Hydrogeochemistry and 

Simulated Solute Transport, 

Piceance Basin, 

Northwestern Colorado

Prepared in cooperation with the 

U.S, Environmental Protection Agencyi
Hydrogeochemistry and 

Simulated Solute Transport, 

Piceance Basin, 

Northwestern Colorado

By STANLEY G. ROBSON and GEORGE J. SAULNIER, JR. 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1196

Prepared in cooperation with the 

U.S. Environmental Protection Agency

The hydrogeochemistry of the oil-shale-rich 

area is described and a digital model is used 

to simulate the possible effects of proposed 

oil-shale mining on ground-water quality

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Library of Congress Cataloging in Publication Data

Robson, Stanley G.

Hydrogeochemistry and simulated solute transport, Piceance basin, northwestern Colorado.

(Geological Survey Professional Paper 1196)

Prepared in cooperation with the U.S. Environmental Protection Agency.

Bibliography: p. 63

Supt. of Docs, no.: I 19.16

1. Oil-shale industry Environmental aspects Colorado Piceance Creek watershed. 2. Hydrology ColoradoPiceance Creek watershed. 3. Geochemistry Colorado Piceance Creek watershed. I. Saulnier, George J., 

joint author. II. United States Environmental Protection Agency. HI. Series: United States Geological 

Survey Professional Paper 1196

TD19S.04R62 628.1*6836 80-607137

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page

Conversion factors ................................................................... V

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

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

The natural hydrologic system .................................................. 3

Stratigraphy .............................................................................. 3

Aquifer characteristics .......................................................... 7

Fracturing ........................................................................... 7

Aquifer limits .................................................................... 7

Elevation and thickness of bedding in aquifers .. 7

Hydraulic conductivity .................................................. 10

Porosity and compressibility ...................................... 13

Ground-water recharge, discharge, and potentiometric

heads ........................................................................................ 15

Ground-water recharge and discharge .................... 18

Potentiometric surfaces .................................................. 20

Geochemical system ................................................................ 25

Chemical reactions .................................................................. 26

Dissolution .......................................................................... 27

Precipitation ...................................................................... 29

Ion exchange .................................................................... 29

Oxidation ............................................................................ 30

Reduction ............................................................................ 30

Page

Distribution of trace constituents ...................................... 31

Arsenic ............................................................. .. .. . 31

Barium ................................................................................. 31

Boron ...................................................................... . . . 31

Fluoride .................................................................. .».- . 31

Iron ........................................ -- - 32

Lithium ................................................................................. 32

Strontium ............................................................................. 33

The simulated hydrologic system .............................................. 33

Model description .................................................................... 33

Model calibration .................................................................... 34

Hydrologic knowledge gained through modeling ........ 42

Model simulations ............................................................................ 44

Tract C-a .................................................................................... 46

Mine dewatering .............................................................. 46

Mine flooding .................................................................... 50

Tract C-b .................................................................................... 53

Mine dewatering .............................................................. 53

Mine flooding .................................................................... 58

Summary ............................................................................................ 60

References .......................................................................................... 63

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of study area .............................................................................................................................................. 2

2. Generalized geologic sections through the Piceance basin .................................................................................................... 5

3-22. Maps showing:

3. Extent of surficial fracturing ............................................................................................................................................. 8

4. Structure contours on the base of the Uinta Formation .......................................................................................... 9

5. Saturated thickness of the Uinta Formation ................................................................................................................ 10

6. Structure contours on the top of the Mahogany zone ................................................................................................ 11

7. Saturated thickness of the Parachute Creek Member above the Mahogany zone .......................................... 12

8. Saturated thickness of the Mahogany zone ................................................................................................................. 13

9. Saturated thickness of the interval from the base of the Mahogany zone to the base of the R-6 zone .. 14

10. Structure contours on the base of the lower aquifer .................................................................................................. 15

11. Saturated thickness of the Parachute Creek Member below the base of the R-6 zone .................................. 16

12. Lateral and vertical hydraulic conductivity for layer 1 ............................................................................................ 17

13. Lateral and vertical hydraulic conductivity for layer 2 .................................................. . .................................. 18

14. Lateral hydraulic conductivity for layer 3 .................................................................................................................... 19

15. Vertical hydraulic conductivity for layer 3 .................................................................................................................... 20

16. Lateral hydraulic conductivity for layers 4 and 5 ...................................................................................................... 21

17. Vertical hydraulic conductivity for layers 4 and 5 .................................................................................................... 22

18. Potentiometric surface of the upper aquifer .................................................................................................................. 23

19. Potentiometric surface of the lower aquifer .................................................................................................................. 24

20. Difference in potentiometric heads between the upper and lower aquifers ...................................................... 25

21. Estimated dissolved-solids concentrations in water recharging the aquifer system ...................................... 27

22. Major faults and joints in the northern part of the Piceance basin .................................................................... 28

23. Graph showing distribution of differences between calculated and measured values of potentiometric head for

layers 1 and 5 ............................................................................^^ 35

m
IV CONTENTS

Page

FIGURE 24-30. Maps showing:

24. Measured and calculated dissolved-solids concentrations for layer 1 .................................................................. 37

25. Measured and calculated dissolved-solids concentrations for layer 2 ................................................................. 38

26. Measured and calculated dissolved-solids concentrations for layer 3 .................................................................. 39

27. Measured and calculated dissolved-solids concentrations for layer 4 .................................................................. 40

28. Measured and calculated dissolved-solids concentrations for layer 5 .................................................................. 41

29. Precipitation recharge distribution .................................................................................................................................. 43

30. Rate of dissolution of saline minerals at the base of the aquifer system .......................................................... 44

31. Graph showing cumulative ground-water discharge distribution and dissolved-solids

concentrations along Piceance and Yellow Creeks ...................................................................................................................... 45

32-34. Maps showing:

32. Model-calculated drawdown in layer 5 produced by pumping mine near tract C-a at 5 cubic feet per

second ......................................................................................................^ 48

33. Model-calculated decrease in dissolved-solids concentrations in layer 2 produced by pumping

mine near tract C-a at 5 cubic feet per second ........................................................................................................ 49

34. Model-calculated change in dissolved-solids concentrations in layer 5 produced by pumping

mine near tract C-a at 5 cubic feet per second ........................................................................................................ 50

35-37. Graphs showing:

35. Change in model-calculated ground-water discharge and dissolved-solids concentrations in

Yellow Creek valley produced by pumping mine near tract C-a at 5 cubic feet per second .................. 51

36. Change in dissolved-solids concentrations in aquifers adjacent to abandoned mine near track

C-a after 60 years of leaching............................................................................................................................................ 52

37. Change in dissolved-solids concentrations with time in aquifers adjacent to abandoned mine for

leaching rate of 1.3X 105 pounds per day.............................................. ...........-- ...........- - . - . ..... - 53

38-40. Maps showing:

38. Model-calculated increase in dissolved-solids concentrations in layer 2 after 60 years of leaching

from abandoned mine at 1.3X105 pounds per day ........t ............................................................................................... 54

39. Model-calculated drawdown in layer 5 produced by pumping mine in tract C-b at 15 cubic feet

per second ............................................................................................................_ 55

40. Model-calculated change in dissolved-solids concentrations in layer 5 produced by pumping

mine in tract C-b at 15 cubic feet per second .................................................................. .........- . ......... ....... 56

41-43. Graphs showing:

41. Change in model-calculated ground-water discharge and dissolved-solids concentrations in

Piceance Creek valley produced by pumping mine in tract C-b at 15 cubic feet per second .................. 57

42. Change in dissolved-solids concentrations in aquifers adjacent to abandoned mine in tract C-b

after 60 years of leaching ................................................................................................................................................ 58

43. Change in dissolved-solids concentrations with time in aquifers adjacent to abandoned

mine in tract C-b for leaching rate of 1.3X105 pounds per day........................................................................ 59

44. Map showing model-calculated increase in dissolved-solids concentrations in layer 5 after 60 years of

leaching from abandoned mine at 1.3 X105 pounds per day ............. 60

45. Graph showing model-calculated dissolved-solids concentrations in ground-water discharge to 

Piceance Creek valley produced after 60 years of leaching from abandoned mine 

in tract C-b at 1.3X105 pounds per day......................................................................................................................... 61

TABLE

TABLES

Stratigraphic units in Piceance basin .........................................

Ground-water mass balance ...........................................................

Mineral-dissolution rates .................................................................

Distribution of total pumpage by model layer for tract C-b

Page

6

36

42

53
CONTENTS

CONVERSION FACTORS

For the reader who may prefer to use metric units rather than inch-pound units, the 

conversion factors for the terms used in this report are listed below:

Multiply inch-pound unit By To obtain metric unit

acre 4.047X10"1 hectare

acre-foot (acre-ft) 1.234X10'3 cubic hectometer

barrel 1.59X101 cubic meter

foot (ft) 3.048X10'1 meter

foot per day (ft/d) 3.048X10~l meter per day

foot per mile (ft/mi) 1.894X10'1 meter per kilometer

cubic foot per second (ftVs) 2.832X10'2 cubic meter per second

foot per year (ft/yr) 3.048X10"1 meter per year

gallon per ton (gal/ton) 4.171X10'2 cubic meter per megagram

square inch per pound (in2/lb) 1.450X10"1 kUopascah1

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

pound per day (Ib/d) 4.536X10"1 kilogram per day

pound per day per square mile 1.751X10"1 kilogram per day per

[(lb/d)/mi2] square kilometer

pound per square inch(lb/in2) 6.895 kilopascal

ton, short 9.072X10"1 metric ton

HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, 

PICEANCE BASIN, NORTHWESTERN COLORADO

By STANLEY G. ROBSON and GEORGE J. SAULNIER, JR.

ABSTRACT

The Piceance basin of northwestern Colorado contains rich 

deposits of oil shale in sedimentary rocks of the Parachute Creek 

Member of the Green River Formation. Plans for mining these 

deposits in Federal prototype lease tracts C-a and C-b require 

ground water to be pumped to allow the construction of 

underground retorts for use in extracting the shale oil. The 

possible adverse effects of mine pumping and resaturation of spent 

shale in an abandoned mine were investigated by studying the 

geochemistry of the aquifers and by use of ground-water quality 

modeling techniques.

The aquifers in the basin crop out along the east, north, and west 

margins of the basin and attain a saturated thickness of 2,000 feet 

in the northeast part of the basin. Faults, joints, collapse breccia, 

and solution cavities are the primary source of permeability in the 

otherwise impermeable sandstone and marlstone sedimentary 

rocks. Estimates of the lateral hydraulic conductivity of the 

aquifers range from 0.1 to 2.0 feet per day and of the vertical 

hydraulic conductivity from 7X10"4 to 1.0 foot per day, 

respectively.

Ground-water recharge occurs from infiltration of precipitation 

in the upland areas along the east, south, and west margins of the 

basin. From the recharge areas, ground water moves vertically 

into deeper zones in the aquifers and laterally toward the 

discharge areas along Piceance and Yellow Creeks and their 

tributaries. The steady-state recharge discharge rate of 32 cubic 

feet per second results in hydraulic gradients in the aquifers of as 

much as 130 feet per mile in some parts of the basin.

The saline zone, located at the base of the Parachute Creek 

Member, contains the soluble minerals naholite and halite and is 

the primary source of the dissolved solids in the ground water. 

Dissolution of minerals in the unsaturated zone and in the aquifers 

also contribute dissolved solids to the water. Additional chemical 

reactions occurring in the aquifers include precipitation, ionexchange, and oxidation-reduction reactions. The product of the 

principal chemical reactions is a sodium bicarbonate water with 

oxidized sulfur species in the Uinta Formation and reduced sulfur 

species in the Parachute Creek Member. Carbonate species are 

produced primarily by reduction of sulfur species and dissolution 

of the carbonate minerals calcite, dolomite, and nahcolite. 

Carbonate species are removed from the system in the northern 

part of the basin by calcite precipitation. Relatively large sodium 

concentrations are produced by ion exchange and nahcolite and 

halite (dissolution. Local brines are produced where faults exposed 

saline minerals to circulating ground water.

A mathematical model was used to simulate the ground-water 

quality changes that would occur as a result of mine dewatering or 

leaching of saline minerals from an abandoned mine. The model 

simulates confined density-dependent flow and solute transport in

three dimensions with dispersion in nonhomogeneous, anisotropic 

aquifers under steady or transient ground-water flow conditions. 

An acceptable model calibration was achieved for steady-state 

flow conditions.

Simulations of pumping mines in tracts C-a and C-b indicate 

that the altered direction of ground-water movement near the 

pumped mines will decrease dissolved-solids concentrations in 

ground water near the mines and increase dissolved-solids 

concentrations in some areas downgradient from the mines. 

Simulated long-term pumping from tract C-a at a rate of 5 cubic 

feet per second indicates that ground-water discharge to Yellow 

Creek will be reduced by as much as 2 cubic feet per second and to 

Piceance Creek by as much as 3 cubic feet per second. Simulated 

long-term pumping from tract C-b at 15 cubic feet per second 

indicates that ground-water discharge to Piceance Creek will be 

reduced by as much as 15 cubic feet per second and dissolved-solids 

concentration in the creek will be increased by as much as 300 

milligrams per liter.

Leaching of saline minerals contained in spent shale in an 

abandoned and flooded mine could have an adverse affect on the 

surface- and ground-water quality in the basin. Model simulations 

of mine leaching in tract C-a indicate that the dissolved-solids 

concentration of ground water in a 140-square-mile area downgradient from the tract would be increased by more than 10 

milligrams per liter after 60 years of leaching. Dissolved-solids 

concentrations as much as 40,000 milligrams per liter were 

produced adjacent to the simulated mine. After 60 years of 

leaching, the water quality in Yellow Creek had not yet been 

adversely affected by the degraded quality of the ground water. 

Simulations of mine leaching at tract C-b indicate that the 

dissolved-solids concentration in a 40-square-mile area near the 

tract would be increased by more than 10 milligrams per liter after 

60 years of leaching. The estimated dissolved-solids concentration 

in Piceance Creek increased from 750 to 1,750 milligrams per liter 

in a reach of the creek near tract C-b. Modeling thus indicates that 

equal rates of mine leaching from tracts C-a and C-b will produce 

much different effects on the water quality in the Piceance basin. 

Tract C-a, by virtue of its remote location from perennial streams, 

will primarily degrade the ground-water quality over a large area 

to the northeast of the tract. Tract C-b, by contrast, will primarily 

degrade the surface-water quality in Piceance Creek with only 

localized effects on the ground-water quality.

INTRODUCTION

The Piceance basin of northwestern Colorado (fig. 

1) contains rich deposits of kerogen-bearing dolomitic marlstone (oil shale) and saline minerals in 

lacustrine sediments of the Eocene Green River For-
HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES 

_I I I I

0 5 10 KILOMETERS

FIGURE 1. Location of study area (sections A-A' and B-B' shown in figure 2).
STRATIGRAPHY

mation. The estimated total reserves of the Piceance 

structural basin amount to 1.2 X1012 barrels of shale 

oil (Culbertson and Pitman, 1973) more than the 

total oil consumption of the United States to date 

(1979). The magnitude of these oil-shale deposits has 

resulted in intense exploration by energy companies 

and the leasing by the Federal government of two 

5,000-acre prototype mining tracts, C-a and C-b. 

Present mining plans for tracts C-a and C-b include 

the use of a modified in-situ recovery process that 

will require dewatering of the mining zone as a 

prerequisite to shale-oil extraction. Mine dewatering 

and related mining activities could have a significant effect on the quantity and chemical quality of 

the ground and surface water in the basin. Leachate 

produced by ground-water dissolution of saline minerals in spent shale in an abandoned and flooded 

mine also could degrade the ground- and surfacewater quality on and near the tracts.

The purpose of this study was to investigate the 

hydrogeochemistry of the aquifers in the Piceance 

basin and to evaluate the potential effects of mining 

activities on the chemical quality of the water resources of the basin.

To meet this objective, the extent, depth, thickness, 

and hydrologic characteristics of the aquifers in the 

Piceance basin were estimated. In addition, areal 

variations in dissolved-solids concentrations and the 

occurrence and distribution of major ions and trace 

constituents were described. Mineral reactions, such 

as dissolution of the saline minerals nahcolite (NaCO3) 

and halite (NaCl), and precipitation of other minerals were studied, as were variations in oxidation 

and reduction conditions and ion-exchange reactions 

in the aquifers. Some of these data were used to 

develop and calibrate a three-dimensional mathematical model that is capable of simulating the spatial and temporal changes in potentiometric heads 

and dissolved-solids concentrations in five layers 

representing different depth intervals in the aquifers. By modeling ground-water movement and quality, it is possible to estimate the changes in the 

ground- and surface-water quality in the basin that 

may result from mine dewatering or leaching of 

spent shale in an abandoned and flooded mine.

Primary funding for this work was provided by the 

U.S. Geological Survey through energy research and 

development funds, and Federal program funds. Additional support was provided by the U.S. Environmental Protection Agency through energy research 

and development funds.

THE NATURAL HYDROLOGIC SYSTEM

STRATIGRAPHY

The stratigraphy of the Piceance basin has been 

described previously by Donnell (1961),, Coffin, Welder, and Glanzman (1971), Dyni (1914), and Cashion 

and Donnell (1974). Descriptions of the various stratigraphic units are given in table 1. Generalized 

geologic sections showing the vertical relationships 

of these units are shown in figure 2.

