U.S. Department of the Interior 

U.S. Geological Survey

Hydrology of the Bonneville Salt Flats, 

Northwestern Utah, and Simulation of 

Ground-Water Flow and Solute 

Transport in the Shallow-Brine Aquifer

U.S. Geological Survey Professional Paper 1585

Prepared in cooperation with the 

Bureau of Land Management
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Hydrology of the Bonneville Salt Flats, 

Northwestern Utah, and Simulation of 

Ground-Water Flow and Solute Transport 

in the Shallow-Brine Aquifer

By JAMES L. MASON and KENNETH L. KIPP, JR.

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1585

Prepared in cooperation with the 

Bureau of Land Management

1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Mark Schaefer, Acting Director

The use of firm, trade, and brand names in this report is for 

identification purposes only and does not constitute 

endorsement by the U.S. Government.

Library of Congress Cataloging in Publications Date

Mason, James L.

Hydrology of the Bonneville Salt Flats, northwestern Utah, and simulation of ground-water flow and solute 

transport in the shallow-brine aquifer / by James L. Mason and Kenneth L. Kipp, Jr. 

p. cm. — (U.S. Geological Survey Professional Paper; 1585) 

"Prepared in cooperation with the Bureau of Land Management." 

Includes bibliographical references. 

Supt. of Docs, no.: I 19.16:1585

1. Hydrology—Utah—Bonneville Salt Flats. 2. Ground water flow—Utah—Bonneville Salt Flats. 

I. Kipp, Jr., Kenneth L II. United States. Bureau of Land Management. 111. Title. IV. Series. 

GB1025.U8M38 1997

551.48'09792'43—dc21 97-34524

CIP 

ISBN 0-607-88306-5

For sale by U.S. Geological Survey, Branch of Information Services 

Box 25286, Federal Center 

Denver, CO 80225
CONTENTS

Absttact...................................................................._ 1

Introduction ........................................................................... 3

Purpose and scope ....................................................................................................................................................... 3

Previous investigations................................................................................................................................................ 5

Field work..........................................................................._ 7

Acknowledgments............................................_ 10

Description of study area....................................................................................................................................................... 10

Physical setting.....................................................................................................................................................^ 10

Playamorphology...........................................................................................................................................^ 10

Climate...........................................» 11

Precipitation.....................................................................................................^ 12

Wind......................................................................._ 12

Temperature....................................................................................^ 12

Ground-water hydrology....................................................................................................................................................... 16

Description of ground-water system............................................................................................................................ 16

Principles of variable-density ground-water flow........................................................................................................ 19

Shallow-brine aquifer.................................................................................................................................................. 22

Aquifer properties.............................................................................................................................................. 23

Water-level fluctuation, potentiometric surface, and ground-water movement................................................. 24

Recharge............................................................................................................................................................ 30

Discharge ........................................................ 33

Alluvial-fan aquifer..................................................................................................................................................... 39

Aquifer properties.............................................................................................................................................. 39

Water-level fluctuation and ground-water movement........................................................................................ 40

Recharge and discharge..................................................................................................................................... 42

Basin-fill aquifer.......................................................................................................................................................... 43

Aquifer properties.............................................................................................................................................. 43

Water-level fluctuation and ground-water movement........................................................................................ 44

Recharge and discharge..................................................................................................................................... 44

Brine chemistry and implications for ground-water flow............................................................................................ 45

Major ion chemistry........................................................................................................................................... 45

Isotope chemistry............................................................................................................................................... 49

Salt-transport mechanisms and salt budget............................................................................................................................ 54

Salt transport by surface ponds.................................................................................................................................... 54

Salt transport in solution through the shallow-brine aquifer....................................................................................... 55

Simulation of variable-density ground-water flow and solute transport in the shallow-brine aquifer.................................. 58

Modeling approach...................................................................................................................................................... 58

Description of the simulator of ground-water flow and solute transport........................................................... 58

Application to study area................................................................................................................................... 61

Parameter estimation and available data...................................................................................................................... 69

Aquifer and fluid properties............................................................................................................................... 69

Boundary-condition parameters........................................................................................................................ 70

Initial-conditions for dependent variables......................................................................................................... 76

Model calibration..............................................................^ 78

Calibration criteria............................................................................................................................................. 78

Parameter value and distribution adjustments................................................................................................... 78

Results of calibration......................................................................................................................................... 79

Simulation results........................................................................................................................................................ 84

CONTENTS III
Simulation of variable-density ground-water flow and solute transport in

the shallow-brine aquifer—Continued 

Simulation results—Continued

Dependent variables.......................................................................................................................................... 85

Cumulative amounts.......................................................................................................................................... 88

Balances............................................................................................................................................................ 91

Sensitivity analysis...................................................................................................................................................... 94

Sensitivity to model parameter values and distributions................................................................................... 94

Permeability near south boundary........................................................................................................... 94

Water level in south-boundary ditch ....................................................................................................... 94

Density of recharge flux.......................................................................................................................... 95

Maximum rate of evaporation................................................................................................................. 95

Porosity values........................................................................................................................................ 99

Discussion......................................................................................................................................................... 99

Improved calibration......................................................................................................................................... 101

Limitations of the model............................................................................................................................................. 101

Need for additional study...................................................................................................................................................... 103

Conclusions......................................................................................................................................................^ 104

References Cited ................................................................................. 106

FIGURES

1. Map showing location of the Bonneville Salt Flats study area, northwestern Utah................................................... 4

2. Diagram showing numbering system used for hydrologic-data sites in Utah............................................................ 6

3. Map showing location of weather stations and wells used for water-level measurements, Bonneville

Salt Flats, Utah..................................................._ 8

4. Photograph showing weather station near the center of the salt crust, Bonneville Salt Flats, Utah ......................... 9

5-6. Graphs showing:

5. Annual precipitation at Wendover, Utah, 1912 through 1993........................................................................... 13

6. Monthly precipitation at the three weather stations on or near the Bonneville Salt Flats, Utah,

November 1991 through December 1994 ......................................................................................................... 14

7. Diagram showing average monthly wind direction and velocity for the two weather stations on the Bonneville

Salt Hats, Utah, December 1991 through December 1993........................................................................................ 16

8. Graph showing average yearly air temperature at Wendover, Utah, 1921 through 1994........................................... 17

9. Generalized block diagram showing the relation of aquifers and direction of ground-water flow in the

Bonneville Salt Flats, Utah......................................................................................................................................... 18

10-13. Diagrams showing:

10. Force-intensity vector E for (a) hydrostatic conditions; and (b) hydrodynamic conditions.............................. 20

11. Force-intensity vectors for two fluids of different density acted upon by the same pressure and

gravitational fields............................................................................................................................................. 21

12. Forces acting on an element of fluid 1 and on an element of fluid 2 under hydrodynamic conditions............. 21

13. Interstitial velocity vector and the freshwater head and density-difference components.................................. 22

14. Hydrograph showing water-level fluctuations in well (B-l-17)31acc-l, located near the center of the salt

crust, Bonneville Salt Flats, Utah............................................................................................................................... 25

15-17. Maps showing:

15. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of

the Bonneville Salt Flats, September 1992........................................................................................................ 27

16. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of

the Bonneville Salt Flats, April 1993................................................................................................................. 28

17. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of

the Bonneville Salt Flats, October 1993............................................................................................................ 29

18. Graphs showing estimated daily evaporation and precipitation for 1993, Bonneville Salt Flats, Utah..................... 34

19. Graphs showing estimated daily evaporation and precipitation for 1994, Bonneville Salt Flats, Utah..................... 35

20. Hydrographs showing water-level fluctuations in wells completed in the alluvial-fan aquifer, Bonneville Salt

Flats, Utah .......................................................................................................................^^^ 41

IV HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
21. Photograph showing large surface fracture in vegetation zone along northwestern margin of the play a,

Bonneville Salt Flats, Utah......................................................................................................................................... 42

22. Map showing dissolved-solids concentration in brine from the shallow-brine aquifer, Bonneville Salt Flats,

Utah, September 1992...........................»^ 46

23. Map showing approximate areal distribution of potassium chloride, percentage by weight, in brine from the

carbonate mud of the shallow-brine aquifer, Bonneville Salt Flats, Utah, late summer, 1992.................................. 47

24. Graph showing relation of hydrogen and oxygen isotope ratios in ground water and surface pond, Bonneville

Salt Rats, Utah ........................................... 52

25. Graph showing relation of oxygen isotope ratio to dissolved-solids concentration in ground water, Bonneville

Salt Rats, Utah ................................................. 53

26. Map showing the increase in areal extent of the salt crust from September 1992 to August 1993, Bonneville

Salt Flats, Utah ........................................................... 56

27. Graph showing fluid viscosity as a function of solute mass fraction at 20 degrees Celsius ...................................... 60

28. Diagram of a model cell with associated node, one subdomain, and the associated element.................................... 61

29. Map showing location of model grid and inactive elements for the model of the shallow-brine aquifer,

Bonneville Salt Flats, Utah......................................................................................................................................... 64

30-38. Diagrams showing:

30. Simulation region for the shallow-brine aquifer with boundary-condition types and locations, Bonneville

Salt Flats, Utah.................................................................................................................................................. 67

31. Head distribution specified along the boundaries of the simulation region for the shallow-brine aquifer

during the production season, Bonneville Salt Flats, Utah................................................................................ 72

32. Head distribution specified along the boundaries of the simulation region for the shallow-brine aquifer

during the recovery season, Bonneville Salt Flats, Utah................................................................................... 73

33. Density distribution specified along the boundaries of the simulation region for the shallow-brine aquifer

during both seasons, Bonneville Salt Flats, Utah.............................................................................................. 74

34. Initial-condition density field used in the model of the shallow-brine aquifer and represented by cubic

trend surface, Bonneville Salt Flats, Utah......................................................................................................... 77

35. Final distribution of horizontal permeability used in the calibrated model of the shallow-brine aquifer,

Bonneville Salt Flats, Utah................................................................................................................................ 80

36. Final distribution of effective porosity used in the calibrated model of the shallow-brine aquifer,

Bonneville Salt Flats, Utah................................................................................................................................ 81

37. Final distribution of maximum evaporation rates used in the calibrated model of the shallow-brine

aquifer during the production season, Bonneville Salt Flats, Utah................................................................... 82

38. Final distribution of recharge rates used in the calibrated model of the shallow-brine aquifer during

the recovery season, Bonneville Salt Flats, Utah .............................................................................................. 83

39. Graph showing change in total volume of fluid in the simulation region for the calibrated model of the

shallow-brine aquifer during seven annual cycles, Bonneville Salt Flats, Utah ........................................................ 84

40. Graph showing change in total mass of solute in the simulation region for the calibrated model of the

shallow-brine aquifer during seven annual cycles, Bonneville Salt Rats, Utah ........................................................ 84

41-44. Maps showing:

41. Water-table configuration at the end of the last simulated production season for the calibrated model

of the shallow-brine aquifer, Bonneville Salt Flats, Utah.................................................................................. 86

42. Water-table configuration at the end of the last simulated recovery season for the calibrated model

of the shallow-brine aquifer, Bonneville Salt Rats, Utah.................................................................................. 87

43. Vertically averaged density field at the end of the last simulated production season for the calibrated

model of the shallow-brine aquifer, Bonneville Salt Flats, Utah....................................................................... 89

44. Vertically averaged density field at the end of the last simulated recovery season for the calibrated

model of the shallow-brine aquifer, Bonneville Salt Flats, Utah....................................................................... 90

45. Generalized block diagram showing the direction of annual inflows and outflows of fluid and solute for the

calibrated model of the shallow-brine aquifer, Bonneville Salt Flats, Utah............................................................... 92

46. Graph showing total subsurface solute outflow for the calibrated model of the shallow-brine aquifer and for

each sensitivity simulation, Bonneville Salt Flats, Utah............................................................................................ 99

Contents
TABLES

1. Monthly mean temperature and monthly precipitation and departures from 1961-90 normals at 

Wendover, Utah, compared to monthly mean temperature and monthly precipitation measured at 

two weather stations on the Bonneville Salt Flats, Utah, 1992-94......................................................................... 15

2. Results of chemical analyses of water from selected wells completed in the alluvial-fan aquifer or overlying

lacustrine sediment of the upper part of the basin-fill aquifer, Bonneville Salt Flats, Utah................................... 50

3. Estimated fluid flow and salt transport into and out of the shallow-brine aquifer, Bonneville Salt Flats, Utah..... 57

4. Estimated and simulated annual fluid flow and solute transport into and out of the shallow-brine aquifer,

Bonneville Salt Flats, Utah..................................................................................................................................... 91

5. Simulated fluid and solute inflow to and outflow from the shallow-brine aquifer and pond for the production

and recovery seasons in the final year ................................................................................................................... 92

6. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when permeability 

was doubled along south boundary......................................................................................................................... 95

7. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when specified

level in brine-collection ditch along south boundary was raised during production and recovery seasons........... 96

8. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when density of

infiltrating fluid over salt crust was increased from 1.185 to 1.2 grams per cubic centimeter............................... 97

9. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when density of

infiltrating fluid over carbonate mud was decreased from 1.17 to 1.15 grams per cubic centimeter..................... 97

10. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when evaporation

rates were increased by 20 percent of the calibrated rates...................................................................................... 98

11. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when evaporation

rates were decreased by 20 percent of the calibrated rates..................................................................................... 98

12. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when effective

porosity was increased by 30 percent of the calibrated distribution....................................................................... 100

13. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when effective

porosity was decreased by 30 percent of the calibrated distribution...................................................................... 100

VI HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
CONVERSION FACTORS, VERTICAL DATUM, AND 

ABBREVIATED WATER-QUALITY UNITS

Multiply

acre

acre-foot (acre-ft)

acre-foot per year (acre-ft/yr)

cubic foot (ft3)

cubic foot per foot squared per day (ft3 /ft2 /d)

cubic foot per second (ft3 /s)

cubic foot per day (ftVd)

foot (ft)

foot per day (ft/d)

foot per mile (ft/mi)

foot per second (ft/s)

foot per second squared (ft/s2)

foot squared (ft2)

foot squared per day (frVd)

foot per year (ft/yr)

gallon per minute (gal/min)

inch (in.)

inch per day (in/d)

inch per year (in/yr)

mile (mi)

miles per hour (mi/hr)

mile squared (mi2)

pound per cubic foot (lbs/ft3)

pound per inch squared (lbs/in2)

ton (short, 2,000 pounds)

ton per year (ton/yr)

By

0.4047

0.001233

0.001233

0.02832

0.3048

0.02832

0.02832

0.3048

0.3048

0.1894

0.3048

0.3048

0.0929

0.0929

0.3048

0.0631

25.4

25.4

25.4

1.609

1.609

2.590

16.0185

6.895

0.9072

0.9072

To obtain

hectare

cubic hectometer

cubic hectometer per year

cubic meter

cubic meter per meter squared per day

cubic meter per second

cubic meter per day

meter

meter per day

meter per kilometer

meter per second

meter per second squared

meter squared

meter squared per day

meter per year

liter per second

millimeter

millimeter per day

millimeter per year

kilometer

kilometers per hour

square kilometer

gram per liter

Newton per meter squared

megagram

megagram per year

Contents VII
Degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) by using the following equation:

°F = 9/5(°C)+32. 

Degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) by using the following equation:

°C = 5/9(°F-32).

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929—a geodetic datum derived from a general adjustment of the first-order level nets of the United States and 

Canada, formerly called Sea Level Datum of 1929.

Solar radiation is reported in Langley (ly) units. One ly is equal to 1 calorie per centimeter squared 

and 3.6855 British Thermal Unit (BTU) per foot squared.

Chemical concentration and water temperature are reported only in metric units. Chemical concentration in water is reported in milligrams per liter (mg/L). Milligrams per liter is a unit expressing the 

solute per unit volume, liter (L), of water. One thousand milligrams per liter is equivalent to 1 gram 

per liter (g/L). For concentrations less than 7,000 milligrams per liter, the numerical value is about 

the same as for concentrations in parts per million. For concentrations much larger than 7,000 milligrams per liter, the concentration in milligrams per liter must be divided by water density to obtain 

the equivalent concentration in parts per million. Density of water in grams per cubic centimeter 

(g/cm3 ) at 20°C is obtained by adjusting the specific-gravity measurement with the appropriate conversion factor that corresponds to temperature.

One tritium unit (TU), the more commonly used unit for tritium, is equivalent to 3.2 picocuries per 

liter (pCi/L). One picocurie is the amount of radioactive decay that produces 2.2 disintegrations per 

second in a unit volume (liter) of water.

Viscosity is reported in centipoise units. One centipoise is equal to 0.01 grams per centimeter-second.

The generic units of mass (M), length (L), and time (7) are used to define the units of variables in 

some equations presented in this report. The generic unit abbreviation for length (L) should not be 

confused with a unit volume, liter (L), of water and regular chemical concentrations (g/L) used in the 

text.

VIII HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Hydrology of the Bonneville Salt Flats, Northwestern 

Utah, and Simulation of Ground-Water Flow and 

Solute Transport in the Shallow-Brine Aquifer

By James L. Mason and Kenneth L. Kipp, Jr.

ABSTRACT

The Bonneville Salt Flats is located in the 

western part of the Great Salt Lake Desert of 

northwestern Utah. The salt crust in the Bonneville 

Salt Flats covered an area of 43 square miles in 

September 1992, of which about 34 square miles 

consists of perennial salt more than 1 foot thick. 

The salt crust is the terminus for ground-water 

flow in the area except where this flow has been 

intercepted by brine-collection ditches used for 

mineral production.

For many years, the Bonneville Salt Flats 

has been used for high-speed automobile racing 

because of its uniquely flat surface and accessibility. Potash has been extracted from the underlying 

sodium-chloride-rich brines for commercial use 

since 1917. Three sets of salt-crust thickness measurements made during a 28-year period indicate a 

decrease in salt-crust volume and thickness. For 

these reasons, there appears to be a conflict 

between the primary uses of the playa.

Objectives of this investigation were to 

identify the natural and anthropogenic processes 

causing salt loss from the crust, and where feasible, to quantify these processes. Specific areas of 

study include the transport of salt in solution by 

ground-water flow and wind-driven ponds and the 

subsequent salt precipitation on the surface of the 

playa upon evaporation or seepage of brine into the 

subsurface. Data were collected to make conceptual estimates of these processes and to use in conjunction with variable-density, three-dimensional, 

solute-transport computer simulations.

The ground-water system in the Bonneville 

Salt Flats can be divided into three aquifers: (1) the 

shallow-brine aquifer that contains dense brines 

within the salt crust and underlying carbonate 

mud, (2) the alluvial-fan aquifer that yields water 

from coarse and fine-grained material at the flanks 

of the adjacent mountains, and (3) the basin-fill 

aquifer that is composed of a thick sequence of 

lacustrine sediment in the upper part and a conglomerate at depth. Brine is withdrawn for mineral 

production by seepage from the shallow-brine 

aquifer into brine-collection ditches and by pumping from the conglomerate in the lower part of the 

basin-fill aquifer. Because of possible effects of 

brine withdrawal on the salt crust, data were collected primarily from the shallow-brine aquifer.

Recharge to the shallow-brine aquifer is primarily by infiltration of precipitation that falls on 

the playa surface and dissolves salt from the crust, 

and by subsurface inflow from tne east and north 

that transports a small amount of salt into the shallow-brine aquifer. Evidence indicates that upward 

leakage might be a major source of recharge to the 

shallow-brine aquifer, but data were insufficient to 

quantify this source. Discharge from the shallowbrine aquifer is primarily by evaporation at the 

playa surface, seepage to the brine-collection ditch 

east of the salt crust, subsurface flow to the south 

where it mostly seeps into a brine-collection ditch, 

subsurface flow to the northwestern margin of the 

playa, and downward leakage to the underlying 

lacustrine sediment. Estimated fluid discharge 

was about 19,000 acre-feet in 1992, which was

ABSTRACT 1
substantially more than estimated recharge. Estimated net salt loss was more than 1.3 million tons.

Long-term water-level data indicate that the 

fluid phase of the shallow-brine aquifer appears to 

be in steady state despite seasonal fluctuations that 

can result in yearly fluid imbalances. A steadystate condition is defined as no change throughout 

time. Estimated recharge for 1992, therefore, was 

too small with respect to estimated discharge. Estimated recharge might be larger if a larger specific 

yield is assumed or if upward leakage could be 

quantified. Limited water-level data indicate that a 

small downward driving force exists between the 

shallow-brine aquifer and the underlying lacustrine sediment. Tritium values of essentially 0 tritium units at some locations in the shallow-brine 

aquifer can result only from upward leakage from 

the underlying lacustrine sediment. Discharge 

might be less than estimated. Removal of salt and 

compaction of underlying carbonate mud during 

highway construction might have constricted 

ground-water flow to the south.

Dissolved-solids and potassium concentrations in the brine within the shallow-brine aquifer 

have not changed over the long term. With an estimated net loss of salt from the shallow-brine aquifer, the dissolved-solids concentration must be 

maintained by dissolution of the salt crust. Potassium concentrations probably are maintained by 

diffusion from interstitial fluid within the shallowbrine aquifer and by upward leakage.

Deuterium and oxygen-18 isotope data indicate that there is possible brine movement from the 

shallow-brine aquifer and the underlying lacustrine sediment toward the alluvial-fan aquifer. 

Pumping from the alluvial-fan aquifer has resulted 

in a reversal of the natural hydraulic gradient 

toward the playa.

Extensive flooding of the Bonneville Salt 

Flats during the winter of 1992-93 dramatically 

changed the salt surface. Much of the near-surface 

salt was dissolved and remained in solution long 

into the summer of 1993. When salt was precipitated on the playa surface as the pond evaporated, 

a thin salt crust was deposited in areas beyond the 

usual areal extent of the crust. An estimated 10 to 

14 million tons was dissolved and subsequently

redeposited on the playa surface or infiltrated into 

the shallow-brine aquifer. Some surface salt might 

have been deposited beyond slight topographic 

divides where the salt might be lost from the crust.

A three-dimensional, ground-water flow 

and solute-transport model with variable density 

was developed for the shallow-brine aquifer in the 

Bonneville Salt Flats study area in order to (1) 

develop a fluid and solute balance, (2) evaluate the 

effect of the brine-collection ditches on the salt 

crust, and (3) identify the major and minor solute 

fluxes to and from the system. Preliminary simulations indicated that the ground-water flow system does not reach steady state during production 

or recovery seasons because the response time is 

longer than the seasonal duration of about 6 

months. Because of the transient nature of the 

ground-water system, simulations were carried out 

to a periodic steady state with a repeated sequence 

of production and recovery seasons approximating 

a typical year rather than matching a specific 

period in time.

The conceptual understanding of the major 

flows to and from the shallow-brine aquifer was 

supported by the model. Simulation results indicate a net loss of solute of about 850,000 tons 

through subsurface outflow in a typical year, primarily through seepage to brine-collection ditches 

to the east of the salt crust and along the southern 

boundary. Simulated salt dissolution from 

recharge at the playa surface was almost 5 million 

tons, whereas simulated salt precipitation from 

evaporation was almost 4 million tons. In order to 

reasonably match measured heads along the southern part of the simulation region, permeability was 

decreased from original estimates. Simulated subsurface flow to the south was about one-third of the 

estimated value.

Simulations were made to test the sensitivity 

of the model to changes in permeability, specified 

water level in the brine-collection ditch, density of 

infiltrating fluid from the playa surface, evaporation, and effective porosity. The simulated solute 

transport was most sensitive to an increase in permeability along the southern boundary. All sensitivity simulations resulted in a net annual loss of 

solute.

HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
INTRODUCTION

The Bonneville Salt Flats is located in the western part of the Great Salt Desert of northwestern Utah 

(fig. I). The salt crust on the Bonneville Salt Flats,

r\

which covered 43 mi in late summer 1992, is unique 

for its size and accessibility.

The first documented non-Native American 

explorations of the area were instigated in the early 

1830's by Captain Benjamin Louis Eulalie de Bonneville, for whom the area is named. Captain de Bonneville 

ordered Joseph Reddeford Walker to explore the region 

west of the Great Salt Lake in search of undiscovered 

areas with potential for fur trapping. During his explorations, Walker crossed the salt flats enroute to the 

Sierra Nevada Mountains. In 1835, Captain de Bonneville abandoned the area with very little success in 

acquiring furs. Because of the harsh conditions and 

remoteness of the area, the Bonneville Salt Flats was 

not surveyed until 1926, almost a hundred years later 

(Post, 1992, p. 30).

The salt crust has been used for high-speed automobile racing, and brine associated with the salt crust 

and surrounding playa has been used for mineral production for most of this century. More recently, the 

film-making industry has used this unique area because 

of the contrast of white salt against blue sky and vast 

horizons.

High-speed automobile racing on the Bonneville 

Salt Flats began in 1914. Over the years, the Bonneville 

Salt Flats has proven so desirable for racing that it has 

become one of the most preferred surfaces for speed 

enthusiasts from all over the world. A number of world 

speed records have been, and continue to be, set on the 

Bonneville Salt Flats. Each year, depending on saltcrust conditions, four separate racing events are scheduled. Speed Week, in August of each year, is the largest 

event, usually attracting more than 300 entries. A history of racing on the Bonneville Salt Flats is reported 

by Post (1992). Members of the racing community 

have been concerned about progressive deterioration of 

the quality of the racing surface, as well as the lateral 

extent and thickness of the salt crust. They have suggested that the cause of the deterioration of the salt 

crust is directly linked to brine withdrawal from the 

Bonneville Salt Flats for mineral production.

Mineral production on the Bonneville Salt Flats 

began in the early 1900's when halite (common salt) 

was extracted from the salt crust. Production of potash 

began in 1917 when supplies of potassium salts from 

Germany were interrupted during World War I. By

1920, most of the potassium salts produced in this 

country came from part of the Bonneville Salt Flats that 

passed into private ownership in 1917. Additional lands 

were passed into private ownership during 1933-35. 

Much of the private land is located south of the present 

Interstate Highway 80. When German potassium-salt 

supplies were available again in 1921, production from 

the Bonneville Salt Flats ceased until 1938. A more 

detailed description of the early history of potash production in this area is reported by Gwynn (1996). The 

area from which brine is withdrawn for potash production was expanded in 1963 when mineral leases on 

25,000 acres of Federal land were issued. These Federal leases were renewed in 1983. Presently, the mineral company operates on private land (66 percent), 

Federal land (28 percent), and State land (6 percent). A 

description of the potash-extraction process has been 

written by Bingham (1980).

Other perennial salt crusts are present in remote 

regions of the world, but none match the mostly 

smooth, hard surface, size, and accessibility of the 

Bonneville Salt Flats. Because of these unique characteristics, in 1985 the Bureau of Land Management designated the Bonneville Salt Flats as an Area of Critical 

Environmental Concern. For such an area, management practices must reflect the need for preservation of 

the special feature and balance this preservation with 

the multiple-use concept that allows mineral extraction.

A decrease in thickness and areal extent of salt 

crust has been documented over a 28-year period, from 

1960 to 1988, (Steven Brooks, Bureau of Land Management, written commun., 1989), but no definitive 

data have shown what processes are involved. The 

Bureau of Land Management needs to know the 

dynamics of the salt loss in order to make appropriate 

management decisions concerning the Bonneville Salt 

Flats. This hydrologic investigation was done by the 

U.S. Geological Survey, Water Resources Division, in 

cooperation with the Bureau of Land Management to 

provide them with data and interpretations concerning 

the present movement of salt through the hydrologic 

system. The study began in October 1991 and continued through September 1994.

Purpose and Scope

This report describes the hydrologic system of 

the Bonneville Salt Flats with emphasis on the mechanisms of solute transport. Variable-density, threeINTRODUCTION
r~n

EXPLANATION

Approximate extent of principal salt-crust area

Outline of subsequent figures 

Brine-collection ditch

40°45'

40°42'30"

T. 2S.

R. 19 W.

8 KILOMETERS

Figure 1. Location of the Bonneville Salt Flats study area, northwestern Utah.

4 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
dimensional computer simulations of the near-surf ace 

part of the ground-water system were done to quantify 

both the transport of salt dissolved in subsurface brine 

that leaves the salt-crust area and the salt dissolved and 

precipitated on the land surface.

The study was designed to define the hydrology 

of the brine ground-water system and the natural and 

anthropogenic processes causing salt loss, and where 

feasible, to quantify these processes. Specific areas of 

study include the transport of salt in solution by 

ground-water flow and the transport of salt in solution 

by wind-driven ponds and the subsequent salt precipitation on the surface of the playa upon evaporation or 

seepage into the subsurface. In addition, hydraulic and 

chemical changes in the hydrologic system since previous studies were documented. Data collected during 

this study were used in conjunction with variable-density, three-dimensional, solute-transport computer simulations to better understand and quantify the fluid- and 

salt-transport processes in the ground-water system of 

the Bonneville Salt Flats. Pilot Valley, located northwest of the Silver Island Mountains, contains a playa 

relatively undisturbed by human activities. An objective was to examine Pilot Valley as an analog to the 

Bonneville Salt Flats prior to mineral development; 

however, data were insufficient to make fluid and saltbudget comparisons between the Bonneville Salt Flats 

and Pilot Valley.

The ground-water system in the Bonneville Salt 

Flats study area consists of an alluvial-fan aquifer 

located along the playa margin on the southeast flank of 

the Silver Island Mountains, a deep basin-fill aquifer, 

and a shallow-brine aquifer. Brines that are of economic value are derived from the shallow-brine aquifer 

on the Bonneville Salt Flats. Evaporation of these 

brines contributes to the perennial salt crust on the 

Bonneville Salt Flats. Rainfall on the playa surface 

results in the dissolution of the salt and subsequent 

seepage into the subsurface. For these reasons, the primary emphasis of this study is the flow of brine in and 

out of the shallow-brine aquifer.

Data collected during this study are presented in 

a report by Mason and others (1995). Water-level and 

density data measured in wells located in the Bonneville Salt Flats were used to delineate potentiometric 

surfaces and areal distributions of dissolved solids 

within the shallow-brine aquifer. Data also were collected from wells in Pilot Valley and were presented by 

Mason and others (1995) but are not used in this report. 

The site-numbering system used in Utah to uniquely

identify these wells and other hydrologic-data sites is 

described in figure 2.

Previous Investigations

One of the first references to salt crusts in northwestern Utah was made by Gilbert (1890). While Gilbert was mapping the various shoreline features of 

Pleistocene-age Lake Bonneville, he perceived that 

these features did not represent level water surfaces. 

Many of the shoreline features west of the present 

Great Salt Lake were 180 ft higher than features on the 

east side. He inferred that these differences were the 

result of differential upward movement in response to 

the removal of lake water by evaporation.

Nolan (1928) did the first comprehensive study 

of shallow brines in the Great Salt Lake Desert. From 

data obtained from 405 shallow augured holes, Nolan 

mapped the extent of the salt crusts on the Bonneville 

Salt Flats and in Pilot Valley. On the basis of chemical 

analyses from brine samples collected in 1925, he 

delineated magnesium, potassium, and chloride concentrations. He suggested that differences in magnesium and potassium concentrations were the result of 

differences in the chemical composition of surface runoff at different locations in the desert.

Eardley (1962) used carbon-14 dating of the carbonate muds to develop the evaporative history of the 

Great Salt Lake Desert. Through this research, he 

believed that the present salt crust had migrated westward about 25 mi from its original site of deposition. 

The migration was attributed to the isostatic rebound of 

the center part of the basin.

Crittenden (1963) completed a detailed study of 

isostatic rebound resulting from the dwindling of Lake 

Bonneville. Maximum vertical displacement was at 

least 210 ft in the center of the basin. Crittenden estimated that isostatic recovery had reached at least 75 

percent of the theoretical maximum.

Turk (1969) was the first to complete a detailed 

study of the hydrology of the Bonneville Salt Flats; 

however, much of the data collected were limited to 

areas adjacent to brine-collection ditches and evaporation ponds. Turk incorporated these data into a simple 

advective-flow and mixing model to estimate the 

potential for continued potash production. Turk (1970) 

reported the results of a field evaporation experiment 

during which the measured evaporation from surface 

brines decreased with increasing dissolved-solids concentration. Turk (1973) listed the results of aquifer tests

INTRODUCTION
The system of numbering wells in Utah is based on the cadastral land-survey system of the U.S. Government. The number, in 

addition to designating the well, describes its position in the land net. The land-survey system divides the State into four quadrants 

separated by the Salt Lake Base Line and the Salt Lake Meridian. These quadrants are designated by the uppercase letters A, B, C, 

and D, indicating the northeast, northwest, southwest, and southeast quadrants, respectively. Numbers designating the township and 

range, in that order, follow the quadrant letter, and all three are enclosed in parentheses. The number after the parentheses indicates 

the section and is followed by three letters indicating the quarter section, the quarter-quarter section, and the quarter-quarter-quarter 

section—generally 10 acres for a regular section1 . The lowercase letters a, b, c, and d indicate, respectively, the northeast, northwest, 

southwest, and southeast quarters of each subdivision. The number after the letters is the serial number of the well within the 10- 

acre tract. A number having all three quarter designations but no serial number indicates a miscellaneous data site other than a well, 

such as a location for a core sample. Thus, (C-l-17)18bbb-2 designates the second well constructed or visited in the northwest 1/4 

of the northwest 1/4 of the northwest 1/4 of section 18, T. 1 S., R. 17 W.

Sections within a township

R. 17 W.

Tracts within a section

Section 18

T. 

1 x

S.

6

7 

Well

• 

18

49

30

31

5

8

17

2TJ\

2§\

32

——— 9.

4

9

16

x21

,28

33

3\

10

15

22

^X

X

6 miles 

7 kilomete

\2

1\

14

23

26

\ 

35

1

12

K

24

25

X

\^

»rs —— -

Well

(C-1-17)18bbb-2

Figure 2. Numbering system used for hydrologic-data sites in Utah.

'Although the basic land unit, the section, is theoretically 1 square mile, many sections are irregular in size and shape. Such sections are 

subdivided into 10-acre tracts, generally beginning at the southeast corner, and the surplus or shortage is taken up in the tracts along the north and 

west sides of the section.

6 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
designed to determine the hydraulic properties of the 

shallow-brine aquifer and further elaborated on the 

results of the modeling for continued brine production. 

Turk and others (1973) describe the lithology of the 

sediments that comprise the shallow-brine aquifer and 

the implications on ground-water flow. Turk (1978) 

reported the use of satellite imagery in an attempt to 

determine the areal extent of the seasonal redistribution 

of salt on the playa surface. In this last report, Turk also 

described the use of a three-dimensional, advective, 

ground-water flow model to estimate the zone of influence resulting from brine withdrawal by brine-collection ditches within the shallow-brine aquifer.

Lines (1978 and 1979) completed two reports, 

the first listing data collected from July 1975 through 

June 1977, and the second containing the interpretations derived from those data. Lines described the morphology of the playa surface and its seasonal variation. 

Lines divided the playa surface into three separate 

zones, the outer carbonate zone composed mainly of 

clay-sized authigenic carbonate minerals, the sulfate 

zone composed mainly of authigenic gypsum, and the 

chloride zone composed of crystalline halite. More 

importantly, Lines was the first to estimate the hydrologic budget for the shallow-brine aquifer of the Bonneville Salt Flats. For 1976, he estimated that discharge 

exceeded recharge by about 2,000 acre-ft. Lines suggested that the difference between total recharge and 

total discharge resulted in a decrease in ground-water 

storage in the shallow-brine aquifer. Lines estimated 

that most of the recharge was derived from infiltration 

of precipitation directly on the playa surface or from 

winter surface ponds. The primary forms of discharge 

included evaporation from the playa surface and seepage to brine-collection ditches located east of the salt 

crust and south of Interstate Highway 80. Lines also 

made comparisons to Pilot Valley, a mostly undisturbed 

playa to the northwest.

Field Work

Preliminary field work began in 1991. The primary data-collection period was the spring of 1992 

through the fall of 1993. The study area is a unique 

hydrologic environment that poses many accessibility 

and logistical problems when the playa surface is wet. 

Field work on the Bonneville Salt Flats presented many 

problems with regard to travel on the playa surfaces 

and to data collection because of the corrosive nature of 

the brine. Travel on the playa surface was restricted

during late fall through spring. Winter surface ponding 

and soft, muddy conditions on the remainder of the 

playa surface prevented normal travel. Generally, light 

all-terrain vehicles were used when feasible. Even during summer months, travel can be difficult. Newly 

formed deposits of thin salt concealed underlying soft 

mud that could easily entrap a 4-wheel drive truck. 

Time of year and selection of new well sites had to be 

chosen carefully when heavy equipment was used for 

drilling. Even with careful planning, deceptively soft 

surface conditions resulted in equipment becoming 

entrapped at times, with one drill rig stuck for 2 days.

In the initial phases of field work, existing monitoring wells were inventoried to determine which 

wells were suitable for continued monitoring. Fortyeight additional monitoring wells were completed in 

the shallow-brine aquifer where existing wells had 

been damaged or where additional data were needed. A 

trailer-mounted auger with hollow- or solid-stem capabilities was used to complete most of the shallow wells. 

Some shallow wells were hand augured when the playa 

surface would not support anything heavier than a light 

all-terrain vehicle. The completion of deep, nested piezometers and individual monitoring wells in the alluvial-fan and basin-fill aquifers required the use of 

larger drilling equipment with mud-rotary or hollowstem auger capabilities. The network of monitoring 

wells on the Bonneville S alt Flats is shown in figure 3.

Water levels were measured in all monitoring 

wells for 2 years. After each water-level measurement 

was completed, brine was bailed from the well casing 

for temperature and specific-gravity (density) measurements. Ideally, water-level measurements were made 

every 3 months. Ponding during winter months and 

cold temperatures made travel on the playa extremely 

difficult and hazardous. During the winter of 1992-93, 

the areal extent of the surface pond was extensive and 

the depth was more than 1 ft in several places. Under 

these conditions, some monitoring wells were underwater or at the pond surface. For various other reasons, 

not all wells were measured during each measurement 

period. During the measurement periods in the spring 

of 1993, not all monitoring wells were accessible as 

would have been the case under normal conditions. 

During the summer measurement periods, some monitoring wells were inadvertently missed because of the 

difficulty in locating white well casing on the vast 

white salt crust.

In an attempt to quantify the amount of salt being 

redistributed during the winter months through dissoluINTRODUCTION
EXPLANATION

o

30 

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O

m 

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O

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(/)

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w

z 

o

3D

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40°45'

Brine-collection ditch 

Type of site sampled

Shallow monitoring well completed to a depth of 25 

feet or less. Number by symbol indicates number 

of wells represented if more than one. Letter 'H' by 

symbol indicates water-level recorder installed on 

well

Monitoring well completed to a depth of 63 feet or 

more

Multiple monitoring wells completed at different 

depths within a single borehole. Number by symbol 

indicates number of wells represented

Weather station

Multiple data-collection site. Symbols in parentheses 

indicate type of data-collection sites at this location

Brine-collection-ditch pumping station

R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000, 1979 

Universal Transverse Mercator projection, 

Zone 12

R. 16W. 2

_________L

6 MILES

6 KILOMETERS

Figure 3. Location of weather stations and wells used for water-level measurements, Bonneville Salt Flats, 

Utah.
tion and precipitation, measurements from the top of 

the well casing to the salt crust were made at the same 

time as water level measurements during 1993. From 

the differences in these measurements over time, the 

amount of salt dissolved in the winter of 1992-93 and 

precipitated in the summer of 1993 was estimated.

A continuous water-level recorder was installed 

and maintained on well (B-1-17)3 lace-1, which was 

completed to a depth of 14 ft and located near the center of the salt crust. Six other wells completed at varying depths that ranged from 2.5 to 495 ft were also at 

this location. Two weather stations, installed and maintained by the Utah Climate Center, recorded data necessary to estimate actual evaporation from the playa 

surface.

Brine samples from wells on the Bonneville Salt 

Flats were collected for chemical and isotopic analysis. 

In most monitoring wells, yield was very low, thus preventing the withdrawal of three casing volumes. A minimum of one casing volume was pumped from each 

well before sampling and where feasible, three casing 

volumes were pumped. The distribution of samples 

was designed to examine the variation in chemical constituents areally, vertically within the ground-water 

system, and temporally. Twelve wells, originally sampled by Lines (1978, 1979), that were still usable were 

sampled again to determine any long-term variation. At 

most of the sampled wells completed in the shallowbrine aquifer, two samples were collected in early summer and fall. From these analyses, a seasonal variation 

for various constituents can be determined. All wells 

completed in the lacustrine sediment below the shallow-brine aquifer and at the northwest margin of the 

playa were sampled. Pore fluid from cores collected 

during drilling of some wells also was analyzed.

