40°00'

39°40'

108°30' 108°00'

Cathedral Bluffs

Roan

Plateau

Hogback

Gran

Piceance d

Creek dome Well 13U

Black Sulphur Creek

Hunte r Creek

Ryan Gulch

Yellow Cre ek

Piceance C reek

White River

EXPLANATION

Study-area boundary

(Piceance and Yellow 

 Creek watersheds)

Geologic structures Monitoring well network

Fold axis

Fault or fracture

Generalized direction of 

 groundwater flow in 

 the bedrock aquifers

Base from Environmental Research Systems Institute 

(ESRI) digital data, 2009, 1:24,000 and 

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

Universal Transverse Mercator, Zone 13 North

Meeker

Rifle

EXPLANATION

Study area

Piceance structural basin

Rio Blanco County

Meeker DENVER

Garfield County

0 5 MILES

0 5 KILOMETERS

Chemistry and Age of Groundwater

in the Piceance Structural Basin, Rio Blanco County, 

Colorado, 2010–12

Figure 1. Fourteen monitoring wells were sampled multiple times for a wide variety 

of chemical and isotopic constituents to characterize the chemistry and age of 

groundwater in bedrock aquifers. [Geologic structure from Hail and Smith (1994, 1997).]

Key Findings

• Concentrations of dissolved solids in 

groundwater exceeded the Federal secondary 

drinking-water standard of 500 milligrams 

per liter (mg/L) but generally were less than 

1,500 mg/L, except near the regional discharge area where concentrations in some 

wells exceeded 4,000 mg/L.

• Methane concentrations in groundwater 

ranged from less than 0.0005 to 387 mg/L.

• The methane was predominantly biogenic 

in origin, although the biogenic methane was 

mixed with thermogenic methane in water 

from seven wells.

• Three BTEX compounds (benzene, toluene, 

and ethylbenzene) were detected in water 

from six wells, but none of the concentrations 

exceeded Federal drinking-water standards.

• Groundwater from the upper aquifer system 

had ages ranging from less than 1,000 years 

to about 20,000 years, whereas groundwater 

from the lower aquifer system had ages 

greater than about 10,000 years, and most 

ages were greater than 20,000 years.

Key Implications

• The presence of thermogenic methane and 

BTEX compounds in the aquifers indicates a 

connection and vulnerability to chemicals in 

deeper geologic units in the study area. 

• Hydrocarbon migration in the area is complex, so collection of baseline data prior to 

gas-well installation and collection of timeseries data after gas-well installation is the 

best way to understand hydrocarbon migration in the study area.

• The presence of old groundwater in the aquifers has important management implications 

with respect to contaminant flushing times 

and water availability.

Fourteen monitoring wells were sampled by the U.S. Geological Survey, in 

cooperation with the Bureau of Land Management, to better understand the chemistry 

and age of groundwater in the Piceance structural basin in Rio Blanco County, 

Colorado (fig. 1), and how they may relate to the development of underlying naturalgas reservoirs (McMahon and others, 2013). Natural gas extraction in the area has 

been ongoing since at least the 1950s, and the area contains about 960 producing, 

shut-in, and abandoned natural-gas wells.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3047

September 2013 Printed on recycled paper
Mancos

Shale 

Mesaverde

Group

Upper

Tertiary

Uinta

Formation

Fort

Union

Formation

Cretaceous

Eocene

Garden Gulch

Member of the

Green River Formation 

Wasatch

Formation

Parachute Creek

Member of the 

Green River

Formation

Paleocene

Oil shale

Major 

natural-gas 

reservoir

Upper

aquifer

system 

Mahogany zone

confining layer 

Lower

aquifer

system

Saline zone

(nahcolite

and halite)

NOT TO SCALE

Figure 2. The primary bedrock aquifers are in the 

Uinta Formation and Parachute Creek Member of 

the Green River Formation, whereas natural gas 

commonly is produced from deeper geologic units 

such as the Mesaverde Group. [Figure modified 

from Johnson and Rice (1990) and Johnson and 

others (2010).]

Discrete-depth samplers were used to collect 

water samples from most of the monitoring 

wells. Photographs by Peter McMahon (left) and 

Richard Moscati (above), U.S. Geological Survey.

The Study Area

Monitoring well (foreground) located in the 

vicinity of a gas-well drilling pad (background). 

Photograph by Judith Thomas, 

U.S. Geological Survey.

