentration being less than 0.5 TU, is consistent with 

the water isotopic data for that sample (fig. 9A), which indicate All but two of the dated water samples collected for this it contained a large fraction of water from the Mesaverde Group. study contained a component of young (less than 50 years) The tritium and isotopic data for water from grflds4 indicate that groundwater on the basis of tritium, chlorofluorocarbon, and sample contained a larger fraction of young groundwater than did 

sulfur hexafluoride data (table 1). Two samples (from grflds9 water from grflds9 (table 1 and fig. 9A). The other sample consistand grflds24) consisted entirely of old groundwater (greater than ing entirely of old groundwater (from grflds24, with a tritium 

50 years), 3 samples were mixtures of old and young groundwater concentration less than 0.5 TU) had water isotopic values similar 

(from grflds8, grflds22, and grflds23), and 12 samples consisted to snowmelt (table 1), indicating that not all the old groundwater 

entirely of young groundwater (table 1). Fractions of old and in the study area was derived from the Mesaverde Group. 

young groundwater in the samples were estimated from tracer- Grflds24 was located west of the zone with low dissolved-oxygen 

tracer plots like the one shown in figure 10. Samples consisting concentrations (fig. 4).

Figure 9.  Stable isotopic compositions of water from domesitic wells (A) sampled for this study and (B) sampled during previous 

studies, compared to the stable isotopic compositions of snow from Grand Mesa, Colorado and produced waters from the 

Mesaverde Group.

–20 –18 –16 –14 –12 –10 –8 –6 –4 –2 0

–160

–20 –18 –16 –14 –12 –10 –8 –6 –4 –2 0

–160

–140

–120

–100

–80

–60

–40

–20

0

–140

–120

–100

–80

–60

–40

–20

0

δ2H[H O], in per mil relative to 2

Vienna Standard Mean Ocean Water

δ18

O[H2

O], in per mil relative to

Vienna Standard Mean Ocean Water

δ18O[H2O], in per mil relative to

Vienna Standard Mean Ocean Water

A B

Less than 1 milligram per liter methane

Greater than or equal to 1 milligram per liter methane

Domestic wells, (A) this study and (B) previous studies 

 (U.S. Geological Survey, 2010b)

EXPLANATION

Mesaverde Group produced waters

Samples from southwest Wyoming (U.S. Department of Energy, 2011)

Samples from Garfield County, Colorado (U.S. Geological Survey, 2010b)

Snow from Grand Mesa, Colorado (U.S. Geological Survey, unpublished data)

grflds27

grflds4

grflds9

Global Meteoric 

Water Line

Mesaverde

Group

produced waters

Grand Mesa snow

DIET1WW

LLOY2WW

SCHW1WW

Grand Mesa snow

Mesaverde

Group

produced waters

LANG13WW

grflds8

Global Meteoric 

Water Line
28    Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

The apparent ages of the young fraction of groundwater ranged from 20 to 39 years on the basis of tritium, 

chlorofluorocarbon, and sulfur hexafluoride data (table 1), and 

generally exhibited a positive relation with depth of the well screen 

below the water table (fig. 11), although the pattern contained 

considerable scatter. The two samples containing the oldest water 

(from grflds9 and grflds24) were among the deepest samples collected for this study. The age-depth patterns in figure 11 correspond 

to apparent vertical groundwater velocities on the order of 75 to 

2,000 millimeters per year (mm/yr), or recharge rates on the order 

of 15 to 400 mm/yr, assuming an aquifer porosity of 20 percent. 

Such variability in recharge could be expected in the study area 

because of its diverse land use and topographic settings. Higher 

recharge might occur in the vicinity of ponds, irrigation ditches, or 

intermittent stream channels compared to natural settings or interchannel areas. Higher recharge also might be expected at higher 

elevations as annual precipitation is generally greater there than at 

lower elevations. Aquifer zones with high recharge and relatively 

large nitrogen inputs at the land surface could be particularly 

vulnerable to nitrate contamination if the groundwater is oxic. 

The age of the younger fraction in a mixture of two young waters 

cannot be resolved with existing groundwater-age tracers. Therefore, the possibility exists that the young fractions themselves were 

mixtures containing some groundwater that was even younger than 

what is reported here. Generally, the effects of mixing on groundwater ages are reduced as the length of the screened interval of the 

sampled well decreases.

Apparent ages based on the tritium and helium-3 data were 

noticeably younger than the ages based on chlorofluorocarbon 

and sulfur hexafluoride data (table 1 and fig. 11). This could 

result from incomplete confinement of tritiogenic helium-3 in 

the aquifer, as discussed in the Groundwater Age section. For 

example, four of the samples dated with the tritium/helium-3 

method (from grfldn9, grflds1, grflds2, and grflds10) also were 

dated with the chlorofluorocarbon method. The median tritium/

helium-3 apparent age for those four samples was 6 years and 

the median chlorofluorocarbon apparent age was 26 years, supporting the notion that incomplete confinement of tritiogenic 

helium-3 biased the tritium/helium-3 ages young. Thus, the 

tritium/helium-3 apparent ages are considered minimum ages.

Chloride/Bromide Mass Ratios

Chloride/bromide mass ratios can provide information on 

fractions of water and chloride contributed to the samples from 

different sources. Potential sources examined here included 

young, dilute groundwater recharge, water from the Mesaverde 

Group, and leachate from sewage and/or animal-waste sources 

Figure 11.  Apparent ages of the young fraction of water 

from domestic wells sampled for this study, based on tritium, 

chlorofluorocarbon, and/or sulfur hexafluoride measurements, 

except for the samples that plot in the field labeled “minimum 

tritium/helium-3 ages,” whose ages were based on tritium and 

helium-3 measurements and may be biased young because of 

incomplete confinement of tritiogenic helium-3.