The quantity of brine obtained from land on Federal leases was estimated from data collected at the 

pumping station, which lifts the brine to a higher elevation in the collection ditch, south of Interstate Highway 

80 (fig. 3). A continuous recorder was used to measure 

the brine level in the ditch upstream from the pumping 

station. Data from the recorder were used to determine 

the time the pump was in operation and the brine level 

at time of pumping. A vibration-time-totalizing meter 

was installed on the pump to monitor total length of 

time the pump was in operation. Instantaneous discharge, specific-gravity, and temperature measurements were made periodically. The instantaneous 

discharge measurements were compared to the level in 

the ditch from which the pump lifts the brine. Pump

discharge is larger when the brine level in the ditch is 

high as compared to when the brine level in the ditch is 

low. From these data, a rating curve was developed that 

relates brine level to discharge when the pump is in 

operation.

The two weather stations that were installed by 

the Utah Climate Center are located at a multiple-well 

site near the center of the salt crust and to the northwest 

on the playa margin (fig. 3). These additional stations 

were designed to collect data necessary to determine 

precipitation, wind direction and velocity, air temperature, relative humidity, solar and net radiation, surface 

heat flux, and latent and sensible energy for the determination of actual and potential evaporation. The station on the playa margin northwest of the salt crust was 

put into service in November 1991. The weather station 

on the salt crust (fig. 4) was put into service in September 1992. These weather stations were serviced by personnel from the Utah State Climate Center.

rrtt

Figure 4. Weather station near the center of the 

salt crust, Bonneville Salt Flats, Utah.

INTRODUCTION 9
Acknowledgments

The authors express their appreciation to Reilly 

Wendover, a division of Reilly Industries. Reilly Wendover granted permission to drill monitoring wells on 

their property and allowed access to these wells and 

existing wells for water-level measurements and sampling for chemical analysis. Reilly Wendover granted 

permission to monitor their pumping station on the collection ditch and provided data.

Comments and suggestions provided by members of the technical review committee established by 

the Bureau of Land Management were greatly appreciated. Special appreciation goes to Blair F. Jones, U.S. 

Geological Survey, for numerous suggestions regarding the brine chemistry of the Bonneville Salt Flats, and 

to Brenda L. Coleman and Daniel M. Webster, U.S. 

Geological Survey, for x-ray diffraction identification 

of clay-size minerals and extraction and chemical analysis of pore fluid.

DESCRIPTION OF STUDY AREA

Physical Setting

The Bonneville Salt Flats study area is in the 

western part of the Great Salt Lake Desert, which is in 

the Great Basin part of the Basin and Range Physiographic Province (Fenneman, 1931, pi. 1). The Great 

Basin is a hydrologically closed region with no surface 

outlet. The only means for water to leave the Great 

Basin is by evaporation to the atmosphere. Typical 

topography of the Basin and Range Province consists 

of nearly north-trending mountain ranges separated by 

desert basins. The mountain ranges and separating 

basins are generally 5 to 15 mi wide and about 50 mi 

long, but the Bonneville Salt Flats is located in the 

western part of a basin that is more than twice as wide 

as a typical basin in the Basin and Range Province.

The Bonneville Salt Flats is bounded by the Silver Island Mountains to the northwest. The mountains 

rise abruptly from the playa floor to a maximum altitude of more than 7,500 ft. Paleozoic-age carbonate 

rocks and quartzites with some outcrops of Tertiaryage basaltic extrusive rocks are prominent. Several 

remnant Lake Bonneville shoreline features on the 

steep mountain flanks are visible from the Bonneville 

Salt Flats. Although sand and gravel are present in the 

subsurface, the surface features of alluvial-fan deposits

are less prominent on the southeastern flank of the Silver Island Mountains, adjacent to the Bonneville Salt 

Flats, than on the northwestern flank.

Surface runoff, which generally results from 

intense summer thunderstorms, flows in distinct 

ephemeral stream channels from the mountains toward 

the playa. These channels cross the alluvial fans and 

continue toward the playa where most terminate in surface fractures before reaching the playa mud. Flow in 

some of the ephemeral stream channels might reach the 

playa beyond the vegetation at the playa margin and 

contributes to surface ponds. Surface flow reaching the 

playa, however, was not observed during this study.

On the northeast, east, and south margins of the 

Bonneville Salt Flats, the playa surface gradually 

slopes upward for several miles in each direction. The 

surface of the salt crust represents the lowest point of 

the basin, and surface-water flow generally moves 

toward the salt crust. Even slight changes in altitude of 

the land surface, such as less than 1 ft, can create small, 

closed basins that can accumulate water and salt, thus 

impeding flow toward the salt crust. These small, 

closed basins are easily breached when the water level 

rises as a result of precipitation and when strong winds 

move sheets of water across the playa surface. Surfacewater flow that used to move toward the salt crust has 

been interrupted by man-made structures such as the 

brine-collection ditch and associated berm on the east 

side of the salt crust, and Interstate Highway 80 and the 

railroad to the south. Culverts were installed beneath 

Interstate Highway 80 to facilitate surface flow toward 

the salt crust. In 1983, additional culverts were 

installed beneath the roadbed to alleviate extensive 

flooding. A large amount of water again passed through 

these culverts in the spring of 1993 as a result of 

greater-than-normal precipitation in January and 

below-normal temperatures in January and February.

Playa Morphology

The salt crust and associated near-surface deposits in the Bonneville Salt Flats playa are the result of 

desiccation of Lake Bonneville. As with other playas 

that formed by lake desiccation, evaporite mineral 

zonation, as described by Hunt and others (1966, p. 

B46-B48), can be divided into a carbonate zone, composed primarily of aragonite; a sulfate zone, composed 

primarily of gypsum; and a chloride zone, composed 

primarily of halite. As evaporation progresses, the dissolved-solids concentration of the evaporating fluid

10 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
increases. When the fluid reaches saturation with 

respect to a specific mineral, precipitation of that mineral progresses until the fluid is no longer at saturation 

with respect to that mineral, either due to dilution or 

depletion of one of the required ions. The carbonate 

minerals, which are the least soluble, will crystallize 

first, followed by sulfate minerals, and finally the most 

soluble, chloride minerals. In a simplified sense, the 

carbonate minerals will precipitate along the edges of 

the lake where they might interfinger with alluvial 

deposits and across the bottom where they overlie the 

lacustrine sediment. The sulfate minerals precipitate 

over the carbonate minerals and, likewise, the chloride 

minerals precipitate next and overlie the sulfate minerals. Each zone of minerals is transitional with adjacent 

zones and might be interfingered with adjoining zones 

as a result of erosional and mineral dissolution and precipitation processes. The volume of each zone will 

depend on the original composition of the evaporating 

fluid.

Because halite is the most soluble mineral and 

covers a large areal extent of the Bonneville Salt Flats, 

the chloride zone is most responsive to climatic 

changes. Through variability in rainfall on the playa, 

the salt crust can vary in surface texture and areal extent 

from 1 year to the next because of dissolution and precipitation of salt at the surface as described by Lines 

(1979, p. 42-56) and Turk (1978, p. 3-18). No attempt 

was made to delineate the seasonal variation in texture 

and areal extent of the various types of salt surfaces 

during this study.

Lines inventoried five different types of saltcrust surfaces: thin seasonal, pressure-ridge, sedimentcovered, smooth perennial, and rough perennial, of 

which the rough perennial salt crust was not present 

during this study. The lack of rough perennial salt crust, 

which forms in areas where the salt crust does not 

become covered by water, is probably the result of 

more extensive ponding during the winter months of 

this study than was present during Lines' study. The 

more extensive ponding might be the result of 

increased winter precipitation and cool temperatures 

during the time of this study in combination with the 

elimination of surface drainage of brine into the collection ditch. The erosional channels on the west side of 

the brine-collection ditch, described by Lines (1979, p. 

35), have been eliminated by subsequent dredging of 

the brine-collection ditch and building the resultant 

berm along the west side.

The extensive flooding of the Bonneville Salt 

Flats during the winter of 1992-93 dramatically 

changed the salt surface. Much of the near-surface salt 

was dissolved, especially in the southern part of the salt 

crust. When halite was dissolved, crystalline gypsum, 

which was not dissolved, and some insoluble silt interbedded within the salt crust became concentrated in a 

thin layer on the surface of the crust. At many locations, 

ripple marks were visible on this surface at the bottom 

of the pond. The Bonneville Salt Flats remained mostly 

flooded into the late summer of 1993 with the dissolved 

halite gradually precipitating on the playa surface as 

the pond evaporated. Much of the halite subsequently 

was deposited on the salt crust and smooth, thin seasonal halite crust also was deposited in areas beyond 

the usual areal extent of the salt crust.

A 30-in., vertical column of salt was removed 

from the salt crust by Bureau of Land Management personnel in August 1995. A 3-in. layer of crystalline salt 

was removed prior to extraction of the vertical column. 

This removal exposed the undulating, ripple-marked 

surface that was the result of salt dissolution and precipitation in 1993. Five additional, undulating surfaces 

could be delineated throughout the column. These surfaces indicate that salt dissolution and precipitation has 

occurred during the formation of the salt crust.

The pressure-ridge salt crust was prominent to 

the east of the Bonneville racetrack during late summer 

months of 1990-92. This type of salt surface generally 

forms where thin salt crust has precipitated over the 

carbonate mud. Additional accumulation of salt from 

evaporating subsurface brine causes lateral forces that 

rupture the thin crust. Subsequently, one section of the 

salt crust will thrust over an adjoining section of crust. 

Because of the short period of time in the summer of 

1993 for evaporation from subsurface brine, the pressure-ridge salt crust did not develop where thin salt 

formed over the carbonate mud. Localized areas of 

pressure-ridge salt did develop, however, over the 

smooth perennial salt crust where gypsum or thin layers of sediment had accumulated at the bottom of the 

pond during the winter. Newly precipitated salt is susceptible to the formation of pressure ridges because of 

its lack of cohesion with the underlying gypsum or sediment.

Climate

Climate plays an integral part in controlling the 

movement of water on the surface and within the 

ground-water system of the Bonneville Salt Flats. PreDESCRIPTION OF STUDY AREA 11
cipitation, temperature, wind, and evaporation are all 

part of the mechanism that drives these hydrochemical 

systems. Climatic data have been collected from 1912 

to 1994 at the U.S. National Weather Service station in 

Wendover, about 10 mi southwest of the center of the 

Bonneville Salt Flats. Data were collected at the playamargin station from January 1992 through 1994 and at 

the salt-crust station from November 1992 through 

August 1994.

Precipitation

Precipitation at the Wendover climatic station 

averaged 4.78 in/yr from 1912 through 1993, (data 

were incomplete in 1994) (U.S. Weather Bureau, 1936, 

1957,1961-66, and 1965; U.S. Environmental Science 

Services Administration, 1966-71; U.S. National Oceanic and Atmospheric Administration, 1971-94). 

Annual precipitation has varied from a minimum of 

1.62 in. in 1992 to a maximum of 10.13 in. in 1941 (fig. 

5). Lines (1979, p. 15) identified five wetter-than-normal and drier-than-normal periods from 1912 through 

1973, ranging in length from 12 to 14 years. Precipitation records for 1974 through 1993, indicate the wetterthan-normal period beginning in 1962 lasted through 

1983, with an average annual precipitation of 5.59 in. 

The period from 1984 to 1993 defines a drier-than-normal period, with an average annual precipitation of 

3.64 in.

Lines (1979, p. 14) assumed that these wet and 

dry periods recorded at the Wendover station also 

occurred on the Bonneville Salt Flats on the basis of 15 

months of concurrent record with temporary stations 

on the Bonneville Salt Flats and in Pilot Valley. The 

limited record from the playa-margin station on the 

southeast flank of the Silver Island Mountains indicates 

that area might receive more precipitation than the area 

near the Wendover station. In 1992, the playa-margin 

station recorded 3.05 in. of precipitation, and the Wendover station recorded 1.62 in. of precipitation. The 

greatest discrepancy was for January through May 

1992 when 1.44 in. fell at the playa-margin station and 

only 0.31 in. fell at the Wendover station. During the 35 

months for which concurrent record exists, the playamargin station recorded greater monthly precipitation 

than the Wendover station during 24 months (fig. 6). 

The salt-crust station was not in service during much of 

1992, but in 1993, 4.64 in. of precipitation were 

recorded, 1.41 in. more than at the Wendover station. 

Monthly precipitation at the three weather stations is

listed in table 1, along with the monthly departures 

from the 1961-90 normals at Wendover, Utah.

Wind

Wind is the primary mechanism for moving ponded water over the surface of the Bonneville Salt Flats. 

The two weather stations installed for this study were 

equipped with recording anemometers. For the period 

of record, the median of the average monthly wind 

directions at both sites was North 9° East (fig. 7). The 

median of the average monthly velocities was 6.5 mi/ 

hr. During the spring and summer months (April 

through September) of 1992-94, the median of the 

average monthly wind directions was North 23° East. 

The median of the average monthly velocities for the 

spring and summer months was 7.8 mi/hr. The average 

monthly wind velocity ranged from 5.7 to 9.1 mi/hr 

from April through September. Maximum velocities 

recorded during spring and summer months generally 

exceeded 50 mi/hr during the period of record. In 

August and September 1993 and June 1994, maximum 

velocities exceeded 40 mi/hr.

Temperature

Air and soil temperatures affect water evaporation from above and below land surface. Air temperature has been recorded at the Wendover weather station 

since 1921 (fig. 8). Not including 6 years of missing 

data, the average annual temperature at the Wendover 

weather station was 52.4°F. The average annual temperature has been as high as 56.3°F in 1934 and as low 

as 48°F in 1994. Typically, high average annual air 

temperatures occur during periods of less-than-normal 

precipitation, and low average annual air temperatures 

occur during periods of greater-than-normal precipitation.

Air and soil temperature were recorded at the 

two weather stations on the Bonneville Salt Flats in 

order to estimate evaporation. Monthly air and soil 

temperature at the playa-margin station were slightly 

higher than at the salt-crust station. Except in the fall, 

average monthly soil temperatures were always 2 to 10 

degrees warmer than average monthly air temperature. 

The highest average monthly temperatures occurred 

during August (70° to 90°F), and the lowest average 

monthly temperatures occurred during January (15° to 

25 °F). Average monthly air temperature at the two stations was about the same as at the Wendover station for 

1992-94 (table 1). Wet and dry bulb temperature also

12 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
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DESCRIPTION OF STUDY AREA 13
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1.4

1.2

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0.

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11 IL > o

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1993

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Figure 6. Monthly precipitation at the three weather stations on or near the Bonneville Salt Flats, Utah, November 1991 th
Table 1. Monthly mean temperature and monthly precipitation and departures from 1961-90 normals at Wendover, 

Utah, compared to monthly mean temperature and monthly precipitation measured at two weather stations on the 

Bonneville Salt Flats, Utah, 1992-94.

[°F, degrees Fahrenheit; —, no data available; M, partial data missing; NA, departure not calculated because of missing data]

Wendover

Temperature

January

February

March

April

May

June

July

August

September

October

November

December

January

February

March

April

May

June

July

August

September

October

November

December

January

February

March

April

May

June

July

August

September

October

November

December

Mean

23.6

37.7

47.2

57.5

65.8

71.1

76.6

77.1

66.0

55.2

33.9

24.4

15.4

24.5

42.5

48.6

63.7

64.5

71.3

73.0

64.5

50.7

30.4

21.5

30.7

31.1

44.8

52.4

64.4

73.0

79.4

78.4

67.3

50.0

32.3

28.8

Departure

-4.5

3.3

5.8

7.0

5.0

.7

-3.2

.4

.0

2.8

-4.3

-4.4

-11.4

-9.2

.3

-2.1

2.9

-6.4

-8.4

-3.7

-1.1

-1.3

-8.2

-1.1

3.9

-2.6

2.6

1.7

3.6

2.1

-.3

1.7

1.7

-2.0

-6.3

1.2

Precipitation 

(inches)

Total

.01

.14

.15

.0

.01

.26

.2

.33

.0

.26

.14 M

.12 M

1.0

.13 M

.19 M

.04

.13 M

.37

.6

.01

.24

.51

.01

.0

.02

.01

.47

.29 M

.33

.0

.15

.35

.88

.06 M

.13 M

—

Departure

1992

-.33

-.22

-.27

-.43

-.84

-.35

-.05

-.09

-.23

-.21

NA

NA

1993

.76

NA

NA

-.52

NA

-.28

.29

-.45

-.14

-.04

-.37

-.27

1994

-.22

-.31

.02

NA

-.57

-.65

-.16

-.11

.50

NA

NA

NA

Playa margin

Temperature

22.8

39

49.1

59.7

68.2

72.5

78.7

84.1

65.5

54.6

34.3

24.8

17.6

25.0

42.5

48.9

64.4

65.2

71.6

73.5

64.9

51.1

30.7

26.8

31.5

31.9

45.3

52.5

—

74.1

80.8

79.2

70.3

51.8

32.7

29.9

Precipitation 

(inches)

.44

.52

.25

.01

.22

.37

.05

.03

.06

.57

.27

.26

.97

.0

.0

.06

.54

.9

.57

.06

.34

1.43

.01

.01

.22

.25

.74

1.51

—

.0

.04

.14

.0

.28

.15

.03

Salt crust

Temperature

—

—

—

—

—

—

—

—

—

55.7

34.8

25.4

19.5

27.6

44.9

49.9

64.0

64.4

69.5

71.7

62.9

50.8

30.2

26.4

30.5

32.0

44.8

52.1

63.7

72.7

79.7

78.2

—

—

—

—

Precipitation 

(inches)

—

—

—

—

—

—

—

—

—

.47

.12

.10

.36

.3

.42

.14

.25

.74

1.37

.04

.05

.94

.02

.01

.11

.12

.85

1.23

.6

.0

.0

.11

—

—

—

—

DESCRIPTION OF STUDY AREA 15
340

NORTH 

350° 0° 10C

330 30°

320 40°

310 50°

60°

Median velocity and direction for 

spring and summer months 7.8 miles 

North 23° East

Median velocity and direction 

6.5 miles per hour, North 9° East

260C

250C

240°

230d

220°

210°

200°

1123456789 

WIND VELOCITY, IN MILES PER HOUR

100C

110°

130C

140C

150°

190° 180° 170° 

SOUTH

160°

EXPLANATION

D Playa margin 

• Salt crust

Figure 7. Average monthly wind direction and velocity for the two weather stations on the Bonneville Salt Flats, 

Utah, December 1991 through December 1993.

were recorded at the two weather stations on the Bonneville Salt Flats because they are needed for calculating actual evaporation by the Bowen-ratio method 

discussed later in this report.

GROUND-WATER HYDROLOGY

Description of Ground-Water System

In most of the western Great Salt Lake Desert, 

ground water is present in three distinct aquifers. The 

basin-fill aquifer generally is the largest in volume of 

the three aquifers. Because the alluvial-fan aquifer is in 

close proximity to sources of recharge, such as seepage

through consolidated rock and infiltration of surface 

runoff, it generally is the source of the most usable 

water. The shallow-brine aquifer, where present, generally is in near-surface deposits and can have a thin 

ephemeral salt crust or a thick perennial salt crust, such 

as on the Bonneville Salt Flats. The spatial relation of 

these aquifers as they occur in the Bonneville Salt Flats 

is shown diagrammatically in figure 9.

The basin-fill aquifer generally is composed of 

several hundred feet of fine-grained, lacustrine sediment in the upper part and the underlying conglomerate 

yields brine to wells. These wells are completed to 

depths of more than 1,000 ft. The conglomerate is more 

than 800 ft thick (Lines, 1979, p. 57), and overlies Tertiary-age volcanic rocks or pre-Tertiary-age rocks. The

16 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
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GROUND-WATER HYDROLOGY 17
m

EXPLANATION

Direction of ground-water movement

Consolidated 

rock Alluvial-fan 

aquifer Lacustrine 

sediment of the 

upper part of the basin-fill aquifer Shallow- brine aquifer

Figure 9. Generalized block diagram showing the relation of aquifers and direction of ground-water flow in the Bonneville Salt Flats, Utah.
thick lacustrine sediment between the conglomerate 

and the overlying shallow-brine aquifer previously was 

not considered part of the basin-fill aquifer. For the purposes of this report, the lacustrine sediment will be 

included as the upper part of the basin-fill aquifer.

Along the southeast flank of the Silver Island 

Mountains, the alluvial-fan aquifer in the Bonneville 

Salt Flats study area readily yields brackish water to 

wells. Sand and gravel of the alluvial-fan aquifer are 

interbedded with the carbonate mud of the shallowbrine aquifer and the lacustrine sediment of the upper 

part of the basin-fill aquifer. The vertical extent of these 

sands and gravels of the alluvial-fan aquifer is 

unknown; therefore, the degree of hydraulic connection with the conglomerate in the basin-fill aquifer at 

depth also is unknown. As the layers of the alluvial-fan 

aquifer extend toward the playa, the coarser material 

becomes intermixed with fine-grained sediment. Wells 

completed in these zones would be less likely to yield 

substantial amounts of water.

The shallow-brine aquifer is composed of a 

halite and gypsum crust at the surface and is surrounded by carbonate mud, both laterally and immediately beneath the salt crust. The transition zone 

between the shallow-brine aquifer and the alluvial-fan 

aquifer is difficult to identify at land surface, but the 

boundary between the aquifers is assumed to occur at 

the limit of vegetation or at a slight change in the slope 

of the land surface. On the basis of water levels in 

wells along the northwestern margin of the playa, the 

shallow-brine aquifer might be hydraulically connected to the alluvial-fan aquifer. The thick sequence of 

fine-grained, lacustrine sediment, generally several 

hundred feet thick, in the upper part of the basin-fill 

aquifer probably restricts the vertical movement of 

brine between the shallow-brine aquifer and the more 

porous conglomerate in the deeper part of the basin-fill 

aquifer.

Ground-water density in these three aquifers 

varies from less-dense brackish water in the alluvialfan aquifer to dense brines in the shallow-brine aquifer. 

Ground-water density also can vary within an aquifer. 

Because of the widely varying ground-water density, 

additional features describing ground-water flow need 

to be considered in order to properly describe groundwater movement. Variable-density ground-water flow 

is somewhat more complicated than uniform-density 

flow because of buoyancy effects. For density variations caused by solute-concentration variations, quantification of ground-water flow is no longer obtainable

from the gradient of a scalar potential field such as 

potentiometric head.

Principles of Variable-Density GroundWater Flow

Much of the following discussion of variabledensity, ground-water flow is described in Hubbert 

(1953,1956). These principles were originally used by 

Hubbert to describe the flow of petroleum under hydrodynamic conditions and later applied to ground-water 

flow.

The state of ground-water motion is a result of 

the energy conditions that prevail. A particle of water at 

any point possesses a potential energy per unit mass, 

more commonly known as potential, O. If the potential 

field across a region is variable, work will be required 

to move an element of water from a point of lower 

potential to one of higher potential. In this case, an 

unbalanced force is exerted upon the fluid by its environment, driving it in the direction of the most rapid 

decrease of potential.

The potential at a given location is the amount of 

work that would be required to move a unit mass of 

water from a reference position and state to a position 

and state of the location considered. Total work consists of work against gravity and work against pressure. 

For the slow, creeping motion of ground-water flow, the 

kinetic-energy term can be neglected. Then the potential can be expressed by the following equation:

(1)

where

O is the potential (EIM);

g is the gravitational constant (Z/T2);

z is altitude (L);

z0 is the reference standard altitude (L);

p is the fluid pressure (F/L2); 

PQ is the fluid reference standard pressure (F/L2); 

and

p is the fluid density (M/L3 ).

Because the choice of the reference state is arbitrary, sea level and atmospheric pressure can be 

selected. Thus, the potential depends on the altitude, 

pressure, and fluid density that can, in principle, be 

measured at any location in the subsurface. It can be 

determined at any point capable of being occupied by a 

fluid of given density, whether or not that fluid is actually present. Thus, freshwater potentials have values

Principles of Variable-Density Ground-Water Flow 1 9
not only in space saturated with freshwater, but also in 

space occupied by salt water and vice versa. The potential at a given point is related to the height above standard datum to which the fluid will rise in a manometer 

installed at that point. That is,

<E> = gh (2)

where

h is the height of fluid above standard datum 

(potentiometric head) (L).

The value of potential is the amount of work 

required to lift a unit mass of fluid to the height, h, 

against the gravitational force.

The force exerted on a unit mass of fluid at any 

given point is related to the potential field. Specifically, 

the force is obtained from the gradient of the potential 

field by

-i

where 

E

E = -VO = g-Vpp

is the force vector on a unit mass of fluid 

(F/M);

); and

(3)

-i V is the spatial gradient operator (L 

g is the gravitational vector (L/T 2).

The fluid is in a state of hydrostatic equilibrium 

when E = 0, that is, <E> is a constant. The physical meaning of the force-intensity vector E comes from equation 

3 that states at a given point, a unit mass of fluid will be 

acted upon by a force E that is the sum of a gravitational force g and a pressure-gradient force Vp divided 

by the fluid density p. The force-intensity vector is 

obtained by vector addition shown in figure 10.

In ground-water flow, only rarely does E have a 

magnitude greater than a small fraction of the vector g. 

When the pressure-gradient force is tilted somewhat 

from vertical, the force-intensity vector may point 

upward or downward, and the fluid will have a driving 

force in the direction of E. The resulting fluid movement can be characterized by a specific discharge or 

volumetric fluid flux vector <jr. For a homogeneous fluid 

in an isotropic porous media, q points in the direction 

of E with a scalar parameter of proportionality K 

known as hydraulic conductivity. The relation is given 

by Darcy's law

q = -KVh

-Vp/p -V/7/P

E = 0

(a)

Figure 10. Force-intensity vector E for (a) hydrostatic 

conditions; and (b) hydrodynamic conditions.

3,,2

where

q is the volumetric fluid-flux vector (LJ/LZ-T);

and 

K is the hydraulic conductivity (LIT).

If a second fluid of density P2, which is greater 

than that of our initial fluid, is considered, a similar 

force-vector diagram can be drawn as in figure 10. The 

resulting force-intensity vectors are shown for two fluids in the hydrodynamic case (fig. 11). Note that, for an 

isotropic porous medium, the denser fluid sinks relative 

to the less dense fluid.

Equation 3 can be written for each of the two fluids. Because the pressure field experienced by both fluids is the same, the force-intensity vector for fluid 2 

then can be expressed in terms of that for fluid 1.

E2 = g + ?)*,-.]

P9/

(5)

(4)

A vector diagram (fig. 12) is useful to visualize 

this equation.

At a given point in space, the force per unit mass 

acting on fluid 2 depends on the density of fluid 2 and 

the force per unit mass that would act on fluid 1 if it 

were at that location. When EI is neither zero nor vertical, E2 will be tilted from vertical in the direction of 

the horizontal component of E±. Fluid 2 will move in 

the same horizontal direction as fluid 1 but in a downward vertical direction relative to fluid 1.

If fluid 1 is freshwater and fluid 2 is saline water 

of a given density, the defining equation 1 for the force20 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
-Vp/p

Figure 11 . Force-intensity vectors for two fluids of 

different density acted upon by the same pressure and 

gravitational fields.

2) [E.-g]

Figure 12. Forces acting on an element of fluid 1 and on 

an element of fluid 2 under hydrodynamic conditions.

intensity vector is substituted into equation 5, and a 

potentiometric head is defined as

(6)

then equation 7 relates the potentiometric head values 

for the two fluids.

— 

P2, P2-Pl «2~————— 

Pi 2 Pi

(7)

This equation gives the equivalent freshwater 

head as a function of the in-place saline-water head. 

Note that the altitude of the measurement point must be 

known to make the conversion. The heads are the 

heights above datum that each of the two fluids would 

rise in a manometer installed at the measurement point.

By combining equations 3 and 6, an equation 

giving the force on a parcel of water as a function of 

potentiometric head is obtained as

E = -gVh (8)

Similarly, equations 5 and 7 can be combined to 

express the force on a parcel of saline water as a function of the potentiometric-head gradient for a parcel of 

freshwater. That is,

T-T • 1 \~71 E2 7 = -g— W Vflil Pi (9)

The component of E2 in the vertical direction 

can be given by

P2-P1 (10)

where

e is the unit vector in the vertical direction (-).

For simplicity, if an isotropic medium with equal 

viscosity of the fresh and saline water is assumed, 

Darcy's Law combined with equation 7 gives the velocity of the saline water as a function of the equivalent 

freshwater head.

6JJ,

. Pl-Pl^ + ——— Vz 

Pi

(11)

k

£

where

v2 is the interstitial velocity of the fluid (LIT); 

is the permeability (L2); 

is porosity (-); and 

|i is the fluid viscosity (FIL2 -T).

The vector diagram of equation 11 (fig. 13) 

shows that although the gradient of the equivalent 

freshwater-head field provides the horizontal orientation of the interstitial velocity vector, there is a vertical

Principles of Variable-Density Ground-Water Flow 21
component dependent on the density difference. This is 

why gradients of any given head field based upon any 

chosen reference density will not give the correct direction of flow. Because of the density variation from the 

reference value, the vertical component of the flow vector must be included to obtain the correct flow-velocity 

vector.

-V/7

-[(p2 -

Figure 13. Interstitial velocity vector and the freshwater 

head and density-difference components.

Similar to the force-intensity vector, the component of the velocity vector in the vertical direction z is 

given by

v, = -

ep.

+

Pi (12)

where

vz is the vertical interstitial velocity of the fluid

0/7).

In a variable-density ground-water flow system, 

ground-water flow causes changes in the solute-concentration field, which also results in changes to the 

density field. This causes the ground-water flow field to 

change with time. The only practical way to account for 

the coupling between the ground-water flow field and 

the solute-concentration field is by use of numerical 

simulation. Evaluation of a ground-water system based 

on analysis of an approximate constant-density model 

will lead to erroneous flow rates and directions at locations where the driving forces, as a result of buoyancy 

effects, are large relative to the driving forces that result 

from hydraulic gradients based on a homogeneous 

fluid.

Shallow-Brine Aquifer

The shallow-brine aquifer in the Bonneville Salt 

Flats has been defined by Turk (1973, p. 8) and Lines 

(1979, p. 65) as comprising a halite and gypsum crust

that is surrounded by near-surface carbonate mud to a 

depth of 15-25 ft. Most of the brine withdrawn for potash production is derived from the shallow-brine aquifer. Precipitation of halite from evaporating brine 

sustains the salt crust that provides the racing surface. 

For these reasons, the water and salt budgets for the 

shallow-brine aquifer were examined in more detail 

than those of the other aquifers in the Bonneville Salt 

Flats.

The carbonate mud of the shallow-brine aquifer 

is composed primarily of aragonite, calcite, dolomite, 

quartz, feldspar, and clay. X-ray diffraction techniques 

were used to determine mineral composition of core 

samples. The most abundant clay minerals are the 

smectites, which expand as a result of hydration when 

in contact with water. The remaining clays are illites, 

which are non-expanding when in contact with water. 

Individual halite and gypsum crystals can occur within 

the carbonate mud at some locations where surface 

ponding readily occurs, and the infiltrating brine results 

in halite and gypsum crystallization under evaporating 

conditions. In the upper part of the carbonate mud, 

interbedded layers of oolitic sand are present. These 

layers vary in thickness and lateral extent and have the 

ability to transmit large amounts of brine, depending on 

the degree of connection between layers.

In areas of thick salt crust, the upper few inches 

are generally composed of tightly compacted crystalline halite. Ridges of soft, crystalline halite develop 

over fractures in the hard surface salt during the summer months when brine comes to the surface and evaporates. Beneath the hard, near-surface crust, halite 

crystals are large and well formed with little or no 

cohesion between the crystals. The large crystals are 

the result of slow growth in a stable environment. This 

zone in the halite crust is very porous and has the ability to readily transmit brine. Gypsum and insoluble 

clays may be interspersed throughout the halite crust. 

During winter months when extensive surface ponds 

are present, such as the winter of 1992-93, several 

inches of the upper halite crust can dissolve. Generally, 

the gypsum and insoluble clays concentrate on the submerged surface of the halite crust. In the summer 

months when evaporation occurs, halite precipitates 

over the gypsum and clay. Thus, depending on the history of salt dissolution and precipitation, the halite 

crust might contain thin, stratified layers of gypsum 

and insoluble clays. Beneath the halite crust, a gypsum 

layer generally separates the halite crust from the carbonate mud. This layer varies in thickness and in some

22 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
locations is difficult to distinguish from the loose halite 

and the underlying carbonate mud.

North of Interstate Highway 80, the salt crust on 

the Bonneville Salt Flats has a lense-shaped cross section in an east-west direction. On the margins, a thin 

crust of gypsum generally is present. This crust might 

be covered with clay, silt-sized material, or halite, if 

winter ponds cover the area and subsequently precipitate halite upon evaporation.

Vertical fractures in the carbonate mud can be 

seen readily along the exposed sides of brine-collection 

ditches. These fractures are assumed to be present 

throughout much of the shallow-brine aquifer. The 

fractures can be up to an inch wide, and the spacing 

between fractures is variable with 1 ft a common interval (Turk and others, 1973, p. 69). This report states 

that the fractures might have a hexagonal pattern in 

map view and probably coincide with the hexagonal 

pattern of the fractures on the surface of the salt crust. 

These fractures are probably the principal conduits by 

which brine is transmitted in the shallow-brine aquifer.

Aquifer Properties

The transmissive properties of an aquifer generally depend on the type of porous material it contains, 

and can vary considerably throughout the aquifer. The 

porous material in the shallow-brine aquifer of the 

Bonneville Salt Flats varies from very transmissive 

crystalline halite and gypsum in the salt crust to poorly 

transmissive carbonate mud underlying the salt crust. 

Fractures and interbedded layers of oolitic sand within 

the carbonate mud, however, can result in areas of 

larger horizontal and vertical transmissivity, depending 

on the degree of connection between these fractures 

and oolitic sand layers.

During a period of several years, numerous aquifer tests were completed to determine the transmissive 

properties of the shallow-brine aquifer in the Bonneville Salt Flats. The first tests were completed in 1960 by 

personnel at the Utah State University Engineering 

Experiment Station, in cooperation with the Utah 

Department of Transportation, to determine aquifer 

properties in the area designated for construction of 

Interstate Highway 80 so that appropriate methods of 

construction could be determined. Twelve of the 

reported transmissivity values are included in a report 

by Turk (1973, table 9, fig. 13). About 70 aquifer tests 

were completed by personnel of Kaiser Aluminum 

Corporation and L. J. Turk during 1965-67, and the 

results are reported by Turk (1973, table 9). Lines completed four aquifer tests to determine the aquifer properties of the carbonate mud by excluding the influence 

of the very transmissive salt crust. The results of these 

tests, and those that Turk rated as "good" or better, 

were incorporated into the transmissivity map of Lines 

(1979, fig. 33). Lines (1979, p. 67) stated further that 

the transmissivity map was based on the results of aquifer tests that were completed during a 12-year period 

and therefore, transmissivity values mapped in some 

areas might not accurately represent those areas in 

1976. Because no new aquifer tests were completed 

during this study with which to compare previous tests, 

Lines' statement is assumed to be true for this study.

The use of 2-in. casing in the shallow monitoring 

wells completed during this study precludes the use of 

any aquifer-test analysis that imposes a stress to the 

aquifer by pumping. Falling-head or slug tests easily 

could have been done, but the results would not be representative of the aquifer if the borehole did not intersect any fractures.

Turk (1973, p. 9) reported that transmissivity 

values increased from less than 100 ft /d at the margin

S)

of the salt crust to more than 13,000 ft /d near the center of the salt crust. The highest transmissivity value 

was determined for an area south of Interstate Highway 

80, outside of the present study area. Transmissivity 

values reported by Turk that lie within the present study 

area generally do not exceed 5,000 ft2/d. Turk noticed 

that the transmissivity values increased from the margin to the center of the salt crust, with the highest values coinciding with the highest concentrations of 

potassium and magnesium and the thickest part of the 

salt crust. He attributed the high transmissivity values 

to the permeable salt crust, fractures within the underlying carbonate mud, and thin layers of oolitic sand. In 

areas where the salt crust is thickest, transmissivity values are high because the permeable salt crust makes up 

a larger part of the shallow-brine aquifer.

Lines (1979, table 5) reported the results of four 

aquifer tests that were designed to stress the carbonate 

mud so that the effects in the overlying salt crust could 

be evaluated. Transmissivity values determined from 

these tests ranged from 490 to 8,100 ft2/d (Lines, 1979, 

p. 67). Data collected during the tests show that the salt 

crust and underlying carbonate mud are hydraulically 

connected and that the vertical hydraulic-conductivity 

values for the aquifer range from 30 to 140 ft/d (Lines, 

1973, p. 70).

The porosity of the carbonate mud of the shallow-brine aquifer on the Bonneville Salt Flats has been

Shallow-Brine Aquifer 23
reported in previous studies for the mineral company. 

Turk (1969, p. 104) reported that porosity averaged 45 

percent as determined from numerous wet and dry 

bulk-density measurements. This value fits into the 

normal range for clay of 40 to 70 percent (Freeze and 

Cherry, 1979, p. 37). During sample collection for this 

estimate, any fractures, if present, would not have been 

preserved. The estimate, therefore, is representative of 

only the intergranular pore spaces (primary porosity) 

and not the fractures (secondary porosity), which are 

the principal avenues for flow in the shallow-brine 

aquifer. Effective porosity, which is the amount of 

interconnected pore space or fractures available for 

fluid transmission, could not be determined accurately. 

Effective porosity derived from fractures, such as in 

fractured crystalline rock, is generally several orders of 

magnitude smaller than effective porosity derived from 

intragranular pore spaces (Domenico and Schwartz, 

1990, p. 26). In the shallow-brine aquifer, however, 

with its large fracture apertures and relatively dense 

fracture spacing along with the porous salt crust and 

oolitic sand layers, effective porosity is probably in the 

same order of magnitude as the primary porosity value 

mentioned above.

An average specific-yield value of about 10 percent was reported by Turk and others (1973, p. 69). 

This estimate was derived from tests measuring gravity 

drainage of brine from saturated material obtained 

from the shallow-brine aquifer. Like the porosity estimate mentioned previously, if samples for the tests 

measuring specific yield did not include any fractures, 

then the estimate for specific yield might not be representative of the shallow-brine aquifer.

Values for storage coefficient determined from 

the results of aquifer tests in the shallow-brine aquifer 

varied from 1.2 x 10' 1 to 4 x 10"5 (Turk, 1973, table 9). 

These values are indicative of both unconfined and confined conditions. Of the 22 storage-coefficient values 

reported by Turk, only 10 are representative of wells 

located in the present study area. Similar storage-coefficient values were determined from four aquifer tests 

completed by Lines (1979, p. 68) and ranged from 3.8 

x 10 to 4.1 x 10"4. These values are indicative of semiconfined and confined conditions. Aquifer tests completed by Lines were designed to exclude pumping 

from the salt crust and, therefore, are representative of 

the carbonate mud.

Lines (1979, p. 70) attributed the larger storagecoefficient values reported by Turk to the large amount 

of brine pumped from the salt crust where the aquifer is

under unconfined conditions and to flow in fractures 

and through the oolitic sand. Confined conditions are 

more prevalent where the salt crust is absent, fractures 

are less frequent, and the oolitic sand is absent. These 

conditions exist where shallow monitoring wells were 

hand augured along the north line of wells between the 

salt crust and the brine-collection ditch (fig. 3). The 

near-surf ace, carbonate-mud deposits were hard and 

dry. Brine encountered 2 to 3 ft below land surface rose 

in the borehole to within a few inches of the surface.

Water-Level Fluctuation, Potentiometric Surface, 

and Ground-Water Movement

Water levels in wells fluctuate seasonally 

throughout the shallow-brine aquifer on the Bonneville 

Salt Flats as a result of variations in evaporation at the 

play a surface, precipitation on the play a surface, and 

brine withdrawal for mineral production. Water-level 

fluctuations in well (B-1-17)31 ace-1, located at the 

center of the salt crust, are shown in figure 14. The 

overall water-level trend during late spring through 

early fall is downward with sharp variations that are the 

result of periods of very high temperatures and high 

evaporation or, conversely, intense thunderstorms. 

Water levels rose abruptly during October and November in both 1991 and 1992; the rise in October 1993 

was not as abrupt. These rises were probably the result 

of drastically reduced evaporation that resulted from 

cooler temperatures combined with more precipitation. 

Data are insufficient to determine whether these rises 

can be attributed solely to decreased evaporation.