Water-bearing zones in the Uinta and 

Green River Formations are the primary bedrock 

aquifers within the study area. In this study, the 

bedrock aquifers were divided into an upper 

aquifer system in the Uinta Formation and 

Parachute Creek Member of the Green River 

Formation above the Mahogany zone and a lower 

aquifer system in the Parachute Creek Member of 

the Green River Formation below the Mahogany 

zone (fig. 2). The Mahogany zone is a regionally 

extensive confining layer that separates the upper 

and lower bedrock aquifer systems.

The aquifers are important sources of water 

for people living and working in the area, and to 

streams and springs in the watersheds that support 

a variety of plant and animal communities. The 

study area contains rich energy resources in the 

forms of oil shale and natural gas (Johnson and 

others, 2010; Johnson and Rice, 1990), as well 

as mineral resources such as nahcolite (a sodium 

bicarbonate mineral) (Brownfield and others, 

2010). Commercial development of the oil-shale 

resource has not begun as of 2013, although 

research on oil-shale development in the area is 

ongoing. Commercial production of natural gas 

in the Piceance structural basin has occurred since 

at least the 1950s and is expected to expand in the 

foreseeable future (Bureau of Land Management, 

2013). Gas wells commonly produce from the 

Mesaverde Group (fig. 2) at depths greater than 

10,000 feet below land surface (Colorado Oil and 

Gas Conservation Commission, 2012), although 

some gas also has been produced from geologic 

units above and below the Mesaverde Group. The 

base of the bedrock aquifers generally is less than 

2,500 feet below land surface.
The Study Design

Dissolved Solids in Groundwater

Figure 3. Concentrations of dissolved solids and biogenic 

methane, and groundwater ages, increased as groundwater 

in the upper and lower bedrock aquifer systems moved 

from high-elevation recharge areas to regional discharge 

areas. Thermogenic methane and BTEX compounds were 

detected in groundwater near some gas wells and fault 

or fracture zones. Biogenic methane was produced locally 

in the aquifer systems, whereas thermogenic methane 

was produced in deeper geologic units below the aquifers. 

[Topography and aquifer boundaries modified from Day 

and others (2010). Gas well data downloaded from the 

Colorado Oil and Gas Conservation Commission (2012) 

on December 4, 2012. Chemistry and ages from this study.]

Mahogany zone

Piceance

Creek

Cathedral Bluffs

Saline zone (nahcolite and halite)**

Fault or

fracture

zone

Lower Aquifer system**

Upper Aquifer system*

Gas wells completed in

deeper geologic units*** SW

NE

1

2 2

3

4

Groundwater-flow direction

5

5

Deeper geologic units

NOT TO SCALE

3

Groundwater Chemistry and Age

2

4

5

< less than

> greater than

mg/L milligrams per liter

EXPLANATION

Dissolved solids: <1,000 mg/L

Biogenic methane: <0.1 mg/L

Modern recharge

Dissolved solids: <1,500 mg/L

Biogenic methane: <1 mg/L

Age: <1,000 to 20,000 years

Dissolved solids: <1,500 mg/L

Biogenic methane: >30 mg/L

Age: 10,000 to 30,000 years

Dissolved solids: >4,000 mg/L

Biogenic methane: >30 mg/L

Age: >50,000 years

Thermogenic methane and BTEX near 

 gas wells and fault or fracture zones

1

* Uinta Formation and Parachute Creek Member of Green River Formation above Mahogany zone

** Parachute Creek Member of Green River Formation below Mahogany zone

*** 962 producing, shut-in, and abandoned gas wells in the study area as of December, 2012

concentration of 47,600 mg/L. High concentrations of bicarbonate and 

chloride in that sample are indicative of deeply circulating groundwater 

that interacted with nahcolite and halite minerals present at depth in 

what is known as the saline zone of the Parachute Creek Member. 

Only one water sample from the upper aquifer system had a chemical 

constituent (fluoride) concentration that exceeded a Federal primary 

drinking-water standard, whereas several samples from the lower aquifer system exceeded the drinking-water standards for fluoride (4 mg/L) 

and barium (2 mg/L).

smaller set of constituents to examine temporal changes in water 

quality. The chemical and isotopic constituents were selected to 

provide information on the overall chemical quality of groundwater, 

occurrence and distribution of chemicals that could be related to 

the development of underlying natural-gas reservoirs, and to better 

understand groundwater residence times in the flow system.