The Green River Formation conformably overlies 

the Wasatch Formation in Piceance basin. The Green 

River Formation is composed of a basal sandstone 

unit (Douglas Creek Member) containing some limestone and shale that is overlain by a finely laminated 

shale and barren marlstone (Garden Gulch Member) 

containing some sandstone and limestone.The Garden Gulch Member is overlain by a large amount of 

kerogen-rich dolomitic marlstone (Parachute Creek 

Member). The Douglas Creek and Garden Gulch 

Members undergo a f acies change near the northern 

and eastern edge of the basin where the units occur 

as heterogeneous layers of shale, sandstone, and 

barren marlstone and are mapped as the Anvil Points 

Member by Donnell (1961). The Anvil Points, Garden 

Gulch, and Douglas Creek Members yield little or no 

water to wells and, in this study, are considered to be 

an impermeable base to the overlying aquifer system.

The Parachute Creek Member of the Green River 

Formation contains economically valuable oil shale, 

large volumes of ground water, and soluble saline 

minerals. The most common saline mineral in the 

Parachute Creek Member is nahcolite, with halite 

occurring in lesser concentrations. Nahcolite predominately occurs as large crystal rosettes and nodules as much as 3 ft in diameter (Dyni, 1974). Nahcolite also occurs in relatively thin beds of almost 

pure nahcolite and as disseminated crystals in zones 

of varying thickness. The sediments containing the 

majority of the saline minerals are located near the 

center of the basin at the base of the Parachute Creek 

Member. This saline zone is relatively impermeable 

except in areas where fractures have allowed ground 

water access to the soluble minerals. A stratigraphic 

horizon containing numerous vugs, cavities, and 

collapse breccia occurs in areas where nahcolite has 

been dissolved from the formation. This horizon, 

referred to as the leached zone (Kite and Dyni, 1967), 

extends from either the base of the Parachute Creek 

Member or the top of the saline zone to, and in some
Potentiometric

Layer 1~—•—.— Layer 2

6000

5000 -5000

4000 4000

Note: Model layers are described 

in section "Elevation and 

thickness of bedding in 

aquifers"

*0 6 MILES

I IT I I I 1

0246 KILOMETERS 

VERTICAL EXAGGERATION X 21 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

SECTION A-A'
7000

6000

5000

os

4000

Note: Model layers are described 

in section "Elevation and 

thickness of bedding in 

aquifers"

^•43 4000

0 6 MILES0246 KILOMETERS 

VERTICAL EXAGGERATION X 21 

NATIONAL GEODETIC VERTICAL DATUM OF 1929

SECTION B-B'

FIGURE 2. Generalized geologic sections through the Piceance basin (trace of sections located in figure 1).
HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

TABLE 1. Stratigraphic units in Piceance basin

[Modified from Coffin, Welder, and Glanzman, 1971]

Geologic age

Holocene and 

Pleistocene

Eocene

Geologic unit 

and thickness

Alluvium, 

0-140 ft

Uinta 

Formation, 

0-1,400 ft

Formation Gree Rivr 

Parachute 

Creek 

Member, 

500- 

1,400ft

Anvil Points 

Member, 

0-1,800 ft

Garden Gulch 

Member, 

0-900 ft

Douglas Creek 

Member, 

0-800 ft

Lithologic and hydraulic characteristics

Heterogeneous clay, sand, and gravel.

Coarse- to fine-grained, poorly sorted, silty sandstone, siltstone with some barren marlstone. 

Little or no primary porosity remains due to calcium carbonate and silica cementation. Some 

secondary porosity due to fractures. Hydraulic conductivity is relatively small.

Kerogenaceous dolomitic marlstone with some thin ash beds. The unit is lacustrine, with a 

varved appearance and can be divided into four parts. The lower, relatively impermeable 

part of the unit contains saline minerals and is referred to as the saline zone. The leached 

zone overlies the saline zone and has a relatively large hydraulic conductivity due to voids 

produced by dissolution and fractures. The Mahogany zone, the richest oil-shale interval in 

the section, overlies the leached zone and has a relatively small hydraulic conductivity 

throughout most of the basin. The Mahogany zone is overlain by the upper part of the member, which is fractured and water-bearing.

Shale, sandstone, and barren marlstone with minor amounts of siltstone and algal and oolitic 

marlstone. A basin-edge facies which grades into the Garden Gulch and Douglas Creek 

Members. Relatively impermeable.

Shale and barren marlstone with local thin beds of sandstone and limestone. A basin-edge 

facies best developed in the southern part of the basin. Relatively impermeable.

Sandstone with some limestone and shale. Relatively impermeable.

places into, the lower part of the Mahogany zone. 

Alternating layers of marlstone containing more 

than 30 gal/ton on shale oil (rich) and less than 15 

gal/ton (lean) have been identified and sequentially 

numbered by Donnell and Blair (1970) and Cashion 

and Donnell (1972). These alternating rich and lean 

layers occur in the leached and saline zones and are 

used as stratigraphic markers throughout much of 

the basin.

The Mohogany zone is the most consistently rich 

and areally extensive interval of oil shale in the 

Piceance basin, being correlated in outcrops and 

electric logs throughout the basin. The Mahogany 

zone is located in the upper one-third of the Parachute Creek Member and is considered the principal 

mining zone in lease tracts C-a and C-b. The Mahogany zone appears to have fewer fractures than the 

overlying and underlying units. As a consequence, 

the hydraulic conductivity of the zone is generally 

smaller than in the adjacent units. Because of this, 

Coffin, Welder, and Glanzman (1971) and Weeks, 

Leavesley, Welder, and Saulnier (1974), considered 

the ground-water system in the basin to be a twoaquifer system with the Mahogany zone being a 

leaky confining layer between the upper and lower 

aquifers. The part of the Parachute Creek Member 

above the Mahogany zone consists of alternating 

rich and lean layers of oil shale that are overlain by 

the Uinta Formation.

The Uinta Formation consists of discontinuous 

layers of silty sandstone, siltstone, and barren marlstone, and is exposed at the surface throughout much 

of the Piceance basin. The Uinta Formation is about 

1,400 ft thick near the northern end of the study area 

(fig. 2) but the thickness varies considerably due to 

the steep, dissected topography in the basin. The 

Uinta Formation is hydrologically connected to the 

upper part of the Parachute Creek Member but is less 

permeable except where fractures locally enhance 

water movement (Saulnier and Ford, 1977).

Alluvial sediments that occur in the major stream 

valleys in the basin are derived locally from the 

Green River and Uinta Formations. The width of the 

alluvium generally is less than 0.5 mi and the thickness ranges from 0 to about 140 ft. Where saturated, 

the alluvium can serve as a source of recharge to the
AQUIFER CHARACTERISTICS

bedrock aquifers or a sink for discharge from the 

bedrock aquifers depending on local differences in 

potentiometric heads between the alluvial aquifers 

and the bedrock aquifers.

AQUIFER CHARACTERISTICS

The basic aquifer terminology for the Piceance 

basin was proposed initially by Coffin, Welder, and 

Glanzman (1971). The hydrologic system, as proposed, consists of two aquifers separated by a confining layer.

The upper aquifer consists of the saturated rocks 

above the Mahogany zone, and the lower aquifer 

consists of the permeable, saturated rocks below the 

Mahogany zone. The lower aquifer is considered to 

be confined by the Mahogany zone, which acts as a 

leaky confining layer. The Douglas Creek and Garden 

Gulch Members and the saline zone are relatively 

impermeable and form the base of this hydrologic 

system.

FRACTURING

The marlstones in the Parachute Creek Member of 

the Green River Formation have little or no primary 

porosity. Wide variations in hydraulic properties exist 

over short distances because fractures, solution cavities, and collapse breccia are the main source of 

secondary porosity and permeability (Weeks and 

others, 1974). The lean marlstones are more brittle 

than the rich marlstones and, thus, tend to be more 

fractured and more permeable than the rich units. 

Onsite observations indicate that vertical fractures 

may be confined to particular beds or layers, or may 

pass vertically through many beds or layers. The 

primary direction of fracturing is approximately west 

northwest as indicated by the mapped surficial fracturing in the area (fig. 3) and by the work of Wolff, 

Bredehoeft, Keys, and Shuter (1974). Onsite observations by G. J. Saulnier, Jr., and the work of Welder 

(1971) indicate the existence of an extensive network 

of surficial joints and minor faults subordinate to the 

major faults in the area. The alinement of individual 

fractures varies from normal to subparallel to the 

strike of the major faults (fig. 3). The fracture permeability that occurs in wells located away from the 

major fault zones indicates that joints and minor 

faults may occur at depth in a pattern analogous to 

that indicated at the surface.

The alinement of the major fracture systems is 

similar to the alinement of the structural trough at 

the base of the leached zone and the alinement of the 

surface-water drainage system. The tectonic activity 

that shaped the Piceance basin may have affected 

the distribution of fracture permeability in the bedrock aquifers and predisposed the surface-water drainage system to erode channels either along or perpendicular to the structure and major fracture alinement.

AQUIFER LIMITS

The aquifer system is truncated by topography on 

the eastern, northern, and western margins of the 

basin where the water-bearing units crop out. The 

southern boundary of the study area coincides with a 

ground-water divide extending along the Roan Plateau 

to Cathedral Bluffs (Coffin and others, 1971). South 

of this divide, ground water flows toward the Colorado River. North of this divide, ground water flows 

toward Piceance and Yellow Creeks, the two major 

streams draining the study area. The lateral extent 

of the aquifers considered in this study coincides 

with the area studied by Weeks, Leavesley, Welder, 

and Saulnier (1974).

ELEVATION AND THICKNESS OF BEDDING IN AQUIFERS

The vertical variations in potentiometric head and 

ground-water quality that occur in Piceance basin 

prompted the authors to use a five-layer system to 

describe the hydrology and geochemistry (fig. 2). 

Each of the layers was subsequently represented as a 

layer in a mathematical model of the ground-water 

system.

Layer 1 consists of the saturated part of the Uinta 

Formation. The base of layer 1 slopes toward the 

northwestern part of the basin and ranges in elevation from about 5,500 toabout 8,000 ft (fig. 4). The 

saturated thickness of layer 1 exceeds 800 ft in the 

northwestern part of the basin and diminishes to 

zero around a structural high between Piceance Creek 

and Dry Fork, and around the basin margins (fig. 5).

Layer 2 consists of the saturated part of the Parachute Creek Member above the Mahogany zone. The 

base of layer 2 (top of the Mahogany zone) slopes 

toward two structural lows in the northwestern and 

central parts of the basin and ranges in elevation 

from about 5,000 to about 8,000 ft (fig. 6). The saturated thickness of layer 2 varies from zero to more 

than 400 ft in the southern and central parts of the 

basin (fig. 7).

Layer 3 consists of the saturated part of the Mahogany zone. The saturated thickness of layer 3 varies 

from less than 100 to more than 225 ft and is the most 

uniformly thin layer considered in this study (fig. 8).

Layer 4 extends from the base of the Mahogany 

zone to the base of a rich oil-shale unit designated 

R-6 by Cashion and Donnell (1972) and Dyni (1974). 

The saturated thickness of layer 4 varies from less 

than 125 ft in the southwestern part of the basin to
HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

STRIKE OF MAJOR FAULT OR FAULT 

ZONE - Modified from Duncan (1976a-f), 

Hail (1970, 1973, 1974a, b, 1975, 1977), 

O'Sullivan (1974), Pipiringos and 

Johnson (1976), Roehler (1972), and 

Ziemba (1974)

INFERRED STRIKE OF MINOR FAULT 

OR JOINT - Modified from Welder (1971)

108°

[Price Creek

i __MJQFFAT 

L "~^RiC>rBLAN€>6 ICO

Base from U. S. Geological Survey 

State base map, 1969

10

I_

15 MILES

0 5 10 15 KILOMETERS 

FIGURE 3.—Extent of surficial fracturing.

more than 250 ft in the northwestern part of the 

basin (fig. 9).

Layer 5 extends from the base of the R-6 zone to the 

base of the water-bearing materials in the basin. The 

base of layer 5 ranges in elevation from about 4,250

to about 7,500 ft (fig. 10). Thus, a difference of about 

3,250 ft exists between the structural high at the 

western margin of the basin and the structural low in 

the north-central part of the basin. The asymmetry 

of the ground-water basin is apparent as is the extent
AQUIFER CHARACTERISTICS

EXPLANATION

-6000 STRUCTURE CONTOUR - Shows 

elevation of base of Uinta 

Formation. Contour interval 500 

feet. National Geodetic Vertical 

Datum of 1929

 WELL

40°

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES

I I i

0 5 10 KILOMETERS

FIGURE 4. Structure contours on the base of the Uinta Formation.

of the structural high located in the east-central part 

of the basin. The saline zone is found in a 340-mi2 

area in the central part of the basin (fig. 10). The 

saturated thickness of layer 5 varies from zero near 

the outcrops to more than 1,000 ft in the northeastern 

part of the basin (fig. 11).

The total saturated thickness of the five layers 

exceeds 2,000 ft in the northeastern part of the basin 

and generally exceeds 1,500 ft throughout most of the 

central part of the basin. Saturated thickness increases rapidly from zero at the outcrops to more 

than 1,000 ft near the eastern, northern, and western 

margins of the basin.

The aquifer terminology used by Coffin, Welder, 

and Glanzman (1971) and Weeks, Leavesley, Welder, 

and Saulnier (1974) correlates with the layers described above. Layers 1 and 2 represent the upper 

aquifer, layer 3 represents the leaky confining layer, 

and layers 4 and 5 represent the lower aquifer.
10 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

-200—— LINE OF EQUAL SATURATED 

THICKNESS - Interval 200 

feet

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

I I T

0 5 10 KILOMETERS 

FIGURE 5.—Saturated thickness of the Uinta Formation.

HYDRAULIC CONDUCTIVITY

The mathematical model chosen to simulate the 

hydrology and water quality in the Piceance basin 

required a more detailed areal and vertical distribution of hydraulic conductivity than had been presented in reports of earlier studies. New estimates of 

lateral and vertical hydraulic conductivity were made 

using data presented by Picke, Weeks, and Welder 

(1974); Weeks and Welder (1974); Weeks, Leavesley,

Welder, and Saulnier (1974); Wright Water Engineers, 

Inc. (1975); Ashland Oil, Inc., and Shell Oil Co., 

Operator (1976); Campbell (1977); and Dale and Weeks 

(1978); as well as unpublished data on aquifer tests 

performed by F. A. Welder, J. B. Weeks, and G. J. 

Saulnier, Jr. Data from these sources and other information on the structure, stratigraphy, and faulting in the basin were used by the authors to construct 

maps of the horizontal and vertical hydraulic conductivity of each layer. Where aquifer tests or other
AQUIFER CHARACTERISTICS 11

EXPLANATION

-5500—— STRUCTURE CONTOUR - Shows 

elevation of top of Mahogany 

zone. Contour interval 500 feet. 

National Geodetic Vertical 

Datum of 1929. Structure contours 

from Mullens (1976) and modified 

from Austin (1971), Duncan 

(1976a-e), Hail (1974a and 1974b), 

O'Sullivan (1974), and Pipiringos 

and Johnson (1976)

108°

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES

0 5 10 KILOMETERS 

FIGURE 6.—Structure contours on the top of the Mahogany zone.

data were of sufficient density, hydraulic conductivity was estimated on a node by node basis. In less 

well-defined areas, only general estimates of the hydraulic conductivity over large areas were justified. 

The nonhomogeneous character of the fractured 

aquifer makes determination of average aquifer characteristics based on scattered data points extremely 

difficult. As a result, the initial estimates of hydraulic conductivity were modified somewhat based

on results of the model calibration. Although the 

resulting hydraulic conductivity distributions produce reasonable model results, it must be emphasized that the distributions are only estimates and, 

due to the complexity of the fractured aquifers, may 

not be unique in that a different distribution of hydraulic conductivity might be found that also produces reasonable model results and agrees with the 

hydrologic data. The lateral and vertical hydraulic
12 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

 300—— ONE OF EQUAL SATURATED 

THICKNESS - Interval 100 

feet

WELL

108°

Base from U. S. Geological Survey 

State base map, 1969

0

I——

10 MILES

0 5 10 KILOMETERS 

FIGURE 7. Saturated thickness of the Parachute Creek Member above the Mahogany zone.

conductivities estimated for each of the five layers 

are shown in figures 12-17. The figures indicate 

lateral hydraulic conductivities of as much as 2.0 

ft/d along Piceance Creek and in the north-central 

part of the basin. Lateral hydraulic conductivities 

around the margins of the basin range from 0.1 to 0.6 

ft/d. Vertical hydraulic conductivities as small as 7X 

10"4 ft/d occur in the Mahogany zone (layer 3) with 

values as much as 1.0 ft/d in layers 2, 4, and 5.

Aquifer-test data (Ashland Oil, Inc., and Shell Oil 

Co., Operator, 1976, and Sokol, 1976) and work by 

Campbell (1977) indicate that the aquifers are nonhomogeneous and anisotropic as is shown in figures 

12-17. This is due to the differences in the response of 

the rocks to fracturing and is related to the distribution of rich and lean zones and the presence or 

absence of solution cavities and collapse breccia. 