Temperature and specific gravity of the brine in 

each well casing were measured at the time of waterlevel measurement. From these data, density at 20°C 

was calculated. All water-level measurements were 

converted to a potentiometric head by using a reference 

density and equation 7 as described in the "Principles 

of Variable-Density Ground-Water Flow" section of 

this report. The reference density was the average density for the particular set of measurements. Instead of 

using the base of the shallow-brine aquifer to calculate 

the height of the water column in each well as was done 

by Lines (1979, p. 72), the altitude of the midpoint of 

the screened or perforated interval was used. This altitude was assumed to represent the measurement location for the water level. If the screened or perforated 

interval was unknown as for the wells completed by 

Turk (1969), the altitude of the midpoint of the drilled 

depth was used.

24 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
LU 

LU 

Li.

-1.0

LU 

O

I"05

o

z 

<

UJ

> 0

o

GO

o

UJ 

GO

UJ

UJ

-J

DC 

UJ

0.5

1.0

i i i i i i i i r

g JASONDJFMAMJJASONDJFMAMJJA

1991 1992 1993 

Figure 14. Water-level fluctuations in well (B-1-17)31 acc-1, located near the center of the salt crust, Bonneville Salt Flats, 
Average brine density was used as the reference 

density in equation 9 to minimize the magnitude of the 

gravitational term. This means that p2 - Pi was small 

throughout most of the region, and directions of 

ground-water flow are closely approximated using only 

the gradient of the reference potentiometric head. In 

addition, the areal potentiometric maps are based on 

projected head data from various elevations in the aquifer. Flow directions interpreted from these maps are 

correct for the horizontal components only. The maps 

provide no vertical flow information. In the shallowbrine aquifer, vertical flow components were calculated 

to be negligible at the few locations where vertical profiles of head and density data were obtained.

About 60 percent of the wells were completed in 

the upper third of the aquifer, 40 percent in the middle 

third, and only two wells in the lower third. The 

screened-interval length ranged from 2.5 ft to about 25 

ft, with only a few wells having the smallest interval. 

Thus, it was not possible to obtain areally distributed 

data that would quantify any density profiles in the 

shallow-brine aquifer.

The potentiometric surface with respect to an 

average reference density of 1.171 g/cm3 for the shallow-brine aquifer, shown in figure 15, was derived from 

water levels measured at the end of September 1992. 

This potentiometric surface is representative of conditions near the end of the evaporation season, after most 

of the brine has been withdrawn through the ditches. 

The general configuration and altitude of the contours 

are very similar to those mapped by Lines (1979, fig. 

35) for the same time of year. This comparison indicates that there is no long-term change in transient storage in the shallow-brine aquifer. With no long-term 

change in storage, the shallow-brine aquifer appears to 

be in a fluid steady state despite seasonal and yearly 

fluctuations in fluid inflows and outflows.

The horizontal direction of ground-water movement is from a higher to a lower reference potential as 

described in the "Principles of variable-density groundwater flow" section of this report. At the end of the 

summer evaporation and brine-production season, the 

horizontal component of brine movement in the shallow-brine aquifer is from the ground-water divide, 

located along the approximate axis of the salt crust, to 

the northwestern margin of the playa, and to the brinecollection ditches on the east and south. The horizontal 

hydraulic gradient in the area of the salt crust and to the 

north is small. The horizontal hydraulic gradient 

toward the northwestern margin of the playa and

toward the brine-collection ditches, in contrast, 

becomes relatively large near these boundaries as 

shown by the closeness of the contours (fig. 15).

The potentiometric surface with respect to an 

average reference density of 1.162 g/cm for the shallow-brine aquifer in April 1993 (fig. 16) has a different 

configuration than it did in September 1992. The 

slightly lower average reference density is indicative of 

the infiltration of less-dense recharge water. Slight gradients toward the northwestern margin of the playa and 

the brine-collection ditch south of Interstate Highway 

80 are discernible. Noticeable inflow is from the southeast. Much of the center of the playa has a flat potentiometric surface, resulting from the extensive surface 

ponding and lack of withdrawal at the brine-collection 

ditch east of the salt-crust area.

Because of the wet conditions on the playa in the 

summer of 1993 and the preceding winter months, the 

configuration of the potentiometric surface with 

respect to an average reference density of 1.161 g/cm3 

in October 1993 is not the same as in September 1992 

(fig. 17). The influence of the brine-collection ditches 

is clearly visible, but water levels throughout the 

remainder of the playa are much higher. This indicates 

that less evaporation from the shallow-brine aquifer 

occurred during the summer of 1993. Similarly, the 

lower average reference density also is indicative of 

less evaporation.

Where a playa is part of a regional ground-water 

flow system, the playa generally is the terminus of that 

flow system. At the terminus, ground water converges 

from all directions, and the general direction of flow is 

upward where it discharges at the surface and evaporates. Because the Bonneville Salt Flats is the topographic low within the Great Salt Lake Desert, this saltcrust area probably represents the terminus for groundwater flow for this region. As suggested by Harrill and 

others (1988, sheet 2), the Great Salt Lake Desert and, 

ultimately, the Bonneville Salt Flats, is the terminus for 

regional ground-water flow from adjacent basins to the 

south and southwest. With the natural ground-water 

flow direction toward the basin fill beneath the Bonneville Salt Flats, ground-water movement should be 

upward from the basin-fill aquifer into the shallowbrine aquifer.

Water-level and density measurements from 

adjacent wells that were completed in the shallowbrine aquifer and the underlying lacustrine sediment of 

the upper basin-fill aquifer at five locations (fig. 3) were 

examined to determine vertical ground-water flow

26 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
f

m

EXPLANATION

•421 0— Approximate potentiometric contour—Shows 

approximate altitude at which water level would 

have stood in tightly cased wells when 

referenced to an average density of 1.171 grams 

per cubic centimeter. Contour interval variable. 

Datum is sea level

• • • • Approximate location of ground-water divide

• Brine-collection ditch

TM**TMTMTMnTMa!" Playa boundary

• Well—Shallow monitoring well completed to a 

depth of 25 feet or less

40°45'

R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000,1979 

Universal Transverse Mercator projection, 

Zone 12

R. 18 W.

6 MILES

R. 17 W.

6 KILOMETERS

Figure 15. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of the Bonneville 
EXPLANATION

•4214— Approximate potentiometric contour—Shows 

approximate altitude at which water level would 

have stood in tightly cased wells when referenced 

to an average density of 1.162 grams per cubic 

centimeter. Contour interval variable. Datum is 

sea level

———— Brine-collection ditch 

wummmmw playa boundary

• Well—Shallow monitoring well completed to a depth 

of 25 feet or less

40°45'

R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000,1979 

Universal Transverse Mercator projection, 

Zone 12

R. 18 W. 6 MILES

6 KILOMETERS

Figure 16. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of the Bonnevil
.Q 

C

EXPLANATION

—4214—Approximate potentiometric contour—Shows 

approximate altitude at which water level would 

have stood in tightly cased wells when referenced 

to an average density of 1.161 grams per cubic 

centimeter. Contour interval variable. Datum is 

sea level

———— Brine-collection ditch

•""•"•••""•""" Playa boundary

• Well—Shallow monitoring well completed to a 

depth of 25 feet or less

R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000, 1979 

Universal Transverse Mercator projection, 

Zone 12

6 KILOMETERS

Figure 17. Approximate potentiometric surface, referenced to an average density, in the shallow-brine aquifer of the Bonneville 
between the two aquifers. The reference heads were 

determined by converting the measured water levels to 

freshwater equivalent heads. The total vertical driving 

force is then calculated by adding the gradient of reference head with the gravitational force as shown by 

equation 10. The calculated total head gradient is 

assumed to give the direction of the driving force on a 

parcel of fluid located midway between the measurement points, even though the distance between the 

points is rather large. Calculated driving forces from 

limited data from 1993 show that downward movement 

might exist during most of that year. If the direction of 

vertical movement is determined only from the gradient of the reference head, then the direction would be 

upward. When the gravitational force was added, however, the resultant direction for the total driving force 

was generally downward in 1993 when conditions on 

the playa surface were wetter than normal.

When the water level in the brine-collection 

ditch is lowered during the summer, the water level in 

well (B-l-17)23abd-l is lowered sufficiently so that the 

direction of the total driving force between the brine in 

this well and in the adjacent well, (B-l-17)23abd-2, 

completed in the underlying lacustrine sediment of the 

basin-fill aquifer, is upward. A similar condition 

occurred near the end of summer when the water level 

in well (B-l-17)31acc-l was lowered by evaporation. 

The direction of the total vertical driving force on a parcel of fluid midway between the measurement point in 

well (B-l-17)31acc-l, completed in the shallow-brine 

aquifer, and well (B-l-17)31acc-7, completed in the 

lacustrine sediment of the basin-fill aquifer, was 

upward.

The calculated downward movement of brine 

from the shallow-brine aquifer into the lacustrine sediment of the upper basin-fill aquifer might be a pressure 

response to the withdrawal of brine from deep wells 

completed in the conglomerates at depth. Despite 

water-level measurements that indicate downward 

movement, additional evidence, presented later in this 

report, indicates that over the long term there might be 

considerable upward movement of brine from the 

lacustrine sediment of the upper basin-fill aquifer into 

the shallow-brine aquifer, particularly near the center 

of the salt-crust area.

The sensitivity of the calculated downward total 

driving force was analyzed with respect to density at 

the five locations. The calculated total driving force is 

dependent on both water-level and density measurements. To test for density, water-level measurements

for the respective wells were not varied. Generally, the 

density of the brine in the wells completed in the shallow-brine aquifer would have to decrease by 0.02 to 

0.05 g/cm3 and likewise, the density of the brine in 

wells completed in the lacustrine sediment of the upper 

basin-fill aquifer would have to increase by 0.02 to 0.05 

g/cm3 in order to reverse the driving force. Density 

measurements of brine from the deeper wells do not 

vary as much as in the shallower wells. The calculated 

total driving force, therefore, is unlikely to reverse 

direction as a result of variability in density from the 

deeper zone. At times, density measurements of brine 

in wells completed in the shallow-brine aquifer have 

changed sufficiently to cause a reversal in the direction 

of the total driving force, but water-level measurements 

also have changed and thus canceled the density variation.

Recharge

Sources of recharge to the shallow-brine aquifer 

include infiltration of precipitation on the playa surface, horizontal subsurface inflow, and possibly 

upward leakage from the underlying lacustrine sediment in the upper basin-fill aquifer. Recharge from 

infiltration of precipitation on the playa surface, which 

dissolves salt from the crust, is the largest source of 

recharge. Horizontal subsurface inflow contributes 

only a small amount of recharge and salt to the shallowbrine aquifer because of low permeabilities and small 

hydraulic gradients along the eastern and northern 

boundaries of the study area. Insufficient data were 

available to quantify upward leakage. Other possible 

sources of recharge to the shallow-brine aquifer include 

subsurface inflow from the alluvial fan along the northwestern margin of the playa and surface runoff that 

reaches the playa from the Silver Island Mountains. 

Because of the direction of ground-water flow and 

hydraulic gradients, recharge from these sources is 

considered to be insignificant compared to recharge 

from infiltration of precipitation.

From hydrographs of observation wells, Turk 

(1973, p. 13) suggested that rainfall in excess of 0.05 

in. during winter and 0.1 in. during summer contributes 

to recharge in the area of the thick salt crust. Lines 

(1979, p. 85) suggested that minimal recharge occurred 

during winter months because the salt crust is often 

completely saturated and flooded on the surface. Additional winter precipitation was thought to accumulate 

at the surface until it evaporated or could infiltrate and 

replace brine that had moved laterally in the underlying

30 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
carbonate mud. Lines estimated recharge through the 

salt crust for 1976 by comparing precipitation to waterlevel changes for a monitoring well on the salt crust. 

Eighteen major recharge events were identified that 

resulted in a cumulative rise of 3.8 ft during the year. 

Lines assumed that this recharge occurred over the 

entire salt crust and that the specific yield of the aquifer 

was 10 percent, thus resulting in a total recharge of 

about 9,700 acre-ft.

Turk (1969, p. 146) states that lateral subsurface 

inflow adds some recharge to the system but is inconsequential compared to recharge from precipitation. 

Turk (1973) does not mention recharge other than from 

precipitation on the playa surface. Lines (1979, p. 84- 

86) stated that almost all recharge was from direct infiltration of precipitation with minor amounts coming 

from infiltration of runoff from the Silver Island Mountains and from subsurface inflow from beyond the limits of the study area. Lines attributed 40 acre-ft of 

recharge to subsurface inflow for 1976.

Subsurface lateral inflow is possible only from 

the east and from the northeast as shown by the potentiometric surfaces (figs. 15 and 17). During other seasons of the year when water levels in the shallow-brine 

aquifer are higher (fig. 16), hydraulic gradients along 

the northeastern and eastern margins of the study area 

were very small. Although water-level data from these 

margins are sparse, a maximum horizontal hydraulic 

gradient of 1 ft/mi could be assumed based on water 

levels measured in September 1992. When this hydraulic gradient was multiplied by a boundary length of 15 

mi and a transmissivity value of 500 ft2/d (Lines, 1979, 

fig. 33), estimated subsurface lateral inflow was 60 

acre-ft during 1992.

Observations during this study indicate that runoff from the Silver Island Mountains contributes to 

recharge along the northwestern margin of the playa. 

Although the amount of recharge is unknown, this 

source probably is not significant. The pH values of 

brine from several shallow wells along the northwestera margin of the playa are noticeably higher than the 

pH values of brine from wells that are on the playa or 

salt crust. This difference indicates that runoff from the 

Silver Island Mountains either infiltrates the alluvial 

sediments, similar to the probable recharge mechanism 

described for the alluvial-fan aquifer, or that it reaches 

the playa margin before infiltrating. Despite the horizontal hydraulic gradient toward the northwestern margin of the playa, recharge along this margin probably 

results in decreasing brine density and increasing pH

values but probably does not contribute any substantial 

amount of recharge to the playa.

Recharge during this study was mostly from 

infiltration of water through the playa surface that 

resulted from intense thunderstorms during the summer months and rainfall and snowfall during the winter 

months, and is evident from the potentiometric surfaces 

as shown (figs. 15, 16, and 17). The location of the 

ground-water divide at the center of the salt crust during September 1992 (fig. 15) was the result of transient 

drawdown in the brine-collection ditches and of 

ground-water flow toward the northwest margin of the 

playa. Because ground-water flow was away from the 

center of the salt crust, recharge to this area of the shallow-brine aquifer during the summer months was possible only by direct infiltration from the surface or 

upward leakage from the underlying lacustrine sediment. As a result of these summer thunderstorms, water 

levels are increased temporarily, thus increasing evaporation until water levels return to previous levels. 

Water that has infiltrated into the shallow-brine aquifer 

from these thunderstorms, therefore, causes only a 

short-term change in storage that offsets the total 

amount of water removed by evaporation and seepage 

to brine-collection ditches.

The potentiometric surface in April 1993 (fig. 

16) reflects the influence of recharge during the preceding winter months. Water levels are high throughout the 

center of the playa; only water levels in the southeastem corner of the study area are higher where the horizontal component of ground-water flow is toward the 

center of the playa. Along the southern and northwestern boundaries, the horizontal component of groundwater flow was away from the center of the playa. 

Although less obvious, the higher heads in the center of 

the playa are the result of infiltration from the playa 

surface, upward leakage, and possibly a small amount 

of subsurface inflow from the southeast. The potentiometric surface for August 1993 (fig. 17) also reflects the 

summer transient drawdown in the brine-collection 

ditches. The higher water levels in the central part of 

the playa were the result of infiltration of water on the 

playa surface from a large thunderstorm and diminished evaporation from the shallow-brine aquifer during the preceding months.

The bulk of recharge to the shallow-brine aquifer 

probably occurs from late autumn, when evaporation is 

negligible, to the following spring, when evaporation 

increases. Actual recharge or, more appropriately, 

actual change in storage in the shallow-brine aquifer

Shallow-Brine Aquifer 31
from any autumn to spring depends on the amount of 

brine removed through the brine-collection ditches or 

subsurface outflow, the amount of water removed by 

evaporation, and the amount of water available for infiltration from precipitation on the playa surface during 

the winter months.

An estimate for recharge was made by calculating the change in storage for the shallow-brine aquifer 

from September 1992 to late May 1993. This method 

for estimating recharge does not differentiate between 

recharge on the salt crust or carbonate mud, nor does it 

differentiate between surface and subsurface inflow. 

Subsurface inflow is assumed to be insignificant compared to infiltration from the playa surface. The change 

in storage was calculated by subtracting water-level 

measurements for September from those for May. 

Some of the May water levels were above land surface, 

thus indicating a fully saturated aquifer at that location 

and ponded water at the surface. These water levels 

were set to land surface for the change in storage calculation; otherwise, water on the surface would have been 

included. The differences between the two sets of 

water-level measurements were contoured. The area 

contoured was delineated by Interstate Highway 80 to 

the south, the 4,215-ft topographic contour along the 

northwestern margin of the playa, and an arbitrary 

north-south line 1 mi east of the easternmost part of the 

brine-collection ditch, east of the salt crust. This area is

fy

about 80 mi . The contoured volume is calculated to 

obtain the change in storage. The calculated volume 

was multiplied by a specific-yield value of 10 percent 

to obtain the estimated change in storage in the shallow-brine aquifer for September 1992 to the end of 

May 1993. The change in storage of 8,300 acre-ft was 

assumed to be equal to total recharge for that period. 

Most of this recharge probably occurs in the beginning 

of the 9-month period during which the two sets of 

water-level measurements were taken.

An areally averaged recharge rate of 0.16 ft/yr 

for this period was calculated by dividing the recharge, 

8,300 acre-ft, by the area, 80 mi2 , about 30 mi2 less 

than the area used by Lines. If average recharge is 

assumed to occur throughout the larger area, then total 

recharge would be 11,300 acre-ft. This is comparable 

to the 11,700 acre-ft estimated by Lines (1979, p. 85). 

Lines attributed 9,700 acre-ft to infiltration through the 

salt crust and 2,000 acre-ft to infiltration through the 

carbonate mud.

Infiltration of surface brine that is depleted in 

most chemical constituents other than sodium and

chloride and the lack of evaporation during the winter 

months results in the dilution of the brine in the shallow-brine aquifer with respect to many chemical constituents, most notably magnesium and potassium. 

Recharge can be estimated by calculating the quantity 

of infiltrating brine necessary to dilute potassium concentrations in brine from wells sampled in the late summer of 1992 and June 1993. The mass-balance equation 

used to describe the mixing that occurred and to make 

the recharge estimates is

where

C\ is the concentration in August 1992 (mg/L); 

<2i is the initial volume of brine with concentration 

Q (ft3 ); 

C2 is the concentration of infiltrating fluid in

January 1993 (mg/L);

Q2 is the volume of infiltrating fluid (ft ); and 

CT, is the resultant concentration in June 1993 

(mg/L).

Because the unknown is the volume of infiltrating fluid, the equation is rearranged to solve for Q2 :

(14)

The initial volume of brine containing concentration C\ was assumed to be the volume of fluid in a 

saturated column of aquifer material surrounding each ' 

well. This volume is calculated by multiplying the 

length of the column of water in each well, based on 

September 1992 water-level measurements, by 1 ft2 

representing the area of the surrounding aquifer material, and by a specific yield of 10 percent. This volume 

is substituted into equation 13 along with the appropriate concentrations (Mason and others, 1995, table 4) to 

determine the amount of fluid necessary for the measured dilution of potassium.

Within this method, estimates for recharge 

ranged from 0.01 to 1.6 ft for the 14 wells that show a 

dilution in potassium concentration. If the three 

extreme values are disregarded, the remaining 11 values range from 0.12 to 0.5 ft, with a median value of 

0.25 ft, which equates to 12,800 acre-ft of recharge in 

80 mi2 . This agrees favorably with the average value of 

0.16 ft, 8,300 acre-ft, determined by the method that 

uses the change in storage of the shallow-brine aquifer. 

Recharge, therefore, ranged from 8,300 to 12,800 acreft for 1992. Both methods assume a specific yield of 10 

percent in the shallow-brine aquifer. A larger value for

32 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
specific yield, which is quite possible, would increase 

both estimates accordingly.

Infiltration of brine from the winter surface pond 

is a dynamic process that is difficult to quantify. Data 

used in the estimates mentioned previously are localized, instantaneous measurements and might not reflect 

the constantly changing nature of the system. Water 

levels used in the estimate for change in storage might 

not represent the absolute minimum or maximum values. Similarly, chemical data from both sampling periods might not be representative of maximum and 

minimum potassium concentrations. Furthermore, the 

potassium concentration assumed to represent the infiltrating fluid (surface-pond sample, January 1993) 

might not be representative of the potassium concentration throughout the entire pond at the time of sampling 

or during the entire period in which most of the infiltration occurs.

Discharge

Primary sources of discharge from the shallowbrine aquifer include evaporation from the playa surface, withdrawal from the brine-collection ditches, and 

lateral and vertical subsurface outflow. Evaporation, 

withdrawal from the brine-collection ditch located east 

of the salt crust, and subsurface outflow to the south are 

the largest sources of discharge; the last two components are the most controllable from a management 

perspective.

Evaporation generally is the largest source of 

discharge on any playa and is very difficult to quantify. 

Any estimated rate of evaporation determined at a specific site and then applied to the entire playa surface 

might result in substantial error if that rate is not truly 

representative of all types of surfaces on the playa. 

Evaporation depends on several factors including type 

of playa surface, depth to ground-water surface, density and temperature of evaporating water, atmospheric 

conditions, and solar radiation.

Numerous techniques for estimating evaporation 

have been developed and applied to playa surfaces in 

the past. Turk (1969, p. 180-196, and 1970, p. 1209- 

1215) studied the decrease in evaporation from a brine 

surface resulting from increasing brine density, but he 

did not correlate these results to evaporation of brine 

from beneath the playa surface. Lines (1979, p. 86-89) 

stated that the amount of halite on the playa surface is 

related directly to the delicate balance between 

recharge through the surface and discharge through the 

surface by evaporation. Lines observed that about 0.25

in. of halite built up over the thick salt crust during 

1976 following evaporation of the flooded playa surface. This buildup of halite was considered to represent 

the amount of evaporation of brine from the shallowbrine aquifer (1,800 acre-ft) that exceeded recharge 

through the thick salt crust (4,900 acre-ft) during the 

summer months. Lines, therefore, estimated 6,700 

acre-ft of brine evaporated from the area of thick salt 

crust during 1976.

Estimates for evapotranspiration made in earlier 

studies, or more appropriately evaporation because of 

the sparsity of vegetation on the Bonneville Salt Flats, 

were based on water-budget methods or simple meteorological techniques. With the development of sophisticated instrumentation since the previous studies, data 

that are necessary for evaporation estimates can now be 

collected using mass-transfer and energy-balance techniques. These techniques measure, either directly or 

indirectly, the transfer of water from the liquid to gaseous phase and are independent of fluid density. Evaporation is generally referred to in terms of actual and 

potential evaporation. Actual evaporation is the amount 

of water evaporated under any given conditions, 

whereas potential evaporation is the maximum amount 

of water that could be evaporated if water was given 

freely to the evaporating surface, such as evaporation 

from a pond. The Bowen ratio method, which was used 

to estimate actual evaporation on the Bonneville Salt 

Flats, is an energy-balance method that is dependent on 

measurements of temperature and humidity gradients. 

The Penman method was used to estimate potential 

evaporation and the Bowen ratio method was used to 

estimate actual evaporation. An explanation of both 

methods, the required instrumentation, and its application in Pilot Valley are presented in Malek and others 

(1990).

Much of the playa surface of the Bonneville Salt 

Flats remains flooded during the spring and possibly 

into the early summer months. Because an overall flow 

balance for the shallow-brine aquifer is an objective of 

this report, it is important to distinguish between evaporation from standing water and from playa surfaces. 

Evaporation from the playa surface would be included 

as discharge from the shallow-brine aquifer, whereas 

evaporation from a surface pond would not be 

included.

Actual and potential evaporation for 1993 and 

1994 were determined from data collected at the two 

weather stations, located on the salt crust and playa 

margin (figs. 18 and 19). When the large surface pond

Shallow-Brine Aquifer 33
0.2

0

t M if) /! ', i [[ J | f

» 'i I' ''4l ! '* ' 'ri' 1 '); I* '^

./! '/\ !'•' I,'''•/ -VTf 1 ''* 

' Vli * **.' u ' '.i

April May June July August September October 

Figure 18. Estimated daily evaporation and precipitation for 1993, Bonneville Salt Flats, Utah.

0.8 

0.6 

0.4 

0.2 

0

was present through June of 1993, actual evaporation 

approached the potential evaporation rate at the station 

on the salt crust. Actual evaporation from a water surface is higher than from a bare land surface as can be 

seen by the difference in actual evaporation at the saltcrust station compared to the playa-margin station for 

the same period. Both actual and potential evaporation 

generally rise after precipitation has fallen on the playa 

surface (figs. 18 and 19). This is more noticeable in 

spring and early summer months when water remains 

on the surface from winter surface ponding or the nearsurface material is still nearly saturated.

Total evaporation from the shallow-brine aquifer 

in the Bonneville Salt Flats was estimated from data 

collected at the two stations during 1993 and 1994. 

Estimated total evaporation for both years might be 

representative of minimum and maximum estimates for 

evaporation from the Bonneville Salt Flats. During the 

primary months for evaporation in 1993, monthly mean 

temperatures at both stations were 2 to 8° F below the

1961-90 normal mean monthly temperature at Wendo ver. Total precipitation for the same months in 1993 

was virtually the same as the total 1961-90 normal 

monthly precipitation at Wendover. A large, single-day 

rainfall of more than 1.3 in. measured at the salt-crust 

station, however, was more than four times the normal 

monthly precipitation for July. This storm resulted in 

surface ponding shortly after the extensive surface 

pond remaining from winter had evaporated. Total 

evaporation from the shallow-brine aquifer in 1993, 

therefore, is smaller than in most years. Total evaporation from the shallow-brine aquifer in 1994, in contrast, 

is probably larger than in most years.

The area of the Bonneville Salt Flats from which 

total evaporation from the shallow-brine aquifer was 

estimated is larger than the area used in estimating 

recharge. Both areas are bounded on the south by Interstate Highway 80 and have the same arbitrary boundary 

to the east of the brine-collection ditch. The northwest

34 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Measured precipitation 

Actual evaporation 

Potential evaporation

April May June July August 

Figure 19. Estimated daily evaporation and precipitation for 1994, Bonneville Salt Flats, Utah.

boundary used for delineating evaporation is at the 

playa margin, whereas the boundary used in estimating 

recharge was limited to the area of measured seasonal 

change in storage in the shallow-brine aquifer. The area 

used for estimating evaporation was about 114 mi2, and 

the area used for estimating recharge was about 80 mi2. 

No water-level or chemical data were available to 

extend the area used in the recharge estimates to coincide with the area used in the evaporation estimate.

Net evaporation from the shallow-brine aquifer 

for each season was calculated at the two weather stations. The daily mean values for actual evaporation 

were totaled and total rainfall for the same period of 

time was subtracted. Evaporation of water on the playa 

surface from rainfall is subtracted from total evaporation because it is not part of the evaporation from the 

shallow-brine aquifer. If any rain falling on the playa 

surface does infiltrate into the subsurface, it is assumed 

to recirculate to the surface and evaporate in a short 

period of time. A net actual evaporation rate was then

calculated by dividing the total net evaporation by the 

number of days in which data were collected during 

each year.

The number of days is critical for calculating a 

net actual evaporation rate from the shallow-brine aquifer at the salt-crust station. Because actual evaporation 

is higher when a surface pond is present on the playa 

surface than when a surface pond is not present, only 

those days after the surface pond has evaporated should 

be used in the calculation for total evaporation from the 

shallow-brine aquifer. Net actual evaporation rates at 

the salt-crust station for 1993 and 1994 were 0.0006 

ft/d and 0.0018 ft/d, respectively. Net actual evaporation rates at the playa-margin station for 1993 and 1994 

were 0.0013 ft/d and 0.0017 ft/d, respectively. Net 

actual evaporation rate for 1993 at the salt-crust station 

is considerably lower than at the playa-margin station, 

and both stations have a similar rate for 1994. The large 

difference in net actual evaporation rates is the result of

Shallow-Brine Aquifer 35
the large amount of rainfall on the salt crust in July 

1993 (fig. 18) and lower-than-normal temperatures.

Net actual evaporation rates were not adjusted in 

response to varying depths to water. Net actual evaporation rates were assumed to apply to a uniform depth 

of water despite the fact that evaporation declines with 

increasing depth to water. At the salt-crust station, the 

ground-water level was close to land surface, whereas 

at the playa-margin station, the ground-water level can 

be several feet below land surface. In areas where the 

water beneath the carbonate mud is close to land surface, the evaporation rate from this surface could be 

much higher than the rate calculated at the playa-margin station. In areas where the depth to water was more 

than that at the two stations, such as near the brine-collection ditches, actual evaporation would be less than 

what was estimated. During the evaporation of the surface pond, no upward leakage of brine from the shallow-brine aquifer was assumed. If this movement 

occurs, estimated total evaporation is less than what 

actually occurs.

Total evaporation from the shallow-brine aquifer 

was estimated for each year by applying different net 

actual evaporation rates to the three different playa surfaces: thick salt, carbonate mud, and thin salt over carbonate mud. The area of thick salt is defined as the area 

where the salt-crust thickness is more than 1 ft (Steven 

Brooks, Bureau of Land Management, written commun., 1989). This area is assumed to remain constant 

for the years in which estimates were made and was 

calculated to be 34 mi2. Because of the transient nature 

of thin salt precipitated on carbonate mud, the areas 

used to represent carbonate mud and thin salt over carbonate mud varied in each year. Carbonate mud and 

salt crust were delineated from satellite imagery for 

August 1993. The area of thick salt was subtracted 

from the total salt-crust area in 1993 to obtain the area 

of thin salt. The appropriate rates calculated previously 

were used where thick salt and carbonate mud are 

present.

In their study of evaporation in Pilot Valley, 

Utah, Malek, and others (1990, p. 23) suggest that 

where a thin salt crust is present there is a high resistance to water-vapor transfer; thus, evaporation is less 

than where a bare mud surface is present. This is especially true when the crust separates from the underlying 

mud surface upon drying and subsequent shrinkage. 

Because of the transient distribution of salt along the 

margins of the crust, a long-term data-collection station 

could not be established where a perennial layer of thin

salt would form. A net actual evaporation rate, therefore, had to be assumed for each year and applied to the 

area where a thin salt crust was present.

Evaporation from the shallow-brine aquifer in 

the area of thick salt was estimated by multiplying 34 

mi2 by a rate of 0.0006 ft/d for 114 days. This number 

of days is the length of the evaporation season in which 

the area of thick salt was not covered by a pond. Evaporation from thick salt in 1993 was estimated to be 

1,500 acre-ft. The area for carbonate mud in 1993 was 

56 mi2. This area was multiplied by a rate of 0.0013 

ft/d for 185 days, the assumed duration of the evaporation season. Evaporation from the carbonate-mud surface was estimated to be 8,600 acre-ft. The area of thin

r*

salt was 24 mi , which was larger than in most years as 

a result of the extensive surface pond during the preceding winter months. An intermediate net actual evaporation rate of 0.0009 ft/d was assumed for the area of 

thin salt. This rate was deemed appropriate if rainfall 

was assumed to be between the amounts measured at 

the two weather stations. Evaporation from thin salt 

was estimated to be 2,100 acre-ft during 149 days of 

evaporation. Total evaporation for 1993 from the shallow-brine aquifer was estimated to be 12,200 acre-ft.

No satellite imagery data for 1994 were 

obtained; therefore, the area for carbonate mud and thin 

salt on the playa surface could not be delineated for that 

period. For the purpose of estimating total evaporation 

in 1994, the distribution of thin salt in September 1992, 

obtained from satellite imagery data for that period, 

was assumed to be more representative of 1994 conditions than the thin-salt distribution in 1993. Much of 

the thin salt that covered a large area in 1993 probably 

redissolved during the winter months and coalesced 

into a smaller salt-crust area by the summer of 1994, 

thus decreasing the area of thin salt.

The method used to estimate total evaporation 

from the shallow-brine aquifer in 1994 was the same as 

that used in 1993. The higher rate derived from data 

collected at the salt-crust station was used for 138 days 

of evaporation. Estimated evaporation from thick salt 

was 5,400 acre-ft. Evaporation from the carbonate-mud 

surface was calculated for an area of 71 mi for 185 

days. Evaporation from the carbonate mud was estimated to be 14,300 acre-ft. As mentioned previously, 

the actual evaporation rate from thin salt overlying the 

carbonate mud is probably lower than that for carbonate mud. A net actual evaporation rate of 0.0011 ft/d 

was assumed to be representative for 1994. This rate 

was multiplied by an area of 9 mi2 for a period of 150

36 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
days, which was assumed to be an intermediate number 

of days for evaporation from thin salt. Evaporation 

from thin salt was estimated to be 1,000 acre-ft. Total 

evaporation from the shallow-brine aquifer in 1994 was 

estimated to be 20,700 acre-ft.

Withdrawal from the brine-collection ditch east 

of the salt crust is a major form of discharge from the 

shallow-brine aquifer as shown by the potentiometric 

contours in figure 16. Withdrawal from the brine-collection ditch was estimated to be about 1,500 acre-ft in 

1992 and about 2,500 acre-ft in 1993. About 750 acreft of the amount estimated for 1993 were withdrawn 

during January through March, when more than the 

usual area of the Bonneville Salt Flats was flooded. 

Much of the estimated 750 acre-ft, therefore, could 

have been derived from surface drainage from the 

extensive surface pond south of Interstate Highway 80, 

where berms are less prominent, rather than from the 

shallow-brine aquifer. During 1993, this brine withdrawal from the shallow-brine aquifer probably was 

more than 1,750 acre-ft. The estimates for brine withdrawal in 1992 and 1993 are considerably more than 

the 680 acre-ft reported by Lines (1979, table 6).

Brine withdrawal from the brine-collection ditch 

was estimated by monitoring the pumping station 

located in the NW1/4, SW1/4, and SE1/4 of section 18, 

T. 1 S., R. 17 W. (fig. 3). Because of the lack of topographic relief on the Bonneville Salt Flats, canals do 

not have a natural slope and a head gradient is necessary in order to induce brine to flow to the primary 

evaporation ponds where the potash extraction process 

is begun. At the pumping station, the brine is pumped 

from a lower to a higher altitude, thus creating the artificial gradient. The level from which the brine was 

pumped was continuously recorded. Instantaneous discharge measurements were made periodically at the 

pump and correlated to the pumping level. From the 

relation of pump discharge to pumping level, the continuous record of pumping level, and hours of pump 

operation, an estimate of brine withdrawal can be 

obtained.

Lateral subsurface outflow from the shallowbrine aquifer to the south potentially might be another 

major cause of discharge. Lines (1979, p. 89) estimated 

that 2,000 acre-ft of brine migrated from the Bonneville Salt Flats north of Interstate Highway 80 to the collection ditch south of the highway in 1976. His estimate 

was made by using a form of Darcy's Law

Q = TiL (15)

where

Q is the ground-water flow (ft3/d);

T is the transmissivity (ft2/d);

/ is the horizontal hydraulic gradient 

(dimensionless); 

and

L is the length of cross section (ft).

The two components with the most uncertainty 

in this equation are transmissivity and horizontal 

hydraulic gradient. Transmissivity values are determined at specific sites and then extrapolated over a 

larger area. Similarly, horizontal hydraulic gradient 

might vary along the length of the cross section being 

considered. Lines used a transmissivity value of 6,000 

ft2/d that was based on values determined from aquifer 

tests completed prior to construction of Interstate Highway 80 and an aquifer test completed just north of the 

highway after construction. Turk (1978, p. 75) has suggested that highway construction probably has reduced 

the transmissivity of the aquifer material in this area. 

During the construction, the salt crust was removed 

along the highway corridor and replaced with fill for a 

roadbed. Not only was the highly transmissive salt 

removed, but the use of heavy equipment might have 

compacted the underlying carbonate mud and thus 

destroyed much of the fracture permeability of the 

aquifer. No new aquifer-test data were collected along 

the Interstate Highway 80 corridor during this study 

because of possible uncertainty in test results. Pumping 

of brine in sufficient amounts necessary to obtain measurable effects in appropriately spaced observation 

wells was questionable. Furthermore, disposal of 

pumped brine at a sufficient distance necessary to prevent adverse effects in observation wells probably was 

not possible in this area because of the low topographic 

relief.

Subsurface outflow to the south was estimated 

where the hydraulic gradient indicated flow toward a 

brine-collection ditch located south of Interstate Highway 80. A vertical cross section oriented parallel to the 

brine-collection ditch was divided into three sections. 

This division allows the use of a spatial variation in 

transmissivity and a spatial and temporal variation in 

hydraulic gradient in the calculation of flow through 

the cross section. Transmissivity values presented by 

Lines (1979, fig. 33) were used in the calculations 

despite the questions presented previously. A hydraulic 

gradient was determined for each of the three cross sections from water-level data collected during this study 

and computed potentiometric heads referenced to an 

average fluid density for each measurement period. The

Shallow-Brine Aquifer 37
estimated subsurface outflow to the south for 1992 

ranged from 1,800 to 2,300 acre-ft. A range is reported 

because of the variability in each hydraulic gradient 

used in the calculations. An estimate was not made for 

1993 because the wet condition of the playa and high 

water levels are not representative of long-term 

ground-water flow. Any decrease in simulated transmissive properties of the shallow-brine aquifer in this 

area would decrease the amount of subsurface outflow. 

Seepage to the brine-collection ditch located south of 

Interstate Highway 80 is needed to obtain a better 

understanding of this component of subsurface outflow. Although this measurement would be difficult 

because of the dimensions of the ditch and the low flow 

velocity, the uncertainty in the measurement probably 

would be less than the uncertainty in the estimate given 

above. Flow in this brine-collection ditch, which is on 

private property, could not be measured because of its 

inaccessibility.

Lateral subsurface outflow to the northwest from 

the shallow-brine aquifer was estimated by Lines 

(1979, p. 90) to be 70 acre-ft in 1976. Lines made this 

estimate by using Darcy's Law and an estimated transmissivity value of 500 ft2/d, a value that is representative of the area along the northwestern edge of the salt 

crust. Lines stated that recharge from precipitation balanced discharge from evaporation along this margin. 

Seepage from the shallow-brine aquifer toward the 

northwestern margin of the playa, and possibly to the 

alluvial-fan aquifer, therefore, was replaced by brine 

that had migrated through the aquifer from near the 

ground-water divide (fig. 15). Because the hydraulic 

gradient is small along this margin of the playa, the 

amount of subsurface outflow also is small. Water-level 

data collected during this study indicate that the 

hydraulic gradient is less than the value used by Lines 

in his estimate for seepage to the northwest. If the 

transmissivity value is assumed to be representative of 

this area, then seepage toward the northwestern margin 

of the playa was about 40 acre-ft for 1992.

Vertical leakage, upward or downward, between 

the shallow-brine aquifer and the underlying lacustrine 

sediment of the basin-fill aquifer had not been examined in any of the previous studies. The vertical-driving-force analysis, presented in the "Water-level 

fluctuation, potentiometric surface, and ground-water 

movement" section in this report for the shallow-brine 

aquifer, was extended to estimate vertical leakage 

through the bottom of the shallow-brine aquifer. Equation 12, which is described in the "Principles of variable-density ground-water flow" section of this report,

was used to calculate the vertical interstitial velocity at 

five locations where head data were available in both 

the shallow-brine aquifer and the underlying lacustrine 

sediment of the basin-fill aquifer. An average vertical 

interstitial velocity was multiplied by the area where 

leakage was assumed to occur. The calculated vertical 

interstitial velocity at wells (B-l-17)23abd-l and -2 

was not included in the estimate for downward leakage 

because of the local bias from the influence of the ditch. 

Any estimate of total vertical leakage must be considered to be a crude approximation because of the simplifying assumptions necessary in making the 

calculations, discussed in the following paragraphs, 

and extrapolating these calculations over a large areal 

extent using data that are limited spatially and temporally.

Permeability in the vertical direction used in the 

calculation for interstitial velocity was the harmonic 

mean of estimated permeabilities for the shallow-brine 

aquifer and the underlying lacustrine sediment of the 

basin-fill aquifer because the vertical direction is generally perpendicular to stratification, and vertical 

movement will be controlled by the smallest permeability. Permeability for the shallow-brine aquifer at 

each site was calculated by converting the transmissivity reported by Lines (1979, fig. 33) to hydraulic conductivity and then permeability. Hydraulic 

conductivity, which was obtained by dividing the transmissivity by the aquifer thickness of 25 ft, was multiplied by viscosity and divided by density of the brine at 

each site and by the gravity constant. Viscosity was 

assumed to be equal to 1.4 centipoise for all calculations. No data were available for describing the permeability of the lacustrine sediment of the basin-fill 

aquifer. Permeability, therefore, was assumed to range 

from 1.0 x 10' 16 to 1.0 x 10' 14 ft2, the upper range for 

unweathered marine clay and the lower range for silty 

sand (Freeze and Cherry, 1979, p. 29).