EXPLANATION

Year of recharge

 All young water (less than 50 years)

Significant fraction of old water (greater than 50 years) 

 or loss of chlorofluorocarbon-113

Domestic-well samples

Piston-flow line shows the expected concentrations 

 of tritium and chlorofluorocarbon-113 in groundwater 

 if they were unaffected by mixing

Mixing line defined by old water and water recharged 

 in 1985, and percentage (%) is the fraction of young 

 (1985) water in the mixture 

25%

Concentration of chlorofluorocarbon-113,

in parts per trillion by volume

Concentration of tritium, in tritium units

(decayed to 2010)

2000 1985 1990

1980

1960

1970

50%

75% 25%

Piston-flow line

50

40

30

20

10

0

0 20 40 60 80 100

Figure 10.  Concentrations of tritium and chlorofluorocarbon-113 

in water from domestic wells sampled for this study.

Apparent age of young fraction of water, in years

0

20

40

60

80

100

120

0 10 20 30 40 50 60

grflds9

2,000 mm/yr

Minimum

tritium/helium-3

ages

grflds27

grflds24

75 mm/yr

South of Colorado River

Apparent age greater than 50 years

North of Colorado River

EXPLANATION

Domestic wells

Apparent vertical groundwater velocities of 75 to 

 2,000 millimeters per year (mm/yr), and correspond 

 to recharge rates of about 15 to 400 mm/yr, assuming 

 an aquifer porosity of 20 percent

Depth to midpoint of well screen, in meters below water table
Groundwater Mixing    29

(fig. 12). The most likely sources of sewage and animal waste 

in the vicinity of the sampled wells were septic systems and 

livestock/pets, respectively. Halite may have been an additional 

source of chloride in at least one of the samples, but halite was 

not used in the source analysis.

The source analysis examined two scenarios: mixing 

between (1) young, dilute groundwater and sewage and/or 

animal waste and (2) young, dilute groundwater and water from 

the Mesaverde Group. Two mixing lines were used to bracket 

the range of chloride/bromide ratios in the end-member waters 

(fig. 12). The fraction of water contributed to the mixture by each 

end member was calculated according to equations 5 and 6:

Cls

 = (ClEM1)(f) + (ClEM2)(1-f) (5)

 (Cl/Br)s

 = Cls/[(BrEM1)(f) + (BrEM2)(1-f)] (6)

where

 Cls

 is the chloride concentration in the sample,

 ClEM1 and ClEM2 are the chloride concentrations in end 

members 1 and 2, respectively,

 f is the fraction of end member 1 in the mixture,

 (Cl/Br)s

 is the chloride/bromide ratio in the sample,

and

 BrEM1 and BrEM2 are the bromide concentrations in end 

members 1 and 2, respectively.

The fraction of chloride contributed to the mixture by each end 

member was calculated according to equations 7 and 8:

 Chloride from end member 1 = (ClEM1)(f)/ Cls

 (7)

 Chloride from end member 2 = (ClEM2)(1-f)/ Cls

 (8)

Most of the samples consisted of more than 90 percent 

young, dilute groundwater. But relatively large contributions of water and chloride from sewage and/or animal-waste 

sources were apparent in some samples collected north of the 

Colorado River and large contributions of water and chloride 

from the Mesaverde Group were apparent in some samples 

collected south of the river (fig. 12). Some samples may have 

contained water and chloride from more than two sources.

Samples from grflds9 (this study) and LLOY2WW and 

SCHW1WW (U.S. Geological Survey, 2010b) may have 

derived more than 90 percent of their water and chloride from 

the Mesaverde Group, based on the mixing scenarios shown in 

figure 12, whereas the sample from grfldn1 may have derived 

more than 90 percent of its water and chloride from sewage 

and/or animal-waste sources. Some samples (from grflds4, 

grflds27, DIET1WW, and LANG13WW) appeared to derive 

a large fraction of their water from dilute recharge but only a 

small fraction of their chloride content. The samples from grflds4 

and grflds27, for example, derived about 98 percent of their 

water and only 3 percent of their chloride from dilute recharge, 

based on the mixing scenarios illustrated in figure 12. The sample 

from grflds8 derived about 64 percent of its water and less than 

1 percent of its chloride from young, dilute recharge. The young 

fraction of water in grflds8 was estimated to be about 50 percent 

on the basis of age tracers (table 1). These results indicate the 

Mesaverde Group was an important source of chloride (and possibly other constituents as well) in several of the samples collected 

south of the river, even when the actual fraction of water from the 

Mesaverde Group in the sample was small. This is because of the 

large effect that elevated chloride concentrations in water from 

the Mesaverde Group have on the mixing calculations. These 

interpreted mixing fractions could vary if the actual end-member 

compositions differed from what is shown in figure 12, or if other 

end members not identified here contributed water and chloride to 

the samples.