Insufficient data exist to estimate the lateral anisotropy in layers 1, 3, 4, and 5; as a result, these layers 

were assumed to be isotropic [hydraulic conductivity 

in the re-direction (.KV)=hydraulic conductivity in the 

y-direction (Ky)]. A ratio of Kx to Kv of 1.1 was 

estimated for all of layer 2 based on the work of 

Campbell (1977). The ratio of lateral (K*) to vertical
AQUIFER CHARACTERISTICS

EXPLANATION

13

•ISO—— LINE OF EQUAL SATURATED 

THICKNESS - Interval 25 

feet. Modified from Donnell 

and Blair (1970)

WELL

108°

\

Base from U. S. Geological Survey 

State base map, 1969

0 

I——

10 MILES

0 5 10 KILOMETERS 

FIGURE 8.—Saturated thickness of the Mahogany zone.

(Kz) hydraulic conductivity varies from 2 in layers 1 

and 2 to as much as 100 in layer 3, and 500 in layers 4 

and 5.

POROSITY AND COMPRESSIBILITY

Few data have been collected to determine the 

porosity and compressibility of the rocks in Piceance 

basin. Aquifer tests of unfractured sections in the 

saline zone performed in 1973 (Wolff and others, 

1974) and 1976 (Dale and Weeks, 1978) indicate that 

these sections are virtually impermeable and likely 

have little effective porosity. By contrast, relatively

large porosities occur as a result of voids in collapse 

breccia and solution cavities.

Porosity estimates by the authors and by Banks 

and Franciscotti (1976) have been made by solving 

an equation (Lohman, 1972, p. 9) relating porosity to 

the storage-coefficient data obtained from aquifer 

tests in Piceance basin. This technique resulted in 

varied porosity estimates. From such results, Banks 

and his coworkers estimated the porosity in the Parachute Creek Member of the Green River Formation 

to be between 2 and 4 percent. A porosity of 1.6 

percent was obtained from an analysis of a core
14 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

 250—— LINE OF EQUAL SATURATED 

THICKNESS - Dashed where 

approximately located. 

Interval 25 feet. Modified from 

Donne 11 and Blair (1970)

108°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 5 10 KILOMETERS

FIGURE 9. Saturated thickness of the interval from the base of the Mahogany zone

to the base of the R-6 zone.

sample collected by Campbell and Olhoeft (1977). In 

an effort to use a reasonable yet conservative estimate of basinwide porosity, the authors chose a 

value of 1 percent. A small value of porosity was 

chosen so the rate of ground-water movement shown 

by subsequent modeling would not be too conservative and, thus, cause the potential for degradation 

of ground-water quality around the simulated mine 

tracts to be underestimated. About 6.7 million acre-ft 

of ground water are in storage in the Green River and

Uinta Formation aquifers in the basin if the average 

porosity of the formations is 1 percent. If the average 

porosity is 4 percent, the volume of water in storage 

would be about 27 million acre-ft. Better data on the 

porosity of the aquifers are needed before a more 

accurate estimate of the volume of ground water in 

storage can be made.

Podio (1968) determined that the compressibility of 

water-saturated oil shale from the Green River Formation ranged from 2.6X10"7 to 2.85X10"7 in2/lb
GROUND-WATER RECHARGE, DISCHARGE, AND POTENTIOMETRIC HEADS

EXPLANATION

 5500—— STRUCTURE CONTOUR - Shows 

elevation of base of lower 

aquifer. Contour interval is 

variable. National Geodetic 

Vertical Datum of 1929

SALINE ZONE 

WELL

108°

15

Base from U. S. Geological Survey 

State base map, 1969

0 

I——

10 MILES

0 5 10 KILOMETERS 

FIGURE 10.—Structure contours on the base of the lower aquifer.

under confining pressures ranging from 0 to 10,000 

lb/in2. Using these formation compressibilities, a 

porosity of 1 percent and water compressibility of 

3.1X10"6 inVlb, the specific storage of the oil-shale 

aquifers* was calculated to be 1.3X10"7 per foot using 

equations of DeWiest (1969, p. 15). This compares 

favorably with the mean specific storage of 5X10"7 

per foot calculated from the storage-coefficient data 

presented by Weeks, Leavesley, Welder, and Saulnier 

(1974, p. 28).

GROUND-WATER RECHARGE, DISCHARGE 

AND POTENTIOMETRIC HEADS

The part of the Piceance structural basin considered in this investigation is a closed ground-water 

basin in which no cross-boundary flow of ground 

water occurs. Infiltrating precipitation is the principal source of ground-water recharge. This recharge 

causes both downward and lateral ground-water movement toward the north-central part of the basin. In
16 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

j~'~\-. 

EXPLANATION

 400 LINE OF EQUAL SATURATED 

THICKNESS - Interval 200 feet 

feet

WELL

Base from U. S. Geological Survey 

State base map, 1969

0

I——

10 MILES

0 5 10 KILOMETERS 

FIGURE 11. Saturated thickness of the Parachute Creek Member below the base of the R-6 zone.

the central part of the basin, potentiometric-head 

gradients are opposite those near the basin margins 

and ground water moves upward from deeper to 

shallower strata, ultimately discharging into Piceance and Yellow Creeks and their tributaries. Some 

ground water also is discharged by evapotranspiration from vegetation and wetlands in the stream

valleys. Comprehensive descriptions and estimates 

of the sources and rates of recharge to and discharge 

from the aquifers in Piceance basin have been reported (Weeks and others, 1974). The reported estimates generally were used in this investigation and 

are reviewed briefly to provide continuity in this 

report.
GROUND-WATER RECHARGE, DISCHARGE, AND POTENTIOMETRIC HEADS 17

40°

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY

Lateral Vertical

Base from U. S. Geological Survey 

State base map, 1969

10 MILES 

_I I I

10 KILOMETERS

FIGURE 12. Lateral and vertical hydraulic conductivity for layer 1.
18 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY

Lateral Vertical

Kx=0.6, Ky=Q.54 KZ = Q.3

Kx =1.2, Ky= 1.08 Kz=0.6

KX =2.Q, Ky = l.8 KZ = 1.Q

MODEL GRID

DATA POINT

108°

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES

0 5 10 KILOMETERS 

FIGURE 13.—Lateral and vertical hydraulic conductivity for layer 2.

GROUND-WATER RECHARGE AND DISCHARGE

Weeks, Leavesley, Welder, and Saulnier (1974) estimated that the principal source of ground-water recharge was snowpack melt water in areas with elevations greater than 7,000 ft. An estimated 33 to 36 

ftVs of aggregate recharge occurs either as direct 

infiltration into the bedrock aquifers or by recharge 

to small alluvial aquifers in the upper reaches of
GROUND-WATER RECHARGE, DISCHARGE, AND POTENTIOMETRIC HEADS 19

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY

40°

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES 

_l

0 5 10 KILOMETERS 

FIGURE 14. Lateral hydraulic conductivity for layer 3.

numerous creeks, which, in turn, recharge the bedrock aquifers. The distribution of recharge is controlled by topography. Areas of higher elevation 

generally receive more precipitation than lower areas. 

As a result, the principal areas of ground-water recharge probably occur near the topographically high 

regions along the eastern, southern, and western 

margins of the basin.

Ground water discharges from the bedrock aquifers by means of: (1) Direct discharge to surface flow 

in Piceance and Yellow Creeks and their tributaries; 

(2) discharge to alluvial aquifers in the major stream 

valleys; and (3) by evapotranspiration from vegetation and wet soils located primarily in the stream 

valleys. Weeks, Leavesley, Welder, and Saulnier (1974) 

estimated that the aggregate discharge is between
20 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY

Kz = 7 x HT 4

Kz = \ x 10-3 to 6 x 10-3

Kz=\ x 10-2 to 5x 10-2

MODEL GRID

108°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

33 and 36 ftVs, with 18 percent of the total discharge 

occurring in the Yellow Creek drainage basin and 82 

percent occurring in the Piceance Creek drainage 

basin. About 80 percent of the surface runoff in the 

basin is supplied by ground-water discharge.

POTENTIOMETRIC SURFACES

Measurements of the elevation of the static water 

level in wells were used to construct the potentiometric-surface maps shown in figures 18 and 19. 

Because of only minimal ground-water development
GROUND-WATER RECHARGE, DISCHARGE, AND POTENTIOMETRIC HEADS 21

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY

K-x — *Vy ~~ 0 • 3 

Kx=Ky=0.5

MODEL GRID

DATA POINT

108°

~^^v\^^r

Base from U. S. Geological Survey 

State base map, 1969

0 10 MILES

0 5 10 KILOMETERS 

FIGURE 16.—Lateral hydraulic conductivity for layers 4 and 5.

as of 1977, the potentiometric surfaces are representative of long-term ground-water conditions in 

the basin. The elevation of the potentiometric surface in the aquifer above the Mahogany zone (fig. 18) 

(layers 1 and 2) ranges from about 5,800 ft near the 

mouths of Piceance and Yellow Creeks at the north-
22 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

ESTIMATED HYDRAULIC 

CONDUCTIVITY, IN 

FEET PER DAY 

Kz-l x lO- 3 to 7 x 10-3

Kz = lx 10-2 to 6xlO- 2 

= l x 10-1 to 5 x io-l

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

FIGURE 17.—Vertical hydraulic conductivity for layers 4 and 5.

ern margin of the basin, to about 7,400 ft in the 

southern part of the basin. Ground-water gradients 

range from 20 to as much as 120 ft/mi, and lateral 

ground-water pore velocities range from about 150 

ft/yr near the confluence of Ryan Gulch and Piceance Creek to about 300 ft/yr in the area south of 

Piceance Creek. These velocities are based on an 

average porosity of 1 percent and are thought by the 

authors to be a reasonable upper estimate of the rate 

of ground-water movement.
GROUND-WATER RECHARGE, DISCHARGE, AND POTENTIOMETRIC HEADS 23

EXPLANATION

6200 —— POTENTIOMETRIC CONTOUR - 

Shows elevation of potentiometric surface. Dashed where 

approximately located. Contour 

interval 200 feet. National 

Geodetic Vertical Datum of 1929

-————— APPROXIMATE LIMIT OF 

UPPER AQUIFER

• WELL

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

FIGURE 18.—Potentiometric surface of the upper aquifer.

The potentiometric surface in the aquifer below the 

Mahogany zone (fig. 19) (layers 4 and 5) is similar to 

that in the upper aquifer. Elevations range from 

about 5,800 to about 7,400 ft. Ground-water gradients 

range from 20 to as much as 130 ft/mi, and lateral 

ground-water pore velocities range from about 90 

ft/yr near the confluence of Ryan Gulch and Piceance Creek to about 140 ft/yr near the confluence of 

Stewart Gulch and Piceance Creek.

Water moves through isotropic aquifers in a direction perpendicular to the potentiometric contours. 

Because the aquifers in the Piceance basin apparently are not significantly anisotropic, the direction of 

water movement is approximately perpendicular to 

the contours. On the basis of the configurations of 

the potentiometric surfaces, water moves from recharge areas near the basin margins toward discharge areas along Piceance Creek and the lower
24 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

-6200— —

EXPLANATION

POTENTIOMETRIC CONTOUR - 

Shows elevation of potentiometric surface. Dashed where 

approximately located. Contour 

interval 200 feet. National 

Geodetic Vertical Datum of 1929

APPROXIMATE LIMIT OF 

LOWER AQUIFER

108°

Meeker^

r4V^

A^YVA^T

40°

— —-7400-——————'

Base from U. S. Geological Survey 

State base map, 1969

0

\——

10 MILES

0 5 10 KILOMETERS 

FIGURE 19.—Potentiometric surface of the lower aquifer.

reaches of Yellow Creek. The configuration of the 

potentiometric surfaces also indicates that water 

discharges along Dry Fork Creek and the downstream reaches of the creeks located between Black 

Sulphur and Willow Creeks.

The differences in the potentiometric heads in wells 

completed above and below the Mahogany zone are 

shown in figure 20. Potentiometric-head differences 

as large as 240 ft have been measured in adjacent

wells completed in the two aquifers. The pattern of 

potentiometric-head differences indicates that the 

potential for downward movement of water exists 

along the eastern, southern, and western margins of 

the basin, along the drainage divide between Piceance and Yellow Creeks, and in the area between 

Piceance Creek and Dry Fork. Potential for upward 

movement of water primarily exists along Piceance 

Creek and the downstream reaches of its tributaries.
GEOCHEMICAL SYSTEM 25

EXPLANATION

MODEL CALCULATED HEAD 

DIFFERENCE BETWEEN 

LAYERS 2 and 4 INDICATES 

POTENTIAL FOR

Upward ground-water flow

Downward ground-water flow

OBSERVATION WELLS SHOWING 

GROUND-WATER FLOW ACROSS 

THE MAHOGANY ZONE - Number 

is potentiometric head difference, 

in feet, where known. Modified 

from Weeks, Leavesley, Welder, 

and Saulnier (1974)

Upward

Downward

None

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 5 10 KILOMETERS 

FIGURE 20.—Difference in potentiometric heads between the upper and lower aquifers.

GEOCHEMICAL SYSTEM

The concentration of dissolved solids in ground 

water in the Piceance basin ranges from 400 mg/L 

(milligrams per liter) near the margins of the basin to 

40,000 mg/L in the north-central part of the basin. In 

addition to these lateral changes in quality, large 

increases in dissolved-solids concentrations also occur with depth in the aquifers. These spatial differences in concentration are due to changes in water 

quality along the ground-water flow paths between 

the points of recharge and the points of discharge. 

These changes include an increase in dissolved-sohds 

concentration, and a change in water type from a 

mixed cation bicarbonate water to a sodium bicarbonate water, oxidation and reduction of sulfur 

species, and relatively large increases in certain trace
26 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

constituents such as strontium and fluoride. The 

changes reflect the geochemical environment through 

which the ground water flows. The magnitude of the 

changes is affected by the type and concentration of 

soluble minerals present in the aquifers, the type of 

clay minerals and amount of organic material present in the aquifers, and the flow path the water 

follows through the aquifers.

In the highland recharge areas, both sandstones 

and siltstones of the Uinta Formation and marlstones of the Parachute Creek Member crop out. The 

soil cover is relatively thin in these areas and snowmelt often infiltrates directly into weathered bedrock 

before moving down into unweathered bedrock. The 

infiltrating water carries dissolved atmospheric gases 

and soluble minerals from the weathered and unweathered surfaces into the upper aquifer. Dissolvedsolids concentrations in the recharge have not been 

directly measured but are indicated by the chemical 

quality of water in springs and shallow wells in the 

upper aquifer. These data indicate that the dissolvedsolids concentrations in the recharge may range 

from 450 to 800 mg/L, as shown in figure 21. Dissolved carbon dioxide in the form of carbonic acid 

increases the solubility of the carbonate cements 

present in the sandstones and the carbonate minerals present in the marlstones. As a result, water 

samples from springs and surface water in the recharge areas indicate that the ground water varies 

from a calcium magnesium bicarbonate type to a 

mixed cation bicarbonate type. Although ground 

water in the recharge areas is generally in an oxidized state, reducing conditions exist at depth as 

indicated by the presence of hydrogen sulfide gas in 

water from springs from deep sources and wells 

producing water from the upper part of the Parachute Creek Member.

As the ground water in the upper aquifer flows 

away from the recharge areas, dissolved-solids concentrations increase and the water changes to a 

sodium bicarbonate type. Large dissolved-solidsconcentration increases occur in the Uinta Formation (layer 1) where concentrations are two to three 

times as great as in the recharging water, and near 

the base of the upper part of the Parachute Creek 

Member (layer 2) where concentrations are 10 or more 

times as great as in the recharging water. The principal source of this increase appears to be the upward 

flow of water with large dissolved-solids concentrations from the lower aquifer into the upper aquifer. Upward flow probably accounts for the larger 

dissolved-solids concentrations near tract C-b and 

in the north-central discharge areas of the basin 

(figs. 20 and 24).

Increases in the dissolved-solids concentrations in 

the lower aquifer probably occur in three ways. First, 

the dissolved solids increase by dissolution of the 

host marlstones as water moves through the aquifer. 

Second, permeable zones produced by fracturing and 

dissolution of saline minerals near the basin margins laterally abut impermeable sections of the saline zone toward the central part of the basin. The 

configuration of the edges of the saline zone is highly 

irregular, resulting in local restrictions to horizontal 

and vertical flow. This restriction to flow appears to 

create zones of almost stagnant ground water that 

slowly dissolves saline minerals from the host rock. 

Third, water moving through fractures and solution 

openings in the lower part of the Parachute Creek 

Member can come in contact with sodium chloride 

and sodium bicarbonate salts in the aquifer. These 

soluble minerals dissolve to produce water with dissolved-solids concentrations in excess of 30,000 mg/L. 

Over geologic time, these processes of dissolution 

have produced the leached zone.

Saulnier (1978) found that the dissolution is presently most active along the edges of the main body 

of the saline zone. Further, dissolution in the saline 

zone is aided by major fractures that penetrate this 

section and allow circulating ground water to contact the soluble salts. This phenomenon is well displayed in a west-northwest-trending zone of fracturing along the northern margin of the basin (fig. 22). 

Here fractures allow saline water to move from the 

lower to the upper aquifer, thereby increasing the 

dissolved-solids concentrations in the upper aquifer. 

This fracture-associated dissolution is reponsible for 

the saline springs in Alkali Flat adjacent to the lower 

reach of Piceance Creek and Stinking Spring along 

the lower reach of Yellow Creek. An analogous situation is the probable cause of the saline water pumped 

from wells that have been drilled through and left 

open to the saline zone. Saline water moving up the 

well bores locally contaminates water in the upper 

aquifer and in the alluvium. In the past, this local 

contamination has given an erroneous indication of 

the extent of saline water in the upper aquifer (Welder and Saulnier, 1978, p. 14).