Vertical interstitial velocity was calculated for 

each set of measurements at the four well sites along 

the axis of the salt crust. Calculated interstitial velocities in a downward direction range from 3.0 x 10 to 

2.05 x 10"5 ft/d when the permeability for the lacustrine 

sediment of the basin-fill aquifer was assumed to be 1.0 

x 10" 16 ft2 . The average downward interstitial velocity 

is 7.32 x 10'6 ft/d. If a permeability of 1.0 x 10" 14 ft2 is 

assumed for the lacustrine sediment, the calculated 

interstitial velocities range from 3.0 x 10"4 to 2.05 x 

10'3 ft/d, with an average velocity of 7.32 x 10"4 ft/d. A 

linear relation clearly exists between assumed perme38 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
ability of the lacustrine sediment and the calculated 

interstitial velocities because the smallest permeability 

value predominates in the calculation of the harmonic 

mean. Total annual downward leakage was estimated 

by multiplying the average interstitial velocity by area 

and 365 days. The area of 34 mi2 for which leakage 

might occur was assumed to be the area where the salt 

crust is more than 1 ft thick. Calculated annual leakage 

ranges from 58 to 5,800 acre-ft in the downward direction. The upper limit is probably too large when compared to other components of inflow and outflow for the 

shallow-brine aquifer. This is especially apparent when 

other evidence supporting upward leakage is presented 

later in the report. The estimated annual downward 

leakage, therefore, was assumed to range from 58 to 

580 acre-ft.

Alluvial-Fan Aquifer

The alluvial-fan aquifer in the Bonneville Salt 

Flats is composed of sand and gravel interbedded with 

the lacustrine sediment that underlies the playa. Specific layers of sand and gravel in the alluvial-fan aquifer 

gradually become mixed with silt and clay-sized particles as the fans extend into the lacustrine sediment. The 

alluvial fans generally are connected along the southeastern flank of the Silver Island Mountains. The extent 

of interbedding into the lacustrine sediment and the 

degree of connection within the alluvial-fan aquifer as 

distance increases from the mountains are unknown.

The alluvial-fan aquifer has been tapped by 27 

wells that withdraw brackish water. This water is used 

in the production of potash. Not all the wells are in 

operation at present. In recent years, only four wells 

have been used. Drillers' logs, which are available for 

12 of these production wells (Stephens, 1974, p. 46- 

47), indicate that sand and gravel layers compose from 

12 to 52 percent of the material penetrated during drilling. This variation occurs in relatively short distances 

between wells. All wells were drilled about the same 

distance from the Silver Island Mountains along a line 

about 3 mi long.

Fourteen monitoring wells, of which 11 were 

nested in 4 boreholes, were completed during this study 

along a line perpendicular to the Silver Island Mountains and the playa margin (fig. 3). This line of wells 

includes an alluvial-fan production well, (B-l18)31 ace-1, that was drilled in 1947. The purposes of 

these monitoring wells were to analyze the effects of 

ground-water withdrawal from the alluvial-fan aquifer,

monitor the water level in the alluvial-fan aquifer, and 

obtain water from the alluvial-fan aquifer for chemical 

analysis.

Aquifer Properties

Results of aquifer tests at two wells completed in 

the alluvial-fan aquifer were reported by Turk (1969, p. 

67, 1973, p. 3). Transmissivity values ranged from 

24,000 to 52,000 ft2/d, and storage-coefficient values 

ranged from 1.4 x 10"4 to 3.4 x 10"4. A longer test was 

completed in 1967. Data from this test indicated that 

leakage from confining layers yielded a substantial 

amount of water with time. Transmissivity values 

determined by using type curves for a leaky aquifer 

ranged from 21,000 to 55,000 ft2/d. Storage-coefficient 

values ranged from 2 x 10"4 to 5 x 10"4. These storagecoefficient values are indicative of confined conditions.

Stephens (1974, p. 18) suggested that the ability 

of fractures and solution channels in the underlying 

carbonate rocks to transmit water to the alluvial-fan 

aquifer might be a contributing factor for these large 

transmissivity values. Brackish water is readily produced from the production wells completed in the 

coarse-grained zones of the alluvial-fan aquifer. A 

pumping rate of more than 1,500 gal/min from a single 

well is not unusual. Lines (1979, p. 76) attributed the 

hydraulic gradient from the center of the playa toward 

the Silver Island Mountains to pumping from the alluvial-fan aquifer. If withdrawal from the alluvial-fan 

aquifer is the cause of this hydraulic gradient, then the 

effects of this pumping extend a considerable distance 

beyond the pumping center to the northeast, along the 

playa margin (Lines, 1979, fig. 35). This suggests that 

the underlying carbonate rocks do not readily yield 

water to the alluvial-fan aquifer, as originally thought.

An aquifer test was done in an attempt to define 

further the aquifer properties of the alluvial-fan aquifer, 

determine the distance from the pumped well in which 

measurable effects could be observed in the lacustrine 

sediment that separates the shallow-brine and alluvialfan aquifers, and determine the vertical hydraulic conductivity of the lacustrine sediment where such effects 

are measured. An unused alluvial-fan production well, 

well (B-1-18)31 ace-1 (identified as FW-20 in earlier 

reports), was pumped for 72 hours at a rate of 1,500 

gal/min. Water levels were measured in 14 monitoring 

wells prior to the test to determine pre-test trends, during the 72-hour pumping period, and for 72 hours after 

pumping had ceased.

Alluvial-Fan Aquifer 39
Analysis of the data indicates that the discharge 

water in the vicinity of the pumped well had moved 

rapidly downward through probable fractures in the 

fine-grained sediment that overlies the coarse-grained 

material of the alluvial-fan aquifer. The artificial 

recharge created from the well discharge affected the 

water levels in two monitoring wells completed in or 

just above the production zone. Because of the interference from the discharge water, the transmissivity and 

storage-coefficient values calculated from the data are 

unreliable. These values, however, are in agreement 

with those determined from previous tests of the alluvial-fan aquifer. Aquifer properties determined from 

tests in earlier studies also might not be representative 

if the discharge water was not drained away from monitoring wells.

Water-level measurements in an adjacent monitoring well, completed in fine-grained lacustrine sediment almost 100 ft above the production zone, were 

analyzed by using the Neuman-Witherspoon ratio 

method (Neuman and Witherspoon, 1972). The value 

for vertical hydraulic conductivity determined from 

these data is almost five orders of magnitude larger than 

the value determined by laboratory analysis of a core 

sample collected during drilling operations. Because 

the laboratory value for vertical hydraulic conductivity 

was small and the time required for the water level in 

adjacent monitoring wells to be affected was short, the 

probable mechanism for transmitting the discharge 

water downward is through vertical fractures. Because 

of the rapid infiltration of discharge water, pumping 

from the production well was insufficient to affect the 

water level in the deep monitoring wells toward the 

playa.

Water-Level Fluctuation and Ground-Water 

Movement

Water levels in wells completed in the alluvialfan aquifer on the margin of the Bonneville Salt Flats 

were reported to be 10-20 ft above land surface when 

the production wells were drilled between 1946 and 

1949 (Stephens, 1974, p. 44-45). At the end of 1965, 

Turk (1969, p. 74) reported that water levels had 

declined to 18-20 ft below land surface. Stephens 

(1974, p. 16) reported that at least one of the wells was 

again flowing in 1972. Water levels in two wells also 

were slightly above land surface in March 1985 (fig. 

20). Present pumped water levels are generally more 

than 20 ft below land surface, and nonpumped water 

levels measured in the unused production wells northeast of the pumping area remain at about 20 ft below 

land surface. Long-term water-level fluctuations in the 

alluvial-fan aquifer are probably the result of the combined effects of reduced withdrawal and increased 

recharge during years of greater-than-normal precipitation.

Water levels in wells completed in the alluvialfan aquifer were at or above land surface when the 

wells were drilled. The potentiometric surface, based 

on resident densities, sloped toward the playa. Because 

of continued withdrawal from the alluvial-fan aquifer, 

water levels have declined below land surface. During 

the present study, the potentiometric surface appears to 

be nearly level with the playa.

Water levels were measured in the line of nested 

monitoring wells, perpendicular to the northwestern 

margin of the playa (fig. 3). These water levels were 

converted to heads on the basis of resident densities in 

each well to determine horizontal and vertical driving 

forces. The upper monitoring wells in the three nests of 

wells, farthest from the playa boundary, are completed 

in the lacustrine sediment of the upper basin-fill aquifer. These wells are included in this discussion because 

they show effects on water levels that result from 

pumping from the alluvial-fan aquifer. Water-level 

measurements for wells at similar depths were referenced to an average density to determine reference 

potentiometric heads. The vertical driving forces were 

calculated using equation 10, described in the "Principles of variable-density ground-water flow" section in 

this report, and water-level and density data from each 

set of nested wells.

Based on the reference potentiometric head distribution, the direction of horizontal flow from the set 

of nested wells closest to the Silver Island Mountains, 

wells (B-l-18)31bda-l, -2, and -3, is toward the salt 

crust. Horizontal flow in the three sets of nested wells 

closest to the salt crust, wells (B-l-18)31dac-l, -2, -3, 

(C-l-18)6abb-l, -2, -3, and (C-l-18)6adc-l and -2, is 

toward the Silver Island Mountains.

As would be expected in a recharge area, the calculated vertical driving forces are downward in the set 

of nested wells closest to the Silver Island Mountains. 

The calculated vertical driving forces also are mostly 

downward in the three sets of nested wells closest to the 

salt crust. The vertical driving force reverses periodically between the upper two wells of the nested set containing wells (B-l-18)31dac-l, -2, and -3. The reversal 

in the vertical driving force for these wells is the result 

of a change in density in water in well (B-l-18)31dac40 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
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Figure 20. Water-level fluctuations in wells completed in the alluvial-fan aquifer, Bonneville Salt Flats, Utah.

2, which is completed at an intermediate depth in relation to the other two wells. The water in this well is 

more dense than water in either of the other two wells 

in this nest. Density in water from well (B-1 -18)31 daco

2 varies from 1.019 to 1.029 g/cm . Density in water 

from wells (B-l-18)31dac-l and -3 was consistently 

1.003 g/cm3 (Mason and others, 1995, table 3). When 

the density of water in well (B-1 -18)31 dac-2 was about

o

1.027 g/cm or more, the vertical driving force was 

upward between wells (B-l-18)31 dac-1 and -2. The 

vertical driving force between wells (B-l-18)31dac-2 

and -3, however, remains downward.

Sensitivity in the direction of the vertical driving 

force between wells (C-l-18)6abb-l, -2, and -3 varies 

with the density of water. The density of water in well

(C-l-18)6abb-l would have to decrease by more than 

0.04 g/cm3 to reverse the direction of the vertical driving force between wells (C-l-18)6abb-l and -2. The 

density of water in well (C-l-18)6abb-2 would have to 

increase by more than 0.05 g/cm3 to reverse the direction of the vertical driving force between wells (C-l18)6abb-l and -2, whereas a decrease of 0.008 g/cm3 

would reverse the direction of the vertical driving force 

between wells (C-l-18)6abb-2 and -3. The density of 

water in well (C-l-18)6abb-3 would have to increase

o

by 0.01 g/cm to reverse the direction of the vertical 

driving force between wells (C-l-18)6abb-2 and -3. 

Density values measured in water from these wells do 

not vary to this degree (Mason and others, 1995, table

Alluvial-Fan Aquifer 41
3). The direction of the vertical driving force, therefore, 

is unlikely to change because of density.

Similar to the wells previously discussed, density of water from wells (C-l-18)6adc-l and -2 would 

have to change considerably from measured values 

(Mason and others, 1995, table 3) to reverse the direction of the vertical driving force. The density of water 

in well (C-l-18)6adc-l would have to decrease by

o

more than 0.04 g/cm and the density of water in well 

(C-l-18)6adc-2 would have to increase by more than 

0.04 g/cm3 .

The horizontal and vertical driving forces toward 

the production zone of the alluvial-fan aquifer are the 

result of pumping and the induced drawdown in potentiometric head in the aquifer. Driving forces in the three 

nests of wells between the production well and the salt 

crust probably would be mostly toward the salt crust 

and upward if potentiometric head in the alluvial-fan 

aquifer were at the pre-development level. Groundwater flow toward the shallow-brine aquifer probably is 

minimal because of the low permeability of the lacustrine sediment that separates the coarse-grained material of the alluvial-fan aquifer from the shallow-brine 

aquifer and the small driving force.

Recharge and Discharge

Probable sources of recharge to the alluvial-fan 

aquifer include surface runoff from the Silver Island 

Mountains and subsequent downward infiltration 

through fractures in the overlying lacustrine sediment, 

subsurface flow from the consolidated rocks in the Silver Island Mountains, and subsurface flow from the 

lacustrine sediment along the playa margin. Results of 

the aquifer tests mentioned in the previous section indicate that surface runoff and subsequent infiltration 

through fractures probably contributes more to the 

recharge of the alluvial-fan aquifer than thought previously. From field inspection of the alluvial-fan surface, 

numerous channels that provide avenues for surface 

runoff are visible. These channels terminate at pits that 

are the surface expression of the fractures (fig. 21). No 

data are available to quantify recharge by seepage 

through these fractures. If this is the principal form of 

recharge to the alluvial-fan aquifer, then the amount of 

recharge could be a large percentage of the precipitation from intense thunderstorms. This type of precipitation is most likely to produce surface runoff.

It is unknown whether this form of recharge to 

the alluvial-fan aquifer was ongoing prior to groundwater development or whether ground-water developFigure 21. Large surface fracture in vegetation zone 

along northwestern margin of the playa, Bonneville Salt 

Flats, Utah.

ment with the declining water levels might have caused 

the fracturing of the lacustrine sediment. If these fractures had existed prior to development of the aquifer 

and if these fractures actually reach the coarser material of the alluvial fan, then the areas where the fractures are visible at the surface should have visible 

evidence of prior spring discharge. Dried organic material is present in one area, thus indicating prior surface 

discharge.

Stephens (1974, p. 10) reported that about 800 

acre-ft/yr recharges the alluvial-fan aquifer along the 

east flank of the Silver Island Mountains adjacent to the 

Bonneville Salt Flats. This estimate is derived by 

assuming that 8 percent of the average yearly precipitation that falls on the east flank of the Silver Island 

Mountains infiltrates into the consolidated rock and 

alluvium and eventually reaches the alluvial-fan aquifer. Turk (1973, p. 4) did not quantify recharge to the

42 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
alluvial-fan aquifer, but did suggest that small amounts 

of rainfall on the Silver Island Mountains preclude 

abundant recharge from precipitation. Turk suggested 

that other sources of recharge include lateral leakage of 

brine from the lacustrine sediment or upward leakage 

from Paleozoic carbonate rocks. Turk (1969, p. 76 and 

1973, p. 4) cited increased dissolved-solids concentration as evidence to support leakage of brine from the 

lacustrine sediment. Elevated water temperature also 

was cited by Turk (1973, p. 4) as evidence to support 

upward leakage from a depth of 1,000 to 2,000 ft.

Stephens (1974, p. 19) estimated discharge by 

evapotranspiration from phreatophytes to be about 

1,200 acre-ft/yr. Lines (1979, p. 63) reported that 800 

acre-ft/yr of water was discharged by pumping. These 

two forms of discharge result in a considerable imbalance in the estimated inflows and outflows for the alluvial-fan aquifer. Lines suggested that the deficit 

between recharge and discharge was compensated by 

water being removed from storage in the aquifer and 

confining layers, leakage of brine from the lacustrine 

sediment resulting from the reversal of the hydraulic 

gradient, and by upward leakage from Paleozoic rocks. 

Although no data are available to quantify leakage 

from each source, Lines suggested that total leakage is 

less than the 800 acre-ft/yr of withdrawal.

Recent discharge from the alluvial-fan aquifer by 

pumping probably is much more than the 800 acre-ft/yr 

reported by Lines (1979, p. 63). Limited data collected 

during 1991-92 indicate that the present rate of withdrawal might be a few thousand acre-ft/yr. With discharge probably much more than previously reported 

and water levels not substantially lower than those of 

30 years ago, recharge to the alluvial-fan aquifer must 

be larger than the 800 acre-ft/yr previously estimated 

(Stephens, 1974, table 3).

Basin-Fill Aquifer

The basin-fill aquifer of the Bonneville Salt Flats 

has been defined in previous investigations as the conglomerate that overlies Tertiary-age volcanic rocks or 

pre-Tertiary-age rocks (Lines, 1979, p. 57). More than 

800 ft of conglomerate has been logged during drilling 

of some deep brine wells used by the mineral company. 

The brine that is withdrawn by these deep wells is used 

mostly in ditches surrounding the evaporation ponds. A 

higher head is maintained in these ditches than in the 

evaporation ponds, which minimizes seepage losses of 

the valuable brine from the ponds (Turk, 1973, p. 5). A

thick sequence of lacustrine sediment separates the 

shallow-brine aquifer from the conglomerate. Drillers' 

logs for the deep brine wells indicate that about 900 ft 

of mostly clay exists between the shallow-brine aquifer 

and the conglomerate of the lower part of the basin-fill 

aquifer (Turk, 1973, appendix B).

Few data are available that describe the hydrologic properties of conglomerate in the lower part of 

the basin-fill aquifer of the Bonneville Salt Flats. Most 

of the data available has been collected by the mineral 

company in the area south of Interstate Highway 80 

and has been reported by Turk (1969 and 1973). More 

recent data, such as water levels and pumping rates, 

were not available. Data collected during this study 

from monitoring wells completed in the lacustrine sediment of the upper basin-fill aquifer are from a limited 

number of sites and are not necessarily representative 

of the lacustrine sediment in the upper basin-fill aquifer 

throughout the Bonneville Salt Flats.

Aquifer Properties

The transmissivity of the conglomerate in the 

lower part of the basin-fill aquifer of the Bonneville 

Salt Flats has been determined from pumping data in 

the deep brine wells operated by the mineral company 

to average about 13,400 ft2/d, and the storage coefficient is about 4 x 10"4 (Stephens, 1974, p. 21). These 

values were provided by personnel of the mineral company. Turk (1969, p. 93) had suggested that the most 

likely range for transmissivity is 2,000 to 8,000 ft2/d 

and for storage coefficient is 2 x 10 to 2 x 10 . A 

regional gravity survey by Cook and others (1964, pi. 

1) indicates that the area where the deep brine wells are 

located coincides with the deepest part of the basin; 

therefore, thickness and transmissivity might not be as 

great in other parts of the basin.

During this study, monitoring wells were completed in the lacustrine sediment of the upper basin-fill 

aquifer in the Bonneville Salt Rats. No fracturing or 

layers of oolitic sand were encountered during drilling 

of completed wells. Cores were collected at depths 

from 48 to 63 ft at two locations on the Bonneville Salt 

Flats. The cores appear to be composed of blue-green 

clay that varied from being very stiff and dry to very 

plastic with interstitial fluid. No tests were done to estimate the permeability of the lacustrine sediment. For a 

discussion of assumed permeability, see "Discharge" 

section of the "Shallow-brine aquifer" section of this 

report, presented earlier. Samples from the cores were 

analyzed by x-ray diffraction techniques. The mineralBasin-Fill Aquifer 43
ogic composition of the lacustrine sediment consists of 

clay, quartz, halite, aragonite, calcite, dolomite, and 

feldspar.

The upper monitoring wells in the three sets of 

nested wells, between the playa boundary and the salt 

crust, also were completed in lacustrine sediment of the 

upper part of the basin-fill aquifer. Because of the proximity to alluvial-fan deposits, the lacustrine sediment 

might have layers of slightly coarser material intermixed with the clay. In this area, the hydraulic properties of these deposits might be greater than those of the 

lacustrine sediment beneath the salt crust.

Water-Level Fluctuation and Ground-Water 

Movement

Water-level data are lacking from wells completed in the conglomerate of the lower part of the 

basin-fill aquifer. Turk (1973, p. 8) reported that the 

water level in a deep well was about 25 ft below land 

surface in 1948. Measurements in two deep wells during 1966-67 varied from 44 to 50 ft below land surface 

(Turk, 1973, table 7). This change in water level 

equates to an average decline of about 1 ft/yr for that 

period of time. Lines (1979, p. 58) reported that water 

levels affected by pumping in nearby wells were lowered as much as 80 ft below land surface in 1976. 

Because of the limited water-level data, the potentiometric surface of the lower part of the basin-fill aquifer 

cannot be determined.

Water-level data are limited spatially for wells 

completed in the lacustrine sediment of the upper part 

of the basin-fill aquifer. Water levels were measured in 

wells (B-l-17)llaac-2, (B-l-17)21add-4, (B-l17)23abd-2, (B-l-17)31acc-7, and (C-l-18)9adc-2, 

which are all completed to depths of 63 ft below land 

surface (Mason and others, 1995, table 3). The lowest 

water levels were measured in well (B-l-17)23abd-2, 

located adjacent to the brine-collection ditch. These 

data indicate that there is a horizontal gradient within 

the lacustrine sediment in the upper part of the basinfill aquifer, from the salt crust toward the east. Data are 

insufficient to determine whether this direction of 

ground-water movement has been induced by withdrawals from the brine-collection ditch. Water levels in 

wells (B-l-17)llaac-2 and (B-l-17)21add-4, located 

near the northern end of the salt crust, are higher than 

those in wells (B-l-17)31acc-7 and (C-l-18)9adc-2, 

located near the center and southern end of the salt 

crust. These data indicate that the direction of groundwater movement within the lacustrine sediment

beneath the shallow-brine aquifer is from the north 

toward the south, beneath the center of the salt crust.

Water levels were measured in nested wells, (Bl-17)31acc-4, -5, -6, and -7, located near the center of 

the salt crust. These wells were completed to depths of 

63 to 495 ft below land surface, all within the lacustrine 

sediment of the upper basin-fill aquifer. These data 

were used to calculate the total driving force between 

wells for each measurement period. Although the calculated total driving force was small for each set of 

measurements, it was always downward. Because the 

Bonneville Salt Flats is the terminus for ground-water 

flow for most of the Great Salt Lake Desert, and probably the terminus for regional ground-water flow, 

ground-water movement within the basin-fill aquifer 

should be upward under undisturbed conditions.

Recharge and Discharge

Possible sources of recharge to the conglomerate 

in the lower part of the basin-fill aquifer of the Bonneville Salt Flats include the alluvial-fan aquifer on the 

western margin of the playa and consolidated rock at 

depth as part of a much larger regional ground-water 

flow system. If ground-water withdrawals from the 

alluvial-fan aquifer were to increase, some of the water 

that might have been available for recharge to the lower 

part of the basin-fill aquifer could be intercepted. This 

would result in lower water levels in the basin-fill aquifer. The thick sequence and low permeability of the 

lacustrine sediment in the upper part of the basin-fill 

aquifer probably prohibits any substantial downward 

leakage from the overlying shallow-brine aquifer, even 

though the driving force is downward.

Discharge from the conglomerate in the lower 

part of the basin-fill aquifer is by pumping from deep 

brine wells and by upward leakage into the thick 

sequence of lacustrine sediment. Stephens (1974, p. 

22) estimated that about 2,900 acre-ft/yr were withdrawn from October 1969 to September 1972. No data 

were available to estimate the present rate of withdrawal. Data also are insufficient to estimate any possible upward leakage.

Because of continued withdrawal from the alluvial-fan aquifer, ground water within the lacustrine 

sediment of the upper part of the basin-fill aquifer 

along the northwestern margin probably migrates into 

the alluvial-fan aquifer as discussed previously. 

Another form of discharge might be upward leakage 

into the shallow-brine aquifer.

44 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Brine Chemistry and Implications for 

Ground-Water Flow

The brine chemistry of the Bonneville Salt Flats 

is a consequence of the desiccation of ancient Lake 

Bonneville. The Bonneville Salt Flats subsequently 

became the natural terminus for ground-water discharge of much of the lake's area. The presence of the 

salt crust is an indicator of ground-water discharge by 

evaporation under these natural ground-water flow conditions. Under these conditions, the dissolved-solids 

concentration in the shallow-brine aquifer increases 

toward the salt crust where the brine comes to the surface and evaporates. Natural ground-water flow toward 

the salt crust on the Bonneville Salt Flats has been 

intercepted in recent decades as a result of anthropogenic causes as explained in previous sections of this 

report.

Major Ion Chemistry

The brine chemistry of the shallow-brine aquifer 

in the Bonneville Salt Flats probably has not changed 

substantially since the Lines (1979) study. The dissolved-solids concentration of brine in the shallowbrine aquifer in the Bonneville Salt Flats ranges from 

about 65,000 mg/L along the northwestern margin of 

the playa to more than 325,000 mg/L in the salt-crust 

area (Mason and others, 1995, table 4). Dissolved-solids concentration in the Bonneville Salt Flats during 

1992 (fig. 22) is virtually the same as that reported by 

Lines (1979, fig. 43). The ground-water flow regime in 

the shallow-brine aquifer has been changed because of 

brine withdrawal for mineral extraction. For the dissolved-solids concentration to remain unchanged, the 

concentration must be maintained by the dissolution of 

salt crust, upward leakage from the underlying lacustrine sediment of the upper part of the basin-fill aquifer, 

or a combination of both.

Because of the hydraulic gradient toward the 

northwestern margin of the Bonneville Salt Flats, Lines 

(1979, p. 91) suggested that the dissolved-solids concentration gradually increases with time along this 

margin. Data collected during this study, which 

included specific-gravity measurements and chemical 

analyses of brine from wells, indicate that seasonal 

influxes of more dilute recharge water from the east 

flank of the Silver Island Mountains have masked the 

proposed increase in dissolved-solids concentration. 

The pH in brine collected from shallow wells along the 

northwestern margin is consistently higher than in

brine from wells closer to the salt crust. This suggests 

mixing of water from another source. The dissolvedsolids concentration in the brine from some wells along 

the northwest margin decreased from the summer of 

1992 to the spring of 1993 (Mason and others, 1995, 

table 3). This slight freshening of the brine can occur 

periodically when precipitation is sufficient to allow 

infiltration before evaporation.

Potassium concentrations in brine from wells 

completed in the shallow-brine aquifer (fig. 23) have a 

distribution similar to that reported by Lines (1979, fig. 

46), with the highest concentrations in the general 

vicinity of the salt crust. Potassium and magnesium 

remain in solution because the density at which potassium chloride and magnesium chloride precipitate cannot be reached until sodium chloride has precipitated. 

The concentration of the potassium and magnesium 

cations becomes diluted as a result of infiltration of 

fresher water after a large thunderstorm during the 

summer months or rain and snow during the winter 

months when evaporation is minimal. The sodium 

chloride concentration, however, is maintained by dissolution of the salt crust. Because of dilution of some 

ions by the infiltration of fresher water, brine samples 

to be used for comparative purposes over a period of 

years must be collected during the same time each year. 

Lines collected samples during September 1976; the 

samples whose potassium concentrations are shown in 

figure 23 were collected during August and September 

1992. Both data sets are representative of late summer 

conditions in which the potassium concentration probably has reached its maximum after several weeks of 

evaporation from the playa surface.

Prior to mineral development, the highest concentrations of potassium and magnesium coincided 

with the terminus of ground-water flow, generally in 

the center of the playa where topography is lowest, natural discharge is by evaporation, and the salt crust is 

thickest. This area coincides with the central part of the 

playa north of Interstate Highway 80 for about 2 mi and 

south of the highway for about 5 mi. Lines (1979, p. 91) 

suggested that brine withdrawal from the shallow-brine 

aquifer for mineral production has affected the areal 

distribution of potassium and magnesium in solution in 

the central part of the area just north of Interstate Highway 80 and along part of the brine-collection ditch east 

of the salt crust. The areas, depleted in potassium, 

remain evident (fig. 23). The percentage of potassium 

chloride by weight just north of Interstate Highway 80, 

however, is larger than the value shown by Lines (1979,

Brine Chemistry and Implications for Ground-Water Flow 45
m

>

-n

o

3D

I

m

3 

m

3D

z 

c

EXPLANATION

• 100 — Approximate line of equal dissolved-solids

concentration—Contour interval 100 grams per 

liter

——— Brine-collection ditch 

"——"» Playa boundary

, Well—Shallow monitoring well completed to a 

depth of 25 feet or less

40°45'-

R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000,1979 

Universal Transverse Mercator projection, 

Zone 12

R. 18 W.

6 MILES

R. 17 W.

6 KILOMETERS

Figure 22. Dissolved-solids concentration in brine from the shallow-brine aquifer, Bonneville Salt Flats, Utah, Septembe
113°45'

O 

3

Q.

0)

40°45'

EXPLANATION

• 0.5 — Approximate line of equal percentage by weight of 

potassium chloride in brine from carbonate mud 

in shallow-brine aquifer—Contour interval 0.5 

percent

——— Brine-collection ditch 

««•"»"— Playa boundary

• Well—Shallow monitoring well completed to a depth 

°'31 of 25 feet or less. Number is percentage of potassium 

chloride

R. 19 w.

Base from U.S. Geological Survey digital line graph data, 1:100,000, 1979 

Universal Transverse Mercator projection, 

Zone 12

R. 18 W. 6 MILES

6 KILOMETERS

Figure 23. Approximate areal distribution of potassium chloride, percentage by weight, in brine from the carbonate mud of the 

Salt Flats, Utah, late summer, 1992.
fig. 46). It is uncertain whether the new value is indicative of a replenishment of potassium, or whether this 

is a seasonal effect in which the concentrating effect of 

evaporation at the salt surface was more prevalent in 

1992 than in 1976. The lower percentages present to 

the east of the brine-collection ditch are representative 

of the brine migrating toward the ditch from east of the 

study area and should not change appreciably throughout time.

Evaporation from the playa surface has a concentrating effect on potassium and magnesium in the 

brine just below the playa surface because these cations 

will remain in solution as sodium chloride is precipitated. In contrast, Lines (1979, p. 94) reported that 

potassium and magnesium concentrations were less in 

brine from the salt crust than in the underlying carbonate mud at four of five locations where wells were completed in both zones. Lines suggested that the lower 

potassium concentrations in brine from the salt crust 

are the result of brine withdrawal for mineral production. He stated that where the lower potassium concentrations were present in the brine, the direction of 

vertical ground-water flow was downward throughout 

the year for water levels referenced to freshwater. If 

vertical driving forces are calculated using the same 

data, the direction of ground-water flow was upward 

during part of the summer at most of these sampling 

sites and downward at all sites during September 1976. 

These data suggest that seasonal effects might be a 

more plausible explanation for the lower potassium 

concentrations in brine from the salt crust.

Brine was collected for chemical analysis in May 

and September 1993, from three wells, (B-1-17)3lacc1, -2, and -3. These wells were completed at different 

depths in the shallow-brine aquifer. The percentage by 

weight for potassium chloride was similar for both 

samples in each of the three wells. The percentage 

potassium chloride in the brine from well (B-l17)31acc-3, completed in the salt crust, was less than 

that in brine from well (B-1-17)31 ace-1, completed in 

the carbonate mud immediately beneath the salt crust. 

This trend is the same as reported by Lines. The percent 

potassium chloride in the brine from the well completed in the salt crust, however, was similar to the 

value for brine from well (B-l-17)31acc-2, completed 

in the deepest part of the shallow-brine aquifer.

The vertical driving force for ground-water flow 

between the salt crust and the carbonate mud beneath 

the salt crust, wells (B-l-17)31acc-l and -3, was 

upward during 1993. Climatic conditions at the Bonneville Salt Flats for the summer of 1993 were wetter 

and cooler than normal (table 1). The playa surface 

remained flooded for most of summer 1993, therefore, 

sufficient evaporation from the shallow-brine aquifer 

had not occurred by late summer to sufficiently lower 

the water level in the well completed in the salt crust to 

reverse the driving force.

Much of Lines' discussion concerning the percentage of potassium chloride in the shallow-brine 

aquifer pertains to the possible effects of brine withdrawal for mineral production. Turk (1969 and 1973), 

was concerned more with evaluating potassium inventory and producibility in the shallow-brine aquifer 

through the use of a simplified mixing model. Turk 

(1969, p. 179) suggested that the percentage of potassium chloride in brine from the shallow-brine aquifer 

would decline to about 0.5 percent in 30 to 40 years. As 

discussed previously, data collected during this study 

indicate that potassium is not being depleted. A possible explanation for the lack of potassium depletion 

throughout time might be the diffusion of potassium 

from pore fluid within the carbonate mud and clays into 

the brine that migrates through fractures within the 

shallow-brine aquifer to areas of discharge. Results of 

chemical analyses of pore fluid reported by Mason and 

others (1995, table 5) show that potassium concentrations in pore fluid are similar to those in samples from 

shallow wells. As brine is withdrawn from the fractures 

by the brine-collection ditch and subsequently replenished by infiltration from the playa surface, diffusion of 

interstitial potassium might occur as the brine migrates 

along its flow path. Another possible source for potassium is through upward leakage from the underlying 

lacustrine sediment of the basin-fill aquifer, despite the 

lack of measured water-level and density data to verify 

this movement. If these are the processes by which 

potassium might be replenished, then the potassium 

resource is unlikely to show any depletion in the next 

several years or possibly decades because of the large 

amount of interstitial fluid in storage within the carbonate mud and clays of the shallow-brine aquifer and the 

lacustrine sediment in the upper part of the basin-fill 

aquifer.

The chemical composition of brine from the 

lacustrine sediment in the upper part of the basin-fill 

aquifer appears to be basically the same as in the overlying shallow-brine aquifer, but the overall dissolvedsolids concentration is considerably lower. Brine samples were collected from four wells at the center of the 

salt crust, wells (B-l-17)31acc-4, -5, -6, and -7, com48 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
pleted to depths of 95,235,495, and 63 ft, respectively. 

The highest dissolved-solids concentration is in brine 

from the well completed just beneath the shallow-brine 

aquifer. The concentration decreases and then 

increases again with depth (Mason and others, 1995, 

table 4). These data indicate that mixing with brine 

from the shallow-brine aquifer might be the cause of 

the increased dissolved-solids concentration in the 

lacustrine sediment of the upper part of the basin-fill 

aquifer, but that mixing probably has not affected the 

brine at depths of more than 63 ft. The apparent 

increase in dissolved-solids concentration in brine 

from the two deepest wells might be the result of 

incomplete well development after well completion. 

Brine collected from four additional wells, (B-l17)llaac-2, (B-1-17)21 add-4, (B-l-17)23abd-2, and 

(C-l-18)9adc-2, all completed to a depth of 63 ft, had 

similar concentrations as the brine from well (B-l17)31acc-7.

Water from the production zone of the alluvialfan aquifer along the northwest margin of the Bonneville Salt Flats has a dissolved-solids concentration that 

ranges from about 6,200 mg/L to more than 8,000 mg/ 

L (Mason and others, 1995, table 4). The dissolved-solids concentration in the water from the nested wells 

located in a line perpendicular to the playa margin 

increases toward the playa. Results of chemical analyses of water from the alluvial-fan aquifer and the lacustrine sediment that separates the alluvial-fan and 

shallow-brine aquifers are listed in table 2.

Turk (1969, p. 76, and 1973, p. 4) and Lines 

(1979, p. 63) have indicated that the brackish water 

now present in the alluvial-fan aquifer has resulted 

from mixing of relatively freshwater with brine from 

the lacustrine sediment since pumping began in the late 

1940's. As mentioned in the previous section, they 

attributed this increase to leakage of brine from the 

adjacent lacustrine sediment. Other probable sources 

for the dissolved-solids concentration in water from the 

alluvial-fan aquifer might include water leaking 

upward along fault zones, terminus of regional flow, 

surface runoff dissolving small amounts of salt deposited by wind on the surface of the alluvial fan prior to 

infiltration through surface fractures, or a combination 

of these possibilities. No data are available to provide a 

definitive answer as to which source is most probable. 

Chemical data that might have been collected when the 

production wells were drilled were not available. This 

type of data would have been useful for determining 

antecedent conditions. Results of chemical analyses of

water from wells (C-l-19)10aba-l and 10bac-l (table 

2) indicate that the water in the alluvial-fan aquifer in 

the pumping area might have become more saline during the 26-year period between samples. The sample 

collected in 1993 had about a 20-percent-higher concentration for all major constituents as compared to the 

sample collected in 1967. Unfortunately, these samples 

were not obtained from the same wells. The difference 

in chemical concentration, therefore, might result from 

a difference in well-completion depth.

On the basis of four pond samples collected and 

analyzed in January, May, June, and July 1993, the 

chemical evolution of the ponded water can be surmised. The analysis for the sample collected in January 

indicates the ponded water quickly attained a dissolved-solids concentration of 272,000 mg/L by dissolving salt from the salt crust (Mason and others, 

1995, table 4). As evaporation of the pond progressed, 

the concentration of dissolved solids increased to more 

than 325,000 mg/L. After the total concentration 

exceeded 300,000 mg/L in May 1993, calcium, 

sodium, sulfate, and chloride concentrations increased 

by only a small percentage, but magnesium and potassium concentrations increased by 4 to 6 times as 

sodium chloride precipitated and evaporation 

decreased the volume of brine. Concentrations of bromide, boron, and strontium also increased as the size of 

the pond shrank from evaporation.

Isotope Chemistry

In selected samples of brine from all aquifers in 

the Bonneville Salt Flats, the stable isotopes of hydrogen and oxygen were analyzed to help determine possible sources for the water within each aquifer. The 

stable isotopes are analyzed by measuring the ratios of 

hydrogen (*W1H) and oxygen ( 18O/16O) in a sample of 

water. The 2H/1H isotope ratios of brackish water and 

brine samples were analyzed using the method 

described by Kendall and Coplen (1985). The 18O/16O 

isotope ratios of brackish water and brine samples were 

analyzed using a modification of the method described 

by Epstein and Mayeda (1953). The stable isotope of 

hydrogen (2H) is commonly called deuterium (D). The 

isotope ratios are reported as differences from an arbitrary standard known as STANDARD MEAN OCEAN 

WATER (SMOW) and are expressed in 5 units permil 

(parts per thousand or o/oo). The 5 values are determined from the equation:

5 = W-Rstandard)IRstandard\ * 1,000 (16)

Brine Chemistry and Implications for Ground-Water Flow 49
Table 2. Results of chemical analyses of water from selected wells completed in the alluvial-fan aquifer

[°C, degrees Celsius; mg/L, milligrams per liter; —, no data; Location: See figure 2 for explanation of numbering system used for hydrologicin a standard referenced to Standard Mean Ocean Water (SMOW), calculated as: [((2H/1H)sample - 2H/1H)standard)/(2H/1H)standard]

ratio in a standard referenced to Standard Mean Ocean Water (SMOW), calculated as: [«18O/16O)standard -

Location

(B-l-18)29ccc-l

(B-l-18)31acd-l

(B-l-18)31bda-l

(B-l-18)31bda-2

(B-l-18)31bda-3

(B-l-18)31dac-l

(B-l-18)31dac-2

(B-l-18)31dac-3

(C-l-18)6abb-l

(C-l-18)6abb-2

(C-l-18)6abb-3

(C-l-18)6adc-l

(C-l-18)6adc-2

1 (C-l-19)2adb-l

(C-l-19)10aba-l

2 (C-l-19)10bac-l

Date

03-29-72

08-18-92

08-17-92

09-07-93

08-17-92

09-07-93

08-17-92

09-07-93

08-18-92

08-18-92

09-07-93

08-18-92

09-07-93

08-19-92

08-19-92

08-18-92

08-19-92

08-19-92

03-29-72

02-05-93

08-04-67

Water 

temperature 

(°C)

28.0

16.0

26.0

20.0

21.0

20.5

—

23.5

18.0

19.0

19.5

18.0

15.0

17.0

20.0

17.0

22.0

16.0

24.5

30.0

31.0

Specific 

gravity, 

field

_

1.001

1.002

1.002

1.000

1.002

1.001

1.002

1.004

1.004

1.029

1.002

1.002

1.068

1.066

1.060

1.062

1.083

—

—

—

Solids, 

residue at 

180°C, 

dissolved 

(mg/L)

6,260

6,330

6,490

_

6,360

—

6,230

—

11,600

37,000

—

7,060

—

100,000

101,000

87,800

97,600

98,300

5,700

8,310

—

pH, 

field 

(standard 

units)

7.7

8.0

8.2

7.6

8.2

7.8

8.1

7.7

8.1

8.1

7.4

8.0

7.8

7.2

7.5

7.5

7.5

7.4

7.5

7.8

—

Alkalinity, 

lab 

(mg/L 

as 

CaCO3)

_

192

180

—

172

—

174

—

317

262

—

243

—

297

224

152

249

316

—

154

—

Calcium, 

dissolved 

(mg/L 

asCa)

91

43

88

—

71

—

75

—

91

310

—

48

—

990

1,100

1,200

1,100

1,200

79

130

100

Magnesium, 

dissolved 

(mg/L 

asMg)

71

42

75

—

69

—

72

—

94

410

—

42

—

1,700

1,500

1,200

1,600

1,400

63

95

80

Sodium, 

dissolved 

(mg/L 

as Na)

2,200

2,100

2,000

—

2,000

—

2,000

—

3,700

12,000

—

2,500

—

31,000

30,000

27,000

31,000

39,000

2,000

2,600

2,100

1 Reported by Lines, 1978, table 3.

2 Reported by Turk, 1973, table 4b, as well FW-5.

where R and Rstandard are tne isotope ratios of sample 

and standard, respectively. A negative 5 value indicates 

that the isotope ratio of the sample is depleted with 

respect to the heavy isotope in comparison to the 

heavy isotope in the standard, or conversely, a positive 

5 value indicates the sample is more enriched than the 

standard.