Figure 12.  Chloride/bromide mass ratios in water from domestic wells sampled for this study and during previous studies. Ratios for 

seawater, halite, and sewage and animal waste are from Mullaney and others (2009). Chloride/bromide mass ratio

Concentration of chloride, in milligrams per liter

1,500

1,200

900

600

300

0

1 10 100 1,000 10,000 100,000

DIET1WW

LLOY2WW

SCHW1WW grflds8 Seawater

Range for sewage 

and animal waste

Mass ratio for potential

halite end member is

greater than 3,000

grlfdn4

grfldn1

50% 10%

90%

10%

grflds4 and grflds9

grflds27

50%

LANG13WW

Young,

dilute

groundwater 90%

 North of the Colorado River, this study

 South of the Colorado River, this study

 South of the Colorado River, previous studies

(U.S. Geological Survey, 2010b)

 Mesaverde Group (Papadopulos and

 Associates, Inc, 2008)

EXPLANATION

 Domestic wells

 Produced water

50%

Mixing lines defined by young, dilute groundwater and sewage

 and animal waste. Percentage (%) is the fraction of young, 

 dilute groundwater in the mixture. Two mixing lines bracket 

 the chloride/bromide mass ratio in sewage and animal waste. 50%

Mixing lines defined by young, dilute groundwater and Mesaverde

 Group water. Percentage (%) is the fraction of young, dilute 

 groundwater in the mixture. Two mixing lines bracket the 

 chloride/bromide mass ratio in Mesaverde Group water. 
30 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Summary of Mixing Results

The water-isotopic, age-tracer, and chloride/bromide data 

indicate most of the sampled groundwater consisted of snowmelt 

recharge with apparent ages less than 50 years old. Some samples 

also contained fractions of water from sewage and/or animalwaste sources, and this was most apparent north of the Colorado 

River. A few samples collected south of the river contained large 

fractions of water from the Mesaverde Group. The samples with 

large fractions of water from the Mesaverde Group also contained 

a component of old (greater than 50 years) water based on the 

age tracers, but not all the old groundwater contained water from 

the Mesaverde Group. The Mesaverde Group appeared to be an 

important source of chloride in several of the samples collected 

south of the river, even when the actual fraction of water from the 

Mesaverde Group in the sample was small.

Sources and Sinks of Nitrate 

in Groundwater

All but two of the samples contained at least a fraction of 

young water, indicating the wells from which those samples were 

collected could have been vulnerable to nitrate contamination 

from surface sources. Figure 13A shows samples with the highest 

measured nitrate concentrations also contained at least a fraction 

of young groundwater. The most likely sources of high-nitrate 

groundwater in the study area included septic systems, animal 

manure, and fertilizer applied to lawns and crops. Several samples 

appeared to contain a component of water from sewage (septicsystem effluent) or animal-waste sources on the basis of chloride/

bromide mass ratios (fig. 12), and one of them (from grfldn1) 

had the second highest measured nitrate concentration (fig. 13A). 

A more precise characterization of nitrate sources was beyond 

the scope of this project, but well established geochemical and 

isotopic tracer techniques are available for such purposes (Böhlke 

and others, 2002; McMahon and others, 2008).

Samples with some of the lowest measured nitrate concentrations were anoxic or contained old groundwater. The low 

nitrate concentrations could reflect denitrification processes in 

the aquifer or low nitrate concentrations in the water at the time 

it recharged the aquifer, or both. The measured nitrate concentrations in anoxic samples where corrected for denitrification affects 

in the aquifer by adding back excess nitrogen gas from denitrification, thus providing an estimate of the nitrate concentration 

in the water at the time it recharged the aquifer. Several of the 

samples with low dissolved-oxygen concentrations, such as the 

samples from grflds6 and grflds8, had substantially larger nitrate 

concentrations at the time of recharge than at the time of sampling 

(table 1, figs. 13A and 13C). The nitrate concentration in water 

from grflds6 at the time of recharge was about 14 mg/L as N, 

which would have exceeded the federal drinking-water standard 

for nitrate (fig. 13C) (U.S. Environmental Protection Agency, 

2010), but denitrification in the aquifer removed 99 percent 

of that nitrate during its 31-year residence time in the aquifer 

(table 1). These data indicate denitrification was an important 

sink for nitrate in parts of the aquifer containing groundwater 

with low concentrations of dissolved oxygen, particularly in 

the zone located south of Silt (figs. 4 and 5). In contrast to the 

samples containing young groundwater, the one sample (from 

grflds24) that contained 100 percent old groundwater and had sufficient gas data to make denitrification corrections was recharged 

with a nitrate concentration of about 1.6 mg/L as N (fig. 13), 

indicating nitrate concentrations in that water were low at the time 

it recharged the aquifer prior to the 1950s.

Sources and Sinks of Methane 

in Groundwater

Sources and sinks of methane in water from the Wasatch 

Formation were examined using hydrocarbon and noble-gas 

tracers. Hydrocarbon molecular and isotopic tracers were used 

to determine whether the high methane concentrations in some 

samples were biogenic or thermogenic in origin and whether 

oxidation processes in the aquifer could be a sink for methane. 

Biogenic methane is derived from the microbial degradation 

of organic matter by methanogenic bacteria in the subsurface. 

Thermogenic methane is derived from the abiotic degradation 

of organic matter at high temperatures and pressures when 

it is deeply buried in the subsurface. Helium concentrations 

and isotopes were used to examine whether the high methane 

concentrations in some samples could have formed within or 

external to the sampled intervals of the Wasatch Formation and 

they were used to provide qualitative constraints on the age of 

groundwater containing the highest methane concentrations.

Hydrocarbon Molecular 

and Isotopic Compositions

Hydrocarbon molecular and isotopic compositions have 

been used in several studies to determine sources of methane 

in groundwater (Bernard and others, 1978; Barker and Fritz, 

1981; Whiticar, 1999; URS Corporation, 2006; Papadopulos 

& Associates, Inc., 2008). Data from this study indicate 

methane in two of the three samples with high concentrations 

(from grflds9 and grflds27) was biogenic in origin, whereas 

methane in natural-gas samples from the Piceance Basin was 

thermogenic in origin (fig. 14A) (Johnson and Rice, 1990). 