CHEMICAL REACTIONS

The principal chemical reactions observed in the 

Piceance basin are dissolution, precipitation, ion exchange, and oxidation reduction. These reactions 

primarily are responsible for the chemical composition of the ground water in the basin. The reactions 

are best described in reference to position in the 

ground-water flow system.
CHEMICAL REACTIONS 27

450

EXPLANATION

ESTIMATED DISSOLVED-SOLIDS 

CONCENTRATIONS, IN MILLIGRAMS PER LITER

108°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 5 10 KILOMETERS 

FIGURE 21.—Estimated dissolved-solids concentrations in water recharging the aquifer system.

DISSOLUTION

Various dissolution reactions occur throughout the 

basin but specific reactions predominate at different 

points along the ground-water flow paths. Mineralogic analyses of samples of oil shale from the Green 

River Formation indicate that about 43 percent of the 

marlstone is composed of calcite and dolomite (Yen 

and Chulingarian, 1976). In the Uinta Formation, 

calcite and dolomite are present as cements in the 

sandstones and siltstones of the unit. Although these 

minerals do not dissolve rapidly, the chemical composition of ground water in recharge areas where 

marlstones are exposed indicates calcite and dolomite are going into solution. Calcium concentrations

usually range from 50 to 100 mg/L and are almost 

double the magnesium concentrations.

Farther down the ground-water flow path (toward 

the center of the basin), other dissolution reactions 

predominate. These reactions involve the evaporative salts, nahcolite and halite, found in the saline 

zone. The minerals are so soluble that they can be 

detected during drilling by measuring the specific 

conductance of water produced during drilling and 

comparing these data to core samples. Cores containing the salts are often of nonuniform diameter 

due to the differing rates of dissolution of adjacent 

segments of the core. Because of the small permeability of this zone (Dale and Weeks, 1978), the minerals can be dissolved only when fractures, wells, or
28 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

MAJOR FAULTS AND JOINTS 

AS SHOWN ON ERTS 

SATELLITE IMAGERY -

Dashed where approximately 

located

SPRING YIELDING SALINE WATER 

WELL YIELDING SALINE WATER

• In9x^ Fracture zone responsible for 

\Spnng \ioc3iupvvardxrn6vernentof 108° 

"N^X saline water

VNl\/oJ ^fc. /

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 5 10 KILOMETERS

FIGURE 22.—Major faults and joints in the northern part of the Piceance basin (modified from

Saulnier, 1978).

adjacent dissolution have exposed the minerals to 

water. Welder and Saulnier (1978) and Dale and 

Weeks (1978) report that water from the leached 

zone in the central part of the basin, where the saline 

zone is thickest, has a specific conductance that 

ranges from 1,100 to 1,600 ^mho/cm (micromhos per 

centimeter at 25°C). These relatively small values

of specific conductance indicate that only limited 

dissolution is occurring at the top of the present 

saline zone. However, the specific conductance of 

water from wells near the perimeter of the saline 

zone indicates that large-scale dissolution occurs 

near the northern margin of the basin and along the 

edges of the saline zone. Dissolution of these sodium
CHEMICAL REACTIONS 29

salts is a contributing factor in the change of water 

from a mixed cation bicarbonate type water to a 

sodium bicarbonate type water. Other factors affecting this change in water type are calcite precipitation and ion exchange, as explained in the following sections.

PRECIPITATION

Examination of drill cores and unweathered outcrops reveals calcite deposits in fractures and vugs in 

the Parachute Creek Member. A fault zone in Ryan 

Gulch contains a 2- to 5-ft wide filling of laminated 

and crystalline calcite. Springs in the northern part 

of the basin sometimes issue from travertine mounds, 

and seeps on cliff faces may flow down "drapings" of 

calcite and travertine. These occurrences are indirect 

evidence that calcite is at or near saturation in most 

ground water in Piceance basin even though maximum calcium concentrations are about 100 mg/L 

and may be as small as 10 mg/L in water from the 

Parachute Creek Member.

A computer program entitled WATEQ (Truesdell 

and Jones, 1974) was used to analyze water-quality 

data for the different aquifer units in the basin to 

determine the stability of various mineral phases in 

the ground-water environment. The calculations 

showed that calcite is at or near saturation in the 

majority of samples and in nearly all samples collected away from the recharge areas. The precipitation 

of calcite is probably determined by the generalized 

reaction:

2HC(V+Ca2+-CaCO3l+H2CO3. (1)

In Piceance basin, the driving mechanism for such a 

reaction is the bicarbonate concentrations (usually 

greater than 300 mg/L) in the water. The bicarbonate is supplied by dissolution of carbonate minerals and nahcolite, by the sulf ate-reduction reaction 

discussed below, and possibly by silicate hydrolysis. 

Other minerals that have been observed to be precipitated secondarily are pyrite, FeSz, and neighbor! te, NaMgF3 . Pyrite is commonly found lining 

vugs, filling fractures, and replacing fossil remains 

in rocks from the Parachute Creek Member, especially below the Mahogany zone. Hydrogen sulfide gas 

often is detected in wells penetrating the Parachute 

Creek Member, indicating that reducing conditions 

are present. At the levels of pH (7 to 8.8) generally 

measured in the Piceance basin, pyrite is stable in 

reduced waters. Pyrite also is found as a primary 

precipitate in the marlstones, indicating that the 

secondary minerals may represent recycled pyrite 

produced by past changes in alkalinity and formation temperature.

Neighborite is a rare mineral and was found only 

in a few samples from the northern part of the basin. 

The mineral consists of soft, yellow to white, translucent crystals found in vugs in the Parachute Creek 

Member. Very little is known about the stability of 

neighborite, but water in the northern parts of Piceance basin locally contains relatively large concentrations of sodium and fluoride. Plots of water 

types and ionic ratios calculated for this part of the 

basin indicate a slight enrichment in magnesium 

due to calcite precipitation. Larger fluoride concentrations may combine with magnesium to form the 

ion pair, MgF3", prior to neighborite precipitation.

ION EXCHANGE

Although the changing of water from a mixed 

cation bicarbonate type in the recharge areas to a 

sodium bicarbonate type in the central discharge 

parts of the basin occurs in both the upper and lower 

aquifers, the change is more pronounced and occurs 

in shorter lateral and vertical distances in the Parachute Creek Member. For example, water from a 

spring issuing from the upper aquifer along upper 

Black Sulfur Creek has a calcium concentration of 61 

mg/L and a sodium concentration of 45 mg/L. A 

sample collected from the lower aquifer at Ryan 

Ridge in the central part of the basin has a calcium 

concentration of 4.2 mg/L and a sodium concentration of 550 mg/L. Dissolution of nahcolite is a 

significant factor in this change in ionic proportions in areas where brines exist, but in other areas 

and in the Uinta Formation, another mechanism 

must account for this change in ionic proportions.

The dolomitic marlstones of the Parachute Creek 

Member contain significant amounts of clay minerals. Dyni (1976) determined that smectite, illite, 

chlorite, and kaolinite occur in these rocks and that a 

progression of clay types occurs from the basin margins to the basin center. Illite is the most common 

clay mineral in the Piceance basin, especially away 

from the saline-mineral depocenter. Ground water 

near the margins of the basin may have calcium 

concentrations of about 100 mg/L as the result of 

dissolution of the dolomitic marlstones. If ion exchange were taking place according to the following 

reaction (X indicates an exchange site):

(2)

the calcium would be removed from solution and 

sodium would become the dominant cation in solution. Although the calcium concentration is not ex-
30 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

cessive, preliminary geochemical model studies indicate that ion exchange may be actively altering the 

water composition in the central part of the basin. 

Another ion exchange mechanism involving dolomite could potentially contribute to the observed 

magnesium enrichment according to the following 

generalized reactions (Vindicates an exchange site):

CaMg(CO3)2+Ca*-MgAH-2CaCO3 , (3)

(4)

This series of reactions could lead to the observed 

precipitation of low magnesium calcite (Desborough 

and others, 1974) and relative magnesium enrichment in the water. Sodium concentrations in the 

water could remain relatively large due to nahcolite 

dissolution and the calcium-sodium exchange illustrated by reaction 2.

OXIDATION

Sulfur is present in ground water in Piceance basin 

as both oxidized and reduced species. Sulfur in the 

Uinta Formation is present as the oxidized sulphate 

ion in concentrations exceeding 500 mg/L. Oxidizing conditions exist in the Uinta Formation due to 

the availability of oxidized water, which occurs as 

precipitation recharge. In areas of upward water 

movement, sulfide species that migrate upward into 

the Uinta Formation from underlying formations 

are oxidized to sulfate. In other areas where upward 

movement of sulfur is not possible, the source of the 

sulfate in the Uinta Formation is unknown. Gypsum 

has been recorded only in discharge areas in the 

Uinta Formation, not in recharge areas or near the 

water table. In the upper parts of the Willow Creek 

drainage basin, springs bring water containing hydrogen sulfide gas to the surface from below the 

Uinta Formation. In this part of the recharge area, 

such upward leakage along fractures may provide 

the source of sulfate for the Uinta Formation.

In the Parachute Creek Member, primary and 

secondary pyrite minerals in the organic-rich marlstones probably provide a sulfur source when exposed to oxidized water in the recharge areas.

REDUCTION

Water from the organic-rich marlstones of the Parachute Creek Member often contains hydrogen sulfide 

gas and smaller sulfate concentrations than found in 

the Uinta Formation. Sulfate concentrations less 

than 10 mg/L are common in samples from the lower

aquifer. During the drilling of 24 test holes in 1975- 

76, hydrogen sulfide gas commonly was released as 

the drill penetrated the Parachute Creek Member.

The source of sulfur in the Parachute Creek Member is not certain but the large amounts of organic 

material present in this unit probably help to maintain a reducing condition throughout much of Piceance basin. The most probable mechanism for 

sulfide production is through the generalized reaction:

organic material+SCV"--

HaS+carbonate species. (5)

Whether biogenic processes are involved in the reduction of sulfate is not clear, but the coincidence of 

aqueous sulfide species and organic material in the 

bedrock occurs throughout the basin. This reaction 

also would act as another contributor to the large 

concentrations of bicarbonate found in water from 

this stratigraphic unit.

Pyrite is commonly found in small quantities as a 

primary and secondary mineral in the Parachute 

Creek Member (Desborough and Pitman, 1974). This 

mineral is slightly soluble in warm water and may 

contribute small amounts of hydrogen sulfide to the 

water through the following reaction in a reducing 

environment:

FeS2+4H++2«f-2H2S+Fe2+ . (6)

The Fe2* ion that would result from reaction in 

equation 6 may be reprecipitated as the black precipitate often observed during the pumping of wells 

producing water from the Parachute Creek Member. 

The Fe2* ion probably combines with the HS~ to 

precipitate FeS, which is unstable in the oxidizing 

environment that occurs when the water reaches the 

surface. This instability causes the precipitate to 

oxidize in the presence of air to form an iron oxide. 

Iron oxide coatings are commonly found on rocks 

that are exposed to the "black water" discharged 

from wells tapping the Parachute Creek Member.

Secondary pyrite is formed where the dissolved 

iron, which went into solution in the recharge areas, 

precipitates out of solution in the discharge areas. 

Drill cores from the Naval Oil Shale Reserve, immediately to the south of the study area, have extensive pyrite coatings in vugs and on fractures. 

These secondary pyrite minerals occur where ground 

water is close to a discharge area.

The product of the principal chemical reactions is a 

sodium bicarbonate water with oxidized sulfur species
DISTRIBUTION OF TRACE CONSTITUENTS 31

in the Uinta Formation and reduced sulfur species in 

the Parachute Creek Member. Carbonate species and, 

therefore, total carbon dioxide in the sytem are produced by atmospheric introduction via recharge, reduction of sulfur species, and dissolution of the carbonate minerals, calcite, dolomite, and nahcolite. 

Carbonate species are removed from the system in 

the northern part of the basin by calcite precipitation. Large sodium concentrations are produced by 

ion exchange and nahcolite and halite dissolution.

DISTRIBUTION OF TRACE CONSTITUENTS

Certain trace constituents occur in ground water in 

the basin in larger than normal concentrations. The 

relative concentrations of these constituents can be 

used as an aid in identifying the source of the water 

in springs and wells. Desborough, Pitman, and Huffman (1974) reported on the mineral residence of 

many of the trace constituents, and Dyni (1976) speculated that smectites and illites served as traps for 

trace constituents soon after the lake sediments were 

deposited. Other sources of trace constituents are in 

primary sulfide, halide, carbonate, and silicate minerals that occur in the basin sediments. The following data on the occurrence and distribution of trace 

constituents in ground water in the Piceance basin is 

excerpted from Saulnier (1978).

Trace constituents that were analyzed for but found 

in less than significant concentrations were beryllium, copper, cadmium, lead, mercury, manganese, 

molybdenum, selenium, and zinc. Trace constituents 

found in significant concentrations to warrant further discussion were arsenic, barium, boron, fluoride, 

iron, lithium, and strontium.

ARSENIC

Concentrations of dissolved arsenic in ground water 

in Piceance basin generally do not exceed 50 fjLg/L 

(micrograms per liter). However, concentrations 

of dissolved arsenic between 10 and 50 Mg/L are 

common in water from wells tapping the Parachute 

Creek Member in the area between Piceance Creek 

and Dry Fork. Arsenic concentrations of 80 and 95 

Mg/L were found in water from two wells located in 

this area. The mineral residence of arsenic is probably authigenic sulfides in the Parachute Creek Member (Desborough and others, 1974).

BARIUM

Dissolved-barium concentrations greater than 500 

Mg/L occurred in water from about one-third of the 

sampled wells completed in the lower aquifer. The

largest concentrations occurred in the brines found 

in the northern part of the basin. Barium concentrations generally are less than 500Mg/L in the upper 

aquifer. Hem (1970) suggests that barium concentrations generally are controlled by the solubility of 

barite (BaSO4). The occurrence of barite as a secondary mineral in the northern part of Piceance basin 

has been reported (G. A. Desborough, written commun., 1975). Calculations using the WATEQ computer program indicate that the brines in the basin are 

slightly saturated with respect to barite and are 

saturated with respect to witherite (BaCOs), which 

also occurs in the area (Milton, 1971). Barium can 

possibly be complexed with chloride in reduced waters 

(Holland, 1967), a distinct possibility in the high 

chloride, reducing waters of the north-central Piceance basin. Upon entering oxidized waters, the 

relatively insoluble barite would then precipitate as 

a secondary mineral, the mode of occurrence observed for barite in this study (Plummer, 1971).

BORON

Concentrations of boron greater than 5,000 ng/L 

are common in water samples from the lower aquifer 

and less common in the upper aquifer. Concentrations of this magnitude occur in water throughout 

the basin but are most common in the northern part 

of the basin where the brines occur.

Boron is commonly concentrated in evaporites, but 

no borate minerals have been reported in the Piceance basin. Shepard and Gude (1973) found that 

boron can occur in potassium feldspars of saline, 

alkaline, lacustrine deposits. Boron has not been 

identified in the feldspars of the Piceance basin, but 

searlsite (Na3Si2O6«H2O), reedmergneritefNaBSiaOs), 

garrelsite [(Ba,Ca,Mg)B2SiO6(OH)3], and leucosphenite 

(CaBaNaBTiaSigOag) have been reported in the Green 

River Formation (Milton, 1971). Little is known of the 

solubility of these minerals. As a result, the mineral 

source of the boron in solution in the Piceance basin is 

unknown at present.

FLUORIDE

The distribution of dissolved fluoride in the upper 

and lower aquifers can be described in relation to the 

abundance of dissolved calcium as well as the presence of fluoride minerals. The amount of fluoride 

ion theoretically in solution at equilibrium can be 

determined by the solubility product (X.p) of fluorite 

(CaF2):

(7)
32 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

Because the fluoride activity is squared, small calcium concentrations would allow large fluoride concentrations to be present in the water. The equilibrium concentration of dissolved calcium fluoride is 15 

mg/L at 18°C. The average concentrations are 38 

mg/L for calcium and 1.1 mg/L for fluoride in water 

from the Uinta Formation, 18 mg/L for calcium and 

7.1 mg/L for fluoride in water from the upper part of 

the Parachute Creek Member, and 10 mg/L for calcium and 22 mg/L for fluoride in water from the 

lower part of the Parachute Creek Member. Larger 

concentrations of calcium in water from the Parachute Creek Member would result in the precipitation 

of fluorite. The reduction of calcium concentrations 

due to ion exchange and calcite precipitation are 

thus important factors in maintaining the observed 

large concentrations of fluoride.

Authigenic cryolite (NasAIFe), fluorite (CaF2>, fluorapatite [CasfPO^F], and neighborite (NaMgFs) 

are present in the Parachute Creek Member (Charles 

Milton, written commun., 1974). Calculations using 

the WATEQ computer program indicate that the 

water is undersaturated with respect to both fluorite 

and cryolite; thus, these minerals probably are dissolving rather than precipitating. As a result, the 

large proportion of fluoride in cryolite could add 

significantly to the dissolved-fluoride concentrations 

found in the ground water. Little is known about the 

kinetics of dissolution of these minerals in the presence of water containing relatively small concentrations of calcium and relatively large concentrations of sodium.