The relation between dD and 518 0 is known as 

the meteoric water line and is determined by the equation:

5D = 8 x 518O + d (17)

where d is the deuterium excess (Dansgaard, 1964, p. 

456). The mean global value for d in freshwater is 10

(Craig, 1961, p. 1702). The value for d may differ 

depending on location and geologic time. Any change 

in the value for d, whether larger or smaller, will 

merely shift the meteoric water line to either side of 

the mean global meteoric water line. The North American meteoric water line (fig. 24) has a d value of 6 

(Coplen, 1993, p. 235).

Evaporation from open-water bodies enriches D 

and 18O in the remaining water so that the slope of the 

line for 5D/518O values is typically between 3 and 6 

(Coplen, 1993, p. 235). In the evaporation trend for the 

Bonneville Salt Flats (fig. 24), the slope of the local 

evaporation line is slightly less than 5. This trend was 

determined from three pond samples collected during 

the winter through early summer of 1993 at varying

50 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
or overlying lacustrine sediment of the upper part of the basin-fill aquifer, Bonneville Salt Flats, Utah

data sites in Utah; O H: The relative difference in permil (parts per thousand) between the isotope ratio of 2H to [H in a sample and the ratio 

x 1,000; 8 O: The relative difference in permil (parts per thousand) between the isotope ratio of 18O to 16O in a sample and the 

( 18O/ 16O)sample)/( 18O/16O)standard] x 1,000; Tritium, total: pCi/L, picocuries per liter detection limit, 5.7 pCi/L.

Potassium, 

dissolved 

(mg/L 

asK)

130

130

120

—

120

—

120

—

200

520

—

170

—

1,300

1,300

1,200

1,400

1,600

120

140

100

Sulfate, 

dissolved 

(mg/L 

as SO4)

240

280

260

_

250

—

260

—

960

3,300

—

370

—

9,000

9,100

7,700

8,700

7,300

190

370

300

Chloride, 

dissolved 

(mg/L 

asCI)

3,400

3,300

3,200

_

3,200

—

3,200

—

5,600

17,000

—

3,700

—

45,000

50,000

41,000

47,000

48,000

3,100

4,600

3,700

Bromide, 

dissolved 

(mg/L 

as Br)

_

1.9

2.5

_

2.4

—

1.8

—

3.2

5.1

—

2.0

—

20

17

13

4.7

19

—

2.0

—

Boron, 

dissolved 

(mg/L 

asB)

0.96

1.5

1.3

_

1.6

—

1.3

—

1.2

4.4

—

2.1

—

7.3

6.5

8.1

11

7.1

—

1.2

—

Strontium, 

dissolved 

(mg/L 

asSr)

_

1.2

2.1

_

1.5

—

1.9

—

5.6

18

—

2.6

—

19

24

25

22

28

—

2.8

—

82H 

(permil)

_

-127.0

-127.0

_

-127.0

—

-128.0

—

-123.0

—

-105.0

-125.0

-125.0

-71.0

-76.0

-82.0

-73.0

-70.5

—

-127.0

—

5180 

(permil)

_

-16.35

-16.35

—

-16.40

—

-16.40

—

-15.70

—

-12.67

-16.10

-16.14

-6.30

-7.00

-8.10

-6.45

-5.85

—

-16.31

—

Tritium, 

total 

(pCi/L)

_

—

—

<5.7

—

<5.7

—

<5.7

—

—

<5.7

—

<5.7

—

—

—

—

—

—

—

~

degrees of pond evaporation (Mason and others, 1995, 

table 4).

Brine from the shallow-brine aquifer on the Bonneville Salt Flats is the most enriched ground water from 

the study area and the SD and SO values plot either 

very close to the local evaporation line or between the 

North American meteoric water line and the local evaporation line (fig. 24). The variation in SD and S 18O values for these samples represent varying degrees of 

mixing with infiltrating water from precipitation on the 

playa surface and enrichment through evaporation. The 

least enriched (most negative) sample from the shallowbrine aquifer was collected from well (B-l-18)34bbb-l, 

located near the northwestern margin of the playa. These 

values are indicative of less evaporation than other samples from the shallow-brine aquifer because of the 

greater depth to water.

The SD and S 18O values for brine from the lacustrine sediment of the upper part of the basin-fill aquifer 

beneath the salt crust are virtually the same as those in 

the overlying shallow-brine aquifer. These values support vertical mixing at the interface of these two aquifers.

The SD and S18O values for brackish water from 

the alluvial-fan aquifer and brine from the lacustrine 

sediment of the upper part of the basin-fill aquifer in the 

vicinity of the northwestern margin of the playa have a 

larger variation than samples from the shallow-brine 

aquifer and the underlying lacustrine sediment in the 

salt-crust area. These SD and SO values form two disBrine Chemistry and Implications for Ground-Water Flow 51
(0

i

o 

O

c

(0 

0)

(•o 

c

(055

-20

-40

-60

-80

- 100

oc

LU 

Q_.-120

0

CO

-140

-160

Well (B-1-18)31 dac-2

Wells near flank of 

_ Silver Island 

Mountains

Nested wells closest to salt crust

O Well completed in alluvial-fan aquifer and 

lacustrine sediment of basin-fill aquifer 

near playa margin

n Well completed in lacustrine sediment of 

basin-fill aquifer beneath the salt crust

A Well completed in shallow-brine aquifer 

• Surface pond

I__I

-22 -20 -15 -10 -5 0 5 

818O, IN PERMIL (Standard Mean Ocean Water)

10

Figure 24. Relation of hydrogen and oxygen isotope ratios in ground water and surface 

pond, Bonneville Salt Flats, Utah.

tinct groups with a single intermediate value. The values that are the most depleted and coincide with the 

North American meteoric water line are from samples 

collected from wells (B-l-18)31acd-l, (B-1-18)3 Ibda1, -2, and -3, and (B-1-18)3Idac-land -3, all located 

very close to the flank of the Silver Island Mountains. 

These waters are indicative of recharge from precipitation, either directly on consolidated rock with rapid 

infiltration, or by runoff into open surface fractures 

with rapid infiltration, as discussed previously. If the 

dissolved-solids concentration, and more specifically 

the sodium and chloride concentrations, in water from 

these wells are the result of leakage from the lacustrine 

sediment, then the 5D and 518O values would have a 

more evaporative signature. The probable causes of the 

high dissolved-solids concentration in these waters are 

deep circulation of meteoric water through regional 

flow or along local faults or the dissolution of salt from

the soil surface before the runoff enters surface fractures.

The single intermediate value was derived from 

water collected from well (B-1-18)31 dac-2, which is 

completed to a depth of 149 ft. The dissolved-solids 

concentration is higher and the 5D and 518O values are 

more enriched in water from this well than from the 

other two wells in this nest of three wells, which are 

completed to depths of 74 and 245 ft. These values are 

indicative of mixing with brine from near the center of 

the playa. The chemically and isotopically distinct 

zones at this location are probably not indicative of a 

natural condition; rather, the tongue of higher dissolved-solids concentrations and more enriched isotope values are probably indicative of transient mixing, 

possibly induced by withdrawal by pumping from the 

alluvial-fan aquifer. The 5D and 518O values in brine 

from wells (C-l-18)6abb-l, -2, and -3, and (C-l18)6adc-l and -2, indicate varying degrees of enrich52 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
ment. These wells are located closer to the salt crust 

and therefore would allow more mixing with the concentrated brine.

In a more useful comparison of the different 

groups of water sampled for D and 18O (fig. 25), the 

values for 518O are compared to dissolved-solids concentration. The trend toward higher dissolved-solids 

concentrations in water that has undergone more evaporation is evident. The sample from well (B-l18)34bbb-l is the outlier from the other groups.

Tritium (3H) concentration also was determined 

in samples collected from the Bonneville Salt Flats. 

Tritium is a radioactive isotope of hydrogen and decays 

with a half-life of 12.43 years. It occurs naturally in the 

atmosphere by the interaction of cosmic rays with 

nitrogen and oxygen. More importantly, the largest 

source for H was from above ground, thermonuclear 

weapons testing between 1952 and 1969. The natural 

level for H in the atmosphere prior to weapons testing

was 10 TU. During above-ground weapons testing, 

atmospheric levels were more than 10,000 TU (Drever, 

1982, p. 347). At the present time (1996), 3H concentrations are again approaching the natural (commonly 

called pre-bomb) level in the atmosphere.

Twenty-five samples were collected for 3H analysis from wells completed in the shallow-brine and 

alluvial-fan aquifers and the lacustrine sediment of the 

upper part of the basin-fill aquifer. One sample, with a 

value of 10 TU, was collected from a winter surface 

pond to verify the 3H concentration for infiltrating 

water. Other values ranged from less than detection 

limits (5.7 pCi/L) to 78 pCi/L (Mason and others, 1995, 

table 4). This range equates to less than 1.8 to 24 TU. 

For purposes of this report, any value below detection 

limits is considered equal to 0 TU. The samples with 

values of 0 TU are from waters that have infiltrated into 

the subsurface prior to the above-ground weapons testing. Samples with values above 10 TU are from water

<D

c

(0

o 

o 

O

co -5 

o

(0

(0

cc

UJ 

Q.

P

to

r -15;-

-20

Nested wells closest to salt crust

Well(B-1-18)34bbb-1

Well (B-1-18)31 dac-2

Wells near flank of 

Silver Island Mountains

A Well completed in shallow-brine aquifer

O Well completed in alluvial-fan aquifer and 

lacustrine sediment of basin-fill aquifer

o Well completed to a depth of 63 feet in 

lacustrine sediment of basin-fill aquifer 

beneath the salt crust

... I ,,,,.,,,, I ......

0 100 200 300 400 

DISSOLVED-SOLIDS CONCENTRATION, IN GRAMS PER LITER

Figure 25. Relation of oxygen isotope ratio to dissolved-solids concentration in ground 

water, Bonneville Salt Flats, Utah.

Brine Chemistry and Implications for Ground-Water Flow 53
that has infiltrated into the subsurface within a few 

years prior to sampling or has infiltrated after aboveground weapons testing and been diluted with prebomb water and possibly with recent water. A more 

precise date for infiltration is not possible because of 

uncertainty in knowing if and to what extent mixing has 

occurred.

Nineteen samples were collected from wells 

completed in the shallow-brine aquifer. A noticeable 

trend exists for these values. Generally, samples with 

the lower 3H values, about 10 TU or less, are from the 

center of the salt-crust area, whereas samples with the 

higher values, from about 14 to 24 TU, are from the 

periphery. This distribution supports the concept that 

most of the infiltration from the surface occurs over the 

salt crust with brine migrating laterally to points of discharge. Wells from which samples having less than 10 

TU were obtained are located along the axis of the salt 

crust. The sample from well (B-1-17)31 ace-1 had a 

value of 0 TU. In order to have 3H values of 0 to less 

than 10 TU, mixing with water from a source of prebomb water is necessary. The only probable source for 

this water entering the shallow-brine aquifer is through 

upward leakage from the lacustrine sediment of the 

upper part of the basin-fill aquifer. This is contradictory 

to water-level data and estimates for downward leakage 

discussed previously in this report. Upward leakage in 

the amount necessary to account for the low 3H values 

is possible if water levels had been slightly lower than 

those measured in 1993 in shallow wells on the salt 

crust and if the permeability of the underlying lacustrine sediment of the upper part of basin-fill aquifer 

were two to four orders of magnitude greater than previously assumed. The required permeabilities are representative of silt rather than clay.

If surface runoff and infiltration through surface 

fractures are a relatively recent source of recharge to 

the alluvial-fan aquifer and to the lacustrine sediment 

of the upper part of the basin-fill aquifer, then traces of 

tritium are expected in these waters. Six samples were 

collected from the line of nested wells completed in 

these zones. None of the samples had a 3H concentration above the detection level (table 2). The lack of 3H 

in this water does not preclude surface runoff as a possible mechanism for the dissolved-solids concentration 

in the alluvial-fan aquifer; rather, it might indicate that 

post-bomb water has not had sufficient time to migrate 

to the sampling points.

SALT-TRANSPORT MECHANISMS AND 

SALT BUDGET

An understanding of the amount of salt contained in the Bonneville Salt Flats is needed in order to 

place salt-budget and transport estimates in proper perspective. An estimate of total salt should be considered 

as a crude approximation because of sparse data and is 

only provided for a sense of scale.

The amount of salt contained in the crystalline 

salt crust north of Interstate Highway 80 was estimated 

from dry density and volume. The dry density of the 

crystalline salt near the surface of the salt crust ranges 

from 98.2 to 123.3 lbs/ft3 (P. Allard, Bureau of Land 

Management, written comrnun., 1995). The average 

dry density from 30 measurements is 109.8 lbs/ft3 . If 

the average dry density is assumed to be representative 

of the entire salt crust, then the salt crust, as it existed 

north of the Interstate Highway 80 in 1988 (S. Brooks, 

Bureau of Land Management, written commun.,1988), 

contained more than 125 million tons of salt. The 

amount of salt dissolved in the brine that fills the pore 

space and fractures of the shallow-brine aquifer north 

of Interstate Highway 80 was estimated to exceed 175 

million tons. This estimate is based on an average 

porosity of 0.45 (Turk, 1969, p. 104), an average aquifer thickness of 20 ft, an average dissolved-solids concentration of 286 g/L (17.85 lbs/ft3 ), and an areal extent 

of 80 mi2, the area where the brine is most concentrated. The amount of salt estimated to have been lost 

from the salt crust in the 28-year period, 1960-88, is 

more than 55 million tons. This estimate is based on the 

change in volume of the salt crust for the 28-year 

period (S. Brooks, Bureau of Land Management, written commun., 1988) and the average dry density.

For both principal mechanisms for transporting 

salt within the hydrologic system of the Bonneville Salt 

Flats, the salt is dissolved in water. The first mechanism 

transports salt by surface ponds blown by wind, or in 

some years, simply by the large areal extent of the surface pond. The second mechanism transmits salt by 

advective ground-water flow and associated dispersive 

transport, which can be the result of natural and (or) 

anthropogenic causes.

Salt Transport by Surface Ponds

Precipitation on the Bonneville Salt Flats during 

the late fall, winter, and early spring generally undergoes little evaporation; thus, what does not infiltrate

54 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
into the subsurface accumulates at the lowest altitude in 

the basin and forms a shallow pond, generally a few 

inches deep but sometimes more than 1 ft deep in some 

areas. Ponds are most likely to be deeper, more extensive, and longer lasting when winter precipitation is 

normal or greater than normal and when temperatures 

are lower than normal. This relatively freshwater provides a mechanism by which salt can be dissolved and 

transported away from the normal bounds of the saltcrust area and deposited on the carbonate mud by evaporation.

As freshwater accumulates on the salt-crust surface, it dissolves underlying halite until the brine 

approaches saturation. Wind can easily move this brine 

over the virtually flat surface. Wind direction at the two 

weather stations was predominantly from the southwest to northeast during the winter months of 1992-94 

when evaporation rates were the lowest. This wind 

moves surface brine from the salt-crust area northeastward toward Floating Island. If the water were to 

remain away from the center of the salt-crust area as it 

evaporated, then there would be a net loss of salt from 

the main body of the salt crust. This does not necessarily constitute a net loss of salt from or a gain to the 

basin, but only a temporary redistribution of salt. In the 

succeeding summer months, this salt can be redissolved by water from thunderstorms and then migrate 

back toward the salt crust.

During 1993, precipitation was greater than normal during January, and temperatures were much 

colder than normal for January and February. Because 

of these conditions, extensive flooding of the Bonneville Salt Flats dramatically changed the salt surface. A 

large amount of surface water, primarily from south 

and east of the study area, migrated toward the playa. 

The density of this inflow, as measured on March 26, 

1993, ranged from 1.014 to 1.030 g/cm3 . By visual 

observations, the surface inflow was not turbid. 

Although this inflow added an unknown amount of salt 

to salt-crust area, the relatively freshwater dissolved 

salt from the southern part of the salt crust and transported it to the north. No data were collected from 

which to estimate total inflow. Much of the salt 

remained in solution into the summer of 1993, when 

the salt precipitated on the playa surface as the pond 

evaporated. Satellite imagery was used to determine 

the areal extent of the salt crust in September 1992 and 

August 1993 (fig. 26). The areal extent of the salt crust 

was 43 mi2 in 1992 and 58 mi2 in 1993. The newly 

redeposited salt extended to the west edge of the brinecollection ditch and to the northeast, about 3 mi beyond 

the brine-collection ditch. Measurements from the top 

of the well to the salt surface were used to estimate the 

thickness of salt dissolved and precipitated. The volume of salt that was dissolved and precipitated was 

estimated from these measurements. An estimated 10 

to 14 million tons of salt were dissolved from the saltcrust area just north of Interstate Highway 80, with 

much of the salt redeposited to the north and east of the 

main body of salt.

The fate of the salt transported into the area north 

and east of the north ends of the brine-collection ditch 

is unknown. Much of this salt might have been redissolved and infiltrated into the subsurface where it could 

migrate to the brine-collection ditch, or this salt might 

have been deposited across a very subtle topographic 

rise where it would be unable to eventually migrate 

back to the salt crust by movement of small surface 

ponds that can form after thunderstorms. Some of the 

salt transported into the area that is influenced by the 

brine-collection ditch to the east of the salt crust plight 

have redissolved, infiltrated into the subsurface, and 

migrated to the brine-collection ditch. No data were 

collected to quantify the amount of salt permanently 

removed from the salt-crust area.

Salt Transport in Solution Through 

the Shallow-Brine Aquifer

The salt and fluid that circulate within the pore 

space of the salt crust and underlying carbonate mud 

are in a constant state of change. Freshwater from rainfall that infiltrates into the shallow-brine aquifer almost 

immediately dissolves salt in order to reach chemical 

equilibrium. Evaporation of the fluid during the summer months concentrates the dissolved salts in the 

remaining fluid until salt precipitates out of solution, 

forming new deposits.

From water levels measured in 1976, Lines 

(1979, p. 79) determined that brine in the shallow-brine 

aquifer moves from the main salt-crust area northwest 

toward the alluvial-fan aquifer, east toward the brinecollection ditches, and south under Interstate Highway 

80 toward a network of brine-collection ditches. Brine 

flows into the study area through the subsurface from 

the east and northeast. In 1992, the same pattern of 

brine movement existed (fig. 15). The amount of brine 

moving across the study-area boundaries in 1992 was 

estimated from aquifer properties derived from tests 

(Turk, 1973, table 9; and Lines, 1979, table 5) and from

SALT-TRANSPORT MECHANISMS AND SALT BUDGET 55
I

D 3J 

O

I

O

m oo O

EXPLANATION

Areal extent of salt crust, September 1992 

Areal extent of salt crust, August 1993 Brine-collection ditch Playa boundary

40°45' - R. 19 W.

Base from U.S. Geological Survey digital line graph data, 1:100,000,1979 Universal Transverse Mercator projection, Zone 12

R. 18 W. 2

_______I

6 MILES R. 17W. R. 16 W.

6 KILOMETERS

Figure 26. Increase in areal extent of the salt crust from September 1992 to August 1993, Bonneville Salt Flats, Utah.
Table 3. Estimated fluid flow and salt transport into and out of the shallow-brine aquifer, Bonneville Salt Flats, Utah

[—, no estimate]

Component

1976 (Lines, 1979) 1992 (this study)

Acre-feet Tons of solute Acre-feet Tons of solute

Recharge

Subsurface inflow from east 40

and northeast 

Infiltration from playa surface 11,740

Total (rounded) 11,800

14,600

14,600

60

8,300-12,800 

8,400-12,900

21,900

21,900

Discharge

Subsurface outflow to south

Subsurface outflow to

brine-collection ditch

Subsurface outflow to northwest

Downward leakage 

Evaporation

Total (rounded)

2,000

680

70

10,900

13,600

780,000

264,000

27,200

0

1,071,000

1,800-2,300

1,500

35

58-580 

15,000

18,400-19,400

700,000-895,000

580,000

13,600

22,600-226,000 

0

1,316,000-1,715,000

horizontal hydraulic gradients derived from contours of 

reference potentiometric heads based on average density (fig. 15). These flow estimates are compared to 

those made by Lines (table 3). Differences in these estimates result from the different method used by Lines to 

calculate reference potentiometric heads and temporal 

variations in recharge from precipitation and evaporation. Variations in precipitation and evaporation occur 

on a seasonal and annual basis, but the long-term conditions have remained virtually unchanged during the 

past 20 years.

No evaporation data were collected for the summer of 1992. As discussed previously, the estimate for 

evaporation from the shallow-brine aquifer of 12,200 

acre-ft in 1993 is probably less than the usual amount 

that occurs on the Bonneville Salt Flats because of 

cooler-than-normal temperature and larger-than-normal precipitation in June and July. The estimate for 

1994 of 20,700 acre-ft is probably more than the typical amount because of hotter-than-normal temperature 

and less-than-normal precipitation. An intermediate 

value of about 15,000 acre-ft might be more representative of evaporation from the playa surface in 1992.

In both estimates, total discharge is more than 

total recharge. Lines (1979, table 6) attributed the difference to a decrease in ground water stored within the 

shallow-brine aquifer. The difference in the estimate 

from this study is considerably larger than the difference stated by Lines, and yet, no long-term decline in 

storage has been observed because the shallow-brine 

aquifer appears to be in a fluid steady state. No longterm change in storage would dictate that fluid recharge 

would have to be about the same as fluid discharge. As 

suggested previously, subsurface flow to the south 

might be less than estimated because of compaction or 

removal of porous material during construction of 

Interstate Highway 80, and downward leakage might 

be offset by upward leakage at the center of the salt 

crust. Even if subsurface flow to the south was assumed 

to be a fraction of the estimate stated in table 3, total 

discharge would remain considerably larger than the 

estimated total recharge. No substantial changes in 

water levels have occurred within the shallow-brine 

aquifer; therefore, more fluid must be entering the aquifer than previously estimated by Lines (1979, table 6). 

On the basis of the ground-water flow directions (fig. 

15), the probable source of fluid in a large enough 

amount to compensate for discharge would be upward 

leakage.

Previous and current estimates of recharge into 

and discharge from the shallow-brine aquifer can be 

used to estimate the amount of salt entering and leaving 

the Bonneville Salt Flats study area (table 3). Subsurface inflow from the northeast and east was estimated 

to be 60 acre-ft in 1992. On the basis of an average dissolved-solids concentration of 268,500 mg/L (16.75

SALT-TRANSPORT MECHANISMS AND SALT BUDGET 57
o

Ibs/ft ) from four wells on the northeastern and eastern 

boundaries, an estimated 21,900 tons of salt per year 

are transported into the study area at these boundaries, 

a small amount of the total salt budget.

An average brine density of 1.18 g/cm3 , which 

equates to a dissolved-solids concentration of 286,200 

mg/L (17.86 Ibs/ft3), was assumed to be representative 

for all fluid exiting the shallow-brine aquifer. This dissolved-solids concentration was used to convert all 

subsurface outflow (table 3) to tons of salt. An estimated 700,000 to 895,000 tons of salt were transported 

out of the study area along the south boundary in 1992. 

The amount of salt that was transported into the brinecollection ditch in 1992 was estimated to be 580,000 

tons. The amount of salt transported from the shallowbrine aquifer into the alluvial-fan aquifer in 1992 was 

estimated to be 13,600 tons.

On the basis of limited data indicating downward 

leakage, the amount of salt that might be transported 

downward to the lacustrine sediment that underlies the 

shallow-brine aquifer is estimated to range from 22,600 

to 226,000 tons. If upward leakage occurs, then the 

estimated net loss of salt would be less because of the 

transport of salt into the shallow-brine aquifer.

Total inflow of salt to the shallow-brine aquifer 

system in 1992 was estimated to be about 22,000 tons, 

but total outflow of salt from the same system was 

about 1.3 million to 1.7 million tons. Lines (1979) estimated that in 1976, total inflow of salt was about 

14,600 tons and total outflow of salt was about 1 million tons.

SIMULATION OF VARIABLE-DENSITY 

GROUND-WATER FLOW AND SOLUTE 

TRANSPORT IN THE SHALLOW-BRINE 

AQUIFER

Modeling Approach

A model is a representation of some, but not necessarily all, features of an actual system (Foley and others, 1990, p. 286). The purpose of a model is to enhance 

visualization and understanding of the structure and 

behavior of the actual system, and to provide a convenient means of experimentation and prediction of the 

effects of various changes to the model parameters. 

Computer models in the physical sciences, such as 

ground-water flow and solute-transport models, are

typically expressed as systems of mathematical equations, and the modeler learns by experimenting with 

different values of independent variables and parameters.

Any numerical simulation of ground-water flow 

and solute transport is necessarily limited in its ability 

to realistically portray hydrologic interactions in the 

field. If, however, a sufficient amount of parameter 

information is available, numerical simulation can be 

useful in quantifying and testing hypotheses of the 

mechanisms and relations in the simulation region. 

This was the role of the numerical simulation of the 

shallow-brine aquifer in the Bonneville Salt Flats study 

area.

Numerical simulation was used as a tool to aid in 

the understanding of the hydrologic environment and 

solute transport in the study area. It helped to quantify 

transport mechanisms, integrate field information, and 

test hypotheses. It also was used to evaluate sensitivity 

of the system to boundary-condition and parameter values and to address the following questions:

1. What quantitative effects does withdrawal of brine 

for mineral production have on the salt crust?

2. What quantitative effects do annual evaporation 

and precipitation have on the salt crust? 

To answer these questions, a three-dimensional, 

ground-water flow and solute-transport model was 

developed for the Bonneville Salt Flats study area. Calculation of an overall ground-water flow and solutetransport balance was a major objective. The mass balance on the salt crust itself would be a consequence of 

the cumulative transport amount at the land-surf ace 

boundary of the model.

The primary goals of the combined groundwater flow and solute-transport modeling are to (1) 

develop a fluid and solute balance for the ground-water 

system in the simulation region, (2) evaluate the effect 

of the brine production from the ditches on the salt 

crust, and (3) identify the major and minor solute fluxes 

to and from the system.

Description of the Simulator of Ground-Water Flow 

and Solute Transport

Simulator refers to the computer code that simulates ground-water flow and solute transport. Model 

refers to the application of the simulator to represent 

the shallow-brine aquifer underlying the study area. 

Solute refers to the dissolved constituents in the ground 

water and is equivalent to dissolved solids. Salt refers

58 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
to the solid-phase mixture of salt minerals, primarily 

halite and gypsum. Salt crust refers to the perennial salt 

deposit on a large part of the playa surface.

The numerical simulation of ground-water flow 

and solute transport for this study area consisted of the 

computation of numerical solutions to the equation of 

saturated ground-water flow coupled with the equation 

for solute transport. For an assumed isothermal situation of variable fluid density, the saturated groundwater flow equation is formed from the combination of 

the conservation of total fluid mass and Darcy's Law 

for flow in porous media. The solute-transport equation 

is formed from the conservation of mass for a single 

solute species.

The three-dimensional Heat and Solute Transport (HST3D) computer code (Kipp, in press) of the 

U.S. Geological Survey was the simulator used to 

model the shallow-brine aquifer underlying the Bonneville Salt Flats. This simulator was chosen because of 

its ability to model variable-density flow and solutetransport in three dimensions. Although this simulator 

can calculate heat and solute transport simultaneously, 

effects of temperature variation on the density of the 

brine were assumed to be negligible for the purposes of 

this study. Therefore, only ground-water flow and associated solute transport were simulated. The flow and 

transport equations are coupled through the interstitial 

pore velocity and the dependence of the fluid density 

and viscosity on pressure and solute-mass fraction.

The HST3D simulator uses some different variables and parameters than are used for ground-water 

flow and solute transport for constant-density fluids. 

They include (1) pressure instead of potentiometric 

head as the dependent variable of flow, (2) mass fraction instead of volumetric concentration as the dependent variable of solute, (3) porosity instead of specific 

yield for storage in unconfined flow systems, (4) conservation of mass rather than conservation of volume 

for the flow equation, (5) permeability instead of 

hydraulic conductivity in Darcy's Law, and (6) energy 

per unit mass instead of potentiometric head at the 

external surface of a leakage boundary.

Pressure is chosen as the dependent variable for 

fluid flow because no potentiometric-head function 

exists for density fields that depend on temperature and 

solute concentration. All pressures are expressed relative to atmospheric pressure.

The partial-differential equation of groundwater flow is based on the following assumptions and 

limitations:

1. Ground water fully saturates the porous medium 

below the water table.

2. Ground-water flow is described by Darcy's Law.

3. The coordinate system is chosen to be aligned 

with the directions of the permeability tensor 

so that this tensor is diagonal for anisotropic 

media.

4. Diffusive fluxes of the bulk fluid caused by density gradients are neglected relative to advective-mass fluxes.

5. Osmotic potential driving forces are neglected.

6. Dispersive-mass fluxes of the bulk fluid from spatial-velocity fluctuations are not included. 

Although Hassanizadeh and Leijnse (1988) 

show that in high-concentration solutions, fluid motion 

is dependent on solute concentration gradients, and 

they present a formulation of Darcy's and Pick's Laws 

that contains density flow and pressure diffusion, their 

equation formulations are not implemented in the 

HST3D simulator. The flow equation is based on the 

conservation of total fluid mass in a volume element, 

coupled with Darcy's Law for flow through a porous 

medium. Thus,

(18)

where:

£ is the effective porosity (-); 

p is the fluid density (M/L3); 

t is the time (7); 

k is the porous-medium permeability tensor

(L2);

(Li is the fluid viscosity (F-77L);

p is the fluid pressure (F/L );

g is the gravitational constant (Z/T ); and

ez is the unit vector in the z-direction (-).

Equation 18 relates the rate of change of total 

mass in the fluid phase to net fluid-inflow rate. The fluid 

consists of pure water plus the dissolved solids. Capacitance is described by effective porosity rather than specific yield. In this report, effective porosity simply will 

be called porosity.

The interstitial or pore velocity, V, is obtained 

from Darcy's Law as:

(19)

where:

v is the interstitial-velocity vector (1/7).

Modeling Approach 59
The equation for conservation of a single solute 

species is based on the following assumptions and limitations:

1. Solute transport by local, interstitial, and velocityfield fluctuations and mixing at pore junctions 

is described by a hydrodynamic-dispersion 

coefficient.

2. Thermal and pressure diffusion are neglected.

3. Forced diffusion by gravitational, electrical, and 

other fields is neglected. 

The mass fraction of solute is considered to be 

the dependent variable for solute transport. It is the 

mass of dissolved solids per mass of solution (solute 

plus water), that is, a mass-based concentration. The 

more widely used concentration term is an amount per 

unit volume of fluid, that is, a volume-based concentration. But volume-based concentration is not conserved 

in a variable-density system. The term "solute concentration" refers to the mass-based concentration or mass 

fraction. In the simulator, the density field is derived 

from the mass fraction. The conservation equation for 

the solute in the fluid phase can be written:

(20)

V • V • /Vw-V-£pvw

where

w is the mass fraction of solute in the fluid phase (-); 

Ds is the mechanical-dispersion-coefficient tensor

(L2/7);

Dm is the effective-molecular diffusivity of the solute

(L2/T); and 

/ is the identity matrix of rank 3 (-).

Equation 20 relates the rate-of-change of solute 

in the fluid phase to the net dispersive and diffusive 

flux, and the net advective flux. The equation is written 

for a unit volume of saturated porous medium.

The conservation equations for ground-water 

flow and solute transport are based upon the conservation of mass, not conservation of volume as in a constant-density flow system. A change in volume occurs 

when fluids containing different amounts of salt are 

mixed. For example, mixing 1 L of freshwater (density 

1 g/cm3 ) with 1 L of brine (density 1.2 g/cm3 ) yields a 

total mass of 2.2 kg of salt water, but the volume is 

2.075 L, not 2.000 L, which is about a 4-percent difference.

Before the two conservation equations can be 

solved, information about the fluid properties, aquifer 

properties, and transport coefficients needs to be 

obtained. The fluid properties are density and viscosity. 

The porous-matrix properties are porosity and permeability. The transport coefficients are solute-dispersion 

tensors and the effective molecular diffusivity of the 

solute. In the HST3D simulator for the present model 

application, density and viscosity are functions of the 

solute-mass fraction. The solute-dispersion tensors are 

functions of the interstitial velocity. The other parameters are either uniform or functions of space within the 

simulation region.

For fluids such as water, a linear-density function is typically adequate for the ranges of pressures 

and solute concentrations encountered. Fluid viscosity 

is strongly dependent on temperature, less dependent 

on solute concentration, and least dependent on pressure. The fluid-density function in the simulator is linear in pressure, temperature, and solute concentration 

(Kipp, in press).

The HST3D simulator uses an empirical equation for viscosity of brines as a function of pressure, 

temperature, and solute concentration. Moderate curvature can be seen in a plot of fluid viscosity as a function of solute-mass fraction at 20°C (fig. 27).

2.5i-r

UJ 

(0

Q2.0 

0.

UJ 

O

1.5

(0 

O

O 1.0 

(0

0.5 i i i i i i i i i i i i i i i

'0 0.05 0.10 0.15 0.20 0.25 

SOLUTE MASS FRACTION, DIMENSIONLESS

Figure 27. Fluid viscosity as a function of solute mass 

fraction at 20 degrees Celsius.

In the HST3D simulator, the mechanical-dispersion coefficient is taken from the work of Scheidegger 

(1961) as presented by Bear (1972, chap. 10). For an 

isotropic porous medium, two parameters describe the

60 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
mechanical-dispersion tensor: the longitudinal dispersivity, (XL(L), and the transverse dispersivity, aT (L). 

Field data have shown that longitudinal dispersivity 

typically is 3 to 10 times larger than transverse dispersivity (Freeze and Cherry, 1979, p. 396; Anderson, 

1979) and that their magnitudes are dependent on the 

scale of observation distance over which the solute is 

transported in a system. The dispersivities are affected 

by the heterogeneity of the permeability distribution 

for the aquifer.

Discretization of the ground-water flow and solute-transport equations and boundary conditions in 

space and time was done using finite-difference techniques (Kipp, in press). This converted the equations 

into sets of simultaneous linear equations at the node 

points in space that are solved numerically for each 

time step.

This flow and transport simulator is based on a 

finite-difference approximation of the partial-differential equations derived by the subdomain collocation 

method described by Cooley (1974), Varga (1962), and 

Patankar (1980). The grid of node points can have variable node spacing, and a point-distributed grid is used 

as opposed to a cell-centered grid (Kipp, 1987). Aquifer properties are specified by zones that are subregions 

bounded by planes of nodes. This means that properties 

change values at planes of nodes rather than at cell 

boundaries. Any full cell may have up to eight subdomains with different aquifer properties. The resultant 

cell property is a volume-weighted average of the subdomain values. A conductance property is an areaweighted average of up to four subdomain values, for a 

full cell, over the relevant cell face. An element is a volume with a node at each of the eight corners. A node 

with its associated cell and one of its subdomains and 

associated element is shown in figure 28.

Aquifer properties are uniform over an element. 

Thus, an element is the smallest zone possible for variation of aquifer properties.

The boundary conditions were discretized on a 

nodal or cell basis. For the specified-pressure boundaries, entire cells have the specified value. For flux and 

leakage boundaries, the face to which the flux or leakage is applied is the entire cell face. Spatial changes in 

flux values or leakage parameters occur at cell boundaries, not at nodes.

Figure 28. Diagram of a model cell with associated 

node, one subdomain, and the associated element.

Application to Study Area

For the Bonneville Salt Flats, the equation of 

ground-water flow had the following additional 

assumptions specific for this application:

1. Ground-water flow is described by Darcy's Law, 

although this is a fractured-clay and fracturedsalt medium.

2. The porous medium and fluid are incompressible 

for this unconfined system.

3. The storage coefficient representing elastic storage effects from fluid and porous-matrix compressibilities is negligible for this unconfined 

system.

4. The fluid viscosity is a function of space and time 

through dependence primarily on solute concentration.

5. Contributions to the total fluid mass from pure 

solute-mass sources within the region are not 

included.

The first assumption was adopted, although it is 

known that the shallow-brine aquifer underlying the 

Bonneville Salt Flats is a fractured medium of carbonate mud and salt crust. There is a slight dependence of 

viscosity on pressure in the function employed, but the 

effect is negligible for the pressures encountered in the 

shallow-brine aquifer.

The ground-water system simulated for the Bonneville Salt Flats is assumed to be isothermal at 20°C, 

so no heat-transport equation is needed. Examination 

of water density as a function of temperature (Weast 

and others, 1964, p. F-4) shows that a 10°C temperature 

change causes a density change of about 0.002 g/cm , 

which is within measurement error. On this basis, temperature effects on density were neglected.

Modeling Approach 61
Consideration of viscosity changes as a function 

of temperature for a brine with a mass fraction of 0.22 

has about a 20-percent viscosity change for a 10°C 

change in temperature from 15 to 25 °C. This means 

that seasonal changes in aquifer temperature do not 

produce major changes in hydraulic conductivity. 

Minor changes in hydraulic conductivity, therefore, 

would be masked by the uncertainty in the hydraulicconductivity distribution. Neglecting temperature 

effects on viscosity does not compromise the realism of 

the simulations.

An evaluation of the potential for induced buoyant thermal convection can be made by evaluating the 

dimensionless Rayleigh number (Ra) for typical aquifer parameters (Nield and Bejan, 1992, p. 147). A stability analysis shows that a Rayleigh number of more 

than about 27 is needed to induce thermal convection 

for an unconfined ground-water system. Using typical 

thermal and hydraulic parameters for the shallow-brine 

aquifer, the flow system is stable under a temperature 

difference of 10°C from the bottom to the top of the 

aquifer (Ra =10). This condition might be possible 

when winter cooling causes a temperature profile that 

increases from top to bottom of the aquifer, assuming 

uniform salinity. The thermal conduction mechanism is 

sufficient to stabilize the adverse density gradient 

caused by the temperature profile.

A similar analysis can be done to evaluate the 

potential for convective circulation resulting from an 

unstable density or solute-concentration gradient with 

the denser brine within the salt crust overlying the 

slightly less dense brine in the carbonate mud. The critical Rayleigh number is also about 27. An unstable 

density gradient condition occurs when summer evaporation causes a denser brine to form at the top of the 

aquifer. Assuming an isothermal system, for a density 

difference of 0.02 g/cm3 between the top and the bottom of the shallow-brine aquifer, static brine is always 

unstable when the denser fluid is on top (Ra = 30,000). 

This is because the molecular diffusivity for the dissolved salt is so small that diffusive transport cannot 

overcome forces for convective transport to stabilize 

the adverse density gradient.

For brine moving near the brine-collection 

ditches and the water-table surface where evaporative 

discharge and precipitation recharge occur, dispersive 

mixing maintains a stable configuration (Ra = 0.05 to 

25) with denser brine on top. In other parts of the aquio

fer where interstitial velocities are less than about 10 

ft/d, convective circulation would be expected when

the denser brine overlies the less dense brine (Ra = 30 

to 800). These results are based on an estimated dispersivity of 1,000 ft, which is characteristic of the flowpath 

length under the salt crust.

For the Bonneville Salt Flats, the equation of solute transport had the following additional assumptions 

specific for this application:

1. The salt mixture is considered to be a single solute 

species.

2. No mechanisms for solute precipitation or dissolution are included.

3. No pure solute sources occur in the fluid or solid 

phases.