Data for the third sample with a high methane concentration 

(from grflds4) plotted between the fields for thermogenic and 

biogenic gases in figure 14A, which could be interpreted as a 

mixture of gases from both sources. As discussed previously, 

water from grflds4 was a mixture of young and old waters. But 

water from grflds4 also contained dissolved oxygen (table 1), 

presumably from the young fraction of water in the sample, so 

the possibility that methane oxidation affected the composition 

of that sample cannot be ruled out. Methane oxidation would 

have the effect of shifting the molecular and isotopic compositions of the sample away from the biogenic field and toward 
Sources and Sinks of Methane in Groundwater 31

the thermogenic field of figure 14A (Whiticar, 1999). Based 

on the dissolved-oxygen data and position of the sample from 

grflds4 in figure 14A relative to the likely mixing scenarios, 

methane in that sample was considered to be biogenic in origin 

but affected by oxidation processes.

Methane oxidation also may have affected hydrocarbon 

molecular and isotopic compositions in samples collected during previous studies (fig. 14B). The median concentration of 

dissolved oxygen in samples with methane/(ethane + propane) 

ratios larger than 200 (biogenic methane) was 1.1 mg/L and it 

was 2.7 mg/L in samples with ratios less than 50 (thermogenic 

methane). In other words, the most oxic groundwater samples 

plotted in the thermogenic field of fig. 14B, indicating methane 

oxidation could have altered the composition of some of those 

samples. Methane oxidation also could be coupled with sulfate 

reduction in groundwater with low concentrations of dissolved 

oxygen (Van Stempvoort and others, 2005), but data needed to 

evaluate that process were not collected as part of this study. 

Despite the probable role of aerobic oxidation as a methane sink 

in the aquifer, several of the samples, such as DIET1WW and 

LANG13WW, were both anoxic and had molecular and isotopic compositions indicative of a thermogenic methane source 

(fig. 14B). So there is evidence for the occurrence of thermogenic methane from the Mesaverde Group, or some other deep 

Concentration of dissolved oxygen,

in milligrams per liter (mg/L)

Concentration of dissolved oxygen,

in milligrams per liter (mg/L)

Concentration of dissolved oxygen,

in milligrams per liter (mg/L)

Concentration of nitrate, 

in milligrams per liter as nitrogen

Concentration of methane, 

in milligrams per liter

Concentration of nitrate at the time of recharge, 

in milligrams per liter as nitrogen

16 102

101

100

10–1

10–2

10–3

10–4

14

12

10

8

6

4

2

0

16

14

12

10

8

6

4

2

0

0 2 4 6 8 10 0 2 4 6 8 10

0 2 4 6 8 10

Detection level

0.5 mg/L

grflds9

grflds4

grflds27

Oxygen levels favoring elevated 

methane concentrations

0.5 mg/L

MCL

grflds6

grflds8

grflds24

MCL

0.5 mg/L

Oxygen levels favoring

elevated nitrate

concentrations

grflds1

grfldn1

grflds6, 

grflds8

A B

C

Young groundwater, less than 50 years

Mixture of young and old groundwater

Old groundwater, greater than 50 years

EXPLANATION

MCL, Maximum Contaminant Level 

 (U.S. Environmental Protection Agency, 2010)

Domestic wells

Figure 13. Measured concentrations of (A) nitrate and 

(B) methane in water from domestic wells sampled for this study, 

and (C) concentrations of nitrate in the samples at the time they 

were recharged (measured nitrate concentration + concentration 

of excess nitrogen gas from denitrification), in relation to 

concentrations of dissolved oxygen. The shaded regions in (A) and 

(B) indicate ranges of dissolved oxygen concentrations in which 

nitrate and methane concentrations were highest, respectively.
32 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

source, in the Wasatch Formation. Like the wells sampled for 

this study that produced high-methane water, DIET1WW and 

LANG13WW were located in the zone of low dissolved-oxygen 

and high methane concentrations south of Silt. Even though the 

DIET1WW and LANG13WW samples appeared to contain 

thermogenic methane, the water-isotopic and chloride/bromide 

data indicate they contained relatively little water from the 

Mesaverde Group.

Helium Concentrations and Isotopes

Although the high concentrations of methane in some samples collected for this study appeared to be biogenic in origin that 

does not necessarily mean the methane was produced in the sampled intervals of the Wasatch Formation. The major-ion, waterisotopic, and chloride/bromide data show that samples containing 

high concentrations of biogenic methane, like the samples from 

grflds9, LLOY2WW, and SCHW1WW, also appeared to contain 

substantial fractions of Mesaverde water and ions. Noble-gas 

measurements indicate the zone of low dissolved-oxygen and 

high methane concentrations in which those wells were located 

(south of Silt) also was an area of high helium-4 concentrations, 

with concentrations up to 4,900 × 10–8 cubic centimeters at standard temperature and pressure, per gram of water (cm3

 STP/g) 

(table 1 and fig. 15). That helium-4 concentration is about 1,000 

times higher than the concentration expected in water equilibrated 

with air at 10oC and 1,800 m elevation, indicating groundwater in 

that area contained a large component of helium-4 derived from 

subsurface (terrigenic) sources. Importantly, there appeared to be 

a positive relation between methane and helium-4 concentrations 

(fig. 16A), implying helium-4 could be a useful tracer for understanding the geologic source of the high methane concentrations.

The primary terrigenic sources of helium-4 are upward 

fluxes from the mantle and decay of uranium and thorium 

in rocks and sediment of the earth’s crust. If terrigenic 

helium-4 in the samples was from mantle sources then the 

co-occurrence of high concentrations of helium-4 and biogenic 

methane (fig. 16A) may just be coincidental because biogenic 

Figure 14. Molecular and isotopic composition of short-chain hydrocarbons in water from domestic wells (A) sampled for this study 

and (B) sampled during previous studies. Biogenic and thermogenic fields modified from Bernard and others (1978) and Whiticar (1999).