Fourteen monitoring wells completed in the bedrock aquifers 

were sampled in this study (fig. 1). Thirteen of the wells were 

selected from an inventory of existing wells and one well was 

installed for this study. In 2011 and 2012, all 14 wells were sampled 

for a wide variety of chemical and isotopic constituents. Five 

of the wells also were sampled quarterly in 2010 and 2011 for a 

Groundwater generally evolved from a mixed-cation-bicarbonatesulfate type water in the upper aquifer system to a sodium-bicarbonate 

type water in the lower aquifer system. Most of the dissolved solids 

in groundwater were produced by dissolution of minerals such as 

dolomite, pyrite, nahcolite, and halite. Ion exchange further altered the 

water chemistry by removing calcium and magnesium from solution 

and replacing them with sodium. Concentrations of dissolved solids 

in both aquifer systems exceeded the Federal secondary drinkingwater standard of 500 mg/L but generally were less than 1,500 mg/L. 

Concentrations increased to more than 4,000 mg/L in the lower 

aquifer system near the regional discharge area at the northern end of 

Piceance Creek (fig. 3). Water from one well completed in the lower 

aquifer system near the regional discharge area had a dissolved-solids 
Figure 4. Most methane in the samples was biogenic, but some samples contained 

a mixture of biogenic and thermogenic methane. Samples from monitoring well 

13U contained 100 percent thermogenic methane similar in composition to some 

natural gas produced in the Piceance structural basin; δ13C, stable carbon isotopic 

composition. [The boundaries of the biogenic and thermogenic methane fields are 

modified from Bernard and others (1976) and Whiticar (1999).]

View looking south in the Hunter Creek drainage, 

Rio Blanco County, Colorado.

Photograph by Peter McMahon, 

U.S. Geological Survey.

Hydrocarbons in Groundwater

Wild horses in the Yellow Creek drainage, 

Rio Blanco County, Colorado. Photograph 

by Kevin Dennehy, U.S. Geological Survey.

76

49 106

112

0.2

150

387

0.1

Biogenic methane

Thermogenic methane

17

Oxidation

0.2

Well 13U

Mixing of biogenic and thermogenic methane

1%

10%

50%

60

1

10

100

1,000

10,000

100,000 Methane/(Ethane+Propane) ratio

–80 –70 –60 –50 –40 –30 –20

δ13C in methane, in per mil

49 Groundwater sample, number is 

 the methane concentration, 

 in milligrams per liter

Natural gas in the Piceance structural 

 basin (Johnson and Rice, 1990)

EXPLANATION

Sample also contained benzene,

 toluene, or ethylbenzene

1% Percentage of thermogenic methane 

 in mixture

Methane concentrations in groundwater 

ranged from less than 0.0005 to 387 mg/L. 

There currently (2013) is no Federal drinkingwater standard for methane, but water containing 

methane at concentrations greater than about 

28 mg/L could result in an immediate explosive hazard if used or stored in confined spaces 

(Eltschlager and others, 2001). Seven of the 

14 wells produced water with methane concentrations greater than 28 mg/L. The methane was 

predominantly biogenic in origin, although the 

biogenic methane was mixed with thermogenic 

methane in water from seven wells (fig. 4). One 

sample (from well 13U) contained 100 percent 

thermogenic methane that had an isotopic composition similar to that of some commercially 

produced natural gas in the Piceance structural 

basin (Johnson and Rice, 1990). The highest 

concentrations of biogenic methane occurred in 

the lower aquifer system (fig. 3) and were probably produced by microbial degradation of oil 

shale contained therein. In contrast, the thermogenic methane was produced by nonbiological 

processes in deeper geologic units below the 

aquifer systems and subsequently moved up into 

the aquifers (Johnson and Rice, 1990).

Three BTEX compounds (benzene, 

toluene, and ethylbenzene) were detected in 

small concentrations in water from six of the 

monitoring wells, but none of the concentrations 

exceeded a Federal primary drinking-water 

standard. Well 13U, for example, contained 0.4 

to 1.1 micrograms per liter (μg/L) of benzene––

much less than the standard, which is 5 μg/L. 

Five of the six wells that contained BTEX also 

contained at least a small amount of thermogenic 

methane. Taken together, the methane and BTEX 

data indicate that deep fluids interacted with 

groundwater in the aquifers.
2 wells

3 wells

(including well 13U)

3 wells

2 Group-1 wells and 1 Group-2 well 18A

1:1

0

1

2

Number of geologic structures in 1,500-foot

radius of monitoring well

0 2 4 6 8 10

Number of gas wells in 1,500-foot radius of monitoring well

Group 1—Groundwater samples that contained BTEX

 compounds or some thermogenic methane

Group 2—Groundwater samples that contained no 

 BTEX compounds or thermogenic methane

EXPLANATION

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108°23' 108°22' 108°21' 108°20' 108°19'

39°53'

39°52'

39°51'

39°50'