The solutes being transported in the Bonneville 

Salt Flats shallow-brine aquifer include several types of 

salt, of which about 90 percent is sodium chloride. For 

modeling purposes, assumption 1 is made. Assumption 

2 means that the simulator does not contain a mechanism for precipitation of salt when brine evaporates, 

nor a mechanism for dissolution of salt when undersaturated water moves through crystalline salt. The lack of 

these mechanisms in the simulator means that when 

formulating a calculation for the amount of crystalline 

salt, it is assumed that brine precipitates salt at a rate 

given by the solute concentration of the evaporating 

fluid at the boundary face, and the infiltration of rain 

and snowfall advects dissolved salt into the aquifer at a 

specified concentration.

The HST3D simulator uses the SI metric system 

internally but accepts input and provides output data in 

English units. The units used for the modeling of the 

Bonneville Salt Flats study area were time in days, 

length in feet, mass in pounds, volumetric flow rate in 

cubic feet per day, mass flow rate in pounds per cubic 

foot per day, and pressure in pounds per inch squared.

Boundary-condition types employed include 

specified pressure, specified solute mass-fraction, 

specified fluid flux, evaporation, leakage, and a free 

surface or water table. All boundary conditions can be 

functions of time.

Certain additions and modifications were made 

to the HST3D simulator for application to the Bonneville Salt Flats simulation region. These modifications 

are described by Kipp (in press). They include:

62 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
1. Addition of an evapotranspiration boundary condition.

2. Modification of the algorithm for location of the 

free surface or water table.

3. Addition of the ability to input data as a linear distribution over a boundary segment.

4. Addition of the ability to input data node-by-node 

and element-by-element from gridded data 

prepared by a geographical information system.

5. Elimination of the partial Gauss reduction used 

for numerical coupling of the difference equations for flow and transport.

6. Computation of data for postprocessing of flow 

rates and cumulative flow amounts.

7. Addition of output of water-table altitude and 

density field for postprocess contour plotting.

The evaporation boundary condition has a piecewise linear function of evaporation flux with depth to 

the water table. The function is non-linear because of 

the sharp breaks in function slope at the land surface 

and at the extinction depth. Evaporation flux decreases 

as the water table drops until it becomes zero at the 

extinction depth.

The fifth item was necessary to prevent simulation failure due to instability of the original partially 

reduced difference-equation system when a cell went 

dry. It also corrected a major mass-balance problem 

associated with cells going dry.

Postprocessing programs were written for the 

totalization of boundary flow rates and individual 

cumulative flows by boundary faces at selected locations, and for the computation of pond volumes and 

fluid mass at the end of each simulated season. The 

simulator calculates cell flow rates and cumulative 

amounts at each cell for each type of boundary condition. These values are written to a file for postprocessing so that total flow rates and cumulative amounts can 

be calculated for any desired part of a boundary face. 

The trapezoidal rule is used for temporal integration of 

the flow rates to obtain the cumulative amounts. The 

volume of the pond that appears during the recovery 

season is calculated by adding the volume of fluid 

between the water table and the altitude of the pond 

bottom over the pond area on a cell-by-cell basis. The 

mass of brine in the pond is calculated by adding the 

density-weighted pond volume using the density of the 

bottom layer of cells in the pond.

The finite-difference approximations to the 

advective-transport terms were chosen to be upstream 

in space. The temporal finite-difference approximations were chosen to be backward in time or fully 

implicit. These choices mean that a certain amount of 

artificial numerical dispersion is introduced into the 

solutions calculated by the simulator. This numerical 

dispersion is a function of the time-step size and the 

mesh spacing (Kipp, in press), and it prevents nonphysical oscillations in the solute-concentration field at the 

cost of artificial smoothing or dispersion of concentration gradients.

The simulation region in the Bonneville Salt 

Flats study area focused on the shallow-brine aquifer. 

The areal extent of the simulation region is shown in 

figure 29.

The simulation region contains the entire area of 

thick perennial salt crust north of Interstate Highway 

80 that is known as the Bonneville Salt Flats. Carbonate mud surrounds the salt crust and extends toward the 

northwest to the Silver Island Mountains and to the east 

into the Great Salt Lake Desert. A thinner, ephemeral 

salt crust forms seasonally on the surface of the carbonate mud. Also, evaporation from the water table precipitates salt in the fractures of the carbonate mud and the 

existing salt crust. For modeling purposes, the term 

"salt crust" will be used to refer to the perennial salt 

deposit encompassing the area where the salt crust is at 

least 1 ft thick. The salt crust plus the thin, ephemeral 

salt deposits make up the total crystalline salt deposit at 

any given time.

The vertical extent of the simulation region represents the thickness of the shallow-brine aquifer from 

the water table at or near the land surface down to the 

contact with the lacustrine sediment of the upper part of 

the basin-fill aquifer. The areal extent of the simulation 

region represents the study area from the alluvial-fan 

aquifer along the flank of the Silver Island Mountains 

on the northwestern and northern boundaries to the 

brine-collection ditch along Interstate Highway 80 on 

the south, and from the alluvial-fan aquifer on the west 

to the limit of available data east of the brine-collection 

ditch, which is located east of the salt crust.

Previous work (Lines, 1979) indicates that the 

shallow-brine aquifer in the Bonneville Salt Flats study 

area was of large areal extent (125 mi2) and rather thin 

vertically (25 ft). However, the large fluid-density variation between the northwestern boundary with the alluo

vial-fan aquifer (1.04 g/cm ) and the shallow-brine

o

aquifer (as much as 1.21 g/cm ) raises the potential for 

density-induced flow. Thus, a variable-density flow and 

solute-transport simulator was needed. The model has 

to be three-dimensional to allow for buoyant effects 

from density contrasts and to include the areal features

Modeling Approach 63
Tl

5

113°45'

EXPLANATION

—I

Area of inactive elements

—— Brine-collection ditch 

Playa boundary

40°45'

T. 1 N.

T. 1 S.

R. 19W.

Base from U.S. Geological Survey digital line graph data, 1:100,000,1979 Universal Transverse Mercator projection,

R. 16 W.

6 KILOMETERS

Figure 29. Location of model grid and inactive elements for the model of the shallow-brine aquifer, Bonneville Salt Flats, Utah.
of the brine-collection ditch and aquifer boundaries. 

Although some density variation was measured 

between brine from the salt crust and brine from the 

underlying carbonate mud, vertical profiles of density 

variation throughout the thickness of the shallow-brine 

aquifer were not obtainable.

The shallow-brine aquifer consists of carbonate 

mud, oolitic sand, gypsum, and halite, and is fractured 

with fissures up to 1 in. in width extending down to 25 

ft with fissure spacing of about 1 ft being common. 

Although the aquifer is fractured, the well known 

assumption of an equivalent porous medium was made. 

This was necessary because data to characterize the 

fracture distribution were insufficient. Also, the density 

of fractures was believed to be high enough to be represented as a distributed porosity and permeability for 

the purposes of this model.

Inflow to and outflow from the simulation region 

were identified as follows. Water and brine enters the 

simulation region by (1) infiltration of precipitation and 

(2) ground-water inflow from the north, east, and west, 

and a small segment at the west end of the southern 

boundary (fig. 15). Brine leaves the simulation region 

by (1) evaporation, which precipitates crystalline salt 

on the playa surface, (2) ground-water outflow to a 

brine-collection ditch along the southern boundary, 

south of Interstate Highway 80, (3) ground-water seepage into the brine-collection ditch east of the salt crust,

(4) ground-water outflow to the adjacent alluvial-fan 

aquifer along the northwestern margin of the playa, and

(5) leakage to the underlying basin-fill aquifer. Ponding 

occurs over the lower areas of the salt crust in response 

to seasonal precipitation and reduction in evaporation 

rate during the winter months.

Review of several years of available data shows 

considerable annual variation of precipitation, evaporation, production from brine-collection ditches, and 

other hydrologic conditions. Model calibration by historically matching periods of actual site conditions was 

not done for the following reasons:

1. Insufficient data were available to define initial 

conditions, for an undisturbed state or for a 

period of current brine production.

2. Boundary conditions show transient variations on 

the time scale of days to weeks.

3. Purposes of modeling this study area involve a 

time scale of years to decades. 

Preliminary simulations show that the groundwater flow system does not reach a steady state during 

the production or recovery seasons because the

response time is longer than a seasonal duration of 

about 6 months. For simulation purposes, a synthetic 

seasonal sequence was formulated representing a typical year split into a 6-month summer-fall season of production and evaporation followed by a 6-month winterspring season of recovery and recharge. For purposes 

of this report, the former season will be denoted as the 

production season and the latter season as the recovery 

season. An average brine-production volume, estimated during this study, was used for the production 

season. The goal was to simulate a periodic steady state 

representing a typical climatic year with average brine 

production from the brine-collection ditch east of the 

salt crust. Then the fluid and solute balances could be 

evaluated to help answer the key questions pertaining 

to model simulations in this study as presented in the 

"Modeling approach" section of this report.

The simulation region was contained in a rectangular parallelepiped of dimensions 76,000 ft by 56,000 

ft by 26 ft in the east-west, north-south, and vertical 

directions, respectively. The land surface is flat to 

within 2 ft over the Bonneville Salt Flats. To simplify 

the geometry, an altitude of 4,215 ft was assumed for 

the carbonate-mud surface and 4,214 ft was assumed 

for the salt-crust surface.The contact with the underlying basin-fill aquifer was assumed to be a horizontal 

plane at an altitude of 4,189 ft.

The mesh or grid was constructed with variable 

node spacing. The horizontal spacing varied from 500 

to 1,000 ft, and the finer divisions were made in the 

vicinity of the brine-collection ditch. One reason for 

the 1,000-ft spacing was to be able to define the permeability, pond area, salt-crust area, recharge flux, evaporation flux, and porosity distribution with realistic 

geometric structure. Altitudes of the node layers were 

4,189, 4,194, 4,199, 4,204, 4,209, 4,214, 4,214.5, and 

4,215 ft. The finer mesh near the top was to accommodate the ponding representation. The total number of 

nodes in the simulation region was 44,032; however, 

only about 36,000 nodes were active. The nodes are 

located at the intersection of the model-grid mesh lines. 

Thus, the mesh lines delineate element boundaries, not 

cell boundaries.

The ground-water flow and solute-transport 

simulator requires specification of boundary conditions 

to determine the numerical solutions for the dependent 

variables of pressure and solute concentration. The 

boundary conditions for the simulation region include 

specified pressure, specified recharge flux for the infiltration of precipitation, evaporation, and leakage from

Modeling Approach 65
an adjacent aquifer or outer aquifer region. The leakage 

boundary condition was chosen for those boundaries 

along which water-level distributions in space are 

known only at a few locations. The location of the different boundary-condition types is shown in figure 30.

At all flux and pressure boundaries, associated 

solute-mass fractions were specified for possible 

advective inflow. Fluid flowing into the simulation 

region advects solute at the specified mass fraction. 

Solute is advected by fluid flowing from the simulation 

region at the current solute concentration at a given 

location on the boundary. For model simulations, no 

boundaries had a specified solute concentration 

because this would have constrained the simulated solute-concentration distribution unrealistically.

The boundary conditions (fig. 30) were specified 

as follows by locations and types: (1) along the south 

and west boundaries, specified pressure; (2) along the 

east, north, northwest, and bottom boundaries, leakage; 

(3) along the top (free-surf ace) boundary, evaporation 

flux during the production season and specified 

recharge flux from the infiltration of precipitation during the recovery season; and (4) specified pressure 

along the brine-collection ditch boundary east of the 

salt crust and interior to the simulation region. Equations defining the boundary-condition types are 

described by Kipp (in press). Actual values associated 

with the various boundary conditions were assigned by 

a variety of methods to be described in the "Parameter 

estimation and available data" section of this report.

A leakage boundary condition was chosen along 

those boundaries where water levels are known only at 

a few locations. This boundary condition does not specify water levels, but allows the simulated water levels 

to rise and fall in response to inflow and outflow across 

these boundaries. Thus, neither fluxes nor water levels 

are specified at the leakage boundaries; therefore, they 

are suitable in regions in which information is sparse. 

Interpolated water levels were used to set the driving 

forces at leakage boundaries by specifying the potential 

energy per unit mass of fluid at the exterior surface of 

the leaky layer.

A limitation of the simulator is that, although 

cells at edges and corners of the active simulation 

region may have two or three boundary faces, only one 

flux or leakage boundary condition can be specified for 

a given cell. For example, along edges where a lateral 

leakage-boundary face may meet a vertical leakageboundary face, one of those faces must be pulled back 

from that edge so that there are not two leakage faces

for that cell. This is inconvenient, but not a serious 

problem for regions with a high density of cells 

because the lost-leakage area would be a small fraction 

of the total-leakage area along that boundary. The 

boundary face chosen to be pulled back from the edge 

was the one containing the greater number of cells, so 

that the lost fraction is minimized.

The brine-collection ditch, located east of the 

salt crust, in the interior of the simulation region is represented as a specified-pressure boundary. It penetrates 

two-thirds of the thickness of the shallow-brine aquifer. 

The areal location of the ditch is a sequence of line segments joining node points. The ditch was located 

approximately to align the segments with the northsouth and east-west axes of the model grid. During the 

recovery season, the brine-collection ditch boundary 

was removed from the simulator. This allowed the 

water table to rise in response to the changes in the 

other boundary conditions, such as at the land surface 

and along the south and east boundaries. To keep a 

specified-pressure boundary for the brine-collection 

ditch would have provided an artificial instant recovery 

of the ditch water level, and it would have become a 

false source of recharge to the shallow-brine aquifer.

The shallow-brine aquifer, simulated as an 

unconfined aquifer, has a free-surf ace boundary condition at the water table. The free surface is allowed to 

move to any cell in a vertical column of cells. Recharge 

from precipitation across the free-surface boundary 

was represented by an areally distributed flux over the 

top surface of the simulation region. This flux was 

applied to the cell containing the water table at each 

areal location. Evaporation also was represented by an 

areally distributed flux applied to the cell containing 

the water table. As explained earlier, this flux depends 

on the water-table depth below land surface.

A transient pond forms seasonally on the Bonneville Salt Flats during the winter months, when the 

water table rises above the land surface in areas where 

the altitude of the land surface is lowest. During model 

development and calibration, an approximate representation of this ponding was incorporated by defining an 

area with a land surface uniformly 1 ft lower than the 

general land surface altitude of 4,215 ft. This pond area 

consists of 36 percent of the land surface of the simulation region. It also approximates the area overlain by 1 

ft or more of perennial salt crust. The lower land surface created a zone in the model representing the seasonal pond. Porosity in this zone was set to unity and 

permeability to four orders of magnitude greater than

66 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
. pressure Specified 

view Side Leakage 

30. Simulation forwith theshallow-brine andBonneville Sa locations, Figure reionaqufer boundary-condition types 

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for the shallow-brine aquifer. This approximated an 

open body of water when the water table rose into this 

zone and provided additional fluid storage for the overall simulation region. Because aquifer properties are 

defined on an element basis, the cells spanning the 

pond-bottom altitude of 4,214 ft have an effective average porosity of about 20 percent. Thus the effective 

pond bottom could be regarded as the cell boundary of 

the lowest cell with porosity of 100 percent. This bottom surface was located at an altitude of 4,214.25 ft. It 

would have been desirable to represent the pond by an 

open body of water whose level responded to net inflow 

through the pond bottom as described by Cheng and 

Anderson (1993). The same approach could also have 

been used to model the brine-collection ditches. However, development of such a fully coupled model for 

open bodies of water with variable-density flow and 

transport was beyond the scope of the study.

Alternatively, representation of the brine-collection ditch, interior to the simulation region, as a leakage 

boundary was not feasible because it penetrated six of 

the nodal layers of the model. The brine-collection 

ditch was assumed to be 17 ft deep, which located the 

ditch bottom at an altitude of 4,198 ft, and corresponds 

to the sixth nodal layer down from the top of the model 

grid.

At the beginning of the study, it was believed 

that the fluid system was at periodic steady state for 

several decades but that the solute in the system could 

be in transient evolution. The salt crust was regarded as 

not in periodic steady state, based on the long-term 

decline in area and thickness. Data obtained later in the 

study show that the solute content of brine in the shallow-brine aquifer also was in a periodic steady state for 

more than 20 years. The model was calibrated to 

achieve this condition.

Simulations were carried out to a periodic steady 

state with a repeated sequence of production and recovery seasonal periods with their associated boundary 

conditions, approximating a typical year. Seasonal 

average values for boundary pressures, recharge flux, 

leakage parameters, and evaporation flux were used. 

This removed the dependence of the results on the initial pressure distribution, and, to a lesser extent, on the 

initial solute-concentration distribution. The initial 

conditions of the water-table configuration and the density distribution were quite uncertain because of the 

lack of data for a typical year or set of years from which 

to obtain an average representative of initial conditions. 

The alternative approach of using a pre-development

initial condition also was discarded because of the lack 

of data to quantify this condition.

A periodic steady state for the flow system 

implies the following at the end of two successive production or two successive recovery seasons:

1. The water-table configuration is the same.

2. The boundary fluid volumetric flow rates are the 

same.

3. The volume of fluid in the system is the same.

A periodic steady state for solute transport also 

implies the following:

1. The density and solute-mass-fraction fields are 

the same.

2. The mass of fluid and solute in and out of the system has a net value of zero for a year of simulation.

The durations of seasonal simulations were 180 

days for the production season and 185 days for the 

recovery season. An automatic time-step control was 

used for each simulation starting at 0.01 days. The 

main control was on the pressure change per time step,

o

which was limited to 0.01 Ibs/in , corresponding to 

about 0.025 ft of change in altitude of the water table. 

Although almost never invoked, the secondary control 

was to limit the concentration change per time step to 

0.01 in solute-mass fraction.

Because the flow simulation was run to obtain a 

periodic steady state, the pressure-field results are independent of the initial-condition pressure distribution. 

Therefore, for simplicity, a hydrostatic pressure with a 

flat water table was chosen as the initial condition for 

the ground-water flow equation. The initial condition 

for the solute-transport equation was a mass-fraction 

distribution derived from a cubic trend surface fit to 

density data (see "Parameter estimation and available 

data" section of this report).

Output files were postprocessed to compute flow 

rates and cumulative flow amounts for each boundary 

segment. Trapezoidal integration in time was used to 

compute cumulative flow amounts from the boundarycell data.

Seasonal evaporation from the shallow-brine 

aquifer was computed as total evaporation for the simulation region less the initial amount stored in the seasonal pond. The pond was totally evaporated during the 

production season so there was no residual pond to 

include in the computation for this season. Similarly, 

seasonal recharge to the shallow-brine aquifer was calculated as the total recharge amount less accumulation 

in the seasonal pond during the recovery season. The

68 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
pond volumes were calculated by numerical integration 

of the volume of water contained in each cell within the 

pond volume. The pond depth at each node within the 

pond area was the height of the free surface above the 

effective pond-bottom altitude of 4,214.25 ft.

The volume of brine stored in the pond at the end 

of the previous recovery season was subtracted from 

total evaporation at the end of the production season 

just completed to get actual evaporation from the shallow-brine aquifer. Similarly, the volume of brine stored 

in the pond at the end of the recovery season just completed was subtracted from total recharge for this 

recovery season to get actual recharge to the shallowbrine aquifer. The mass of solute in the pond at the 

beginning of the production season was subtracted 

from the mass of solute removed from the simulation 

region to obtain the net mass of solute removed from 

the shallow-brine aquifer during this season. Similarly, 

the mass of solute in the pond at the end of the recovery 

season was subtracted from the mass of solute added to 

the simulation region by recharge from infiltration of 

precipitation to obtain the net mass of solute added to 

the shallow-brine aquifer during this season. These 

methods account for the solute in storage in the seasonal pond.

The budget for the crystalline salt was calculated 

from the cumulative-amount results. At the end of the 

production season, the total amount of salt precipitated 

through evaporation was assumed to be the total 

amount of solute advected out through the top boundary of the model by evaporative flow, less the amount 

of salt contained in the seasonal pond at the beginning 

of the production season. This calculation neglects any 

concentrating mechanism occurring in the actual system, so the simulated outgoing solute concentration is 

not necessarily at halite saturation. At the end of the 

recovery season, the total amount of salt dissolved and 

recharged to the shallow-brine aquifer was assumed to 

be the amount advected in through the top boundary of 

the model by the recharge flux less the solute in storage 

in the pond at the end of the season. Because there was 

no salt dissolution mechanism in the simulator, the density or solute concentration of the recharge flux was 

specified. The difference between solute outflow and 

inflow was calculated as the net gain or loss of crystalline salt during the year.

Parameter Estimation and Available Data

For the purposes of this report, parameter estimation refers to the process of selecting values for the 

various model parameters estimated without the use of 

computer simulations. These parameters were not 

adjusted during the subsequent model-calibration procedure. The initial estimates of parameter values 

adjusted during model calibration also are described 

here. Data from previous studies of the Bonneville Salt 

Flats, data collected during this study, and general 

hydrologic information from similar settings were used 

to estimate model parameters. Parameters estimated at 

the beginning of model development define (1) geometry of the aquifer, (2) geometry of the salt crust and 

pond area, (3) aquifer elastic-storage properties, (4) 

heads used to set the specified-pressure boundaries 

(except at the brine-collection ditch), (5) external 

potentials for the leakage boundaries, (6) leakageboundary flow properties and geometry, (7) evaporation extinction depth, (8) densities used to set the associated solute concentrations at the boundaries, and (9) 

aquifer dispersivities. The remaining model parameters 

were established during the model calibration and are 

described in the next section. They include: (1) aquiferpermeability distribution, (2) aquifer-porosity distribution, (3) maximum evaporation-flux distribution, (4) 

precipitation-recharge flux distribution, and (5) brinecollection ditch water levels.

Aquifer and Fluid Properties

All aquifer properties were assumed to be uniform throughout the aquifer thickness because of a lack 

of data indicating otherwise. The aquifer properties for 

conductance (permeability) and storage (porosity) 

were calibration variables. Initial estimates of permeability distribution were based on a contour map of 

transmissivity values by Lines (1979, fig. 33). This 

transmissivity map was digitized into piecewise constant bands of transmissivity, and interpolated bands of 

permeability values were generated for the elements 

formed by the areal nodal mesh by geographical-information-system computer software. Zonation by element was used in the horizontal planes of nodes, with 

each element extending from the land surface to the 

aquifer bottom. This gave the finest possible horizontal 

resolution for permeability at the given mesh definition. The permeability values were set to be uniform 

throughout the thickness of the shallow-brine aquifer 

because no data were available on vertical variation.

Parameter Estimation and Available Data 69
Conversion of transmissivity to permeability was done 

by the following equation:

k = 7.37 x 10-11 (21)

where

r\

k is the permeability (ft );

T is the transmissivity (ft2/d);

(I is the fluid viscosity (centipoise);

p is the fluid density (lbs/ft3 );

r\

g is the gravitational acceleration (ft/s ); and

b is the aquifer thickness (ft).

The density and viscosity distributions used in 

this conversion were calculated from the density distribution used for initial conditions. This density-distribution contour map also was digitized into piecewise 

constant bands. An associated piecewise constant-viscosity distribution was calculated from the bands of 

constant density by converting the density to solutemass fraction, then using the viscosity equation from 

the HST3D simulator to obtain the viscosity value for 

each band. The proper densities and viscosities to use 

in the conversion would be those measured during each 

aquifer test; however, these data were not available. 

The initial horizontal permeability distribution ranged 

from 1.29 x 10' 10 ft2 to 1.15 x l(T9ft2 . Lines (1979, p. 

70) reported that the vertical hydraulic conductivity for 

his aquifer tests on the shallow-brine aquifer ranged 

from 30 to 140 ft/d. The ratio of vertical to horizontal 

hydraulic conductivity for these tests ranged from 

about 0.6 to 0.9 based on an anisotropic hydraulic conductivity aquifer-test analysis. On the basis of these 

tests and the vertical fractures penetrating most, if not 

all, of the thickness of the shallow-brine aquifer, the 

vertical permeability of the equivalent porous media 

was assumed to be 0.8 of the horizontal value for each 

zone.

Storage of the flow system was characterized by 

porosity. Because this aquifer is being simulated as an 

unconfmed flow system, fluid and porous-matrix compressibility were assumed to be negligible. A uniform 

effective porosity of 10 percent was used initially. Specific yield for this unconfined aquifer is assumed equal 

to the effective porosity of 10 percent, however, this 

parameter is not used in the simulator.

The solute-dispersivity values were somewhat 

arbitrarily selected to be 500 ft in both longitudinal and 

transverse directions because no data were available to 

estimate the field values for the study area. The amount 

of numerical dispersion contributed by the mesh spacing used is equivalent to a dispersivity of up to 500 ft 

for flow along a horizontal coordinate direction, and up 

to 2.5 ft for flow in the vertical coordinate direction. 

Estimating the numerical dispersion for oblique flow is 

beyond the scope of this report but probably falls 

within the above limits. The total effective longitudinal 

and transverse dispersivities range from about 500 to 

1,000 ft, which may be unrealistically large for simulating vertical mixing. Longitudinal-dispersivity values 

from field-scale tracer experiments tend to be one-tenth 

or less of the length scale of the flowpath (Gelhar and 

others, 1992). The transverse dispersivity was set equal 

to the longitudinal to represent the hydrodynamic mixing effects of the predominantly vertical fractures that 

are the major flow conduits. A high transverse dispersivity results in a great deal of vertical mixing with flow 

that is predominantly horizontal. For flow that is vertical, the large longitudinal dispersivity also results in 

vertical mixing. The amount of numerical dispersion 

contributed by the largest time-step length used is 

equivalent to a dispersivity value of as much as about 

50 ft, clearly less significant than that caused by the 

mesh spacing.

The fluid-density and viscosity functions are 

described in the "Modeling approach" section of this 

report. For modeling purposes, the system was treated 

as isothermal at 20°C. The range of measured fluid density in the shallow-brine aquifer in the Bonneville Salt 

Flats was 1.04 to 1.2 g/cm . The calculated brine viscosity from this density range extends from 1.25 to 

1.99 centipoise. The latter value is more than twice that 

of freshwater. No laboratory measurements of viscosity 

of the brine were made.

Boundary-Condition Parameters

General methods used for quantifying all boundary conditions of the several types used in the model 

are as follows. The specified-pressure boundary conditions were assigned to segments of the boundaries by 

extrapolation from maps of the measured seasonal 

water tables (figs. 15 and 16) and by using the initialcondition density distribution to calculate pressure 

variation with depth to the aquifer bottom. Hydrostatic 

conditions were assumed to apply throughout each 

specified-pressure boundary segment.

A leakage boundary needs an energy per unit 

mass specified at its external surface. This energy was 

calculated from extrapolation of measured water levels 

near the relevant boundary locations (figs. 15 and 16). 

The lateral thicknesses were chosen to place the exter70 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
nal potentials sufficiently far away from the simulation 

region to lessen their effect on the water-table configuration. Also, the lateral thicknesses were chosen to 

place the outer boundary of the leaky layer at a horizontal location where there were at least a few wells to provide external potential and density data. The 

permeability of the lateral leakage boundaries was 

taken to be a smooth approximation of the horizontal 

permeability for the shallow-brine aquifer in the vicinity of the respective boundary. It was assumed that all 

properties and potentials were uniform over the vertical 

extent of each boundary segment.

The head distributions specified for the various 

boundary segments in the production and recovery seasons, respectively, are shown in figures 31 and 32. The 

head distributions along leakage boundaries were used 

to set the external boundary potentials. The boundarydensity values used for head-to-pressure conversion 

and for specification of boundary-solute concentrations 

are shown in figure 33. Associated solute-mass fraction 

for possible inflow was calculated from the initial-condition density distribution along that boundary. Some 

of the boundary-condition pressure and energy values 

changed from production season to recovery season. 

The following sections describe the various boundary 

conditions by geometric location around the model 

region.

The boundary on the north, located just south of 

the northeastern end of the Silver Island Mountains, 

was defined as leakage. The western half of this boundary is the north edge of the shallow-brine aquifer, possibly near a connection to the alluvial-fan aquifer if it 

exists in this area. North of the eastern half of this 

boundary is the extension of the shallow-brine aquifer 

through the pass between Floating Island and the Silver 

Island Mountains. The northern boundary of the model 

has ground-water flow to or from the northeast and also 

receives runoff from the Silver Island Mountains to the 

north and northwest. The horizontal thickness of this 

leakage boundary was set at 100 ft. The use of such a 

thin leakage layer effectively makes this boundary a 

specified-pressure boundary. Permeability was set at 

1.8 x 10- 10 ft2.

The shallow-brine aquifer was assumed to be in 

direct contact with the alluvial-fan aquifer along the 

northwestern boundary of the simulation region. A 

leakage boundary was used here. Water-table maps 

have shown a depression over part of the alluvial-fan 

aquifer, perhaps caused by pumpage of process water 

for the mineral-extraction plant operation. This implies

that ground water flows out from the shallow-brine 

aquifer to the northwest and west. Water levels from the 

few wells in this boundary area were used to set the 

external potentials for the leakage-boundary segments. 

The horizontal thickness of the leakage boundary was 

set at 100 ft. This boundary is also effectively a specified-pressure boundary because of the thin leakage 

layer. Permeability was set at 1.4 x 10' 10 ft2.

The shallow-brine aquifer also was assumed to 

be in contact with the alluvial-fan aquifer along the 

western boundary of the simulation region. A specified-pressure condition was used along this boundary. 

Water levels from the wells in this area and a linear 

density variation in the horizontal direction were used 

to set the pressure distribution.

The southern boundary, south of Interstate Highway 80, was simulated as a specified-pressure boundary. The middle half of the south boundary is a length 

of brine-collection ditch (fig. 30), whereas the east and 

west parts have no hydrologic or topographic features. 

More wells are present at this boundary, relative to the 

other boundaries, from which to establish heads for the 

pressure distribution.

The water level in the brine-collection ditch at 

the south boundary was not measured during the study 

because this ditch, which is on private property, was 

inaccessible. For the production season, the altitude of 

the head in the brine-collection ditch along the south 

boundary was estimated to be 4,200 ft based on water 

levels in wells to the north. During the recovery season, 

the water level was assumed to be 4,208 ft. The western 

and eastern parts of the south boundary were assigned 

piecewise linear or piecewise constant pressure distributions based on heads (figs. 31 and 32) and densities 

(fig. 33).

The eastern boundary has no hydrologic features, but represents the eastern limit of available data 

from the shallow-brine aquifer. The shallow-brine 

aquifer continues to the east, outside the area of primary interest and where data were not available. The 

connection of the simulation region with the outer 

basin region to the east was represented by a leakage 

boundary. This leakage boundary, with a 2,000-ft horizontal thickness, was used to insulate the simulation 

region from the uncertain spatial distribution of exterior potential. The permeability of this leakage boundary was set at 2.0 x 10" 10 ft2.

The brine-collection ditch, east of the salt crust 

and internal to the simulation region (fig. 30), was represented as a specified-pressure boundary. The areal

Parameter Estimation and Available Data 71
I

D 

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EXPLANATION

Linear variation

N

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4213.50 4212.00 4200.00 4200.00

T 

4211.00

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Figure 31. Head distribution specified along the boundaries of the simulation region for the shallow-brine aquifer durin

Bonneville Salt Flats, Utah.
\ 4213.50

EXPLANATION

Linear variation

N

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4213.50 I 1 4213.50 4208.00 4208.00 I 4208.00 I 4211.00 \ 4215.00 4216.00

Figure 32. Head distribution specified along the boundaries of the simulation region for the shallow-brine aquifer during the recovery season, Bonneville Salt Flats, Utah.
D 

3D 

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CD 00 O CMO O i- iEXPLANATION

Linear variation

1.05 Density—In grams per 

cubic centimeter

N

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1.16

1.16

1.15

1.14 1.18 1.18 1.17 1.16 1.15

Figure 33. Density distribution specified along the boundaries of the simulation region for the shallow-brine aquifer during both seasons, Bonneville 

Salt Flats, Utah.
location of the brine-collection ditch was taken from 

1:24,000-scale quadrangle maps. In the model, the 

areal representation of the brine-collection ditch was a 

sequence of line segments joining node points. Some 

approximations to the location of the ditch were made 

to align the segments with the north-south orientation 

of the model grid. The width of the ditch was estimated 

to vary from 28 ft at the land surface to 15 ft at the bottom. Actual ditch width is not represented in the model. 

The depth was estimated to be 16 ft, which places the 

ditch bottom at an altitude of 4,198 ft. Thus, it penetrates two-thirds of the thickness of the shallow-brine 

aquifer.

In the model, the brine-collection ditch is one 

cell wide, which is 500 ft at the mesh spacing employed 

in its vicinity. Computational limitations prevented 

using a finer mesh at the brine-collection ditch. The 

specified water level or pressure means that the ditch 

has instant drawdown at the beginning of the production season, which is somewhat artificial. Analytical 

solutions show that it takes a few days for the water 

level in the ditch to decline to its specified altitude. The 

gradually sloping water surface in the brine-collection 

ditch is of unknown profile. For simulation purposes, 

the brine-collection ditch in the model was divided into 

three reaches of nearly equal length, and estimates of 

water levels were made to approximate the sloping 

water surface. The initial water-level altitudes were 

4,206.5,4,207.5, and 4,208.5 ft; and these values were 

adjusted during the calibration process. The pressure 

distribution was assumed to be hydrostatic with depth, 

based on the average density of 1.175 g/cm3 observed 

in the actual brine-collection ditch. The boundary condition at the brine-collection ditch east of the salt crust 

was removed to allow the water table to rise during the 

recovery season. This is clearly more realistic than 

imposing a pressure in the ditch during the recovery 

season.

The bottom boundary is at the contact between 

the shallow-brine aquifer and the lacustrine sediment in 

the upper part of the basin-fill aquifer. A leakage condition was assigned to this boundary. The shallowbrine aquifer bottom was assumed to be a horizontal 

plane set at 4,189 ft, which corresponds to an average 

shallow-brine aquifer thickness of 25 ft. Well data, 

locating the material contrast between the shallowbrine aquifer and the underlying lacustrine sediment of 

the upper part of the basin-fill aquifer, show virtually 

no variation of the aquifer bottom throughout most of 

the shallow-brine aquifer. However, along parts of the

northwestern margin of the playa, the lacustrine sediment of the upper part of the basin-fill aquifer was 

encountered about 9 ft below land surface.

A uniform thickness of 38 ft for the semiconfining layer was chosen based on the distance between the 

altitude of the bottom of the shallow-brine aquifer, 

4,189 ft, and the depth of the monitoring wells completed in the lacustrine sediment of the upper part of the 

basin-fill aquifer. The uppermost well drilled into the 

basin-fill aquifer at each of five locations was 63 ft 

below land surface. Thus, the leaky-layer thickness 

does not represent the actual thickness of a semiconfining layer, but, rather, a part of the upper part of the 

basin-fill aquifer. The exterior energy potential was 

determined as a spatial average from data available 

from four wells in the fall of 1992. Wells (B-l17)23abd-l, -2, and -3, near the brine-collection ditch, 

were not used because they are influenced by ditch 

water levels. Data were insufficient to determine a spatial distribution of basin-fill aquifer energy potential. A 

uniform value was based on a water level of 4,215.05 ft 

and a fluid density of 1.11 g/cm3 . As described previously in the "Principles of variable-density groundwater flow" section of this report, the net driving force 

on fluid in the shallow-brine aquifer provides a calculated downward leakage into the lacustrine sediment of 

the upper part of the basin-fill aquifer at the five piezometer nest sites at most times of measurement. However, geochemical and isotopic data suggest upward 

leakage has occurred historically, as discussed previously in the "Brine chemistry and implications for 

ground-water flow" section of this report.

The permeability of the leaky boundary in the 

model at the bottom of the shallow-brine aquifer was 

chosen to be representative of unweathered clays, 

which is the type of material found in the upper part of 

the basin-fill aquifer. The permeability of these clays 

was estimated to range from 1 x 10 to 1 x 10 ft , 

and a permeability of 1.1 x 10" 16 ft2 was chosen.

A water table forms the upper boundary of the 

shallow-brine aquifer. Its position varies in space and 

time in response to the dewatering and rewetting of the 

porous medium. Evaporation and recharge fluxes were 

applied to cells containing the water table or the pond 

surface in the model.

As described previously, a seasonal pond was 

simulated by using a porosity value of 1.0 and a very 

large permeability value that is four orders of magnitude larger than that of the underlying aquifer material. 

Use of any greater value of permeability in the pond

Parameter Estimation and Available Data 75
resulted in numerical instability. The longitudinal and 

transverse dispersivity values used for the pond were 2 

x 10 ft to allow salt mixing to a nearly uniform concentration.

Evaporation occurs at the land surface and from 

the surface pond during the summer months. Maximum evaporation flux from the land surface was estimated on the basis of the average of values calculated 

as net actual evaporation after subtracting rainfall. A 

value of 0.019 in/d (1.6 x 10'3 ft3/ft2/d) was applied 

over the entire upper surface of the simulation region. 

The evaporation rate from the seasonal pond was set at 

0.10 in/d (8.0 x 10"3 ft3/ ft2/d), which was five times the 

maximum evaporation rate from the land surface. This 

higher rate is justified because over open water, the 

actual evaporation rate approaches the potential evaporation rate (figs. 18 and 19). Use of a higher evaporation 

rate over the pond increases the nonlinearity of the 

evaporation function by adding a discontinuity at the 

land surface (pond bottom). An extinction depth of 3 ft 

was chosen on the basis of curves from Ripple and others (1975, p. A-18) for evaporation from bare soils. The 

effective land-surface altitude beneath the pond was set 

at 4,214.25 ft for evaporation calculations. This altitude 

also was used for all calculations of volume of brine in 

the pond.

Recharge at the land surface and from the surface pond during the recovery season is assumed to be 

from infiltration of water that is from direct precipitation on the playa surface. Precipitation during the summer months evaporates either before or soon after 

infiltrating into the subsurface and therefore is not considered to be a substantial component of recharge to the 

shallow-brine aquifer. Winter precipitation is subject to 

less evaporation and provides substantial recharge. At 

the weather station in Wendover, the 1961-90 normal 

monthly precipitation during the 6 months of winter is 

2.25 in. As mentioned in the "Climate" section of this 

report, there appears to be more precipitation at the 

Bonneville Salt Flats weather stations than at the Wendover weather station; however, the period of record is 

too short to know if this is representative over the long 

term. The 2.25 in. of precipitation, when applied to the 

playa, is more than the amount needed for recharge to 

the shallow-brine aquifer; the excess contributes to the 

formation of a surface pond. This amount of precipitation was converted into a seasonal volume of water 

applied to the simulation region. Initially, this recharge 

volume of water was apportioned between two areal 

zones. The corresponding precipitation rates for the

185-day recovery period were 0.034 in/d (2.8 x 10"3 

ft3/ft2/d) for the salt crust and 0.0036 in/d (3.0 x 10'4 

ft3/ ft2/d) for the carbonate-mud surface. This apportionment was based on Lines' (1979, p. 85) estimated 

ratio of 80 percent of recharge occurring on the salt 

crust and 20 percent on the carbonate mud.

The associated density of the recharge fluid initially was assumed to be 1.19 g/cm3 for the salt crust 

and 1.17 g/cm3 for the carbonate-mud surface. These 

values were assumed to approximate the density for the 

brine infiltrating the aquifer. Water infiltrating through 

the salt crust will approach halite saturation more 

quickly than water infiltrating through the carbonate 

mud, where halite crystals are not as prevalent.

Initial Conditions for Dependent Variables

Because the ground-water system was simulated 

as being in a periodic steady state, the simulation 

results will be independent of the initial condition for 

the pressure field. Thus, for simplicity, a hydrostatic 

pressure distribution based on a flat water table at an 

altitude of 4,214.5 ft was used as the initial condition.

The initial condition for the solute-transport 

equation was a concentration distribution derived from 

brine-density data. The initial density distribution was 

developed by fitting a cubic trend surface to measured 

density data. The contoured cubic trend surface for the 

density of brine in the shallow-brine aquifer, in grams 

per cubic centimeter, is shown in figure 34. Density 

data were measured primarily in wells completed in the 

carbonate mud of the shallow-brine aquifer. Some density data were measured in wells completed in the 

perennial salt crust. Brine-density values from the salt 

crust were near halite saturation and those from the carbonate mud were typically below saturation. Other than 

the density variation mentioned previously, the amount 

of density profile data available was limited and failed 

to define a density variation with depth. Furthermore, 

the vertical fractures were presumed to cause vertical 

mixing of brine. Therefore, the initial-condition density distribution was assumed to be uniform throughout 

the aquifer thickness.

To obtain an initial condition for the solute-concentration field, the measured density field was transformed using the equation from Lines (1979, p. 94, fig. 