The original source of the fluoride is uncertain. The 

fluoride may have been added to Lake Uintah by hot 

springs or volcanic ash. Devitrified tuffaceous lenses 

are common in the Green River Formation. Evaporation from the lake could have concentrated the 

fluoride sufficiently to form authigenic fluoride minerals and allow the inclusion of the fluoride ion in 

clay minerals. After induration and basin deformation, circulating ground water reversed the process, 

allowing relatively large concentrations of fluoride 

to be present in the water. At some time after resolution of the fluoride, neighborite (NaMgFs) precipitated in vugs. It appears likely that the formation 

of MgF3 complexes precedes neighborite precipitation. As in the calcium fluoride example above, the 

solubility product indicates that relatively small 

magnesium concentrations would be required to maintain the relatively large fluoride concentrations. In 

the lower aquifer, the smallest concentrations of 

dissolved magnesium are found to coincide with the 

largest concentrations of dissolved fluoride as required by the solubility product calculations.

The absence of significant concentrations of dissolved fluoride in the Uinta Formation is due to the 

combination of relatively larger calcium concentrations than in the Parachute Creek Member and the 

absence of a source of fluoride in the sediments or in 

authigenic minerals. Near some fractures, faults, 

and improperly constructed wells, water in the alluvial aquifer and the Uinta Formation has unusually high concentrations of fluoride for this formation. 

These relatively larger concentrations occur in the 

northern part of the basin where the potentiometric 

head allows upward movement of water from the 

Parachute Creek Member.

IRON

Water with more than 300 Mg/L of dissolved iron is 

common in the upper aquifer. The sandstones and 

siltstones of the Uinta Formation have ironstone 

concretions throughout. These concretions probably 

are the major source for much of the dissolved iron.

The Parachute Creek Member contains variable 

amounts of authigenic pyrite (Trudell and others, 

1970). In the recharge areas and in the warm bicarbonate water found in the lower aquifer where the 

pH normally ranges from 7.5 to 8.5, pyrite either can 

be dissociated to form ferrous ions in solution or the 

ferrous ion can combine with carbonate to form siderite and ferroan [(Mg,Fe)COs]. The presence of 

siderite and ferroan are commonly reported in the 

Parachute Creek Member (Desborough and others, 

1974; Robb and Smith, 1974). Relatively small concentrations of iron occur in water in the Parachute 

Creek Member, indicating that most dissolved iron 

may be reprecipitated as carbonate. As noted above, 

the "black water" produced from many wells in the 

Parachute Creek Member indicates that some ferrous iron remains in solution and is carried into wells 

and subsequently precipitated as FeS.

LITHIUM

Concentrations of dissolved lithium are greater 

than 100 Mg/L in water samples from wells that 

contain brines. Relatively large concentrations of 

lithium also are associated with water that has relatively small dissolved-solids concentrations but relatively large concentrations of other trace constituents. The largest concentrations of lithium occur in 

samples from wells in the central part of the basin. 

Dissolved lithium concentrations as much as 39 and 

42 mg/L have been reported in water from these 

wells. Desborough and others (1974) did not find a 

likely mineral residence for lithium but indicated 

that fluoride and lithium show a positive correlation
THE SIMULATED HYDROLOGIC SYSTEM 33

in water samples from the Piceance basin reported in 

Ficke, Weeks, and Welder (1974). Relatively large 

lithium concentrations also are associated with relatively large concentrations of bicarbonate. The lithium may occur in the evaporite minerals of the Parachute Creek Member either as a carbonate or a 

chloride. In addition, lithium may be,adsorbed on 

clays. It is possible that in predominantly sodium 

waters, sodium could exchange for adsorbed lithium 

in smectites and illites. Because these same waters 

also have relatively small calcium concentrations 

and consequently relatively large fluoride concentrations, the result would be a positive lithium-fluoride correlation.

STRONTIUM

Strontium is a significant trace constituent in the 

upper aquifer system, especially in the Uinta Formation and its associated marlstone tongues. In 

samples from springs in drainage basins tributary to 

Piceance Creek, strontium concentrations increase 

downgradient in a distance of 10 to 15 mi. Thus, 

minerals containing strontium probably are being 

dissolved by the circulating ground water. Calculations using the WATEQ computer program indicate the water generally is supersaturated with respect to strontianite (SrCO3) but not with respect to 

celestite (SrSO4 ). However, celestite is rarely reported 

in mineral analyses in Piceance basin.

The strontianite supersaturation indicates a precipitation reaction that is very slow. Thus, the strontium in solution could be dissolved from carbonates 

or sulfates in the first few miles of the flow system 

and remain dissolved in the water, accumulating due 

to a slow rate on mineral precipitation. The original 

source of the strontium in the Uinta Formation is 

unknown, but it is possible that strontium occurs in 

the carbonate cements or as accessory sulfate in the 

Uinta Formation.

THE SIMULATED HYDROLOGIC SYSTEM

Mining of oil shale from tracts C-a and C-b will 

require water to be pumped from the mines in order to 

prevent flooding. The pumping could cause waterlevel declines in the aquifers and reduce the rate of 

ground-water discharge to streams near the mines. 

In addition, the pumping could alter the location of 

ground-water flow paths, thereby producing changes 

in the chemical quality of the water in the aquifers 

and in local streams. Spent shale also could become a 

source of ground-water quality degradation once the 

mining intervals have been exhausted and the mine

retorts containing the spent shale become flooded. 

Studies by Ward, Margheim, and Lof (1971) and 

Schmidt-Collerus (1974) indicate that between 1 and 

5 percent of the spent shale may be water-soluble 

saline minerals. On abandoning tract C-b, for example, the volume of saline minerals in the spent 

shale might range from 30 to 150 million tons and 

could thus have a serious long-term impact on the 

water quality near the abandoned mine. Mathematical-modeling techniques may be used to evaluate the 

possible effects of various mine-pumping practices 

and subsequent mine flooding. Weeks, Leavesley, 

Welder, and Saulnier (1974) and Sokol (1976) determined some of the possible effects of mine dewatering on the water levels in the upper and lower 

aquifers in Piceance basin; however, they did not 

study the water-quality changes that may accompany the mining activities.

MODEL DESCRIPTION

In order to simulate the possible effects of the 

mines on ground-water quality, a three-dimensional 

mathematical model that simulates solute transport 

through the ground-water system was constructed. 

The model is based on a finite-difference solution 

technique developed for the U. S. Geological Survey 

by INTERCOMP, In*. (1976). The model simulates 

confined, density-dependent flow and solute transport 

with dispersion in nonhomogeneous, anisotropic aquifers with irregular geometry under steady- or transient-flow conditions. The model simulates movement 

of a conservative solute (dissolved-solids concentrations) with dissolution as the only chemical reaction 

term. As a result, the complex geochemical system in 

the basin can only be simulated to the extent that the 

geochemistry affects the dissolved-solids concentrations in the aquifer. This is not a severe restriction, 

however, because dissolution of evaporite minerals 

in the saline zone and dissolution in the unsaturated 

zone are the predominant chemical reactions occurring in the basin, and these reactions dominate the 

effects of the other geochemical reactions. The basic 

equations that describe the ground-water flow and 

material balance of the solute have pressure and 

concentration as the primary dependent variables. 

The independent variables include: (1) Time; (2) aquifer characteristics, such as hydraulic conductivity, 

aquifer extent and thickness, porosity, compressibility, solubility, and dispersivity; (3) fluid characteristics, such as concentration, density, compressibility, viscosity, and flux rate; and (4) related initial 

and boundary conditions.

The node network used in the Piceance basin model
34 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

consists of a 9- by 14-cell array for each of the five 

layers considered in the model. The lateral model 

boundary for any individual layer may vary from the 

boundary of an adjacent layer due to the differences 

in the lateral extent of the layer in each depth interval.

The hydrologic-model parameters are described at 

each node in the model area. The thickness of each 

model layer corresponds to the saturated thickness 

shown in figures 5, 7, 8, 9, and 11. The hydraulicconductivity values assigned to the x, y, and z components for each model layer are shown in figures 12 

to 17. The porosity, potentiometric-head, groundwater quality, and specific-storage data discussed 

previously also were used to define the independent 

variables of the model.

Dissolution was simulated by using the groundwater recharge sources in the model program to 

supply mass to the aquifer. The rate at which the 

mass was supplied corresponds to the rate of dissolution.

MODEL CALIBRATION

In order to assure that the model calculations are 

correct, a calibration procedure was used to check the 

model-calculated potentiometric heads and dissolvedsolids concentrations against the corresponding measured values. The present potentiometric heads and 

dissolved-solids concentrations are representative of 

the long-term ground-water conditions in the basin 

because only minimal ground-water development 

has occurred as of 1977. Because of the lack of transient-flow conditions in the basin, a steady-flow calibration of the model was the only calibration check 

possible. In this type of calibration, the model is used 

to simulate the long-term water-level and waterquality conditions that would result from an undisturbed natural environment. These model results 

are then compared to the measured data as a check of 

the simulation capability of the model. A disadvantage of this type of calibration is that the specific 

storage of the aquifers cannot be tested in the model. 

However, if subsequent model simulations represent 

long-term equilibrium conditions, the specific storage 

will not affect the model results.

Model simulations made using different values of 

dispersivity indicated that the best model results 

were obtained when the simulated dispersion equaled 

the numerical dispersion due to space- and timetruncation errors. Subsequent model simulations were 

made using zero-longitudinal and transverse-dispersivity factors with dispersion due only to truncation

error in the backward finite-difference approximation in space and time. This approach appears to be 

reasonable for two reasons. First, as the node size 

and scale of a solute-transport model increases, the 

model results become less sensitive to dispersivity 

(Robson, 1974). Because of this, similar modelconcentration distributions can be obtained using 

very different values of model dispersivity. Second, 

test simulations were made using a centered-in-spaceand-time finite-difference approximation (which produces no numerical dispersion) and the backward-inspace-and-time approximation (which produces numerical dispersion). Comparison of these two simulations indicated that, at the node sizes used in the 

Piceance basin model, the effects of numerical dispersion would not adversely affect the concentration 

distribution. The centered-in-space-and-time approximation could not be used in the Piceance basin model 

due to problems of solution stability inherent in this 

technique.

The equilibrium potentiometric heads calculated 

by the model for layers 1 and 5 are compared in figure 

23 to the measured potentiometric heads in the upper 

and lower aquifers (figs. 18 and 19). The best agreement between calculated and measured potentiometric heads occurs in model layer 1 where calculated heads are within 10 ft of measured heads at 77 

of the 86 model nodes in this layer. The degree of 

agreement is due partly to the effect of the stream 

valleys of Piceance and Yellow Creeks that tend to 

control the water levels in the upper aquifer near the 

valleys. The deeper the model layer is below the 

stream valleys, the less effect the valleys have on the 

potentiometric head in the layer. As a result, the 

calculated potentiometric heads in layer 5 are within 

10 ft of the measured potentiometric heads at only 39 

of the 104 nodes in this layer (fig. 23). The modelcalculated potentiometric-surface maps for layers 2 

and 4 are nearly identical to the potentiometricsurface maps based on measured data shown on figures 

18 and 19, indicating that the model is able to produce 

a reasonable simulation of equilbrium potentiometrichead conditions. This agreement is further indicated 

by the comparison of measured and calculated 

potentiometric-head differences across the Mahogany zone shown in figure 20. In the areas of greatest 

potentiometric-head difference, away from Piceance 

Creek and Yellow Creek valleys, the agreement between measured and calculated data is good. There is 

less agreement near the streams where only slight 

potentiometric-head differences occur across the 

Mahogany zone.
100

80

60

O

o

2

LL

°40 

oc

LU 

CD

D

20

Calculated head lower 

than measured head

\ 

LAYER 1

Calculated head higher 

than measured head

Calculated head lower 

than measured head

Calculated 

than meas

101-130 71-100 41-70 11-40 0-10 11-40 41-70 71-100 101-130 101-130 71-100 41-70 11-40 0-10 

DIFFERENCE BETWEEN CALCULATED AND MEASURED HEADS, IN FEET

11-40 41-70 

FIGURE 23.—Distribution of differences between calculated and measured values of potentiometric head for layers 1 an
36 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

Because the model calculates dissolved-solids concentrations in addition to potentiometric heads, the 

calibration procedure also compared measured and 

calculated concentration data. The measured and 

model-calculated dissolved-solids concentrations for 

each of the five model layers are shown in figures 24 

to 28. Attempts to manually draw lines of equal 

concentration based on the measured data shown on 

the left of figures 24 to 28 have met with only limited 

success. The data points, although fairly numerous, 

apparently are not adequate to properly define the 

extent of areas of equal concentration. The reason for 

this difficulty became apparent when the model results were mapped. As shown on the right side of the 

figures, the calculated dissolved-solids distribution 

is multimodal, showing many areas of high and low 

concentration in the basin. This complex distribution is difficult to define based only on the measured 

data available. When a comparison is made between 

the measured and calculated dissolved-solids concentrations, a reasonable degree of agreement is 

evident. The best agreement between the measured 

and calculated values occurs in layer 5. This is significant because the rate of dissolution at the saline 

zone has a controlling effect on the concentration 

distribution in layer 5. Thus, the dissolution rate 

simulated in the model must be reasonably correct in 

order for the calculated dissolved-solids concentrations in layer 5 to agree with the measured data.

A third calibration check on the simulation ability 

of the model involved a comparison of the measured 

and calculated quantity and chemical quality of water 

recharging to or discharging from the aquifer system. These data are shown in table 2 with dissolvedsolids concentrations expressed in terms of pounds 

of salt per day.

The estimated-flow data are based on the work of 

Weeks, Leavesley, Welder, and Saulnier (1974) and 

represent hydrologic conditions in the basin prior to 

the Rio Blanco subsurface nuclear detonation which 

altered the streamflow characteristics in Piceance 

Creek after April 1973. Estimated salt-load values 

were computed from water-quality and streamflow 

records for the Piceance and Yellow Creeks gaging 

stations near the White River (U. S. Geological Survey, 1961-75). Since the ground-water basin is in 

equilibrium, the salt load leaving the basin must 

equal the salt load entering the basin. The total salt 

load for the recharge terms was, therefore, estimated 

to be equal to the total salt load for the discharge 

terms. Aquifer dissolution is the rate of dissolution of 

soluble minerals in the aquifer and the saline zone. 

In using the model to calculate the rate of aquifer 

dissolution, it was necessary to simulate a very small 

flow rate (0.2 ftVs), although ground-water flow does 

not actually originate from aquifer dissolution.

Significant agreement exists between the estimated 

and calculated flow and salt-load data shown in 

table 2. The discharge to Yellow Creek shows the 

greatest difference; however, 3.5 ftVs may be a better 

estimate of the ground-water discharge to the Yellow 

Creek drainage area than the 4.5 to 5.9 ftVs previously estimated. The lower estimate is based on the physical setting of lower Yellow Creek, the surface-water 

discharge and salt load in the creek, and sensitivity 

evaluations made using this model.

When the model-calculated ground-water discharge 

and dissolved-solids concentrations to Piceance

TABLE 2.—Ground-water mass balance

Total recharge terms -

Discharge terms:

Piceance Creek ..............

Yellow Creek ..................

Total discharge termsEstimated values

33.4-36.1

27.3-31.6 

5.9- 4.5

33.4-36.1

2.0X105

1.8X105 

0.2X105

2.0X105

Calculated values

Recharge terms: 

Precipitation recharge .... 

Aquifer dissolution ..........

Flow 

(ftVs)

33.4-36.1 

0.0

Salt load 

(Ib/d)

0) 

0)

Flow 

(ftVs)

32.4 

0.2

Salt load

<lb/d)

0.9X105

1 1 VI ft5

32.6

29.0 

3.5

32.5

2.0 X105

1.8X105 

0.2X105

2.0X105

xNo data available.
,640

EXPLANATION

WELL — Number is dissolved-solid

grams per liter. Multiple values a

and bottom of depth interval

-TOO- LINE OF EQUAL DISSOLVED- SOL

Interval, in milligrams per liter, is 

40

108°

Base from U. S. Geological Survey ' 

State base map, 1969

MEASURED CALCULATED

10 MILES 

_I

0 5 10 KILOMETERS 

FIGURE 24.—Measured and calculated dissolved-solids concentrations for layer 1.
EXPLANATION

560 WELL — Number is dissolved-solids 

grams per liter. Multiple values ar

and bottom of depth interval

•700 —— LINE OF EQUAL DISSOLVED-SOLI

Interval, in milligrams per liter, is 

40'

108°

Base from U. S. Geological Survey < 

State base map, 1969

MEASURED

CALCULATED

10 MILES

0 5 10 KILOMETERS 

FIGURE 25.—Measured and calculated dissolved-solids concentrations for layer 2.
40

108°

Base from U. S. Geological Survey 

State base map, 1969

MEASURED

EXPLANATION

,910 WELL — Number is dissolved-solids 

grams per liter

-800- LINE OF EQUAL DISSOLVED-SOL

Interval, in milligrams per liter, is 

CALCULATED

10 MILES

0 5 10 KILOMETERS 

FIGURE 26.—Measured and calculated dissolved-solids concentrations for layer 3.
108°

Base from U. S. Geological Survey 

State base map, 1969

MEASURED

EXPLANATION

•61 ° WELL — Number is dissolved-solids 

grams per liter. Multiple values ar

and bottom of depth interval

•700—— LINE OF EQUAL DISSOLVED-SOLI

Interval, in milligrams per liter, is 

CALCULATED

10 MILES

0 5 10 KILOMETERS 

FIGURE 27.—Measured and calculated dissolved-solids concentrations for layer 4.
EXPLANATION

(980 ^ygLL — Number is dissolved-solids concentration in milligrams per liter. Multiple values are concentration at top 

and bottom of depth interval

40

108°

Base from U. S. Geological Survey I 

State base map, 1969

MEASURED

-800 —— LINE OF EQUAL DISSOLVED-SOLIDS CONCENTRATIONS - 

Interval, in milligrams per liter, is variable

SALINE ZONE

108°

CALCULATED

10 MILES

0 5 10 KILOMETERS 

FIGURE 28.—Measured and calculated dissolved-solids concentrations for layer 5.