Multiple producing

gas wells at a single

well pad

EXPLANATION

Fold axis

Fault or fracture

Oil & gas wells (Colorado Oil and 

 Gas Conservation Commission, 2012)

! Abandoned

Bottom hole location and directional ! Producing

 drill lines (Colorado Oil and Gas 

 Conservation Commission, 2012 

! Bottom hole locations

Directional drilling lines

# Monitoring-well site 13 surrounded 

 by 1,500-foot buffer zone

Geologic structures

Black

Sulphur Cr

Base from Environmental Research Systems Institute

(ESRI) digital data, 2009, 1:24,000 and

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

Universal Transverse Mercator, Zone 13 North 

0

0 0.5 1 KILOMETER

0.5 1 MILE

Figure 5. Some samples that contained BTEX 

compounds or thermogenic methane appeared to be 

more closely associated with gas wells than geologic 

structures (faults, fractures, or fold axes), indicating 

that hydrocarbon migration could be associated with 

gas wells in some locations.

Hydrocarbon Migration

Figure 6. Seven gas wells were located within a 1,500-foot radius of well site 13, 

including an abandoned gas well that was collocated with the monitoring wells. 

Water from all three monitoring wells at the site contained BTEX compounds and 

some thermogenic methane. [Geologic structure from Hail and Smith (1994, 1997).]

well 13U (fig. 6). Other features that were potentially relevant to the hydrocarbon 

chemistry of 13U were located outside the 1,500-foot zone but upgradient from 

13U. These features include several faults or fractures (fig. 1) and an old test 

site for in-situ oil-shale retorting where natural gas was actually injected into the 

bedrock aquifer system in the late 1960s in an attempt to generate oil from the 

oil shale (U.S. Congress Office of Technology Assessment, 1980). The manner 

in which hydrocarbons migrated to well 13U is unclear because the relative 

contributions of nearby gas wells, geologic structures, and legacy contamination 

to hydrocarbon migration are not well understood. Ultimately, collection of 

baseline data prior to gas-well installation and collection of time-series data after 

gas-well installation is the best way to understand the roles of gas wells, geologic 

structure, and legacy contamination in hydrocarbon migration in the study area.

Collection of baseline data prior to gas-well installation and collection of time-series 

data after gas-well installation is the best way to understand the roles of gas wells, 

geologic structure, and legacy contamination in hydrocarbon migration in the study area.

In general, the two most likely migration 

pathways for hydrocarbons in the study area are 

movement along natural geologic structures (faults, 

fractures, or fold axes) and movement through 

improperly sealed boreholes of gas wells. Regardless 

of the migration pathway, the presence of thermogenic 

methane in the aquifers indicates a connection and 

vulnerability to chemicals in deeper geologic units in 

the study area. Most monitoring wells that contained 

BTEX compounds or thermogenic methane had more 

gas wells than mapped geologic structures located 

within 1,500 feet of the wells (fig. 5). Most monitoring 

wells free of BTEX compounds and thermogenic 

methane did not have any gas wells located within 

1,500 feet of the wells. These data indicate that 

hydrocarbon migration could be associated with 

gas wells in some locations, such as at well 13U.

Monitoring well 13U, which contained small 

concentrations of benzene and 100 percent thermogenic methane, had seven gas wells and one mapped 

geologic structure located within 1,500 feet of the well 

(fig. 5). One of those gas wells, which was plugged 

and abandoned in 1990, was located within 60 feet of 
Groundwater Ages

Groundwater ages in the aquifers were evaluated using 

tritium, carbon-14, helium-4, and chlorine-36 data. Collectively, 

the data indicate that groundwater in high-elevation recharge areas 

was essentially modern (recharged since the 1950s) and became 

progressively older as the water moved downgradient in the flow 

system (fig. 3). Helium-4 data indicate that groundwater from above 

the Mahogany zone had ages from less than 1,000 years to about 

20,000 years, whereas groundwater from within and below the 

Mahogany zone had ages greater than about 10,000 years, and most 

Study Limitations

Results of this study should be considered in the context of 

some important limitations related to data availability. Fourteen 

monitoring wells were used to characterize groundwater chemistry 

and age in the 900 square-mile study area. This small number of wells 

represents a limitation in the ability to analyze the spatial variability 

of water chemistry and age in the bedrock aquifers. Few data on the 

chemical and isotopic composition of deep formation water below 

References Cited

Bernard, B.B., Brooks, J.M., and Sackett, W.M., 1976, Natural gas seepage in the 

Gulf of Mexico: Earth and Planetary Science Letters, v. 31, p. 48–54.