45) and the definition of solute-mass fraction. Thus, the 

concentration is given by

c = 1590(p-1.000) (22)

76 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
N

A

EXPLANATION

Density, in grams per cubic centimeter

1.04

Figure 34. Initial-condition density field used in the model of the shallow-brine aquifer and represented by cubic trend surface, Bonneville Salt Flats, Utah
and the mass fraction is obtained from Calibration Criteria

w =

0.99823 + c

(23)

where

c is the solute concentration (g/L);

p is the fluid density at 20°C (g/cm3);

w is the solute-mass fraction (-); and 

0.99823 is the density of pure water at 20°C (g/cm3). 

The resulting solute-concentration field is very 

similar in pattern to figure 34 and will not be shown 

here.

Model Calibration

Model calibration refers to the procedure 

whereby a set of the model parameters, and possibly 

their distributions, are adjusted to obtain an optimal 

match between a set of model-output values and those 

observed in the actual system being simulated. The first 

step in model calibration is to select the set of parameters that are to be adjusted to their optimal values. 

Ranges are then estimated for each selected parameter, 

within which the search for optimal values is carried 

out. Some parameters are distributed in space, so the 

possibility of searching for the optimal spatial distribution arises. Clearly, the multitude of possible spatial 

distributions makes all but a limited trial of a few distributions impractical in the absence of automated 

parameter-estimation techniques.

Model calibration was done in the classical 

method of sequential parameter adjustment, by using a 

series of simulations in the search for optimal parameter values and distributions. It would have been more 

objective to use a numerical optimization procedure on 

a computer for model calibration, but this was beyond 

the scope of the study.

Model calibration consisted of seasonal transient simulations run to periodic steady state. A synthetic steady-state simulation representing average 

conditions for a typical year was attempted, but that 

approach was abandoned because of the inability to 

apportion the annual average boundary-condition values to uniquely represent the production and recovery 

seasons. Simulations to periodic steady state were run 

for seven annual cycles.

Comparisons were made between observed and 

model-computed results at the end of the two seasons 

once periodic steady state was obtained. The following 

criteria were chosen to evaluate the quality of the 

model:

1. Match the brine-production volume from the 

brine-collection ditch to within 10 percent of 

the estimated annual production volume of 

1,500 acre-ft.

2. Balance the outflow volume of fluid during the 

production season with the inflow volume of 

fluid during the recovery season to within the 

global volumetric balance error of the simulation.

3. Obtain a reasonable match between contour patterns of the simulated water-table surface and 

contour patterns of measured water-level data 

at the end of the production season and at the 

end of the recovery season.

4. Create a pond with water depth of a few tenths of 

a foot at the end of the recovery season.

Parameter Value and Distribution Adjustments

Several parameter values and distribution patterns were adjusted to obtain a calibrated model of the 

shallow-brine aquifer under the Bonneville Salt Flats. 

The model parameters that were selected to be adjustable for calibration were (1) aquifer permeability and 

porosity values and distributions; (2) water levels in the 

brine-collection ditch; (3) evaporation-flux values and 

distribution; and (4) precipitation-recharge flux values 

and distribution.

Permeability in the vicinity of the south boundary was lowered by modifying the digitized transmissivity distribution from Lines (1979, fig. 33) and then 

recalculating permeability using the density and viscosity distributions as described in the "Parameter estimation and available data" section in this report. The 

initial transmissivity distribution was modified only 

within its original range of 500 ft2/d to 6,000 ft2/d. During initial simulations, the computed hydraulic gradient 

toward the south boundary was less and extended farther north of Interstate Highway 80 than shown by 

measured water-level data. The transmissivity was 

decreased to one-sixth of its initial value in four rows 

of cells just north of the brine-collection ditch along the 

south boundary and the porosity was decreased from 

0.10 to 0.07 in the same area. By decreasing both

78 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
parameters, the hydraulic gradients from the simulation 

more closely matched gradients calculated from measured water-level values during both the production 

and the recovery seasons. The reduction of these 

parameters is justified as a result of construction of 

Interstate Highway 80 as discussed previously in the 

"Discharge" section for the shallow-brine aquifer.

Porosity was increased from 0.10 to 0.20 in the 

area of the salt crust and the area between the salt crust 

and the brine-collection ditch to the east. This increase 

was to reduce an excessive rise in water levels during 

the recovery season and is believed to be within conceptual limits because of the large number of fractures 

in this area. Permeability for the shallow-brine aquifer 

in these areas was not changed from the initial distribution.

Permeability values were adjusted by a factor of 

two and porosity values by a factor of 1.5 along the 

northwestern margin of the playa to better match the 

hydraulic gradient based on measured water levels in 

this area. The initial simulations gave a hydraulic gradient that was too steep in the vicinity of this boundary. 

With the initial porosity value, simulated water levels 

were too high during the recovery season. Seasonally 

changing potentiometric heads were specified on the 

external boundary of the leakage layer along this 

boundary (figs. 31 and 32). The contour map of horizontal permeability distribution for the calibrated 

model after all of these adjustments is shown in figure 

35. Vertical permeability was set at 0.8 of the horizontal 

value, as mentioned previously. The final porosity distribution included values of 0.20, 0.15, 0.10, and 0.07 

and assigned to the salt crust, northwest carbonate 

mud, east carbonate mud, and south carbonate mud, 

respectively (fig. 36).

The brine-collection ditch levels were estimated 

to vary more than 4 ft during a typical year. Water levels 

measured near the booster pump south of Interstate 

Highway 80 ranged in altitude from 4,206 to 4,211 ft 

during the summer of 1992. The initial set of three 

water-stage levels was adjusted through this range to 

obtain the target brine-production volume previously 

reported. For the calibrated model, the brine-collection 

ditch water levels were 4,208.75, 4,209.75, and 

4,210.75 ft.

During the model-calibration phase, maximum 

evaporation flux was divided into two zones (fig. 37), 

one for the pond/salt-crust area and one for the surrounding carbonate-mud surface. During calibration, 

maximum evaporation rates of 0.019 in/d (0.0016

ft3/ft2/d) from the salt crust and 0.016 in/d (0.0013 

ft3/ft2/d) from the carbonate-mud surface were determined. This distribution represents a reduction from 

the initial uniform maximum evaporation flux. The 

total seasonal volume of precipitation recharge was 

reapportioned among four zones shown in figure 38.

Recharge rates of 0.025 in/d (2.075 x 10'3 

ft3/ft2/d) for the salt crust, 0.012 in/d (J.01 x lO'3

Q ry

ft / ft /d) for the carbonate-mud surface between the 

salt crust and the brine-collection ditch, 0.0078 in/d 

(6.5 x 10"4 ft3/ ft2/d) for the carbonate-mud surface east 

of the brine-collection ditch, and 0.0050 in/d (4.18 x

A o o

10 ft / ft /d) for the remaining carbonate-mud surface 

to the northwest and north were determined as optimal. 

The higher recharge rates occur for the salt crust and 

the area between the salt crust and the brine-collection 

ditch. This is supported by the more fractured nature of 

the salt crust and the carbonate mud in this area where 

crystalline halite has precipitated within the carbonatemud matrix. In addition, ponding on the carbonate-mud 

surface in this area increases the net recharge to more 

than that which applies to carbonate-mud surfaces that 

do not experience ponding. This four-zone distribution 

gave a better match of the simulated water-table configuration to measured water-table values in the vicinity 

of the brine-collection ditch during the recovery season 

and provided sufficient water for ponding in the area of 

the salt crust. Recall that the initial recharge distribution was over two zones with a somewhat larger value 

ovt r the salt crust.

Re suits of Calibration

After seven annual cycles of simulation, the 

marimum absolute water-table configuration changes 

were less than 0.06 ft for two successive production 

periods and less than 0.09 ft for two successive recovery periods. The volume of fluid in the system was the 

same as at the end of the previous cycle to within 0.3 

percent of the change in volume for 1 year. Because the 

ma >s balance on the simulated system had errors of 

abcut 1.5 percent, these results were deemed accurate 

enc ugh to assume that periodic steady state had been 

achieved for the ground-water flow system. The simulated seasonal volume of the brine-collection ditch was 

1,4 51 acre-ft (6.32 x 107 ft3), which was well within the 

calibration criterion of plus or minus 10 percent of the 

estimated volume.

A periodic steady state for the flow system was 

attained after seven cycles of simulation, or 7 years. 

The; length of time required to achieve periodic steady

Model Calibration 79
I

D 33 

OO

m oo 

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EXPLANATION

Horizontal permeability, in 10"10 feet squared

1.0- 2.0

N

Figure 35. Final distribution of horizontal permeability used in the calibrated model of the shallow-brine aquifer, Bonneville Salt Flats, Utah.
EXPLANATION

_ Effective porosity, in percent [ijijijjjijiiil o.o?

|: ,,| o.io^H 0.20

N

o

ft

O0)

Figure 36. Final distribution of effective porosity used in the calibrated model of the shallow-brine aquifer, Bonneville Salt Flats, Utah.
03J 

oo oo•n mDO

O

I

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30

EXPLANATION

Evaporation, in inches per day

0.0160.019 (0.095 for surface pond)

N

A

Figure 37. Final distribution of maximum evaporation rates used in the calibrated model of the shallow-brine aquifer during the 

production season, Bonneville Salt Flats, Utah.
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Model Calibration 83
state implies that the response time for the flow system 

is on the order of a decade. Changes in boundary-condition pressures, fluxes, or leakage parameters can be 

expected to affect the flow system for time on the order 

of a decade provided that these changes are not far from 

the calibrated model values. The response time for the 

solute-transport system is longer than for the flow system. Simulations were not run for a sufficient number 

of cycles to achieve periodic steady state with respect 

to solute concentration. The average residence time 

within the shallow-brine aquifer, however, was estimated to be about 120 years. This was calculated as the 

quotient of the maximum ground-water volume 

divided by the through-flow rate. The through-flow rate 

is the annual inflow rate or the annual outflow rate, 

because both must balance at periodic steady state for 

an annual cycle.

The periodic change in fluid volume for the calibrated model (fig. 39) includes the pond. The approach 

to periodic steady state is clearly seen. The net volume 

change over a year is a loss of 39 acre-ft. But the volumetric balance error of the simulator in the seventh year 

of simulation was 650 acre-ft. Therefore, the deviation 

from an exact steady-state volumetric balance is well 

within the balance error of the simulator. Thus, the second calibration criterion was met to within the accuracy 

of the simulation.

within the mass-balance error of each year of the simulation. Therefore, no further calibration was done to 

remove this upward trend.

123456 

TIME, IN YEARS

Figure 39. Change in total volume of fluid in the 

simulation region for the calibrated model of the 

shallow-brine aquifer during seven annual cycles, 

Bonneville Salt Flats, Utah.

A general increase with time can be seen in the 

periodic change in solute content of the ground water in 

the calibrated model (fig. 40). The total increase during 

the 7-year simulation, however, is only 1.25 percent of 

the solute mass in the simulation region, which is

123456 

TIME, IN YEARS

Figure 40. Change in total mass of solute in the 

simulation region for the calibrated model of the 

shallow-brine aquifer during seven annual cycles, 

Bonneville Salt Flats, Utah.

No statistical measurements were made that 

describe the quality of the match between the simulated 

water-table surface and the measured water-level data. 

This was because only a small set of seasonal measurements were made during a 2-year period. One of those 

years was much cooler and wetter than normal, which 

resulted in unusual conditions on the Bonneville Salt 

Flats. The amount and duration of ponding increased 

dramatically. A semi-quantitative assessment of the 

water-table patterns and water levels shows that the 

simulated water-table surfaces match the measured 

water-level data satisfactorily for the representative 

steady-state conditions and meet the third calibration 

criterion.

The pond at the end of the recovery season covers more than 90 percent of the pond area with a depth 

of more than 0.2 ft and a maximum depth near 0.4 ft. 

The southwestern and southeastern corners of the pond 

area have water depths as shallow as 0.05 ft because of 

boundary-condition influences. Thus the fourth calibration criterion, that of pond formation, is satisfied.

Simulation Results

Results of the simulations from the calibrated 

model include water-table altitudes, density fields, and 

cumulative fluid volume and solute-mass amounts 

through the boundary surfaces. As described in the

84 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
"Modeling approach" section of this report, the goals 

of simulating the solute transport in the shallow-brine 

aquifer under the Bonneville Salt Flats were: (1) 

develop a fluid and solute balance for the shallow-brine 

aquifer in the study region, (2) evaluate the effect of 

brine production from the ditches on the salt crust, and 

(3) identify the major and minor solute fluxes to and 

from the shallow-brine aquifer. The first two goals are 

met by computing (1) a flow balance on the shallowbrine aquifer beneath the Bonneville Salt Flats, (2) a 

solute balance for the simulation region, and (3) a mass 

balance on the total crystalline salt deposit. These 

quantitative balances help to understand how the brine 

production affects the gradual disappearance of the salt 

crust. The third goal is achieved by examination of the 

cumulative flow and transport amounts calculated for 

each boundary face.

The set of seasonal simulations leading to a periodic steady state began with the production season. An 

automatic time-step control was used by the simulator 

starting at 0.01 days and increased to about 20 days 

during the simulation seasons. The main control was on 

the pressure change per time step, which was limited to 

0.01 lbs/in2 and corresponds to about 0.025 ft of watertable altitude change. The secondary control, almost 

never invoked, was to limit the concentration change 

per time step to 0.01 in solute-mass fraction. With these 

time-step control parameters, the overall mass-balance 

error for the brine was within 1.5 percent for production seasons and 0.6 percent for recovery seasons. Similar errors were obtained for the solute balance. These 

errors are presented as percentages of the net outflow of 

brine and net outflow of solute from the simulation 

region. Most of the errors occurred because cells 

became dry or rewetted and because of the nonlinearity 

(piecewise linearity) and explicit treatment of the evaporation boundary condition. To reduce the mass-balance error through use of shorter time steps by 

imposing more restrictive pressure change and soluteconcentration change limits would have made simulation times excessively long.

Dependent Variables

The dependent variables from the simulations 

were fluid pressure and solute-mass fraction. The pressures at the uppermost saturated cells were used to calculate a water-table altitude distribution for 

comparison with measured water levels. The solutemass-fraction field was used to calculate the density 

distribution for the aquifer and pond brine. Other simulator outputs of primary interest were the flow rates 

and cumulative amounts of brine and solute that moved 

through the boundary surfaces of the simulation region 

and the mass-balance calculations for the region.

The water-table configurations at the end of the 

production and recovery seasons are shown in figures 

41 and 42. At the end of the production season, the 

drawdown in the vicinity of the brine-collection ditch 

and along the southern boundary can be seen. Also, the 

gradient toward the northwest with flow to the alluvialfan aquifer is apparent. At the end of the recovery season, the water-table drawdown in the vicinity of the 

brine-collection ditch has not recovered completely. 

Setting the water level in the brine-collection ditch 

along the south boundary at 4,200 ft during production 

and 4,208 ft during recovery has maintained a watertable slope to the south along the southern boundary. 

One significant result of calibration is that the high 

transmissivity values at the southern boundary interpreted by Lines (1979, fig. 33) appear inconsistent with 

the calibrated permeability values converted to equivalent transmissivity. To match the measured water levels 

and obtain similar head gradients along the south-ditch 

boundary, the transmissivity had to be reduced by a factor of one-sixth.

Net cumulative flow through the northwestern 

boundary reverses during the recovery season and 

enters the shallow-brine aquifer. The approximation for 

a seasonal pond, which is described in the "Model 

approach" section of this report, does result in the formation of a pond at the end of the recovery season with 

a water depth of as much as 0.4 ft as mentioned in the 

"Model calibration" section of this report.

At the end of both seasons, a comparison was 

made between the simulated vertical leakage through 

the bottom boundary at the five nested-well locations 

and the vertical leakage calculated from observed water 

levels and brine densities (see "Discharge" section of 

the "Shallow-brine aquifer" section of this report). In 

general, all the calculated specific-discharge fluxes 

were downward. The simulated specific-discharge 

fluxes agreed with the calculated fluxes to within factors of 1.2 to 3 for both seasons at all but one location. 

At wells (B-l-17)23abd-l, -2, and -3, the simulated 

flux was greater than the calculated flux by a factor of 

7 during the recovery season. That is the only site with 

an observed hydraulic gradient that was sometimes in 

the upward direction. The simulated specific-discharge 

flux at the bottom boundary at the four other locations 

was always downward at the end of the seasons.

Simulation Results 85
m 

CD O

m

EXPLANATION

-4213— Water-table contour—Shows altitude, in 

feet, of model-computed water-table 

configuration. Contour interval variable. 

Datum is sea level

N

Figure 41. Water-table configuration at the end of the last simulated production season for the calibrated model of the 

Bonneville Salt Flats, Utah.
EXPLANATION

•421 3— Water-table contour—Shows altitude, in 

feet, of model-computed water-table 

configuration. Contour interval variable. 

Datum is sea level

N

CO

I 

a5'

3D 

<D 

(A

I

Figure 42. Water-table configuration at the end of the last simulated recovery season for the calibrated model of the 

Bonneville Salt Flats, Utah.
The simulated vertical interstitial velocity above 

the aquifer bottom at the five nested-well locations was 

variable with depth. At the end of the production season 

there was upward flow between all cell layers at three 

locations, where wells (B-l-17)! laaa-1 and -2, (B-l17)21add-3 and -4, and (C-l-18)9adc-l and -2 are 

located. At the location for wells (B-l-17)23abd-l, -2, 

and -3, there was downward flow between all cell layers. These wells are located adjacent to the brine-collection ditch, which is probably influencing the flow 

direction. At the location for wells (B-1-17)3lace-1 

and -7, the flow was downward from cell layers 4 to 1 

and upward from cell layers 4 to 6. The reason for the 

split-flow directions at this location is unknown. At the 

end of the recovery season there was downward flow at 

only two locations, where wells (B-1-17)1 laaa-1 and - 

2 and (B-1-17)3lace-1 and -7 are located. At the location for wells (C-1-18)9adc-1 and -2, there was upward 

flow from cell layers 1 to 4 and downward flow from 

cell layers 7 to 4. This location is at the southwestern 

end of the salt-crust/pond area, but there are no known 

hydrologic features to account for the split flow here.

An inspection of the simulated velocity fields at 

the end of each season revealed no ground-water interstitial velocities that were unusually large. Maximum 

interstitial velocities were less than 2 ft/d except within 

the pond and through the pond bottom. Interstitial 

velocities ranged from 10~4 to 10~2 ft/d throughout most 

of the simulation region.

Density fields are calculated by the HST3D simulator (Kipp, 1987) from the solute-mass-fraction 

fields using the linear relation of density as a function 

of solute-mass fraction. Because the simulated density 

field had only minor variation throughout the thickness 

of the shallow-brine aquifer (less than 0.001 g/cm3 ), 

results for the density fields are presented as averages 

throughout the aquifer thickness. The vertically averaged density fields at the end of the last simulated production and recovery seasons are shown in figures 43 

and 44. The essentially uniform density with depth is 

mostly caused by the large amount of vertical mixing 

that occurs from using a high transverse dispersivity 

(500 ft), which enhances vertical transport by the 

mostly horizontal flow in the system. In areas where 

vertical flow was large, there was also a large amount 

of vertical mixing because the longitudinal dispersivity 

also was 500 ft.

The density field appears to be nearly the same 

at the end of the production and recovery seasons. The 

only difference was a slightly larger mound with deno

sity greater than 1.18 g/cm near the east-central edge 

of the salt crust at the end of the final production season 

as compared to the recovery season. The decrease of 

density toward the northwest is about the same as the 

initial condition and is maintained by runoff of fresher 

water from the Silver Island Mountains that provides 

recharge through the alluvial-fan aquifer, especially in 

the recovery season. The initial zone of density greater 

than 1.19 g/cm3 has disappeared and the zone of deno

sity greater than 1.17 g/cm has enlarged to approximately the outline of the pond and salt crust. The 

density contours from the initial condition in the vicinity of the brine-collection ditch have moved to reflect 

the influence of the ditch. A zone of density greater 

than 1.17 g/cm3 in the southeastern corner of the simulation region is a remnant of the initial-condition density distribution and is probably not realistic. Few data 

were available in this area to constrain the least-squares 

cubic trend surface. This remnant also indicates that the 

solute distribution has not reached periodic steady state 

to the same degree as the flow system at the end of 

seven annual cycles.

Cumulative Amounts

In order to present the simulated cumulative 

amounts of fluid and solute that flowed into and out of 

the shallow-brine aquifer, the volume of the region 

being considered needs to be defined. For accounting 

purposes, this volume is called the control volume. For 

flow and transport budget purposes, the control volume 

is defined as being bounded by the land surface, the 

bottom of the shallow-brine aquifer, and the northern, 

northwestern, western, southern, and eastern boundaries used in the formulation of the simulation region 

(fig. 30). The control volume includes the crystalline 

salt crust on the land surface and interbedded crystalline salt within the carbonate mud above the water table 

and the brine in the simulated pond. Water in the brinecollection ditch in the interior of the simulation region 

is excluded. The transfer of salt between the solid and 

liquid phase within the shallow-brine aquifer is 

enclosed with parentheses and not included in the totals 

for fluid and solute entering and leaving the control volume.

The estimated and simulated cumulative 

amounts of fluid and solute that passed through each 

boundary of the shallow-brine aquifer during a year are 

listed in table 4. During a typical year, most of the 

ground-water flow leaves the simulation region through 

evaporation and enters by infiltration from the playa

88 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
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0)

CO

IS-D=» 

O) W"

2 J5

0) U_

s =

II

u. co

Simulation Results 89
I

o

3) 

O

O 

O

OT

Z 

O

EXPLANATION

•1.08—— Density contour—Shows density, in grams 

per cubic centimeter, of the vertically averaged 

model-computed density field. Contour 

interval variable

Brine-collection ditch

Figure 44. Vertically averaged density field at the end of the last simulated recovery season for the calibrated mode

aquifer, Bonneville Salt Flats, Utah.
Table 4. Estimated and simulated annual fluid flow and solute transport into and out of the shallow-brine aquifer, Bonneville 

Salt Flats, Utah

t—, no estimate; -, indicates direction of movement is opposite of designated category; NA, not applicable because salt not leaving shallow-brine aquifer;

(), indicates value not added to total]

Components

Current study

Acre-feet Tons of solute

Calibrated simulation

Acre-feet Tons of solute

Recharge

Subsurface inflow from east

and northeast

Subsurface inflow from west 

Infiltration from playa surface

Total (rounded)

Discharge

Subsurface outflow to south 

Subsurface outflow to

brine-collection ditch 

Subsurface outflow to northwest 

Downward leakage 

Evaporation

Total (rounded)

60

8,300-12,800

8,400-12,900

1,800-2,300 

1,500

35

58-580 

15,000

21,900

NA

21,900

700,000-895,000 

580,000

13,600

22,600-226,000 

NA

18,400-19,400 1,316,000-1,715,000

9

11,830

11,930

705

1,450

315

27

10,050

12,550

27,750

-750 

(4,712,000)

27,000

285,100

525,000

50,590

11,000

(3,742,000)

872,000

surface. Significant amounts of brine are removed by 

the brine-collection ditch, located east of the salt crust, 

and net outflow through the southern boundary. Most 

of the outflow along the south boundary was toward the 

brine-collection ditch located south of Interstate Highway 80. There is net subsurface outflow of brine 

through the northwestern boundary. The remaining 

boundaries have minor amounts of subsurface flow. 

The sum of the minor inflow amounts is about 1.2 percent of the total annual inflow amount for the shallowbrine aquifer, and the sum of the minor outflow 

amounts is about 0.2 percent of the total annual outflow 

amount. The boundary-flow amounts for brine entering 

and leaving the simulation region are shown in a generalized block diagram (fig. 45). Note that along the 

western boundary, the net flow is into the shallow-brine 

aquifer during the recovery season, and the net solute 

flow is out.This is because the average density of the 

outflow is greater than that of the inflow.

For the calibrated model, the change in fluid volume between the end of the production season and the 

end of the recovery season was 14,500 acre-ft (6.3 x 

108 ft3) or about 5 percent of the volume in the simulation region at the end of the recovery season. The pond 

volume at the end of the recovery season was 2,440 

acre-ft (1.07 x 108 ft3), which is about 17 percent of the

annual volumetric change of brine in the simulation 

region. The amount of water that evaporates from and 

recharges to the seasonal pond is listed in table 5 and 

shown in figure 45. The net flow of brine is nearly zero 

(4 percent of the average through flow, and 0.2 percent 

of the peak volumetric inventory) because the groundwater system is in periodic steady state.

The cumulative solute-mass amounts that passed 

through each boundary during the production and 

recovery seasons are listed in table 5. An imbalance of 

125,000 tons/yr of solute, which represents about 0.1 

percent of the peak inventory during the year, is lost 

from the shallow-brine aquifer. More solute is transported into and out of the seasonal pond than the sum 

of solute produced by the brine-collection ditch and 

that which is transported through the southern boundary. The boundary-flow amounts for solute also are 

shown in figure 45.

Balances

For a discussion of simulation balances, amounts 

of fluid flow and solute transport are listed for the production and recovery seasons. The control volume for 

accounting purposes, however, is slightly different than 

the one defined previously. Because the model simulates primarily advective fluid flow and associated solSimulation Results 91
Table 5. Simulated fluid and solute inflow to and outflow from the shallow-brine aquifer and pond for the production and 

recovery seasons in the final year

[-, indicates direction of movement is out of shallow-brine aquifer; (), indicates value not added to total]

Production season

Component

South

Ditch

Northwest

Bottom

Evaporation (pond) 

Evaporation (aquifer) 

West

North

East

Recharge (pond) 

Recharge (aquifer)

Total (rounded)

Subsurface outflow

Subsurface inflow

Acre-feet

-650

-1,450

-380

-13

(-2,440) 

-10,050 

8

17

35

(0) 

0

-12,500

2,490

60

Tons of solute

-264,500

-525,000

-52,500

-5,250

(-933,500) 

-3,742,000 

270

4,050

11,400

(0) 

0

-4,574,000

847,000

15,700

Recovery season

Acre-feet

-53

0

64

-14

(0) 

0 

1

-9

45

(2,440) 

11,830

11,860

76

111

Tons of solute

-20,550

0

1,910

-5,750

(0) 

0 

-1,020

-2,450

14,750

(938,300) 

4,712,000

4,699,000

29,800

16,700

EXPLANATION

315 Upper number in acre-feet 

50,590 Lower number in tons of solute

9 — 

-750

1,450 ^

525,000 .p

t

t

/ 705

r 285,100

27

11,000

Figure 45. Generalized block diagram showing the direction of annual inflows and outflows of fluid and solute for 

the calibrated model of the shallow-brine aquifer, Bonneville Salt Flats, Utah.

92 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
ute transport into and out of the shallow-brine aquifer, 

the simulated seasonal pond, although part of the simulation region, is defined as a separate volume, external 

to the control volume. Transfers of fluid and solute to 

and from the simulated pond are not included in the 

totals that apply to the control volume and therefore are 

enclosed in parentheses.

As mentioned previously, global balance errors 

for cumulative flow and solute-transport amounts for 

the simulation region were 1.5 percent for the production season and 0.6 percent for the recovery season. 

This gives an annual balance error of about 2 percent. 

Because the aquifer system is in periodic steady state, 

net brine outflow during the production season is 

replenished by an excess of precipitation that recharges 

the aquifer during the recovery season in the winter and 

spring. A volumetric imbalance of about 5 percent for 

the shallow-brine aquifer exclusive of the seasonal 

pond is shown by the difference in totals for the two 

seasons listed in table 5. This represents the degree to 

which periodic steady state was not achieved.

The net subsurface outflow of solute is mostly 

replenished by dissolution of crystalline salt, both from 

the crust on the playa surface and from crystalline salt 

within the carbonate mud. This gives a solute inventory 

for the shallow-brine aquifer that is nearly in periodic 

steady state. A solute-mass imbalance of about 3 percent can be seen in table 5.

In all simulations (including the sensitivity cases 

discussed later in this report), the amount of solute 

entering the shallow-brine aquifer through recharge is 

slightly more than the amount leaving by solute transport in the subsurface. Thus, the average brine density 

(solute in storage) is slowly increasing during the 

cycles of simulation. This indicates that the specified 

densities of the recharging precipitation are slightly too 

high. An adjustment could have been made, but the 

drift in density is within the solute-balance error of the 

model. In fact, the drift in density could be considered 

to be a result of cumulative model error.

The evaporating brine precipitates salt at the 

playa surface and within interstitial pore space and 

small fractures above the water table within the carbonate mud and in the salt crust. The simulator calculates 

seasonal solute-flow amounts, which can be translated 

to changes in the total crystalline-salt deposit due to 

evaporation and precipitation fluxes during the year. 

Although the simulator has no salt-precipitation mechanism, the amount of salt accumulated as precipitate 

can be approximated by totaling the solute removed

from the saturated ground-water region by the evaporation flux. This approximation neglects any effects of 

concentrating the brine near the water table as evaporation occurs.

Recharge by precipitation (rainfall and snowfall) 

dissolves salt from the crust and crystalline halite 

within the carbonate mud as it infiltrates down to the 

water table. Although the simulator lacks a salt-crust 

dissolution mechanism, a solute-recharge amount 

equal to a salt-dissolution amount for the recovery season was approximated by specifying the fluid density 

(solute concentration) of the recharging precipitation. 

This concentration must be estimated from density 

measurements of brine samples from the wells near the 

salt-crust surface. The recharging fluid is not necessarily at halite saturation and concentrations range from 

1.15 to 1.20 g/cm3 . This gives a significant degree of 

uncertainty for estimating average incoming density.

The difference between the solute entering the 

shallow-brine aquifer and the seasonal pond through 

dissolution of crystalline salt during recharge in the 

recovery season and the solute leaving through salt precipitation upon evaporation during the production season at periodic steady state represents the crystallinesalt budget primarily for the salt crust overlying the 

simulation region. The crystalline-salt budget (table 5) 

indicates that for the calibrated model there is a net loss 

of crystalline salt of almost 975,000 tons during an 

average year of which 850,000 tons are by subsurface 

outflow. From model output, the global solute massbalance error during the year is about one-quarter of 

this amount, so the loss of crystalline salt is partially 

masked by the simulator balance error.

The model adequately simulates the effects of 

brine withdrawal for mineral production on the solute 

and salt-crust balance of the system. Model simulations, which use the average rate of withdrawal from 

the brine-collection ditch estimated during this study, 

do not refute the conceptual understanding that brine 

withdrawal is a major cause to the loss of salt from the 

playa surface. Other than the cycling of fluid and solute 

through the playa surface with each season, subsurface 

fluid flow and solute transport to the south boundary 

and to the brine-collection ditch east of the salt crust are 

the largest losses of salt from the simulation region 

(tables 4 and 5).

Simulation Results 93
Sensitivity Analysis

A sensitivity analysis is an evaluation of the 

effects of changes in individual model parameters on 

simulation results, including specific values and distribution patterns. In principle, the pattern of distributed 

parameters could be varied, but sensitivity to variation 

of distribution patterns, and to distribution patterns 

themselves, can be difficult to quantify. A sensitivity 

analysis provides an indication of the uncertainty of the 

calibrated model as a function of the uncertainty of 

estimation of the parameters. Observations of the sensitivity of the model to variations in parameters were 

made throughout the calibration process. A more 

methodical analysis was done at the end of model calibration for selected parameters.

Sensitivity to Model Parameter Values and 

Distributions

The following parameters were selected from 

those with greater uncertainty in estimation, or with 

higher observed sensitivity during the calibration process. The set of parameters for sensitivity evaluation 

included:

• permeability near south boundary;

• south-boundary ditch water level;

• density of infiltrating precipitation;

• evaporation rates; and

• porosity values.

The response of the simulations to the parameter 

variations was evaluated relative to the results obtained 

for the calibrated model. Because the calibration simulations were run for seven annual cycles to reach a periodic steady state, the sensitivity simulations also were 

run for seven cycles. In some cases, this was not long 

enough to reach periodic steady state for the solute distribution; however, a periodic steady state for brine 

flow was reached for all simulations.

The seasonal simulations for sensitivity were 

combined into total annual results for comparison and 

evaluation of the cumulative flow and solute-transport 

amounts. In the following discussion of sensitivity 

results, percentage changes in annual flow amounts and 

annual solute-transport amounts will be highlighted. 

Each percentage change will be expressed relative to 

the corresponding flow or solute-transport amount for 

the calibrated model. The minor boundary flows and 

solute transport will not be discussed, although on a

percentage basis some of them exhibited large changes. 

Results are compared to those for the calibrated model 

and appear in a separate table for each sensitivity simulation.

Permeability Near South Boundary

Permeability near the south boundary was 

increased by a factor of 2. The water-table surface 

dropped at least 0.1 ft within about 10,000 ft of the 

brine-collection ditch along the south boundary during 

both seasons. The maximum drop was about 0.6 ft during the production season and 0.4 ft during the recovery 

season. There was an associated reduction in watertable gradient. Subsurface outflow at the southern 

boundary increased about 110 percent for the year 

(table 6). Solute production from the brine-collection 

ditch east of the salt crust decreased by about 6 percent. 

Pond volume at the end of the recovery season 

decreased to 1,960 acre-ft (8.54 x 107 ft3) or about 7 

percent. Crystalline-salt accumulation from evaporation of seasonal ponds and from the shallow-brine aquifer during the production season decreased about 7 

percent, and crystalline-salt dissolution from recharge 

decreased 0.3 percent. Total net annual loss of crystalline salt increased 33 percent and total net subsurface 

outflow of solute increased 24 percent. The change in 

solute inventory in the ground water at the end of the 

recovery season was less than 1 percent. Net subsurface 

outflow of fluid increased 22 percent for the simulation 

region.

Water Level in South-Boundary Ditch

The water level in the south-boundary ditch was 

raised to 4,203 ft during the production season and to 

4,210 ft during the recovery season. The resulting 

water-table rise of 1 ft or more was restricted to within 

1,000 ft of the brine-collection ditch along the south 

boundary for both seasons. The flow amount and solute-transport amount through the south boundary 

decreased 27 percent during the year (table 7). Solute 

production from the brine-collection ditch east of the 

salt crust increased by about 3 percent. The volume of 

the pond at the end of the recovery season increased 

about 5 percent. Total net annual loss of crystalline salt 

decreased about 6 percent. The change in solute inventory in the ground water at the end of the recovery season was less than 1 percent. Net subsurface outflow for 

the simulation region decreased 6 percent.

94 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Table 6. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when permeability 

was doubled along south boundary

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Component

Total (rounded)

Calibrated simulation Sensitivity simulation

Acre-feet Tons of solute Acre-feet

11,930 4,739,000 12,430

Tons of solute

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

9

8

80

(2,440)

11,830

-750

1,600

26,150

(938,300)

4,712,000

9

8

80

(1,960)

12,330

-750

1,540

25,960

(749,100)

4,883,000

4,910,000

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

705

1,450

315

27

(2,440 )

10,050

12,550

2,400

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

1,460

1,360

315

26

(1,960)

9,780

12,940

3,060

584,000

491,200

50,850

10,780

(745,900)

3,629,000

4,766,000

1,110,000

1,257,000

Density of Recharge Flux

Because of the uncertainty associated with the 

solute concentration of the infiltrating precipitation, the 

density associated with the precipitation-recharge flux 

applied to the salt crust was varied from the calibratedmodel value of 1.185 g/cm3 . The density was increased 

to 1.21 g/cm for the salt crust, but held at the calibrated value of 1.17 g/cm3 for the carbonate-mud surface. The water-table configuration remained 

essentially the same as for the calibrated model with 

maximum changes of less than 0.02 ft. Recharge of dissolved solute to the shallow-brine aquifer increased 9 

percent (table 8). Crystalline-salt accumulation from 

evaporation increased 4 percent. Ditch-production volume was virtually unchanged. Net subsurface outflow 

for the simulation region was unchanged, but there was 

a 1-percent increase in net outflow of solute. The total 

net annual loss of crystalline salt increased 25 percent; 

however, the solute mass in the ground-water system 

gradually increased 3 percent, which is somewhat more 

than a single season mass-balance error but within 

seven times the seasonal error. This means that the

gradual increase in solute mass during the simulation 

was within the overall mass-balance accuracy.

The density of the recharge flux was decreased 

from 1.17 to 1.15 g/cm3 for the carbonate-mud surface, 

while the density for the salt crust was held at the calibrated value of 1.185 g/cm3 . Again, the water-table 

configuration remained essentially the same. Solute 

production from the brine-collection ditch east of the 

salt crust decreased 5 percent (table 9). Recharge of 

solute to the shallow-brine aquifer decreased 5 percent. 

The amount of solute transported out through the 

southern boundary decreased less than 1 percent for the 

year. Net subsurface outflow for the simulation region 

was virtually unchanged, but there was a 4-percent 

decrease in net transport of solute out of the system. 

The annual net loss of crystalline salt decreased 21 percent, and the solute mass in the ground-water system 

decreased 1 percent.

Maximum Rate of Evaporation

Maximum evaporation flux was increased by 20 

percent of the calibrated rates throughout the simulation region. Water levels during the production season

Sensitivity Analysis 95
Table 7. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when specified level in 

brine-collection ditch along south boundary was raised during production and recovery seasons

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Component

Calibrated simulation Sensitivity simulation

Acre-feet Tons of solute Acre-feet Tons of solute

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

Total (rounded)

80

(2,440) 

11,830

11,930

-750

1,600

26,150

(938,300)

4,712,000

4,739,000

80

(2,580) 

11,700

11,800

-750

1,560

26,030

(991,100)

4,639,000

4,666,000

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

705

1,450

315

27

(2,440)

10,050

12,550

2,400

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

518

1,500

315

27

(2,580)

10,010

12,370

2,260

211,900

545,500

50,860

10,560

(985,700)

3,723,800

4,543,000

792,000

920,600

dropped more than 0.1 ft for most of the region and 

more than 0.2 ft to the southwest, southeast, and west 

of the brine-collection ditch east of the salt crust. During the recovery season, water levels did not recover as 

much as in the calibrated model and were more than 0.1 

ft lower in the pond area and more than 0.2 ft lower in 

the three areas listed previously. Pond volume at the 

end of the recovery season decreased 46 percent (table 

10). Flow and solute-transport amounts through the 

south boundary decreased about 7 percent. Removal of 

solute from the shallow-brine aquifer by evaporation 

increased 12 percent. Production brine volume and solute mass from the brine-collection ditch decreased 3 

percent. Total net annual loss of crystalline salt 

decreased by about 3 percent, and total net outflow of 

solute decreased 6 percent. The change in solute inventory in the ground water at the end of the recovery season was a decrease of 1 percent. The net subsurface 

outflow for the simulation region decreased 7 percent. 

Maximum evaporation flux was decreased by 20 

percent of the calibrated rates throughout the simulation region. The water table rose more than 0.1 ft 

throughout the region during the production season and

rose more than 0.2 ft in the area of the salt crust. There 

was about a 0.3-ft rise in the southeastern area. During 

the recovery season, the water table in the pond area 

was within 0.1 ft of that of the calibrated model. The 

water table was more than 0.2 ft higher outside the 

northwestern margin of the pond and west of the brinecollection ditch and was more than 0.3 ft higher in the 

southeastern area. Pond volume at the end of the recovery season increased about 54 percent (table 11). Flow 

of brine and associated solute transport through the 

south boundary increased 8 percent for the year. Evaporation amounts from the shallow-brine aquifer 

decreased 18 percent for the brine and 14 percent for 

the associated solute transport. Solute production from 

the brine-collection ditch east of the salt crust increased 

4 percent. Recharge to the shallow-brine aquifer 

decreased 11 percent. Total net annual loss of crystalline salt increased 2 percent, and net subsurface outflow of solute increased 7 percent. Solute inventory in 

the ground water at the end of the recovery season was 

increased by 1 percent. Net subsurface outflow for the 

simulation region increased 8 percent. Clearly, there is 

a high sensitivity to evaporation rate.