Biogenic

Thermogenic

Oxidation

grflds9

grflds27

grflds4 

Mixing Mixing

DIET1WW

SCHW1WW

LLOY2WW

LANG13WW

δ13C[CH4], in per mil relative to

Vienna Peedee Belemnite

δ13C[CH4

], in per mil relative to

Vienna Peedee Belemnite

Methane/(ethane+propane)

104

103

102

101

100

104

103

102

101

100

–90 –80 –70 –60 –50 –40 –30 –20 –10 –90 –80 –70 –60 –50 –40 –30 –20 –10

Domestic wells, (A) this study and (B) previous studies

 (U.S. Geological Survey, 2010b)

Concentration of dissolved oxygen less than 2 milligrams 

 per liter

Concentration of dissolved oxygen greater than or 

 equal to 2 milligrams per liter

Natural gas, Piceance Basin (Johnson and Rice, 1990)

Wasatch Formation

Williams Fork Formation

Cretaceous coal

EXPLANATION

Mixing between potential biogenic and thermogenic 

 end members

δ13C[CH4

], Stable carbon isotopic composition of methane

A B
Sources and Sinks of Methane in Groundwater 33

Figure 15. Concentrations of helium-4 in water from domestic wells sampled for this study. Geology from 

Tweto (1979). Geologic structure modified from URS Corporation (2006) and Hoak and Klawitter (1997).

grfldn1

grfldn2

grfldn3

grfldn4

grfldn5

grfldn6

grfldn7 grfldn8

grfldn9

grfldn10

grflds1

grflds2

grflds4 grflds5

grflds6

grflds7

grflds8

grflds9

grflds10

grflds11

grflds20

grflds21

grflds22

grflds23

grflds24

grflds27

Rifle

Silt

Divide Creek anticline

Rifle-Grand hogback syncline

Unnamed syncline

Government Creek

Ma mm Creek

Col orado River

Divide Creek

107°50' 107°45' 107°40'

39°35'

39°30'

39°25'

Base from U.S. Geological Survey digital data, 2010, 1:100,000

Universal Transverse Mercator projection, Zone 13

Geology from Tweto (1979)

Geologic structure modified from URS Corporation (2006)

and Hoak and Klawitter (1997)

EXPLANATION

Geologic unit

Quaternary deposits

Tertiary units

Cretaceous units

Fault (approximate)

Helium-4, cubic centimeters at standard 

 temperature and pressure, per gram of 

 water × 10–8

Less than 10

10 to less than 1,000

Greater than or equal to 1,000

Not measured

0 5 KILOMETERS

0 5 MILES

Axis and direction of plunge 

 of anticline or syncline

Direction of dip of limb 

 of anticline

Direction of dip of limb 

 of syncline
34 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

methane is not likely to form under the high temperature and 

pressure conditions in the earth’s mantle (Scott and others, 

2004). Helium-3/helium-4 ratios can be used to differentiate mantle and crustal sources of 4

He. Crustal sources of 

helium typically have helium-3/helium-4 ratios about 0.02 

times the ratio in air (0.02RA), whereas mantle helium typically has ratios that are about 7RA (Mamyrin and Tolstikhin, 

1984; Ozima and Podosek, 1983). In figure 16B, helium-3/

helium-4 ratios were plotted in relation to the relative amount 

of helium-4 derived from air-water equilibrium with respect 

to total helium-4 in the sample. An x-axis value of 0 indicates 

essentially all the helium-4 in the sample was from terrigenic 

sources. An x-axis value of 1 indicates all the helium-4 was 

from the atmosphere. Most of the samples had x-axis values 

near 1, indicating a predominantly atmospheric source for the 

helium (samples with x-axis values greater than 1 probably 

were affected by degassing at the time of recharge or during 

sampling). Of those samples, some had helium-3/helium-4 

ratios equal to 0.98RA, consistent with helium from waterair equilibration, and some had ratios greater than 0.98RA. 

In the latter case, this is likely explained by contributions 

of tritiogenic helium-3 to those samples from tritium decay 

because all of them contained tritium (line 2 in fig. 16B). The 

data indicate essentially all the helium-4 in the high-methane 

samples from grflds4 and grflds9 was from crustal sources 

(x-axis value near 0 and helium-3/helium-4 ratio near 0.02RA

(0.009RA to 0.017RA)). One sample, from grflds21, that had a 

large component of terrigenic helium-4 (x-axis value less than 

0.2) also had a helium-3/helium-4 ratio larger than 0.02RA

(0.174RA). That sample could contain a small component of 

mantle helium. In general, the data in figure 16B indicate the 

primary terrigenic source of helium-4 in the samples was the 

crust, with a helium-3/helium-4 ratio close to 0.02RA.

Figure 16. (A) Methane concentrations in relation to helium-4 concentrations in water from domestic wells sampled for this study and 

(B) measured helium-3/helium-4 ratios corrected for excess air in relation to the relative amount of helium-4 derived from air-water equilibrium 

with respect to total helium-4 in the sample, corrected for excess air.