Brownfield, M.E., Mercier, T.J., Johnson, R.C., and Self, J.G., 2010, Nahcolite 

resources in the Green River Formation, Piceance Basin, Colorado, in U.S. Geological Survey Oil Shale Assessment Team, eds., Oil-shale assessment of the Piceance 

Basin, Colorado: U.S. Geological Survey Digital Data Series DDS–69–Y, chap. 2, 

57 p., accessed February 14, 2012, at http://pubs.usgs.gov/dds/dds-069/dds-069-y/.

Bureau of Land Management, 2013, Reasonable foreseeable development scenario 

for oil and gas activities in the BLM White River Field Office, Rio Blanco, Moffat, 

and Garfield Counties, Colorado: accessed June 13, 2013, at http://www.blm.gov/co/

st/en/BLM_Programs/land_use_planning/rmp/white_river/documents/RFD.html.

Colorado Oil and Gas Conservation Commission, 2012, Colorado Oil and Gas 

Information System: accessed October 13, 2012, at http://cogcc.state.co.us/.

Day, M., Hansen, E., Gulliver, T., and McKinzie, B., 2010, Northwest Piceance 

Creek Basin hydrogeology, in Proceedings of the 30th Oil Shale Symposium, 

Golden, Colorado, October 18–20, 2010, accessed February 21, 2012, at 

http://www.costar-mines.org/oss/30/paper/Paper_14-02-Hansen_Erik.pdf.

Eltschlager, K.K., Hawkins, J.W., Ehler, W.C., and Baldassare, Fred, 2001, Technical measures for the investigation and mitigation of fugitive methane hazards in 

areas of coal mining: Office of Surface Mining Reclamation and Enforcement, 

125 p., accessed February 8, 2013, at http://www.techtransfer.osmre.gov/ARsite/

Publications/Methane%20Manual.pdf.

Hail, W.J., and Smith, M.C., 1994, Geologic map of the northern part of the Piceance 

Creek basin, northwestern Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I–2400, 1 map.

Hail, W.J., and Smith, M.C., 1997, Geologic map of the southern part of the Piceance 

Creek basin, northwestern Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I–2529, 1 map.

Johnson, R.C., Mercier, T.J., Brownfield, M.E., Pantea, M.P., and Self, J.G., 2010, An 

assessment of in-place oil shale resources in the Green River Formation, Piceance 

Basin, Colorado, in U.S. Geological Survey Oil Shale Assessment Team, eds., 

Oil-shale assessment of the Piceance Basin, Colorado: U.S. Geological Survey 

Digital Data Series DDS–069–Y, chap. 1, 197 p., accessed February 14, 2012, at 

http://pubs.usgs.gov/dds/dds-069/dds-069-y/.

Johnson, R.C., and Rice, D.D., 1990, Occurrence and geochemistry of natural 

gases, Piceance Basin, northwest Colorado: American Association of Petroleum 

Geologists Bulletin, v. 74, p. 805–829.

McMahon, P.B., Thomas, J.C., and Hunt, A.G., 2013, Chemistry and age of groundwater in bedrock aquifers in the Piceance and Yellow Creek watersheds, 

Rio Blanco County, Colorado, 2010–12: U.S. Geological Survery Scientific 

Investigations Report 2013–5132, 89 p.

U.S. Congress Office of Technology Assessment, 1980, An assessment of oil shale 

technologies: accessed January 25, 2013, at http://www.princeton.edu/~ota/disk3/

1980/8004_n.html. 

Whiticar, M.J., 1999, Carbon and hydrogen isotope systematics of bacterial formation 

and oxidation of methane: Chemical Geology, v. 161, p. 291–314.

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

Old groundwater ages have important implications 

from a water management perspective.

ages were greater than 20,000 years. Groundwater ages in the lower 

aquifer near the regional discharge area at the northern end of Piceance 

Creek appeared to be greater than 50,000 years. The old groundwater 

ages have important implications from a water management 

perspective. The parts of the aquifers with long groundwater residence 

times could have century- to millennium-scale flushing times if they 

were contaminated. The presence of old groundwater in parts of the 

aquifers also indicates that these aquifers may not be useful for largescale water supply because of low recharge rates.

the aquifers were available, thus limiting the ability to understand 

their spatial variability and recognize if water from deeper formations 

entered the bedrock aquifers. Similarly, few data on the composition 

of natural gas from the study area were available. Such data would 

be useful for understanding the variability in gas composition and for 

possibly identifying which of the producing gas zones contributed 

thermogenic methane to the bedrock aquifers.

Contact Information

For further information, contact:

Peter McMahon (pmcmahon@usgs.gov)

Judith Thomas (juthomas@usgs.gov)