M

2 oCC

1
42 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

and Yellow Creek valleys are corrected for evapotranspiration losses and surface runoff, the dissolved-solids concentrations of the mean-annual 

streamflow may be estimated. The model-calculated 

estimates for these values were 1,600 mg/L for Piceance Creek and 2,400 mg/L for Yellow Creek. Using 

streamflow records for Piceance Creek and Yellow 

Creek at the near White River gaging stations (U.S. 

Geological Survey, 1961-75), the dissolved-solids 

concentrations of mean-annual streamflow in Piceance Creek was calculated to be 1,620 mg/L and in 

Yellow Creek to be 2,450 mg/L. The close agreement 

between the estimates indicates that the model correctly calculates the dissolved-solids concentration 

in ground-water discharge to streams in the basin as 

measured at the basin boundary. Based on this model study, about 90 percent of the ground water and 

dissolved solids in the basin is discharged in Piceance Creek drainage, and the remaining 10 percent is 

discharged in Yellow Creek drainage.

HYDROLOGIC KNOWLEDGE GAINED THROUGH MODELING

The successful calibration of the model has demonstrated that the model is a reasonably valid tool for 

evaluating the hydrogeochemistry of the Piceance 

basin. Such an evaluation commonly involves model 

calculations of the effects of various stresses on the 

aquifers and streams, but of equal importance is the 

added hydrologic knowledge gained by constructing 

and calibrating the model.

Different rates and distributions of precipitation 

recharge were tried during model calibration until a 

configuration was found that produced a good match 

between the measured and calculated potentiometric 

surfaces. The resulting recharge configuration is 

shown in figure 29 and, in most areas, is similar to 

the distribution of October to April precipitation in 

the area (U.S. Weather Bureau, 1960) and the recharge rates and distribution calculated by Weeks, 

Leavesley, Welder, and Saulnier (1974). The recharge 

rate in the area between Dry Fork and Piceance 

Creek is larger than that previously estimated 

(Weeks and others, 1974), due in part to a lack of 

potentiometric-head data in this area at the time of 

the earlier work.

Prior to the construction of this model, few data 

were available on the rate and distribution of dissolution of the saline minerals occurring in the aquifers or in the saline zone. By using the model to 

evaluate various dissolution rates, it was possible to 

determine rates that appear reasonable and produce

the proper dissolved-solids concentration in the aquifers. It was found that the areas of greatest dissolution occur (fig. 30) either near the margins of the 

saline zone or along a northwest-trending fracture 

system near the northern part of the basin (fig. 22). 

These findings were reached independently of similar findings discussed previously (see discussion in 

"Geochemical System section") and support the previous conclusions. These results suggest that the 

relatively small permeability of the horizontal bedding in the sediments limits rapid dissolution to 

areas where (1) fracturing allows additional groundwater movement through the soluble minerals, or (2) 

ground water is able to contact the soluble minerals 

by lateral movement into the margins of the saline 

zone.

The rate of dissolution occurring in the aquifers 

was found to be much less than the dissolution rate 

at the saline zone (table 3). This is consistent with 

core-drilling results that commonly show negligible 

saline-mineral content above the saline zone (Dyni, 

1974). Model results indicate that about lX105 lb/d of 

dissolution occurs at the saline zone while about 

7X103 Ib/d of dissolution occurs in the aquifers.

TABLE 3.—Mineral-dissolution rates

Source of dissolved solids 

in ground water

Dissolution rate 

(Ib/d)

Saline zone and layer 5 

Layer 4 ..............................

Layer 3 ..............................

Layer 2 ..................__..

Layer 1 ......__..............

Unsaturated zone ..........

100,000

4,000

300

2,000

600

90,000

As precipitation recharge percolates through the 

unsaturated sediments of the Uinta Formation overlying the aquifer, soluble minerals are dissolved and 

ultimately carried into the aquifer. This form of dissolution is the second largest source of dissolved 

solids in the Piceance basin aquifers, totaling about 

9x104 Ib/d. The distribution of this source is indicated on figure 21. The areas with recharge concentrations of 450 and 800 mg/L contribute an average 

salt load of 240 (lb/d)/mi2 to the aquifers and the 

area with recharge concentrations of 500 mg/L contributes an average of 150 (lb/d)/mi2.

The model-calculated ground-water discharge to 

the streams and alluvial aquifers in Piceance and 

Yellow Creek valleys is plotted in figure 31 to show
THE SIMULATED HYDROLOGIC SYSTEM 43

EXPLANATION

RECHARGE RATE, IN INCHES 

PER YEAR

40C

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

FIGURE 29.—Precipitation recharge distribution.

the cumulative distribution from upstream to downstream reaches of the valleys. The rate of groundwater discharge in the Piceance Creek valley is relatively steady in a reach extending from 4 to 31 mi 

upstream from the White River. The dissolved-solids 

distribution, by contrast, shows markedly larger 

concentrations of dissolved solids in the groundwater discharge in two reaches 0 to 9 mi and 24 to 28 mi 

upstream from the White River. These two reaches

overlie two areas with relatively large rates of salinemineral dissolution in the saline zone (fig. 30). A 

similar increase in the dissolved-solids concentration of ground-water discharge to Yellow Creek occurs 10 to 14 mi upstream from the White River, and 

this reach also overlies an area of rapid dissolution 

in the saline zone. These model results indicate that, 

if evapotranspiration and underflow in the alluvium 

can be accounted for, it might be possible to make an
44 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

Base from U. S. Geological Survey 

State base map, 1969

EXPLANATION

DISSOLUTION RATE, IN POUNDS 

PER DAY PER SQUARE MILE

10 MILES 

_i

10 KILOMETERS.

FIGURE 30.—Rate of dissolution of saline minerals at the base of the aquifer system.

onsite determination of the relative rate of salinemineral dissolution in the saline zone simply by 

monitoring changes in the quality of surface water 

during relatively low flows along Piceance or Yellow 

Creeks.

MODEL SIMULATIONS

The ground-water model developed as part of this 

investigation is capable of calculating the potentiometric head and dissolved-solids concentrations that 

result from a given set of ground-water recharge and
MODEL SIMULATIONS 45

1500

1/3

z

p 1250

< tr

CC LU

"

o

C/5

Q

O

C/}

Q

1000

750

500

10 15 20 25 30 

DISTANCE UPSTREAM FROM WHITE RIVER, IN MILES

35

FIGURE 31.—Cumulative ground-water discharge distribution and dissolved-solids concentrations along Piceance and Yellow

Creeks.

discharge conditions. If the recharge and discharge 

conditions include ground-water pumping near an 

oil-shale mine, the model results will show the effects 

of this pumping on the potentiometric surface and 

dissolved-solids concentration in the aquifers. The

model thus provides a means of estimating the effects of various hypothetical mining practices and 

subsequent mine flooding on the water resources of 

the basin. 

The scope of these model simulations is limited by
46 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

the structure of the model and the availability of 

detailed information about the various mining practices and subsequent mine flooding to be evaluated. 

The model cannot simulate both confined and watertable storage coefficients; as a result, it will not 

simulate transient water-table conditions as might 

occur near a mine being actively dewatered or flooded. Because of this limitation,jsubsequent model simulations represent equilibrium flow conditions and 

thus are not affected by the difference between watertable and confined storage coefficients.

In the ground-water model described by Weeks, 

Leavesley, Welder, and Saulnier (1974), the confined 

and water-table storage coefficients were taken into 

consideration, allowing the model to simulate the 

transient changes in potentiometric heads produced 

by mine dewatering in tracts C-a and C-b. These 

model results adequately describe transient conditions during mine dewatering and no attempt was 

made to duplicate these results using the threedimensional model. Their results indicate that although complete equilibrium conditions may not occur for several centuries after pumping begins, the 

major change in head occurs within 20 to 30 years 

after a pumping stress is imposed. Therefore, the 

equilibrium conditions simulated in the present 

model probably are reasonable approximations of 

conditions that would exist in the aquifer at least 

30 years after a steady pumping stress commences.

Insufficient data exist to completely describe the 

future operations of the two mines to be developed in 

tracts C-a and C-b. As a result, the model simulations are not intended to predict the future hydrologic conditions in the Piceance basin but rather are 

intended to show the hydrologic conditions that 

would result if a hypothetical stress were placed on 

the aquifers near the mining tracts. The simulated 

mines near tracts C-a and C-b are both 2.2 mi2 in 

area and are located as shown in figures 32 and 39.

The choice of an aquifer porosity of 1 percent also 

affects the model-simulation results. This conservative value was used in the model so the rate of 

ground-water movement calculated by the model 

would tend to be too rapid rather than too slow. The 

model results, thus, tend to depict a more rapid movement of zones of differing chemical quality than 

would occur if the effective porosity of the aquifer is 

larger than 1 percent. In this respect, the model will 

simulate conditions closer to a "worst case" situation 

rather than a "best case" situation of contaminant 

movement.

TRACT C-a

Current (1978) plans for mining oil shale in tract 

C-a involve the use of modified in-situ retorting procedure in which rubble columns of oil shale are 

created and retorted in situ. Only a small part of 

raw shale is removed to provide void space for the 

rubble and the spent shale is left in place in rubble 

columns after retorting is completed. Mining 

activities are to begin with a 10-year modular development phase followed by about 30 years of commercial mining. The interval to be mined is contained 

in a 3.4 mi2 area in the northern part of the tract and 

extends from near the top of the Mahogany zone 

(model layer 3) to the base of the R-4 zone (Gulf Oil 

Corp.-Standard Oil Co., 1977). The R-4 zone is a kerogen-rich oil-shale zone located near the middle of the 

geologic strata included in model layer 5. Ground 

water will be pumped to prevent flooding of the mine 

workings. Mining will occur when the water level 

near the mine is maintained at a level about 1,000 ft 

below the initial water level in the tract. The rate of 

ground-water pumping will increase as the size of the 

mine expands from the small-scale modular phase to 

full-scale production. Plans for the modular-development phase indicate that water pumped from the 

mine will be injected into the bedrock aquifers 

around the mine to prevent adverse water-level 

changes away from the tract. Plans for injection 

during the commercial development phase are not 

well defined at this time (1978).

MINE DEWATERING

The model was first used to evaluate the long-term 

effects of pumping a simulated mine near tract C-a. 

It was assumed that water would be pumped from the 

mine (or wells adjacent to the mine) at a rate which 

would produce about 1,000 ft of potentiometric-head 

change in the mine and that no pumped water would 

be returned to the aquifer. It also was assumed that 

the water pumped from the mine would be entirely 

removed from the hydrologic system in the Piceance 

basin and, thus, have no further effects on the basin. 

Although this assumption may not be realistic in 

terms of the actual operational procedure to be used 

at the mines, it allows the model to simulate the 

hydrologic impacts of various dewatering practices 

and subsequent mine flooding without the results 

being masked by effects of mine-water disposal in 

the basin. An initial model simulation indicated
MODEL SIMULATIONS 47

that, when only layer 5 is pumped, vertical (downward) leakage of water through the sediments near 

tract C-a would adequately lower the heads in model 

layers 2, 3, and 4 without the need for pumping from 

these layers. The Uinta Formation near tract C-a is 

not considered to be a significant aquifer; as a result, 

layer 1 is not considered in the model at tract C-a.

The model results indicate that a long-term pumping rate of about 5 ftVs would be required to produce 

1,000 ft of potentiometric-head change in a mine near 

tract C-a with a 2.2 mi2 area. The area affected by 

water-level declines in excess of 1 ft increases from 

about 370 mi2 in layer 2 to 480 mi2 in layer 5 (fig. 32). 

Because these changes represent equilibrium conditions in the basin, the rate of pumping from the mine 

would be balanced by an equal reduction in groundwater discharge to Yellow Creek and Piceance Creek 

valleys of about 2 and 3 ftVs, respectively.

Simulations of various mine-pumping rates indicate that (1) pumping the mine will decrease dissolved-solids concentrations in the water near the 

.mine, and (2) the greater the pumping rate the greater the decrease in dissolved-solids concentrations. 

Pumping the mine will increase the vertical (downward) and lateral rates of ground-water movement 

toward the mine. Because the dissolved-solids concentrations in water in the shallow aquifers is less 

than the concentrations in water in the deeper aquifers, increased downward movement would bring 

more water with fewer dissolved solids into the deeper 

part of the aquifer, thereby reducing the dissolvedsolids concentrations in the deeper aquifers. A second factor contributing to these model results is the 

rate of dissolution of the saline minerals, which is 

specified in the model as a function of time. The 

longer ground water remains in contact with a 

source of dissolution, the greater the dissolved-solids 

concentrations will become. The increased rate of 

water movement near the mine would reduce the 

contact time with local sources of dissolution and 

thereby would contribute to the reduced dissolvedsolids concentrations. The successful calibration of 

the model supports the assumption that the rate of 

saline-mineral dissolution is a function of time. However, dissolution as a function of water velocity or 

dissolved-solids concentration can occur, but data 

are not available to determine if these factors have a 

significant effect on the rate of dissolution in the 

Piceance basin.

The areal distributions of changes in dissolvedsolids concentrations that would be produced by 

pumping the mine near tract C-a at a rate of 5 ftVs 

are shown in figures 33 and 34. The effects of the 

mine pumpage on the dissolved-solids concentrations would vary considerably depending on the 

depth of the aquifer. In layer 2, for example, a decrease in dissolved-solids concentrations would occur in a 240-mi2 area around the simulated mine (fig. 

33). In layer 5, the altered rates of ground-water flow 

would produce a decrease in dissolved-solids concentrations in a 140-mi2 area extending from the mine to 

Piceance Creek. A small area of increase in dissolved-solids concentrations of less than 50 mg/L 

would occur to the east of the mine, and a large area 

of increase of more than 400 mg/L would occur 

downgradient (northeast) from the mine. The changes in dissolved-solids concentrations for layers 3 and 

4 would be of intermediate values between those 

shown for layers 2 and 5. Model results based on 

other pumping rates indicate that as the mine pumping rate increases or decreases, the magnitude of 

change in dissolved-solids concentrations will increase or decrease, and the size of the area affected by 

these changes also will increase or decrease.

Under equilibrium conditions, the volume of water 

pumped from the mine would be exactly balanced 

by a reduction in the rate of natural discharge from 

the basin. If the mine were pumped at 5 ftVs, groundwater discharge in Yellow Creek drainage would be 

reduced from 3.5 to 1.4 ftVs and the ground-water 

discharge in Piceance Creek drainage would be reduced from 29 to 26 ftVs. The distribution and chemical quality of cumulative ground-water discharge in 

Yellow Creek valley before and after the mine is 

pumped are shown in figure 35. The model does not 

separate the baseflow, underflow, or evapotranspiration components of ground-water discharge. As a 

result, the surface flow in Yellow Creek is not directly 

calculated by the model. However, the model-calculated cumulative ground-water discharge figures 

give an approximation of surface-water flow and 

chemical quality without taking into consideration 

the effects of precipitation runoff, evapotranspiration, or ground-water underflow in the valley alluvium.

The most upgradient area of ground-water discharge to Yellow Creek valley occurs 20 to 23 mi upstream from the confluence with the White River 

under the initial or present-day conditions. After the 

basin reaches equilibrium with the pumping from the
48 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

•/O- LINE OF EQUAL WATER LEVEL 

DECLINE - Interval, in feet, 

is variable

AREA OF SIMULATED MINE

108°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 10 KILOMETERS

FIGURE 32.—Model-calculated drawdown in layer 5 produced by pumping mine near tract C-a at

5 cubic feet per second.

mine near tract C-a, the most upgradient area of 

ground-water discharge to Yellow Creek valley will 

be shifted downgradient, occurring 9 to 13 mi upstream from the White River. The dissolved-solids 

concentrations will increase from 1,100 to 1,900 

mg/L 8 mi upstream from the White River as a result 

of the loss of the water that initially discharged into 

the upper reaches of the valley. 