Concentration of helium-4, in cm3 STP/g

(

3Hes

-

3Hea)/(4Hes

-

4Hea) (x10–7

)

4

Heeq/(4Hes-4Hea)

Concentration of methane, in milligrams per liter

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

5

10

15

20

10–8 10–7 10–6 10–5 10–4

102

101

100

10–1

10–2

10–3

10–4

grflds10

grflds21

grfldn4

grflds8 grflds6

grflds11

grflds4 (10/2009)

grflds4 (4/2009)

grflds9 (4/2009

and 10/2009)

Methane detection level 

Mantle

helium-3 input 

grflds4 and grflds9

Slope reflects addition 

of 5 tritium units of 

tritiogenic helium-3 to line 1

grflds21

y-intercept (R) = 0.02RA

(typical crustal value) 

0.98RA All helium-4 in sample from the atmosphere 

ASW

Line 2

Line 1

EXPLANATION

cm3 STP/g, cubic centimeters at standard temperature and pressure, 

 per gram of water

ASW, helium-4 concentration in air-saturated water at 10 degrees 

 Celsius and 1,800 meters elevation

RA, helium-3/helium-4 (3

He/4He) concentration ratio in air

North of Colorado River

Domestic wells

South of Colorado River (sample dates included for grflds4 and grflds9)

3

He, helium-3 concentration

4

He, helium-4 concentration

The subscript “eq” is equilibrium, “s” is sample, and “a” is excess air

A B
Sources and Sinks of Methane in Groundwater 35

Figure 17. Concentrations of helium-4 in water from domestic 

wells sampled for this study in relation to depth below the water 

table to the midpoint of the well screen.

Concentration of helium-4, in cm3 STP/g x 10–8

Depth to midpoint of well screen, in meters below water table

0

20

40

60

80

100

120

140

0 1,000 2,000 3,000 4,000 5,000 1x10–8 cm3 STP/g·m

1x10–7 cm3

 STP/g·m

4x10–7 cm3

 STP/g·m

1x10–6 cm3

 STP/g·m

grflds9

grflds8

grflds4

North of Colorado River

South of Colorado River

EXPLANATION

Domestic wells

cm3

 STP/g, cubic centimeters at standard temperature 

 and pressure, per gram of water

cm3

 STP/g·m, cubic centimeters at standard temperature 

 and pressure, per gram of water, per meter

Helium-4 concentration gradients

The three most likely sources of crustal helium-4 are 

(1) helium-4 produced within the aquifer itself from the decay 

of uranium and thorium, (2) helium-4 produced at depths 

below the aquifer and then transported into the aquifer by 

diffusion or advection, and (3) helium-4 stored in aquifer 

minerals that is released from the surfaces of relatively freshly 

fractured or weathered rocks (Torgersen, 1980; Kulongoski 

and others, 2005). Helium-4 concentrations plotted in relation to depth of the well screen below the water table indicate 

most of the samples plotted within an envelope of helium-4 

concentration gradients ranging from about 1 × 10–8 to 

1 × 10–7 cm3 STP/g·m (fig. 17). Most of those samples were 

recharged in the past 50 years (fig. 11), and they contained 

only very small fractions of water from the Mesaverde Group 

on the basis of water-isotopic and chloride/bromide data 

(figs. 9 and 12). The data are consistent with in-situ production 

of small amounts of helium-4 in the Wasatch Formation from 

uranium and thorium decay during the short residence time of 

those samples in the aquifer (crustal source 1). Samples from 

grflds8 and grflds9, which had the highest helium-4 concentrations and largest fractions of water from the Mesaverde 

Group on the basis of chloride/bromide data (fig. 12), represented larger gradients ranging from about 4 × 10–7 to 

1 × 10–6 cm3 STP/g·m. The disparity in helium-4 gradients 

between the samples with small and large fractions of water 

from the Mesaverde Group could indicate high helium-4 in the 

latter group of samples was contained in much older groundwater, such as in the Mesaverde Group or some other relatively deep geologic unit, and subsequently transported into 

the Wasatch Formation (crustal source 2). The other possible 

source of high helium-4 in water from grflds8 and grflds9, 

release from relatively freshly fractured or weathered rocks in 

the aquifer (crustal source 3), cannot be ruled out but seems 

less likely because of the Mesaverde-like composition of those 

water samples.

Concentrations of crustal helium-4 in the samples can 

be used to qualitatively constrain groundwater age in the flow 

system using the following equations (Andrews and Lee, 

1979; Stute and others, 1992):

4 13

14

1 19 10

2 88 10

1

He U

Th

n

n

sol s = × × ×

+ × × ( − )

−

−

 Λ { . [ ]

. [ ]}

(9)

and

age

He

J

nz

He

crustal

w

sol

=

+

4

4

100 

(10)

where 4

Hesol is the rate at which helium-4 enters solution 

from uranium and thorium decay, in cubic 

centimeters at standard temperature and 

pressure, per gram, per year (cm3 STP/g∙yr);

s is rock bulk density, in grams per cubic 

centimeter (g/cm3);

Λ is the fraction of helium-4 produced in the rock 

that is released into solution, unitless (Λ is 

assumed to equal 1 for aquifers; Andrews 

and Lee, 1979; Torgersen and Clarke, 1985);

[U] and [Th] are uranium and thorium concentrations in the 

aquifer material, in parts per million (ppm);

n is porosity, unitless;

4

Hecrustal is the concentration of crustal helium-4 in the 

sample, in cubic centimeters at standard 

temperature and pressure, per gram of water 

(cm3 STP/g);

J is the helium-4 flux across the base of the 

aquifer, in cubic centimeters at standard 

temperature and pressure, per square 

centimeter, per year (cm3 STP/cm2∙yr);

z is the distance from the well screen to the base 

of the aquifer, in meters;

and

w is the density of water (about 1 g/cm3).
36 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

Table 2. Modeled helium-4 ages, in years, of water from well grflds9, Garfield County, Colorado.