96 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Table 8. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when density of 

infiltrating fluid over salt crust was increased from 1.185 to 1.21 grams per cubic centimeter

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Calibrated simulation Sensitivity simulation

Component Acre-feet Tons of solute Acre-feet Tons of solute

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

Total (rounded)

80

(2,440) 

11,830

11,930

-750

1,600

26,150

(938,300)

4,712,000

4,739,000

9

7

80

(2,460) 

11,830

11,930

-745

1,400

26,000

(982,900)

5,167,000

5,194,000

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

705

1,450

315

27

(2,440)

10,050

12,550

2,400

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

695

1,450

315

29

(2,460)

10,060

12,550

2,390

292,100

526,500

50,850

12,300

(976,700)

3,866,000

4,748,000

855,100

1,307,000

Table 9. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when density of 

infiltrating fluid over carbonate mud was decreased from 1.17 to 1.15 grams per cubic centimeter

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Calibrated simulation Sensitivity simulation

Component Acre-feet Tons of solute Acre-feet Tons of solute

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

Total (rounded)

80

(2,440) 

11,830

11,930

-750

1,600

26,150

(938,300)

4,712,000

4,739,000

11 

10 

84

(2,430) 

11,860

11,960

-290

2,280

27,500

(929,400)

4,486,000

4,515,000

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

705

1,450

315

27

(2,440)

10,050

12,550

2,400

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

700

1,440

312

25

(2,420)

10,060

12,540

2,370

281,300

500,500

49,550

10,340

(926,100)

3,710,000

4,552,000

812,200

779,300

Sensitivity Analysis 97
Table 10. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when evaporation 

rates were increased by 20 percent of the calibrated rates

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Component

Recharge

West 

North 

East 

Recharge (pond) 

Recharge (aquifer)

Total (rounded)

Discharge

South 

Ditch 

Northwest 

Bottom 

Evaporation (pond) 

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow 

Total net loss to crystalline salt

Calibrated simulation

Acre-feet Tons of solute

9 -750 

8 1,600 

80 26,150 

(2,440) (938,300) 

11,830 4,712,000

11,930 4,739,000

705 285,100 

1,450 525,000 

315 50,590 

27 11,000 

(2,440) (933,500) 

10,050 3,742,000

12,550 4,614,000

2,400 844,700 

974,800

Sensitivity simulation

Acre-feet

15 

26 

100 

(1,310) 

12,980

13,120

653 

1,400 

300 

25 

(1,310) 

11,240

13,620 

2,240

Tons of solute

356 

6,910 

32,890 

(501,400) 

5,129,000

5,169,000

267,100 

507,000 

48,360 

10,360 

(500,600) 

4,190,000

5,023,000

792,700 

939,800

Table 11. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when evaporation 

rates were decreased by 20 percent of the calibrated rates

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Component

Recharge

West 

North 

East 

Recharge (pond) 

Recharge (aquifer)

Total (rounded)

Discharge

South 

Ditch 

Northwest 

Bottom 

Evaporation (pond) 

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow 

Total net loss to crystalline salt

Calibrated simulation

Acre-feet Tons of solute

9 -750 

8 1,600 

80 26,150 

(2,440) (938,300) 

11,830 4,712,000

11,930 4,739,000

705 285,100 

1,450 525,000 

315 50,590 

27 11,000 

(2,440) (933,500) 

10,050 3,742,000

12,550 4,614,000

2,400 844,700 

974,800

Sensitivity simulation

Acre-feet

3 

-13 

54 

(3,770) 

10,520

10,560

760 

1,510 

334 

29 

(3,770) 

8,600

11,230 

2,590

Tons of solute

-2,150 

-4,520 

17,450 

(1,446,000) 

4,184,000

4,195,000

304,600 

546,500 

54,220 

11,760 

(1,422,000) 

3,212,000

4,129,000

906,300 

996,000

98 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Porosity Values

The porosity value for each of the four zones was 

increased by 30 percent of the calibrated value. The 

water table at the end of the production season rose 

more than 0.1 ft in the pond area and the southeastern 

area and more than 0.2 ft in the vicinity of the brinecollection ditch along the southern boundary. At the 

end of the recovery season, the water table in the vicinity of the brine-collection ditch east of the salt crust and 

in the southeastern area was more than 0.2 ft lower than 

in the calibrated model. At the end of the recovery season, the water table was lower than the calibrated 

model by more than 0.2 ft for most of the region except 

near the brine-collection ditch east of the salt crust. 

Pond volume at the end of the recovery season 

decreased about 11 percent (table 12). Flow and solutetransport amounts through the southern boundary 

decreased about 1 percent. Solute production from the 

brine-collection ditch east of the salt crust increased 3 

percent. Increasing the porosity increased the inventory 

of solute in the ground-water system from the beginning of the simulation. This increase persisted throughout the approach to periodic steady state and resulted in 

a 28-percent increase. The net annual loss of crystalline 

salt increased by 1 percent.

Porosity values were decreased by 30 percent of 

the calibrated values. Water levels at the end of the production season were more than 0.2 ft lower near the 

brine-collection ditch along the southern boundary and 

about 0.1 ft lower elsewhere except near the brine-collection ditch east of the salt crust, where levels were 

mostly unchanged. After the recovery season, water 

levels were more than 0.1 ft lower in the area of the 

pond and more than 0.4 ft higher in the southeastern 

area. At the brine-collection ditch east of the salt crust, 

water levels were about 0.5 ft higher than for the calibrated model. Pond volume at the end of the recovery 

season decreased about 15 percent (table 13). Flow and 

solute-transport amounts through the southern boundary increased 2 percent for the year. Production from 

the brine-collection ditch east of the salt crust 

decreased 4 percent. The inventory of solute in the 

ground-water system decreased 28 percent. The net 

annual loss of crystalline salt decreased by about 7 percent. Note that the water levels specified for the ditch 

along the southern boundary and the brine-collection 

ditch east of the salt crust are probably not realistic with 

this variation in porosity.

Discussion

Total net subsurface outflow of solute from the 

shallow-brine aquifer for each sensitivity simulation 

and for the calibrated model is shown in figure 46. 

Because the solute content of the shallow-brine aquifer 

is assumed to be in periodic steady state, total subsurface outflow of solute is equal to total subsurface inflow 

of solute plus whatever amount of solute from the dissolution of crystalline salt is necessary to balance total 

subsurface outflow.

: 

~ 1,000

»

j^ 800

§

>•" 600 

_ 

- T-" 400

3

3 200

\

'. \

» ;

| j

Calibrated modl

,

* 1

doubled ermeability south boundary ear 

Specified head increased inditch cnnth hnnnHaru nnn 

•i 

J)

infiltrating Density of fluid increased

S — FDensity of infiltrating decreased fluid increased aporation by 20 percent

.„.„ ,,,

aporation decreased by 20 percent

6 ' '

Porosity increased by 30 percent

-

-_

.

'-_

'-

Porosity decreased by 30 percent

Figure 46. Total subsurface solute outflow for the 

calibrated model of the shallow-brine aquifer and for each 

sensitivity simulation, Bonneville Salt Flats, Utah.

Subsurface outflow of solute increased the most 

for the simulation where the permeability in the vicinity of the southern boundary was increased. It was 

moderately increased for the simulation of decreased 

evaporation. Moderate decreases in subsurface outflow 

of solute occurred in the simulations where water levels 

in the brine-collection ditch along the southern boundary were increased, evaporation flux was increased, and 

density of recharging fluid was decreased. The other 

sensitivity simulations resulted in little change in the 

amount of subsurface outflow of solute.

Subsurface outflow of brine and associated solute transport was most sensitive to the permeability in 

the vicinity of the southern boundary. This was caused 

mostly by the simulated low water levels maintained in 

the brine-collection ditch along that boundary. The 

annual loss of crystalline salt was most sensitive to the 

permeability in the vicinity of the southern boundary 

and to the density (solute concentration) of recharge 

infiltrating from the playa surface. Unfortunately, measurements of the density for this recharge could not be

Sensitivity Analysis 99
Table 12. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when effective 

porosity was increased by 30 percent of the calibrated distribution

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Calibrated simulation

Component

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

Total (rounded)

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

Acre-feet

9

8

80

(2,440)

11,830

11,930

705

1,450

315

27

(2,440)

10,050

12,550

2,400

Tons of solute

-750

1,600

26,150

(938,300)

4,712,000

4,739,000

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

Sensitivity simulation

Acre-feet

10

8

82

(2,180)

12,110

12,210

696

1,500

316

27

(2,180)

10,300

12,840

2,440

Tons of solute

-490

1,890

26,980

(831,700)

4,801,000

4,829,000

281,800

541,000

49,810

10,840

(833,000)

3,815,000

4,698,000

855,100

984,700

Table 13. Relation of fluid and solute components for calibrated simulation and sensitivity simulation when effective 

porosity was decreased by 30 percent of the calibrated distribution

[-, indicates direction of movement is opposite of designated category; (), indicates value not added to total]

Calibrated simulation

Component

Recharge

West

North

East

Recharge (pond)

Recharge (aquifer)

Total (rounded)

Discharge

South

Ditch

Northwest

Bottom

Evaporation (pond)

Evaporation (aquifer)

Total (rounded)

Total net subsurface outflow

Total net loss to crystalline salt

Acre-feet

9

8

80

(2,440)

11,830

11,930

705

1,450

315

27

(2,440)

10,050

12,550

2,400

Tons of solute

-750

1,600

26,150

(938,300)

4,712,000

4,739,000

285,100

525,000

50,590

11,000

(933,500)

3,742,000

4,614,000

844,700

974,800

Sensitivity simulation

Acre-feet

8

5

73

(2,800)

11,480

11,570

717

1,390

318

28

(2,810)

9,720

12,160

2,370

Tons of solute

-1,310

788

24,210

(1,083,000)

4,547,000

4,571,000

290,100

503,500

53,870

11,390

(1,080,000)

3,645,000

4,504,000

835,200

905,000

100 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
made. The inventory of solute in the shallow-brine 

aquifer was most sensitive to aquifer-porosity distribution. For a given configuration of boundary-condition 

water levels and a similar water table, the volume of 

brine and associated solute content varied directly with 

aquifer porosity.

All of the sensitivity simulations resulted in a 

total net loss of crystalline salt over the average year. 

This supports the hypothesis that the present configuration of average water levels in the brine-collection 

ditches east of the salt crust and along the southern 

boundary contribute substantially to the loss of crystalline salt from the Bonneville Salt Flats.

Improved Calibration

It was possible, because of the sequential 

nature of the model-calibration procedure, that one or 

more of the sensitivity tests would produce a model that 

is more closely calibrated to the Bonneville Salt Flats 

system. Two of the above sensitivity simulations produced such a result: (1) the simulation with lower density for recharge through the carbonate-mud surface, 

and (2) the simulation with lower porosity values. The 

improvement obtained was a reduced rate of the gradual increase of solute in the shallow-brine aquifer during the simulations. The recovery of the water table in 

the carbonate-mud area east of the brine-collection 

ditch also was improved with the lower porosity values.

Limitations of the Model

The hydrologic system of the Bonneville Salt 

Flats is complex and cannot be uniquely defined with 

the available data. In this respect, it is similar to most 

other basin-scale hydrologic systems. The model of the 

shallow-brine aquifer described in this report is based 

on the numerical solution of finite-difference approximations to the equations of flow and solute transport 

under variable-density conditions. As with every simulator, simplifying assumptions and approximations, as 

presented in the "Modeling approach" section of this 

report, mean that certain mechanisms are not realistically represented. Scarcity of data also means that any 

actual system will not be completely and accurately 

simulated in every feature.

Limitations of the present model of the shallowbrine aquifer in the Bonneville Salt Flats flow system 

include:

• Approximating the pond as a fixed areal 

region of very high permeability and 100-percent porosity rather than as a surface-water 

feature coupled to the ground-water system;

• Having no mechanism for salt precipitation 

and dissolution at the land surface and above 

the water table;

• Representing the brine-collection ditches as a 

specified-pressure boundary rather than as a 

surface-water feature coupled to the groundwater system;

• Approximating flow in the fractured clay and 

salt media as equivalent porous media;

• Being constrained by the maximum horizontal 

mesh dimension to a longitudinal dispersivity 

of 500 ft;

• Accepting a 1.5-percent brine-flow and solutetransport balance error in the simulations;

• Approximating the simulation region as isothermal;

• Extrapolating and interpolating the distributions of porous-media properties, water levels, 

pressures, densities, and boundary-condition 

parameters from very sparse data in some 

areas; and

• Simplification of the transient system into two 

representative seasons, thus excluding system 

changes on a daily or monthly time scale.

The seasonal pond was approximated as a zone 

of the porous-media aquifer with 100-percent porosity 

and very high permeability within the simulation 

region. The development of a method for simulating 

the seasonal pond as a coupled, open body of water to 

the ground-water flow system was beyond the scope of 

this study. The approximation made the free surface of 

the pond vary more than about 0.2 ft throughout most 

of the pond area, with a decline of 0.3 ft in the southwestern and southeastern corners instead of being completely horizontal. However, it is believed that this 

inaccuracy does not significantly affect the overall balance results.

The HST3D simulator has no mechanism for 

concentration of solute in the fluid as evaporation takes 

place. Instead, the solute in the evaporating fluid is lost 

from the simulation region. This is essentially what 

happens when salt is precipitated to the solid phase at 

an evaporation surface, however, the actual system 

experiences a concentrating effect until saturation is 

reached. Data were not available to verify the formaLimitations of the Model 101
tion of denser fluid in the vicinity of the evaporation 

surface; therefore, a solute-concentrating mechanism 

was not formulated for the HST3D simulator. Similarly, no data were available to quantify the salt-crust 

dissolution rates upon application of recharge from 

precipitation. Without these data, a dissolution mechanism could not be formulated accurately in the simulator. The primary effect of the lack of a soluteconcentrating mechanism is that a layer of denser brine 

at the evaporating water-table surface does not form in 

the simulations. Because little vertical density variation 

was measured in the shallow-brine aquifer, and in view 

of the overall model objectives, lack of this mechanism 

is not a serious limitation to the model. The effect of a 

lack of a dissolution mechanism means that the densities of recharge from precipitation had to be estimated 

and specified as a model-boundary condition. In view 

of the sensitivity of the rate of net annual loss of crystalline salt to this parameter, it is unfortunate that definitive data could not be measured.

Simulation of brine-collection ditches as open 

bodies of water coupled to the ground-water flow system, similar to the pond, would have been more desirable. The consequence of representing the brinecollection ditches as specified-pressure boundaries was 

an instantaneous drawdown of the water table at each 

brine-collection ditch at the start of the production season. This approximation was of little consequence on 

the overall seasonal balances for the system because 

the response time for drawdown was estimated to be a 

few days, which is short relative to the 6-month production season.

Representing the fractured salt crust and carbonate mud that form the shallow-brine aquifer as equivalent porous media was necessary because data were not 

available to characterize the medium as a fractured system. Furthermore, no simulator of variable-density, 

ground-water flow was available that simulates fractured systems. Again, for the purposes of obtaining volumetric brine and solute-mass balances, this 

approximation is regarded as acceptable. However, for 

time of travel calculations and flow-path location, the 

potential for serious error is substantial.

The maximum horizontal mesh dimension of 

1,000 ft constrained the minimum longitudinal dispersivity to 500 ft. Because the HST3D simulator treats 

dispersion as isotropic, this value of longitudinal dispersivity also applied to solute transport associated 

with vertical recharge flows. With an aquifer thickness 

of only 25 or 26 ft, this dispersivity value produced

rapid mixing of any solute-concentration profiles over 

the aquifer thickness. Mixing occurs on the time scale 

of a few days; therefore, during a 6-month simulation 

season, the model does not sustain any vertical density 

gradients that might be present in the actual shallowbrine aquifer. However, as mentioned in the "Modeling 

approach" section of this report, numerous vertical 

fractures traverse the thickness of the shallow-brine 

aquifer, which probably produce a high degree of vertical mixing. Furthermore, field data show no significant gradients of density distribution with depth 

beneath the salt crust in the shallow-brine aquifer.

Having a global mass-balance error for flow and 

solute transport of about 1.5 percent for the production 

season and about 0.6 percent for the recovery season 

sets a limit on the accuracy of the simulations. Accuracy limitations mean that a periodic steady-state flow 

and solute transport could be achieved only to within 

about 2 percent. The development of more implicit 

algorithms for the evaporation and the water-table 

movement calculations for drying and rewetting of 

cells, which could have reduced this problem, was 

beyond the scope of the study. Simulation times would 

have become excessive if more stringent time-step controls had been applied to reduce the global mass-balance error.

Although temperature measurements were made 

when brine samples were withdrawn from the wells, 

insufficient spatial and temporal data on thermal properties and boundary conditions were available to simulate heat transport in the shallow-brine aquifer. A 

consequence of neglecting heat transport is that thermal-buoyancy effects were not represented in the 

model. No data were obtained to identify if this flow 

mechanism exists at the study site in the shallow-brine 

aquifer.

Because every model of a basin-scale aquifer 

system is based on insufficient data to some degree, 

data interpolation and extrapolation need to be done 

over large distances relative to the model dimensions. 

Some data must be estimated from offsite information. 

These limitations restrict the accuracy with which the 

model represents the actual system being studied. In 

the present study, no data were available to quantify the 

permeability of the lacustrine sediment beneath the 

shallow-brine aquifer. Permeability of the shallowbrine aquifer had to be estimated in the northwestern, 

northern, and northeastern parts of the simulation 

region. Measurement of water level and flow in the 

brine-collection ditch along the southern boundary,

102 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
which is on private property, was not possible. Because 

most of the precipitation and evaporation data obtained 

in the study area were collected during either an unusually cool, wet year or hot, dry year, evaporation rates 

were either less than or greater than normal. Estimates 

of typical conditions had to be made from short data 

records obtained near the end of the study period. 

These are examples of sparse data used to formulate the 

model of the shallow-brine aquifer.

Simplification of the transient system into two 

representative seasons with representative parameters 

for boundary conditions means that no historical period 

of record could be matched by the simulation. It was 

impractical to collect data on the time scales of days 

and weeks for this study. Lack of long-term data on the 

historical variability of the climatic environment during previous years also led to the average season 

approximation. However, this simplification is consistent with the goals of the simulations, which involved a 

time scale on the order of years to decades.

The set of boundary-condition types and parameters and the parameter distributions internal to the 

model are not a unique calibration, as is true for all 

field-scale ground-water models. A different combination of parameter values and boundary-condition types 

may yield similar results for calibration criteria, including production from brine-collection ditches and watertable configurations.

NEED FOR ADDITIONAL STUDY

The hydrology of the playa and associated salt 

crust in the Bonneville Salt Flats is complex because of 

its transient nature. Estimation of fluid and solute 

fluxes in this type of transient system is difficult 

because the data represent only small segments of time. 

Monitoring of water levels in wells, brine chemistry, 

and evaporation needs to be continued over the long 

term in order to detect changes in the hydrologic system and to revise estimates of fluid flow and solute 

transport. Data collected and the interpretations made 

from them during this study have added to the understanding of the hydrologic system, but uncertainties 

remain.

Estimated and simulated subsurface flow to the 

south, which primarily seeps into a brine-collection 

ditch located on the southern boundary of the study 

area, was based on limited water-level data from wells 

and aquifer-test data prior to road construction. Estimated subsurface flow to the south was about three

times more than the simulated amount. The water level 

and flow in this brine-collection ditch, located on private property, need to be monitored in order to quantify 

more accurately subsurface flow to the south.

The contradicting evidence for upward and 

downward leakage through the bottom of the shallowbrine aquifer needs to be resolved. Limited water-level 

data that were collected during wetter-than-normal 

conditions on the playa surface indicate downward 

movement, whereas, low H values in brine from the 

shallow-brine aquifer at a few locations indicate 

upward movement. Additional nests of wells need to be 

completed in the shallow-brine aquifer and in the 

underlying lacustrine sediment of the upper part of the 

basin-fill aquifer and water levels need to be measured 

over a longer period of time in order to obtain a better 

spatial and temporal data distribution. Additional samples for 3H analysis that are more spatially and temporally distributed need to be collected in order to define 

more accurately the extent of mixing with pre-bomb 

water. Samples should be collected for chlorofluorocarbon analysis to verify that low 3H values are truly prebomb water.

More water is discharged by evaporation than by 

any other mechanism, yet data were collected at two 

weather stations for only two years each and represent 

atypical conditions. Monitoring at weather stations 

needs to be continued and data correlated with longterm records at the station in Wendover to establish 

long-term average evaporation and variation. An additional station needs to be located on the carbonate-mud 

surface where ground-water levels are closer to the 

playa surface and where winter ponding is unlikely. 

Formation of a thin salt crust might interfere with evaporation from the carbonate-mud surface.

Future modeling of the shallow-brine aquifer in 

the Bonneville Salt Flats needs to be based on removing the major limitations of the present model, most of 

which would require modifications to the simulator. 

Seasonal pond formation and brine-collection ditches 

need to be simulated as surface-water features coupled 

to ground-water flow. A mechanism for salt dissolution 

and precipitation at the playa surface and above the 

water table needs to be formulated. This would allow 

for a realistic simulation of the density of infiltrating 

fluid rather than specifying this density. In order to use 

realistic dispersivities on the order of 5 to 50 ft for the 

present mesh spacing, one of the newer numerical techniques such as flux-corrected-transport or total-variation-diminishing algorithms need to be implemented in

NEED FOR ADDITIONAL STUDY 103
the simulator. Also, the simulator needs to be made 

more implicit with respect to the movement of the 

water-table surface in order to reduce mass-balance 

error. With a better understanding of vertical flow 

between the shallow-brine aquifer and the underlying 

lacustrine sediment, new simulations might result in a 

different mass balance.

CONCLUSIONS

The Bonneville Salt Flats study area, located in 

northwestern Utah, contains a perennial salt crust surrounded by a mud playa. The combination of the saltcrust playa and the underlying ground water results in 

a complex and dynamic hydrologic system that is susceptible to ever-changing daily, seasonal, and yearly 

climatic conditions.

Objectives of this investigation were to identify 

the natural and anthropogenic processes causing salt 

loss from the crust and, where feasible, quantify these 

processes. Specific areas of study include transport of 

salt in solution by ground-water flow, and transport in 

solution by wind-driven ponds and the subsequent salt 

precipitation on the surface of the playa upon evaporation or seepage of brine into the subsurface. Data were 

collected to make conceptual estimates of these processes and to use in conjunction with variable-density, 

three-dimensional, solute-transport computer simulations. Because of the seasonal variability in salt transport within a given year and from one year to the next, 

any rigorous quantification was beyond the scope of 

this study. In addition, no continuous long-term data 

are available with which to assess trends in climate and 

mineral production. It was possible, however, to estimate relative quantities of salt being transported 

through different pathways. Uncertainties exist in estimates for advective flow and solute transport because 

they were made from spatially and temporally limited 

data.

The ground-water system in the Bonneville Salt 

Flats study area can be divided into three aquifers, the 

shallow-brine aquifer, alluvial-fan aquifer, and basinfill aquifer. The shallow-brine aquifer in the Bonneville 

Salt Flats has the most immediate influence on the 

playa surface. Because of brine withdrawal for mineral 

production from the shallow-brine aquifer, this aquifer 

was studied in more detail.

The shallow-brine aquifer in the Bonneville Salt 

Flats appears to be in steady state with respect to longterm ground-water flow despite seasonal and yearly

fluctuations. The potentiometric surface for the shallow-brine aquifer in September 1992 is virtually the 

same as the potentiometric surface shown by Lines 

(1979) for the fall of 1976. Recharge and discharge, 

therefore, must be approximately equal during a longterm period.

Recharge to the shallow-brine aquifer is primarily through infiltration of water from precipitation on 

the playa surface, subsurface flow from the east and 

north, and possibly through upward leakage. Infiltration of water dissolves salt from the crust, whereas subsurface flow the east and north along with upward 

leakage, if it occurs, transports salt into the shallowbrine aquifer. Recharge by infiltration for 1992 was 

estimated to range from 8,300 to 12,800 acre-ft. This 

estimate would be larger if a larger specific yield for the 

shallow-brine aquifer was assumed. Because of small 

horizontal gradients and low permeability values, subsurface flow from the east and north was estimated to 

be only 60 acre-ft. Although possible upward leakage 

could not be quantified, there is evidence, direct and 

indirect, to suggest that it occurs.

Discharge from the shallow-brine aquifer primarily occurs as evaporation from the playa surface, seepage to the brine-collection ditch east of the salt crust, 

subsurface flow to the south where it primarily seeps 

into another brine-collection ditch, subsurface flow to 

the northwestern margin of the playa, and downward 

leakage to the underlying lacustrine sediment. Salt is 

precipitated on the playa surface upon evaporation. 

Evaporation for 1992 was estimated to be 15,000 acreft, subsurface flow to the south ranged from 1,800 to 

2,300 acre-ft, seepage to the brine-collection ditch east 

of the salt crust was 1,500 acre-ft, downward leakage 

ranged from 58 to 580 acre-ft, and subsurface flow to 

the northwestern margin was 40 acre-ft. Estimated net 

salt loss through subsurface flow out of the shallowbrine aquifer was more than 1.3 million tons. The estimated seepage to the brine-collection ditch east of the 

salt crust was considerably more than the 680 acre-ft 

estimated by Lines (1979).

Estimated total discharge is considerably larger 

than estimated recharge. This inequality contradicts the 

known fluid steady-state condition. Estimated subsurface flow to the south might be less than was estimated. 

This estimate was based on hydraulic gradient obtained 

from water-level data collected during this study and 

transmissivity values determined prior to construction 

of Interstate Highway 80. Removal of salt crust and 

compaction of underlying carbonate mud during con104 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
struction probably has constricted ground-water flow to 

the south.

Limited water-level data from 1993, which are 

representative of wetter-than-normal conditions on the 

playa surface, indicate that a small downward driving 

force existed between the shallow-brine aquifer and the 

underlying lacustrine sediment of the upper part of the 

basin-fill aquifer. Tritium values in brine from the shallow-brine aquifer ranged from 0 to 24 TU. Values of 

less than 10 TU, which are less than the value for recent 

precipitation, would indicate mixing with older prebomb water. The only possible source for such water is 

upward leakage from the underlying lacustrine sediment. Upward leakage, in the quantities necessary for 

mixing, is possible if water-level measurements in the 

shallow-brine aquifer were slightly lower than those 

measured in 1993 and if permeability values were four 

orders of magnitude larger than those assumed for the 

underlying sediment. If upward leakage is proven to be 

a substantial amount, then estimated net loss of salt 

from the shallow-brine aquifer would be less.

A comparison of long-term data indicate that dissolved-solids and potassium concentrations in brine 

from the shallow-brine aquifer have not changed appreciably since previous studies. Despite the net loss of 

salt by subsurface flow from the shallow-brine aquifer, 

the high dissolved-solids concentration is maintained 

mostly by dissolution of the salt crust and possibly 

upward leakage. Horizontal subsurface flow into the 

shallow-brine aquifer contributes only a small amount 

of salt.

The potassium concentration is unaffected by 

dissolution and precipitation of the salt crust. The 

potassium concentration decreases with infiltration of 

brine depleted with respect to potassium during the 

winter months but increases again during the summer 

months by the concentrating effect of evaporation. Possible sources for the replenishment of potassium that is 

lost through mineral production include diffusion from 

interstitial fluid and upward leakage.

Deuterium and oxygen-18 values for brine in the 

shallow-brine aquifer plot between the North American 

meteoric water line and the local evaporation line. This 

indicates that mixing occurs between highly evaporative and meteoric waters that have infiltrated into the 

shallow-brine aquifer.

Deuterium and oxygen-18 data from a line of 

nested wells perpendicular to the northwestern margin 

of the playa indicate that mixing occurs between brackish water from the alluvial-fan aquifer and brine from

the shallow-brine aquifer or the underlying lacustrine 

sediment. A high dissolved-solids concentration and a

1R

more evaporative signature for D and O in saline 

water from one well as compared to these values in 

water from the other two wells in a single nest would 

be indicative of induced mixing. Pumping from the 

alluvial-fan aquifer probably has induced the mixing.

Extensive flooding of the Bonneville Salt Flats 

during the winter of 1992-93 dramatically changed the 

salt surface. Much of the near-surface salt was dissolved and remained in solution into the summer of 

1993, when the salt precipitated on the playa surface as 

the pond began to evaporate. A thin salt crust was 

deposited in areas beyond the extent of the crust 

mapped in 1992. An estimated 10 to 14 million tons 

was dissolved and subsequently redeposited on the 

playa surface or infiltrated into the shallow-brine aquifer. An unknown amount of surface salt might have 

been deposited beyond slight topographic divides 

where the salt might not be able to coalesce with the 

main salt body by subsequent dissolution and overland 

flow.

A three-dimensional ground-water flow and solute-transport model with variable density was developed for the shallow-brine aquifer in the Bonneville 

Salt Flats, Utah. The goals for the computer simulation 

of the shallow-brine aquifer in the Bonneville Salt Flats 

were: (1) to develop a fluid and solute balance, (2) to 

evaluate the effect on the salt crust of the brine production from the ditches, and (3) to identify the major and 

minor solute fluxes. The first two goals were met by 

computing (1) a flow balance on the shallow-brine 

aquifer beneath the Bonneville Salt Flats, (2) a solute 

balance for the simulation region, and (3) a mass balance for the total crystalline salt deposit. These quantitative balances help to understand how the brine 

production affects the gradual disappearance of the salt 

crust. The third goal is achieved by examination of the 

cumulative flow and transport amounts calculated for 

each boundary face.

Preliminary simulations indicated that the 

ground-water flow system does not reach steady state 

during production and recovery seasons because the 

response time is longer than the 6-month duration of 

each season. Because of the transient nature of the 

ground-water system, simulations were carried out to a 

periodic steady state with a repeated sequence of production and recovery seasons approximating a typical 

year rather than matching a specific period in time. Calibration criteria included matching the estimated

CONCLUSIONS 105
annual production from the brine-collection ditch, balancing volumetric fluid flows for periodic steady state, 

obtaining a reasonable match between simulated watertable contours and measured water-level data, and sufficient recharge to create a surface pond at the end of 

the recovery period. The overall mass-balance error for 

the brine was 1.5 percent during production seasons 

and 0.6 percent during recovery seasons. Similar errors 

were obtained for the solute balance. With these small 

mass-balance errors, the model adequately simulates 

the effects of brine withdrawal for mineral production 

on the solute and salt-crust balance of the shallow-brine 

aquifer and does not refute the conceptual understanding that brine withdrawal is a major cause for the loss 

of salt from the crust.

Simulation results indicate a net loss of salt of 

about 850,000 tons during a typical year through subsurface flow from the simulation region. This loss is 

primarily through seepage to the brine-collection ditch 

east of the salt crust and subsurface flow primarily to 

the brine-collection ditch along the southern boundary.

In order to reasonably match water levels along 

the southern part of the simulation region, permeabilities were reduced to one-sixth of previously estimated 

values. Simulated annual subsurface flow to the south, 

therefore, is 700 acre-ft of fluid and 285,000 tons of 

solute, about one-third of the estimated amount.

Simulated downward leakage from the shallowbrine aquifer was small. No upward leakage into the 

shallow-brine aquifer was simulated for the parameters 

used at this boundary.

Simulated fluxes through the playa surface were 

larger than at any other boundary. Simulated crystalline 

salt dissolution by infiltration at the playa surface was 

less than 5 million tons. Dissolution of crystalline salt 

occurs in the salt crust, in the thin salt overlying the carbonate mud, and in the crystalline salt in the carbonatemud matrix. Simulated salt precipitation by evaporation was less than 4 million tons. The difference mostly 

can be attributed to the net loss of solute from the aquifer.

Sensitivity simulations were made to test the 

effect of uncertainties in spatially distributed data on 

the cumulative annual amounts for fluid and solute 

transport. Changes were made in permeability, specified water level in the brine-collection ditch along the 

southern boundary, density of infiltrating fluid from the 

playa surface, evaporation rate, and porosity. All sensitivity simulations resulted in a net loss of solute 

through subsurface flow from the shallow-brine aquifer. The model is most sensitive to an increase in permeability near the south boundary.

Limitations to the model include approximating 

surface ponding as a fixed region of very high permeability and 100-percent porosity, inability to simulate 

salt dissolution and precipitation, approximating flow 

in fractured clay and salt media as equivalent porous 

media, and being constrained to a longitudinal dispersivity of 500 feet.

REFERENCES CITED

Anderson, M.P., 1979, Using models to simulate the movement of contaminants through groundwater flow systems: Critical Reviews in Environmental Control, v. 9, 

no. 2, p. 97-156.

Bear, Jacob, 1972, Dynamics of fluids in porous media: New 

York, American Elsevier, 764 p.

Bingham, C.P., 1980, Solar production of potash from brines 

of the Bonneville Salt Flats: in Gwynn, J.W., ed., Great 

Salt Lake, a Scientific, Historical and Economic Overview, Utah Geological and Mineral Survey Bulletin 16, 

p. 229-243.

Cheng, Xiangxue, and Anderson, M.P., 1993, Numerical 

simulation of ground-water interaction with lakes 

allowing for fluctuating lake levels: Groundwater, v. 31, 

no. 6, p. 929-933.

Cooley, R.L., 1974, Finite element solutions for the equations of ground-water flow: Technical Report Series HW, Hydrology and Water Resources Publications No. 

18, Reno, Nevada, Desert Research Institute, 134 p.

Cook, K.L., Halverson, M.D., Stepp, J.C., and Berg, Jr., 

J.W., 1964, Regional gravity survey of the northern 

Great Salt Lake Desert and adjacent areas in Utah, 

Nevada, and Idaho: Geological Society of America Bulletin, v. 75, no. 8, p. 715-740.

Coplen, T.B., 1993, Uses of environmental isotopes: in 

Alley, W.M., ed., Regional Ground-Water Quality, New 

York, Von Nostrand Reihold, p. 227-254.

Craig, Harmon, 1961, Standard for reporting concentrations 

of deuterium and oxygen-18 in natural water: Science, 

v. 133, p. 1702-1703.

Crittenden, M.D., 1963, New data on the isostatic deformation of Lake Bonneville: U.S. Geological Survey Professional Paper 454-E, 31 p.

Dansgaard, W, 1964, Stable isotopes in precipitation: Tallus, 

v. 16, p. 436-468.

Domenico, P.A., and Schwartz, F.W, 1990, Physical and 

chemical hydrogeology: New York, John Wiley and 

Sons, 824 p.

Drever, J.I., 1982, The geochemistry of natural waters: New 

Jersey, Prentice-Hall, 388 p.

106 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
Eardley, A.J., 1962, Gypsum dunes and evaporite history of 

the Great Salt Lake Desert: Utah Geological and Mineralogical Survey Special 2, 27 p.

Epstein, Samuel, and Mayeda, T.K., 1953, Variation of the 

O-18 content of water from natural sources: Geochemica et Cosmochirnica Acta, v. 4, p. 213-224.

Fenneman, N.M., 1931, Physiography of the Western United 

States: New York, McGraw-Hill, 534 p.

Foley, J.D., van Dam, Andries, Feiner, S.K., and Hughes, 

J.F., 1990, Computer graphics, principles and practice, 

2nd ed.: Reading, Mass., Addison-Wesley, 1175 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: New Jersey, Prentice-Hall, 604 p.

Gelhar, L.W., Welty, Claire, and Rehfeldt, K.R., 1992, A critical review of data on field-scale dispersion in aquifers: 

Water Resources Research, v. 28, no. 7, p. 1955-1974.

Gilbert, G.K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 p.

Gwynn, J.W., 1996, History of potash production from the 

Salduro Salt Marsh (Bonneville Salt Flats), Tooele 

County: Utah Geological Survey Notes, v. 28, no. 2, p. 

1-3.

Harrill, J.R., Gates, J.S., and Thomas, J.M., 1988, Major 

ground-water flow systems in the Great Basin region of 

Nevada, Utah, and adjacent states: U.S. Geological Survey Hydrologic Investigations Atlas HA-694-C.

Hassanizadeh, S.M., and Leijnse, T., 1988, On the modeling 

of brine transport in porous media: Water Resources 

Research, v. 24, no. 3, p. 321-330.

Hubbert, M.K., 1953, Entrapment of petroleum under hydrodynamic conditions: Bulletin of the American Association of Petroleum Geologists, v. 37, no. 8, p. 1954-2026.

————1956, Darcy's Law and the field equations of the flow 

of underground fluids: AIME Petroleum Transactions, 

v. 207, p. 222-239.

Hunt, C.B., Robinson, T.W, Bowles, W.A., and Washburn, 

A.L., 1966, Hydrologic basin, Death Valley, California: 

U.S. Geological Survey Professional Paper 494-B, 138

PKendall, Carol, and Coplen, T.B., 1985, Multisample conversion of water to hydrogen by zinc stable isotope determination: Analytical Chemistry, v. 57, p. 1437-1440.

Kipp, Jr., K.L., 1987, HST3D: A computer code for simulation of heat and solute transport in three-dimensional 

ground-water flow systems: U.S.Geological Survey 

Water-Resources Investigations Report 86-4095,517 p.

————in press, Guide to the revised heat and solute transport simulator: HST3D—Version 2: U.S. Geological 

Survey Water-Resources Investigations Report 97- 

4157, 64 p.

Lines, G.C., 1978, Selected ground-water data, Bonneville 

Salt Flats and Pilot Valley, western Utah: U.S. Geological Survey Open-File Report, 14 p.

————1979, Hydrology and surface morphology of the

Bonneville Salt Flats and Pilot Valley playa, Utah: U.S.

Geological Survey Water-Supply Paper 2057, 107 p. 

Malek, Esmaiel, Bingham, G.E., andMcCurdy, G.D., 1990,

Evapotranspiration from the margin and moist playa of

a closed desert valley: Journal of Hydrology, v. 120, p.

15-34. 

Mason, J.L., Brothers, W.C., Gerner, L.J., and Muir, P. S.,

1995, Selected hydrologic data for the Bonneville Salt

Flats and Pilot Valley, western Utah, 1991-93: U.S.

Geological Survey Open-File Report 95-104, 56 p. 

National Oceanic and Atmospheric Administration, 1971-

1993, Climatological data annual summary 1970-1992,

v. 72-94, no. 13.

————1993, Climatological data monthly summary, v. 95, 

no. 1-12.

————1994, Climatological data monthly summary, v. 96,

no. 1-12.

Neuman, S.P., and Witherspoon, P.A., 1972, Field determination of the hydraulic properties of leaky multiple 

aquifer systems: Water Resources Research, v. 8, no. 5, 

p. 1284-1298.

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media: New York, Springer-Verlag, 408 p.

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Desert, in Loughlin, G.F., and Mansfield, G.R., Contributions to economic geology, Part 1—Metals and nonmetals except fuels: U.S. Geological Survey Bulletin 

795, p. 25-44.

Patankar, S.V., 1980, Numerical heat transfer and fluid flow: 

New York, Hemisphere Publishing Corporation, 278 p.

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Technology, Spring 1992, p. 28-37.

Ripple, C.D., Rubin, Jacob, and van Hylckama, T.E.A., 

1975, Estimating steady-state evaporation rates from 

bare soils under conditions of high water table: U.S. 

Geological Survey Water-Supply Paper 2019-A, 39 p.

Scheidegger, A.E., 1961, General theory of dispersion in 

porous media: Journal of Geophysical Research, v. 3, 

no. 2, p. 153-162.

Stephens, J.C., 1974, Hydrologic reconnaissance of the 

northern Great Salt Lake Desert and summary hydrologic reconnaissance of northwestern Utah: Utah 

Department of Natural Resources Technical Publication 42, 55 p.

Turk, L.J., 1969, Hydrology of the Bonneville Salt Bats, 

Utah: Unpublished Ph.D. dissertation, Stanford University, 307 p.

————1970, Evaporation of brine: A field study on the Bonneville Salt Flats, Utah: Water Resources Research, v. 6, 

no. 4, p. 1209-1215.

————1973, Hydrogeology of the Bonneville Salt Flats,

Utah: Utah Geological and Mineralogical Survey 

Water-Resources Bulletin 19, 81 p.

REFERENCES CITED 107
————1978, Bonneville salt crust study: Kaiser Aluminum 

and Chemical Corporation (as of 1988, Reilly Industries), Wendover, Utah, 103 p.

Turk, L.J., Davis, S.N., and Bingham, C.P., 1973, Hydrogeology of lacustrine sediments, Bonneville Salt Flats, 

Utah: Economic Geology, v. 68, p. 65-78.

U.S. Department of Agriculture, 1936, Climatic summary, 

section 20—western Utah, 41 p.

[U.S.] Environmental Science Services Administration,

1967-1970, Climatological data annual summary 1966- 

1969, v. 68-71, no. 13.

[U.S.] Weather Bureau, 1957, Climatic summary of the 

United States—supplement for 1931 through 1952, 45

P-

————1962-1966, Climatological data annual summary 

1961-1965, v. 63-67, no. 13.

Varga, R.S., 1962, Matrix iterative analysis: New Jersey, 

Prentice-Hall, 322 p.

Weast, R.C., Selpy, S.M., and Hodgman, C.D., 1964, Handbook of chemistry and physics (45th ed.): Ohio, Chemical Rubber Co., 1468 p.

108 HYDROLOGY OF THE BONNEVILLE SALT FLATS, NORTHWESTERN UTAH
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