The inability of the model to simulate transientflow conditions involving both confined and watertable storage coefficients limits its use in evaluating 

the effects of injection of ground water pumped from 

a mine. Although the pumping-injection procedure is 

primarily a transient ground-water flow problem, a 

special case exists in which the injection rate is 

controlled so as to maintain equilibrium conditions 

in the remainder of the ground-water basin outside of 

the mine. The simulation of these large-scale equili-
MODEL SIMULATIONS 49

EXPLANATION

•100—— LINE OF EQUAL DECREASE IN 

DISSOLVED-SOLIDS CONCENTRATIONS - Interval 100 milligrams per liter

AREA OF SIMULATED MINE

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 10 KILOMETERS

FIGURE 33. — Model-calculated decrease in dissolved-solids concentrations in layer 2 produced by 

pumping mine near tract C-a at 5 cubic feet per second.

brium conditions would be within the capability of 

the model were it not for the importance of smallscale conditions within the mine. The chemical quality 

of the injected water, for example, is determined by 

small-scale conditions that cannot be properly simulated with this model. If the chemical quality of the 

injected water is predetermined, the model can be 

used to show the effects of injecting water of different 

chemical quality into the equilibrium ground-water

flow field in the Piceance basin. Separate model 

simulations of this special case of the injection 

problem were not made because virtually identical 

model simulations are required to simulate the 

effects of spent oil-shale leaching in an abandoned 

and flooded mine. The mine-leaching simulations 

can be considered to represent injection simulations 

in which the dissolved-solids concentrations of the injected water are greater than that of the ground water.
50 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

——200—— LINE OF EQUAL CHANGE IN 

DISSOLVED-SOLIDS CONCENTRATIONS - Interval 

200 milligrams per liter

AREA OF CONCENTRATION 

INCREASE

AREA OF CONCENTRATION 

DECREASE

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES 

_l

0 10 KILOMETERS

FIGURE 34.—Model-calculated change in dissolved-solids concentrations in layer 5 produced by 

pumping mine near tract C-a at 5 cubic feet per second.

MINE FLOODING

The next group of model simulations were made to 

investigate the effects of flooding of an abandoned 

mine near tract C-a containing large quantities of 

spent shale in underground retorts. In these simulations, it was assumed that the radial flow of water 

toward the mine, during the time the cone of depression around the mine would be filling, will effectively 

halt any migration of ground water away from the

saline minerals in the spent shale in the mine. In 

effect, it was assumed that no poor-quality ground 

water would move away from the abandoned mine 

until the aquifer returns to the equilibrium conditions that occurred prior to pumping the mine. This is 

thought by the authors to be a reasonable assumption even though in actuality some movement might 

occur before complete equilibrium exists. For these 

simulations, leaching is assumed to begin when leachate moves into the aquifers outside of the mine. The
MODEL SIMULATIONS 51

2100

O

H 1700

< cc

CC LU 

I- I1300

Sd 

oi

o

CO 

CO

900

1 1 1 1 1 1 1 1 1 1 1

\

\

\

D= O 

LLJ LU

occ u

^5

UJ D

I I I I \ I I I I I

4 8 12 16 20 

DISTANCE UPSTREAM FROM WHITE RIVER, IN MILES

24

FIGURE 35.—Change in model-calculated ground-water discharge and dissolved-solids concentrations 

in Yellow Creek valley produced by pumping mine near tract C-a at 5 cubic feet per second.

simulations differ from those simulating mine pumping in that the model calculates transient (timevarying) concentration conditions that would occur 

in an equilibrium or steady-state flow system.

As mentioned previously, spent oil shale may contain soluble saline minerals in concentrations ranging from 1 to 5 percent by weight. The rate of dissolution of these minerals by ground water will

be affected by indeterminate factors, such as retorting temperatures, shale particle size, and permeability of the retorts. Management of the oil-shale mines 

during and after the retorting operations can drastically affect these factors and, thus, the rate of dissolution that will occur in the abandoned and flooded 

mine. 

The model was used to determine the effects of
52 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

various mine-leaching rates since the actual rate to 

be expected is not known. The change in dissolvedsolids concentrations after 60 years of leaching, shown 

in figure 36, is calculated from initial conditions in 

the basin aquifers prior to the effects of mine pumping. As shown, the mine leachate would have the 

most pronounced effect on the water quality in the 

layer with the largest hydraulic conductivity (layer 

2) and the least effect in the layer with the smallest 

hydraulic conductivity (layer 3). Dissolved-solids 

concentrations as much as 50,000 mg/L would occur 

in layer 2 adjacent to the mine after 60 years of leaching at rates that are thought to be possible for an 

actual mine in tract C-a.

The longer the mine leaching continues, the more 

pronounced the water-quality changes in the aquifer 

will become. The change in dissolved-solids concentrations in each model layer for 60 years of constant 

mine leaching at a rate of 1.3X105 Ib/d is shown in 

figure 37. The large volume of water initially in and 

near the simulated mine containing relatively small 

concentrations of dissolved solids would mix with 

the influx of mine leachate to produce the gradual 

change in water quality shown. Throughout the 60- 

year simulation period, the layers with the largest

hydraulic conductivity show the largest change in 

concentration.

The areal distribution of the water-quality change 

in layer 2 is shown in figure 38. Dissolved-solids 

concentrations will increase by more than 1,000 

mg/L in a 30-mi2 area downgradient from the mine 

after 60 years of mine leaching. Dissolved-solids increases in excess of 10 mg/L will occur in a 140-mi2 

area. As indicated by the changes shown in figure 37, 

the areas affected by water-quality changes in the 

other model layers would be smaller than the areas 

in layer 2. Only minor changes in water quality in 

Yellow Creek will result after 60 years of leaching, 

primarily because water with relatively large 

dissolved-solids concentrations will not have 

reached the main areas of ground-water discharge 

along the creek.

If the mine is managed so as to minimize the rate of 

dissolution of the saline minerals in the abandoned 

retorts, the effects on the ground-water quality could 

be much less significant than those shown by these 

model simulations. Conversely, disregard for spentshale leaching problems could lead to a serious and 

long-term degradation in the ground-water quality 

in Piceance basin.

60,000

50,000

OIJ

C/3 CC

40,000

30,000

</> z 20,000 Q —

10,000

o

0.0 0.5 1.0 1.5 

MINE LEACHING RATE, IN 100,000 POUNDS PER DAY

2.0

FIGURE 36.—Change in dissolved-solids concentrations in aquifers adjacent to abandoned mine 

near tract C-a after 60 years of leaching.
MODEL SIMULATIONS 53

50,000

C/J

2

9 40,000

o-

°K 

oogj

QQ.

-I V)

PQ oi (n~

30,000

=! 20,000

10,000

u

10 20 30 

YEARS

40 50 60

FIGURE 37.—Change in dissolved-solids concentrations with time in aquifers adjacent to abandoned 

mine for leaching rate of 1.3X105 pounds per day.

TRACT C-b

Plans for mining oil shale in tract C-b are similar 

to the planned mining in tract C-a in that a modified 

in-situ retorting procedure will be used. Mining operations are expected to continue for about 60 years 

with the size of the mine increasing about 0.12 

miVyears to an ultimate area of about 7 mi2. The 

mining interval extends from just above the Mahogany zone, in model layer 2, to just below the top of 

model layer 5, a vertical distance of about 500 ft 

(Ashland Oil, Inc., Occidental Oil Shale, Inc., 1977). 

Ground water will be pumped from the mine to lower 

the water level in the mine about 1,400 ft to prevent 

flooding of the mine workings. Plans for disposal of 

the pumped water are not definite at present (1978), 

but possibilities include injection into the aquifers, 

discharge to Piceance Creek, or other off-tract discharge options.

MINE DEWATERING

The model was used to evaluate the effects of longterm pumping from a simulated mine in tract C-b. It 

was assumed that a 2.2-mi2 area mine would withdraw water from model layers 2 through 5 and that 

the pumped water would not be returned to the aquifer. The relative yield of each model layer to the 

simulated mine is shown in table 4. As shown, model 

layers 2 through 5 would contribute water directly to 

the mine, with layers 2 and 5 contributing the 

majority of the water due to the larger hydraulic 

conductivity of these two layers. This is in contrast to 

the model results for tract C-a which indicate that 

water would not flow directly into the mine from 

layers 2 through 4 when layer 5 was pumped to 

maintain the required water level in that layer.

TABLE 4.—Distribution of total pumpage by model layer 

for tract C-b

Model layer Percentage of 

total pumpage

1 ..........................

2 ..........................

3 ..........................

4 .........................

5 .........................

0.0

41.2

3.4

9.9

45.5
54 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

•10——— LINE OF EQUAL INCREASE IN 

DISSOLVED-SOLIDS CONCENTRATIONS - Interval, in milligrams per liter, is variable

AREA OF SIMULATED MINE

108°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

FIGURE 38.—Model-calculated increase in dissolved-solids concentrations in layer 2 after 60 years of 

leaching from abandoned mine at 1.3X105 pounds per day.

It was determined that a long-term mine-pumping 

rate of about 15 ftVs will be needed to maintain the 

required 1,400-ft water-level decline in layer 5 in tract 

C-b. The distribution of pumping shown in table 4 

produced about 800, 900, 1,100, 1,200, and 1,400 ft of 

water-level decline in tract C-b in model layers 1 

through 5, respectively. Water-level declines in layer 

5 in excess of 1 ft will occur in a 560-mi2 area of the 

Piceance basin with declines in excess of 100 ft occurring in a 240-mi2 area in the southern part of the

basin (fig. 39). Progressively smaller areas of equal 

water-level decline will occur in the layers above 

layer 5. In layer 1, for example, declines in excess of 1 

ft will occur in a 410-mi2 area.

Simulations were made using different mine-pumping rates to show the long-term effects of the pumping on the water quality in the aquifers. Results of 

these simulations indicate that, as the mine-pumping rate increases, the dissolved-solids concentration 

near the mine in model layers 1, 2, and 3 will remain
MODEL SIMULATIONS 55

EXPLANATION

LINE OF EQUAL WATER-LEVEL 

DECLINE - Interval, in feet, 

is variable

AREA OF SIMULATED MINE

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

0 10 KILOMETERS

FIGURE 39.—Model-calculated drawdown in layer 5 produced by pumping mine in tract C-b at

15 cubic feet per second.

constant and only a small reduction in concentration 

will occur in layer 4. Concentrations near the mine in 

layer 5, however, show a marked decrease in response 

to increased rates of pumping. A decrease in dissolved-solids concentrations of as much as 1,100 

mg/L will occur in layer 5 when the mine is pumped 

at a rate of 15 ftVs. The decrease in dissolved-solids 

concentrations in layer 5 will occur in a 120-mi2 area 

(fig. 40) around the simulated mine and would be 

adjacent to a 90-mi2 area in which the dissolvedsolids concentration will increase by as much as 50 

mg/L. As discussed previously with respect to tract 

C-a, the change in concentration calculated by the 

model is due primarily to the altered rate and direction of ground-water movement.

Under the equilibrium conditions simulated in 

these model calculations, the volume of water pumped from the mine would be exactly balanced by a 

reduction in the rate of ground-water discharge. The 

proximity of tract C-b to Piceance Creek will cause
56 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

-50-

EXPLANATION

LINE OF EQUAL CHANGE IN 

DISSOLVED-SOLIDS CONCENTRATIONS - Interval, in milligrams per liter, is variable

AREA OF SIMULATED MINE

AREA OF CONCENTRATION 

INCREASE

AREA OF CONCENTRATION 

DECREASE

108°

40°

o

10 MILES

10 KILOMETERS

FIGURE 40.—Model-calculated change in dissolved-solids concentrations in layer 5 produced by 

pumping mine in tract C-b at 15 cubic feet per second.

the reduction in ground-water discharge to affect 

only Piceance Creek. The model does not separate 

the baseflow, underflow, and evapotranspiration 

components of ground-water discharge. As a result, 

the surface flow in Piceance Creek is not directly 

calculated by the model. Therefore, the cumulative 

ground-water discharge figures represent the estimated surface flow in Piceance Creek without consideration for precipitation runoff, evapotranspiration loss, or ground-water underflow in the valley 

alluvium. If the mine were pumped at 15 ft3/s, the 

cumulative ground-water discharge at the mouth of 

Piceance Creek would be reduced 15 ftVs due to loss 

of ground-water discharge to the valley along a reach 

18 to 30 mi upstream from the White River (fig. 41). The 

resulting concentration profile down the valley also
MODEL SIMULATIONS 57

1500

o

r- 1250

< cc

CC LJJ

1000

o

c/j 

Q

750

\

500

O O 

cc °

o

o 5

D 

O

40

30

fc 20

10

\

Jn

10 15 20 25 30 

DISTANCE UPSTREAM FROM WHITE RIVER, IN MILES

35 40

FIGURE 41.—Change in model-calculated ground-water discharge and dissolved-solids concentrations in Piceance Creek valley

produced by pumping mine in tract C-b at 15 cubic feet per second.

is shown on figure 41 and indicates that the dissolvedsolids concentration of the cumulative ground-water 

discharge will be increased from about 1,100 to 1,400 

mg/L in the lower reach of the stream valley as a 

result of pumping the mine in tract C-b. This 

increase would be due to the loss of better quality 

ground-water discharge to the stream valley near

tract C-b, coupled with no change in the rate 

of poorer quality ground-water discharge to the 

lower reaches of the valley. The cumulative 

ground-water discharge and dissolved-solids concentrations presented in figure 41 were computed 

in the same manner as those shown in figure 35 for 

Yellow Creek.
58 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

MINE FLOODING

Model simulations also were made to investigate 

the ground-water quality changes that could occur as 

a result of leaching of spent shale left in an abandoned mine in tract C-b. As for tract C-a, these simulations were based on the assumption that insignificant movement of mine leachate would occur prior to 

the time the water levels around the mine returned to 

near equilibrium (prepumping) conditions. These 

model simulations depict transient concentration conditions that would occur in an equilibrium groundwater flow field.

The rate of dissolution of saline minerals in the 

spent shale in tract C-b will be affected to a major 

extent by mining practices and subsequent steps 

taken prior to the closure of the mine. Specific information about these factors is tenuous at this time 

(1978) and an accurate estimate of the rate of mine 

leaching cannot be made. Consequently, the model 

was used to investigate the effects of various mineleaching rates on the ground-water quality near the 

mine. Model results shown in figure 42 indicate that, 

for a given leaching rate, the largest concentration 

changes will occur in layers 4 and 5. Layer 1, by 

contrast, would be little affected by the range of 

leaching rates considered. At a leaching rate of 1.3X 

105 Ib/d, an increase in dissolved-solids concentrations in excess of 20,000 mg/L would occur in some of 

the water near the mine after 60 years of leaching. A

comparison of the effects of leaching in tracts C-a 

(fig. 36) and C-b (fig. 42) shows that the water quality 

will be more readily degraded near tract C-a. For a 

given leaching rate, the same mass of salt will be 

introduced to the water in each tract, but the larger 

volume of water containing relatively small concentrations of dissolved solids that would move through 

the tract C-b area will produce more dilution. Thus, 

although the water near tract C-b will carry the 

same salt load as that in tract C-a (for identical 

leaching rates), the resultant dissolved-solids concentrations of the water near tract C-b will be less. 

The effect of a given mine-leaching rate on the 

dissolved-solids concentrations in an aquifer will 

become more pronounced the longer the leaching 

continues. This effect is shown for tract C-b in figure 

43. The concentration changes will occur more quickly in the more permeable layers—layers 2, 4, and 5. 

After 60 years of leaching, the concentrations in 

layers 1 and 2 would be at or near an equilibrium 

level, while the concentrations would still increase 

steadily in layers 3,4, and 5. A comparison of data in 

figures 37 and 43 indicates that, for equal mine 

leaching rates, the changes in ground-water quality 

will occur more rapidly near the simulated mine in 

tract C-a than near the simulated mine in tract C-b.

The areal distribution of increased dissolvedsolids concentration in layers 4 and 5 after 60 years of 

mine leaching at a rate of 1.3X105 Ib/d is shown in

CO

Z 

O 

\- 

<

DC

50,000

QO

40,000 -

£ 30,000

Q.

20,000 -

- 10,000

LLJ

0.5 1.0 1.5 

MINE LEACHING RATE, IN 100,000 POUNDS PER DAY

2.0

FIGURE 42,—Change in dissolved-solids concentrations in aquifers adjacent to abandoned mine in tract

C-b after 60 years of leaching.
MODEL SIMULATIONS 59

u

FIGURE 43.—Change in dissolved-solids concentrations with time in aquifers adjacent to abandoned 

mine in tract C-b for leaching,rate of 1.3X105 pounds per day.

figure 44. As indicated in figures 42 and 43, layers 4 

and 5 would be more severely affected by waterquality changes than would the other layers. The 

concentration changes will be primarily localized 

near the mine and along the south side of Piceance 

Creek. The area subject to dissolved-solids 

increases greater than 10 mg/L would be only about 

40 mi2 in size, in contrast to the 140-mi2 area that 

would be affected by identical concentration 

changes produced under similar conditions in tract 

C-a (fig. 38). The reason for the smaller zone of 

influence near tract C-b is two-fold. First, as has 

already been discussed, the larger volume of water 

moving through the aquifers near tract C-b would 

produce more dilution of the mine leachate, thereby 

decreasing the size of the zone of degraded water. 

The second and more significant factor is the 

location of Piceance Creek with respect to tract C-b. 

Ground water moving near tract C-b is able to travel 

only a few miles before being discharged to the 

alluvial aquifer or surface flow in Piceance Creek. As 

a result, the degraded water near the mine would 

affect the surrounding ground-water quality in only 

a limited area before being removed from the aquifer 

system by the surface drainage.

Previous simulations have shown that pumping 

the mine in tract C-b would have a pronounced effect 

on the volume of ground-water discharge to Piceance 

Creek valley. In a like manner, leaching of the mine 

in tract C-b would have a pronounced effect on the 

water quality in the valley. The dissolved-solids concentration profile (fig. 45) shows that an increase in 

dissolved-solids concentration from 750 to 1,750 mg/L 

would occur in the cumulative ground-water discharge near tract C-b. The dissolved-solids concentrations in the cumulative discharge at the mouth of 

Piceance Creek valley would be increased from 1,100 

to 1,450 mg/L as a result of mine leachate from tract 

C-b. The cumulative ground-water discharge calculated by the model would be representative of streamflow if adjustments were made for surface runoff, 

evapotranspiration, and ground-water underflow in 

the valley alluvium as has been described previously 

with respect to tract C-a and Yellow Creek.