[ppm, parts per million; g/cm3, grams per cubic centimeter; 4Hesol, helium-4 solution rate; cm3 STP/g∙yr, cubic centimeters at standard temperature and pressure, 

per gram of water, per year; 4Hecrustal, concentration of crustal helium-4 in the sample; cm3 STP/g, cubic centimeters at standard temperature and pressure, per 

gram of water; J, helium-4 flux across the base of the aquifer; cm3 STP/cm2∙yr, cubic centimeters at standard temperature and pressure, per square centimeter, 

per year; z, distance from well screen to base of aquifer; m, meters. Fixed parameter values in the models: uranium concentration in sediment = 2.2 ppm; 

thorium concentration in sediment = 9.0 ppm; porosity = 0.2; sediment bulk density = 2.6 g/cm3; 4Hesol = 5.42 × 10–12 cm3 STP/g∙yr]

Well 

name

Sample 

date

4

Hecrustal

(10–8 cm3 STP/g)1

J = 0

(cm3 STP/cm2∙yr)

J = 3 × 10–6

(cm3 STP/cm2∙yr)

J = 3 × 10–5

(cm3 STP/cm2∙yr)

z = 670 m z = 1,200 m z = 670 m z = 1,200 m z = 670 m z = 1,200 m

grflds9 10/28/2009 4,250 7,841,328 years 7,841,328 years 185,346 years 325,870 years 18,937 years 33,853 years 1

Recharge temperature assumed to equal 7.9 degrees Celsius.

For the age calculations summarized in table 2, it was assumed 

that ρs = 2.65 g/cm3, [U] = 2.2 ppm and [Th] = 9 ppm, and 

n = 0.2. Concentrations of uranium and thorium in the Wasatch 

outcrop area, derived from aerial gamma-ray contour maps 

(Phillips and others, 1993), were used as estimates of [U] and 

[Th]. Values for J and z in the study area are not known, so 

ranges of values were used based on data from the literature. 

Three values of J were assumed: 0 (no helium flux), 3 × 10–6

(average crustal degassing rate; O’Nions and Oxburgh, 1983), 

and 3 × 10–5 (a high degassing rate, such as was reported 

in a tectonically active area of California; Kulongoski and 

others, 2005) cubic centimeters of helium-4 at standard 

temperature and pressure, per square centimeter, per year 

(cm3 STP 4He/cm2∙yr,). Two values of z were assumed (670 and 

1,200 meters) based on data for the Wasatch Formation reported 

by URS Corporation (2006).

Apparent helium-4 ages were calculated for water from 

grflds9 because it had the highest measured helium-4 concentrations and tritium data indicate it contained no young 

groundwater. The oldest calculated ages (more than 5 million 

years) were obtained for the zero flux condition (table 2), but 

that condition does not seem likely for water from grflds9 

based on the helium-4 concentration gradients in figure 17. The 

higher-flux conditions resulted in ages from about 19,000 to 

326,000 years (table 2), which were substantially older than the 

apparent ages for the other samples from the Wasatch Formation 

(fig. 11) and indicate a deep source for helium-4 in that sample. 

The co-occurrence of high concentrations of methane with 

helium-4 in that sample (fig. 16A) implies the biogenic methane 

also was derived from a deep source. Although the values of J 

selected for this analysis were intended to represent a range of 

possible flux conditions it is not possible to directly evaluate 

their applicability to the study area in the absence of other data 

such as groundwater radiocarbon ages (Kulongoski and others, 

2005). Also, this age analysis did not account for the effects of 

diffusion and advective groundwater flow in the aquifer on the 

observed helium-4 distributions (Stute and others, 1992; Castro 

and others, 2000). Nevertheless, the apparent helium-4 ages and 

concentration gradients are generally consistent with the waterisotopic, major-ion, and chloride/bromide data and, together, 

the diverse geochemical data sets indicate that water, ions, and 

gases in grflds9 were derived from deep sources.

Methane Transport

The water-isotopic and major-ion data indicate the 

Mesaverde Group was the source of water and ions in 

grflds9 (figs. 7 and 9A). But it probably was not the source 

of biogenic methane in grflds9 because methane from the 

Mesaverde Group appeared to be thermogenic in origin 

(Johnson and Rice, 1990; Papadopulos and Associates, Inc., 

2008) (fig. 14B). Deeper zones in the Wasatch Formation were 

a more likely source for the biogenic methane in grflds9. The 

Wasatch Formation extends several hundred meters below 

the depth of grflds9 on the basis of the isopach map of the 

Wasatch Formation published by URS Corporation (2006). 

Water from the Mesaverde Group could incorporate biogenic 

methane from the deep Wasatch Formation before entering the 

shallow Wasatch Formation if it moved through natural fractures or uncemented annular space in gas wells that intersected 

zones enriched in biogenic methane. The north-south trending 

zone where domestic wells produced water with high methane 

and helium-4 concentrations generally coincided with known 

geologic structures such as the Divide Creek anticline, RifleGrand Hogback syncline, and deep faults (figs. 6 and 15), 

and they could be pathways for water and gas migration. This 

convergence of chemistry and geologic structure was noted 

by URS Corporation (2006) and Papadopulos and Associates, 

Inc. (2008). A compilation of data on surface-casing depths 

in 924 gas wells in the study area done by URS Corporation 

(2006) showed a median casing depth of 260 m, which 

is probably several hundred meters above the base of the 

Wasatch Formation in the high methane zone. Presumably, 

the annular space in those wells was uncemented between 

the bottom of the surface casing in the Wasatch Formation 

and the top of cement used to seal the gas-producing interval 

in the Mesaverde Group (URS Corporation, 2006). Some of 

those uncemented intervals could contain high concentrations of biogenic methane and act as pathways for water and 

gas migration.

Well LANG13WW, in contrast to grflds9, had waterisotopic and major-ion compositions similar to that of water 

from the shallow Wasatch Formation (figs. 8, 9B, and 12), 

but it contained thermogenic methane (fig. 14B). Both 

wells contained more than 1 mg/L methane. The data from 
Acknowledgments 37

LANG13WW could be consistent with fluids containing a 

high ratio of gas to water leaking upward from the Mesaverde 

Group, whereas the data from grflds9 appear to be more 

consistent with the leaking fluids containing a low ratio of gas 

to water. Determining whether these apparent differences in 

ratios of gas to water are indicative of different transport pathways was beyond the scope of this investigation. To address 

that question it would be helpful to collect more geochemical 

data that could be used to trace water, ion, and gas movement 

in the area around those wells. Equally important would be the 

collection of detailed geospatial data on water-well, gas-well, 

and fracture locations to determine whether the thermogenic 

gas in water wells could be spatially related to gas wells or 

to fractures. More generally, it would be helpful to collect 

a diverse set of baseline geochemical data in areas prior to 

natural-gas development.

Summary

Previous water-quality assessments reported elevated 

concentrations of nitrate and methane in water from domestic 

wells screened in shallow zones of the Wasatch Formation, 

Garfield County, Colorado. In 2009, the U.S. Geological 

Survey, in cooperation with the Colorado Department of 

Public Health and Environment, analyzed samples collected 

from 26 domestic wells for a diverse set of geochemical tracers for the purpose of determining sources and sinks of nitrate 

and methane in groundwater from the Wasatch Formation. The 

measured constituents included major ions, nutrients, hydrocarbon molecular and isotopic compositions, redox constituents, water isotopic compositions, tracers of groundwater age 

(tritium, chlorofluorocarbons, sulfur hexafluoride, and tritium/

helium-3), and noble gases. Nitrate concentrations ranged 

from less than 0.04 to 6.74 milligrams per liter as nitrogen and 

were highest in young (less than 50 years), oxic (concentrations of dissolved oxygen greater than or equal to 0.5 milligram per liter) groundwater and lowest in anoxic groundwater, 

regardless of age. Methane concentrations ranged from less 

than 0.0005 to 32.5 milligrams per liter. High methane concentrations (greater than 1 milligram per liter) occurred in a 

north-south trending zone of anoxic groundwater located south 

of Silt, Colorado.

Chloride/bromide mass ratios and tracers of groundwater 

age indicate that septic-system effluent or animal waste was a 

source of nitrate in some young groundwater, although other 

sources such as fertilizer also may have contributed nitrate to 

the groundwater. Denitrification removed most of the nitrate 

in anoxic samples. The nitrate concentration in one such 

sample at the time it recharged the aquifer was calculated to 

be about 14 milligrams per liter as nitrogen, which would have 

exceeded the federal drinking-water standard for nitrate, but 

denitrification in anoxic parts of the aquifer removed 99 percent of the nitrate before it reached the sampled well.

Hydrocarbon molecular and isotopic composition data 

indicate that the highest concentrations of methane were 

biogenic in origin. The primary methane sink in the aquifer 

appeared to be methane oxidation on the basis of dissolvedoxygen and methane concentrations and methane stableisotopic data. The north-south trending zone containing high 

methane concentrations also was characterized by high concentrations of helium-4 derived from a relatively deep crustal 

source on the basis of helium-3/helium-4 ratios, helium-4 

concentration gradients in the aquifer, and apparent helium-4 

groundwater ages (possibly greater than 19,000 years in the 

sample with the highest helium-4 and methane concentrations). The co-occurrence of high concentrations of methane 

and helium-4 implies the biogenic methane also was derived 

from a deep source. One sample containing high concentrations of biogenic methane and helium-4 had water-isotopic 

and major-ion compositions similar to that of water from 

the underlying Mesaverde Group, which was the primary 

natural-gas producing interval in the study area. Methane from 

the Mesaverde Group was largely thermogenic in origin so 

biogenic methane in the sample was more likely derived from 

deeper zones in the Wasatch Formation.

Water from the Mesaverde Group could incorporate biogenic methane from the deep Wasatch Formation before entering the shallow Wasatch Formation if it moved through natural 

fractures or uncemented annular space in gas wells that intersected zones enriched in biogenic methane. The north-south 

trending zone where domestic wells produced water with high 

methane and helium-4 concentrations generally coincided with 

known geologic structures such as the Divide Creek anticline, 

Rifle-Grand Hogback syncline, and deep faults, and they could 

be pathways for water and gas migration. A compilation of 

data on surface-casing depths in 924 gas wells done by URS 

Corporation (2006) showed a median casing depth of 260 m, 

which is probably several hundred meters above the base of 

the Wasatch Formation in the high methane zone. Presumably, 

the annular space in those wells was uncemented between 

the bottom of the surface casing in the Wasatch Formation 

and the top of cement used to seal the gas-producing interval 

in the Mesaverde Group (URS Corporation, 2006). Some of 

those uncemented intervals could contain high concentrations of biogenic methane and act as pathways for water and 

gas migration.

Acknowledgments

The authors wish to thank homeowners in Garfield 

County for allowing us to sample their wells. Bryant Jurgens 

and Mark Levorsen provided helpful comments on earlier 

versions of the manuscript.
38 Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009

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This publication is available online at: 

http://pubs.usgs.gov/sir/2010/5215/
McMahon and ohters—Use of Diverse Geochemical Data Sets in Groundwater, Garfield County, Colorado, 2009—USGS/SIR 2010–5215