If the mine in tract C-b is managed so as to minimize the rate of dissolution of the saline minerals in 

the abandoned retorts, the effects on the groundwater quality could be much less significant than 

those shown by the model simulations that used a 

1.3X105|lb/d leaching rate. Conversely, disregard for 

spent-shale leaching problems could lead to a serious 

and long-term degradation of the ground- and surfacewater quality in the basin.

Equal rates of leaching of abandoned mines in 

tracts C-a and C-b will likely produce much different 

effects on the water resources of Piceance basin. 

Tract C-a, by virtue of its remote location from perennial streams, primarily will affect the ground-water 

quality in a large area to the northeast of the tract. 

Tract C-b, by contrast, primarily will affect the surface-water quality in Piceance Creek with only localized effects on the ground-water quality. When long-
60 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

EXPLANATION

•10—— LINE OF EQUAL INCREASE IN 

DISSOLVED-SO LIDS CONCENTRATIONS — Interval, in milligrams per liter, is variable

AREA OF SIMULATED MINE

108°

40°

Base from U. S. Geological Survey 

State base map, 1969

10 MILES

10 KILOMETERS

FIGURE 44.—Model-calculated increase in dissolved-solids concentrations in layer 5 after 60 years of 

leaching from abandoned mine at 1.3X105 pounds per day.

term conditions are considered, ground-water-quality 

degradation from tract C-a will ultimately adversely 

affect the surface-water quality in Yellow and Piceance Creeks. This degradation will not occur soon 

after mine leaching begins but will begin only after 

large-scale and probably irreversibly ground-waterquality degradation has occurred. In tract C-b, the 

surface-water-quality degradation will occur more 

rapidly and may be reversible, to some extent, because large volumes of degraded ground water would 

not be involved.

SUMMARY

Rich oil-shale deposits occur in the Piceance basin 

of northwestern Colorado. The richest zones in the 

basin are contained in the Parachute Creek Member 

of the Green River Formation. Plans for mining 

these deposits include extensive ground-water pumping to allow underground construction of in-situ retorts for extracting the shale oil. To investigate the 

possible impacts of mining on the chemical quality 

of the water resources in the basin, a study of the
SUMMARY 61

1750

DC 

LU 

Q_

1500 -

~ 1250 -

o 1000 -

o 

o

c/j 

o

750 -

500

10 15 20 25 30 

DISTANCE UPSTREAM FROM WHITE RIVER, IN MILES

35 40

FIGURE 45.—Model-calculated dissolved-solids concentrations in ground-water discharge to Piceance Creek valley produced after 

60 years of leaching from abandoned mine in tract C-b at 1.3X105 pounds per day.

geochemistry of the basin was conducted in conjunction with an evaluation of the planned mining activities using a three-dimensional mathematical model 

that simulates solute transport in the ground-water 

system.

The principal aquifers in the basin occur in the 

Uinta Formation, composed of sandstone and siltstone, and in the underlying Parachute Creek Member of the Green River Formation, which consists of 

dolomitic marlstone. These units are permeable as a 

result of secondary porosity produced by joints, 

faults, collapse breccia, and solution cavities. The 

combined saturated thickness of these units varies 

from zero near the outcrops on the eastern, northerti, 

and western margins of the basin to as much as 2,000 

ft in the northeastern part of the basin. The hydraulic conductivity of these units varies considerably 

depending on differences in the amount of secondary 

porosity found in the area. Current estimates indicate a range in lateral hydraulic conductivity of 

0.1 to 2.0 ft/d. Vertical hydraulic conductivities range 

from a low of 7X10"4 ft/d in the Mahogany zone of the

Parachute Creek Member to a maximum of 1.0 ft/d in 

some adjacent layers.

Infiltration of precipitation, primarily in areas 

with elevations greater than 7,000 ft, provides recharge to the aquifers in this closed ground-water 

basin. Ground water moves from the recharge areas 

along the eastern, southern, and western margins of 

the basin to the central and northern parts of the 

basin where it discharges by evapotranspiration and 

surface flow in Yellow and Piceance Creeks. Total 

recharge and discharge in the basin was estimated to 

be about 32 ftVs, with the Piceance Creek drainage 

area receiving about 90 percent of the ground-water 

discharge and Yellow Creek drainage area receiving 

the remaining 10 percent. The flow of ground water 

through the basin is the result of a potentiometric 

gradient that is as steep as 130 ft/mi in some areas. 

The potentiometric head in the discharge area in the 

northern part of the basin is 1,600 ft lower than the 

potentiometric head in the recharge area 26 mi to the 

south. Potentiometric head differences as large as 

240 ft occur between different depth intervals in the
62 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

basin-aquifer system. In recharge areas away from 

major stream channels, the potential exists for downward movement of ground water, while near the 

major stream channels, the potential commonly 

exists for upward movement of ground water. These 

vertical head differences allow water recharging the 

near-surface layers of the aquifer to move to depth in 

the aquifer system and ultimately back to the surface 

near the stream valleys.

Marked differences in the dissolved-solids concentration are found in the ground water. Near the 

recharge areas, ground water with 400 to 800 mg/L of 

dissolved solids is common. In areas subject to upward movement of ground water from the northern 

part of the Parachute Creek Member, dissolvedsolids concentrations range from about 2,000 to 

45,000 mg/L. The source of the dissolved constituents, the direction of ground-water movement, and 

differing rates of dissolution combine to produce a 

complex water-quality distribution in the aquifer.

The two principal stratigraphic sources of dissolved solids found in ground water in Piceance basin are 

the saline zone and the Uinta Formation. The saline 

zone is a 340-mi2 area located in the lower part of the 

Parachute Creek Member that contains nahcolite 

and halite. Ground water comes in contact with these 

soluble saline minerals primarily by moving through 

faults and joints penetrating this zone or by moving 

laterally between bedding layers into the edges of the 

saline zone. Through these two mechanisms, the 

saline zone contributes about lX105 lb/d of dissolved 

solids to the ground water. Infiltrating precipitation 

leaches salts from the unsaturated zone overlying 

the upper aquifer. This source of dissolution introduces about 9X104 Ib/d of dissolved solids into the 

ground water of Piceance basin. The total salt load of 

the ground water is estimated to be 2X105 Ib/d, 

1.9X105 Ib/d of which is supplied by the above 

sources. The remaining dissolved solids are 

produced by dissolution of soluble minerals 

contained in the water-bearing sediments in the 

basin.

Chemical reactions, in addition to dissolution, that 

are thought to be occurring in the basin aquifers 

include precipitation, ion exchange, and oxidationreduction reactions. The product of the principal 

chemical reactions is a sodium bicarbonate water 

with oxidized sulfur species in the Uinta Formation 

and reduced sulfur species in the Parachute Creek 

Member. Carbonate species are produced primarily 

by reduction of sulfur species and dissolution of the 

carbonate minerals, calcite, dolomite, and nahcolite. 

Carbonate species are removed from the system in the 

northern part of the basin by calcite precipitation.

Relatively large sodium concentrations are produced 

by ion exchange and nahcolite and halite dissolution.

Trace constituents detected in ground water in significant concentrations include arsenic, barium, boron, 

fluoride, iron, lithium, and strontium. Lesser concentrations of beryllium, copper, cadmium, lead, mercury, manganese, molybdenum, selenium, and zinc 

were also detected. Primary evaporite minerals and 

primary and secondary sulfide minerals are the likely 

sources of the trace constituents found in the ground 

water.

A three-dimensional mathematical model was used 

to simulate the effects of mining activities on the 

water resources of the basin. The model simulates 

confined, density-dependent flow and solute transport with dispersion in nonhomogeneous, anisotropic aquifers under steady- or transient-flow conditions.

The model was calibrated by comparing the present-day potentiometric heads and dissolved-solids 

concentrations with model-calculated values. Model 

parameters, such as hydraulic conductivity, recharge 

rate, and dissolution rate were adjusted, within the 

constraints of the data, to improve the match between 

the measured and calculated values. An acceptable 

model calibration was achieved for steady-flow conditions. Calibration of transient-flow conditions was 

not possible due to the lack of transient conditions in 

the basin.

The calibrated model was used to investigate the 

possible effects of in-situ mining activities requiring 

large-scale dewatering near the two Federal prototypemine-lease tracts C-a and C-b. This dewatering could 

alter the direction of ground-water flow near the 

mines and consequently affect the chemical quality 

of the ground and surface water in the area.

The simulated mine in tract C-a was pumped at 

various rates and the model results indicated that: (1) 

Pumping the mine will decrease the dissolved-solids 

concentrations in ground water near the mine, and 

(2) increased pumping rates would further reduce the 

dissolved-solids concentrations in the lower aquifer. 

The water-quality improvement would occur because 

the pumping would induce downward movement of 

better quality ground water into deeper zones containing poorer quality water. Downgradient from the 

mine, the dissolved-solids concentrations would increase due to the effects of the altered direction of 

ground-water movement near the mine. At a longterm pumping rate of 5 ftVs, the ground-water discharge to Yellow Creek valley would ultimately be 

reduced by about 2 ftVs, and discharge to Piceance 

Creek valley would be reduced by 3 ftVs. This reduction in flow would be accompanied by an 800-mg/L
REFERENCES 63

increase in dissolved-solids concentrations in part of 

Yellow Creek.

The simulated mine in tract C-b was also pumped 

at various rates. Model results indicate that as the 

mine-pumping rate was increased, the dissolved-solids 

concentrations near the mine would remain fairly 

constant in all but the deepest part of the lower 

aquifer. A decrease in dissolved-solids concentrations of as much as 1,100 mg/L would occur in the 

deepest part of the aquifer if the mine were pumped at 

a rate of 15 ftVs. Downgradient from the mine, the 

dissolved-solids concentrations would increase by as 

much as 50 mg/L in a 90-mi2 area. These concentration changes also would be due to a change in the 

direction of ground-water movement near the mine 

which would allow ground water of better quality to 

migrate into zones containing water of a poorer quality. If the mine were pumped at a long-term rate of 15 

ftVs, the rate of ground-water discharge to Piceance 

Creek valley would be reduced by 15 ftVs. The resulting dissolved-solids concentration in the cumulative 

ground-water discharge would increase from 1,100 to 

1,400 mg/L near the mouth of the creek.

Spent oil shale in abandoned mine retorts could 

contain large volumes of soluble saline minerals. 

From 30 to 150 million tons of saline minerals might 

be present in the spent shale remaining in a mine at 

the site proposed for tract C-b. Dissolution of these 

saline minerals through mine flooding could have a 

serious effect on the chemical quality of the groundand surface-water resources of the Piceance basin.

Various operational procedures used in retorting 

the oil shale can affect the leaching rate of the 

minerals. The solute-transport model was used to 

simulate several mine-leaching rates in order to predict possible effects on the water quality in the basin. 

Model results for both tracts C-a and C-b indicate 

that the dissolved-solids concentrations in the aquifers increase with the rate and the duration of the 

leaching.

The aquifer zones with the largest hydraulic conductivity would have the largest increases in 

dissolved-solids concentrations, while the zones of 

smallest hydraulic conductivity would have the 

smallest increase in dissolved-solids concentrations. 

In tract C-a, a leaching rate of 1.3X105 Ib/d would 

produce dissolved-solids concentrations near the 

mine in excess of 40,000 mg/L after 60 years of mine 

leaching. After 60 years of leaching, a 30-mi2 area 

downgradient from the tract would have an increase 

in dissolved-solids concentrations in excess of 1,000 

mg/L. Only minimal changes in water quality in 

Yellow Creek would occur after 60 years of leaching, 

primarily because ground water with relatively large

dissolved-solids concentrations would not have 

reached the main areas of ground-water discharge 

along the creek.

Model simulations after 60 years of mine leaching in 

tract C-b indicate that dissolved-solids concentrations in excess of 20,000 mg/L would occur near the 

mine with a leaching rate of 1.3X105 Ib/d. Under the 

same conditions, concentrations near tract C-a would 

be in excess of 40,000 mg/L. The discharge of ground 

water containing relatively large concentrations of 

dissolved solids into Piceance Creek near a leaching 

mine in tract C-b will adversely affect the water 

quality in the creek. Model results indicate that in a 

reach of Piceance Creek near tract C-b, dissolvedsolids concentrations in cumulative ground-water 

discharge would increase from 750 mg/L to 1,750 

mg/L after 60 years of leaching.

The model indicates that the ground-water quality 

near tract C-a will be more readily degraded by mine 

leachate than that near tract C-b. The area that 

would be affected by concentration increases in excess 

of 10 mg/L would be about 140 mi2 near tract C-a and 

about 40 mi2 near tract C-b. Equal rates of leaching 

of abandoned mines in tracts C-a and C-b will likely 

produce much different effects on the water resources 

of Piceance basin. Tract C-a, located away from 

perennial streams, primarily will affect the groundwater quality in a large area to the northeast of the 

tract. Tract C-b, in contrast, primarily will affect the 

surface-water quality in Piceance Creek, with only 

localized effects on the ground-water quality.

REFERENCES

Ashland Oil, Inc., Occidental Oil Shale, Inc., February 1977, Oil 

shale tract C-b modifications to detailed development plan: 

89 p.

Ashland Oil, Inc., and Shell Oil Co., Operator, 1976, Oil shale tract 

C-b, first year environmental baseline program, annual summary and trends report, November 1974 through October 1975 

[submitted 'to Area Oil Shale Supervisor, Conservation Division, U.S. Geological Survey, Grand Junction, Colo.]: Denver, 

C-b Shale Oil Project, U.S. Department of the Interior Prototype Oil Shale Leasing Program, 546 p.

Austin, A. C., 1971, Structure contours and overburden on the top 

of the Mahogany zone, Green River Formation, in the northern part of the Piceance Creek basin, Rio Blanco County, 

Colorado: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-309.

Banks, C. E., and Franciscotti, B. C., 1976,1.2 Hydrology analysis, 

in Adams, R.C., Banks, C. E., Bradley, W. S., Brannick, L. L., 

Christian, W. G., Macy, R. L., Robison, J. C. Ill, and Salva, 

M. A.,Technical and cost evaluation of candidate large scale 

open pit oil shale mining methods in Colorado; report prepared for U.S. Department of Interior, U.S. Bureau of Mines 

Contract No. SO241046: Sun Company, Suntech, Inc., Final 

Report, v. II, July 1976, p. 24-56, U.S. Bureau of Mines OpenFile Report 156-77.
64 HYDROGEOCHEMISTRY AND SIMULATED SOLUTE TRANSPORT, PICEANCE BASIN, COLORADO

Campbell, D. L., 1977, Electrical soundings near Yellow Creek, Rio 

Blanco County, Colorado: U.S. Geological Survey Journal 

of Research, v. 5, no. 2, p. 193-205.

Campbell, D. L., and Olhoeft, G. R., 1977, Laboratory measurements of complex resistivity characteristics of oil shales from 

Suterdal no. 1 cavehole, Piceance Creek basin, Colorado: U.S. 

Geological Survey Open-File Report 77-410, 7 p.

Cashion, W. B., and Donnell, J. R, 1972, Chart showing correlation of selected key units in the organic-rich sequence of the 

Green River Formation, Piceance Creek basin, Colorado, and 

Uinta Basin, Utah: U.S. Geological Survey Oil and Gas Investigation Chart OC-65.

——— 1974, Revision of nomenclatures of the upper part of the 

Green River Formation, Piceance Creek basin, Colorado, and 

eastern Uinta Basin, Utah: U.S. Geological Survey Bulletin 1394-G, 9 p.

Coffin, D. L., Welder, F. A., and Glanzman, R. K., 1971, Geohydrology of the Piceance Creek structural basin between the 

White and Colorado Rivers, northwestern Colorado: U.S. Geological Survey Hydrologic Investigations Atlas HA-370.

Culbertson, W. C., and Pitman, J. K., 1973, Oil shale, in Brobst, 

D. A., and Pratt, W. P., eds., United States mineral resources: 

U.S. Geological Survey Professional Paper 820, p. 497-503.

Dale. R. H., and Weeks, J. B., 1978, Hydrologic analysis of the U.S. 

Bureau of Mines' underground oil-shale research-facility site, 

Piceance Creek basin, Rio Blanco County, Colorado: U.S. 

Geological Survey Water-Resources Investigations 78-28, 

35 p.

Desborough, G. A., and Pitman, J. K., 1974, Significance of applied mineralogy to oil shale in the upper part of the Parachute 

Creek Member of the Green River Formation, Piceance Creek 

basin,i Colorado, in Guidebook to the energy resources of the 

Piceance Creek basin, Colorado, Rocky Mountain Association 

of Geologists 25th Anniversary Field Conference: p. 81-89.

Desborough, G. A., Pitman, J. K., and Huffman, Claude, Jr., 1974, 

Concentration and mineralogical residence of elements in 

rich oil shales of the Green River Formation, Piceance Creek 

basin, Colorado, and the Uinta Basin, Utah—A preliminary 

report: U.S. Geological Survey Open-File Report 74-77,14 p.

DeWiest, R. J. M., ed., 1969, Flow through porous media: New 

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