Detailed Study of Selenium and Other Constituents 

in Water, Bottom Sediment, Soil, Alfalfa, and Biota 

Associated with Irrigation Drainage in the 

Uncompahgre Project Area and in the Grand Valley, 

West-Central Colorado, 1991-93

by David L. Butler, Winfield G. Wright, and Kathleen C. Stewart,

U.S. Geological Survey 

Barbara Campbell Osmundson and Richard P. Krueger,

U.S. Fish and Wildlife Service, and 

Daniel W. Crabtree, Bureau of Reclamation

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 96-4138

U.S. GEOLOGICAL SURVEY 

U.S. FISH AND WILDLIFE SERVICE 

BUREAU OF RECLAMATION 

BUREAU OF INDIAN AFFAIRS

Denver, Colorado 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, 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. Geological Survey.

For additional information write to:

District Chief 

U.S. Geological Survey 

Box 25046, Mail Stop 415 

Denver Federal Center 

Denver, CO 80225-0046

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286
CONTENTS

Abstract..............................................................^ 1

Introduction........................................................................................................................................................................... 2

Purpose and Scope....................................................................................................................................................... 3

Description of Study Area........................................................................................................................................... 6

Physiography, Soils, Climate, and Land Uses................................................................................................... 6

Geologic Setting................................................................................................................................................ 7

Irrigation Projects.............................................................................................................................................. 12

Uncompahgre Project.............................................................................................................................. 12

Grand Valley............................................................................................................................................ 13

Salinity-Control Projects......................................................................................................................... 13

Previous Studies.......................................................................................................................................................... 14

Acknowledgments....................................................................................................................................................... 16

Soils and Alfalfa Investigations............................................................................................................................................. 16

Sample Preparation and Analytical Methods.............................................................................................................. 17

Selenium Concentrations in Soils and Alfalfa............................................................................................................. 18

Ground-Water Investigations................................................................................................................................................. 21

Sample Collection and Analytical Methods................................................................................................................ 24

Ground-Water Hydrology............................................................................................................................................ 25

Historical Ground-Water Quality ................................................................................................................................ 28

Major-Ion Chemistry................................................................................................................................................... 28

Review of Selenium Geochemistry............................................................................................................................. 31

Sources of Selenium and Other Trace Constituents.................................................................................................... 35

Occurrence of Selenium in Ground Water................................................................................................................... 37

Distribution........................................................................................................................................................ 37

Seasonal Effects................................................................................................................................................. 40

Mobilization and Transport of Selenium..................................................................................................................... 41

Selenium Oxidation..................................................................................................................................................... 43

Indicators of Microbial Activity.................................................................................................................................. 44

Relation of Selenium to Other Constituents................................................................................................................ 44

Surface-Water Investigations................................................................................................................................................. 47

Sample Collection and Analytical Methods................................................................................................................ 47

Selenium Concentrations and Loads in the Gunnison and Colorado Rivers............................................................... 48

Selenium Concentrations................................................................................................................................... 48

Selenium Loads................................................................................................................................................. 51

Distribution, Magnitude, and Variation of Selenium Concentrations ......................................................................... 54

Uncompahgre Project Area ............................................................................................................................... 54

Tributaries, Canals, and Ditches.............................................................................................................. 54

Uncompahgre River................................................................................................................................. 61

Adjacent Areas in the Lower Gunnison River Basin .............................................................................. 61

Grand Valley...................................................................................................................................................... 61

Magnitude and Variation of Selenium Loads.............................................................................................................. 69

Uncompahgre Project Area ............................................................................................................................... 70

Grand Valley.................................................................................................................................._................. 74

CONTENTS III
Dissolved Solids and Major Constituents and Relations to Selenium........................................................................ 77

Dissolved Solids................................................................................................................................................ 79

Chloride and Evaporative Concentration.......................................................................................................... 82

Other Major Constituents.................................................................................................................................. 84

Other Trace-Constituent Concentrations..................................................................................................................... 87

Insecticides in the Grand Valley.................................................................................................................................. 89

Selenium and Other Constituents in Bottom Sediment........................................................................................................ 90

Sample Collection and Analytical Methods................................................................................................................ 90

Selenium Concentrations............................................................................................................................................. 90

Other Trace-Constituent Concentrations..................................................................................................................... 93

Biological Investigations....................................................................................................................................................... 93

Sample Collection....................................................................................................................................................... 93

Analytical Methods..................................................................................................................................................... 94

Data Analysis............................................................................................................................................................... 94

Data Interpretation....................................................................................................................................................... 94

Selenium Concentrations in Fish Samples and Their Food Items in Irrigated Areas ................................................. 95

UncompahgreProject........................................................................................................................................ 95

Areas Adjacent to the Uncompahgre Project.................................................................................................... 98

Grand Valley...................................................................................................................................................... 100

Selenium Concentrations in Fish Samples from Mainstem Rivers............................................................................. 104

Risk Assessment for Endangered Fish from Selenate and Selenite............................................................................ 107

Other Trace-Constituent Concentrations in Fish Tissues and Fish Food Items in Irrigated Areas............................. 108

Uncompahgre Project Area............................................................................................................................... 108

Grand Valley...................................................................................................................................................... Ill

Other Trace-Constituent Concentrations in Fish Tissues and Fish Food Items from Mainstem Rivers..................... 112

Risk Assessment for Endangered Fish from Four Trace Constituents........................................................................ 113

Organic Contaminants in Fish Tissues........................................................................................................................ 113

Selenium Concentrations in Migratory Birds and Their Food Items.......................................................................... 115

Uncompahgre Project Area Wetlands................................................................................................................ 115

Grand Valley Wetlands...................................................................................................................................... 118

Other Trace-Constituent Concentrations in Birds and Their Food Items.................................................................... 118

Incubation Studies of Bird Eggs.................................................................................................................................. 120

Field Observations at Wetland Sites............................................................................................................................ 120

Water and Bottom-Sediment Bioassays ...................................................................................................................... 122

Water Bioassays................................................................................................................................................. 122

Bottom-Sediment Bioassays.............................................................................................................................. 123

Bioaccumulation of Selenium............................................................................................................................................... 123

Implications for Human Health............................................................................................................................................. 124

Summary................................................................................................................................................................................ 125

References Cited.................................................................................................................................................................... 129

FIGURES

1-6. Maps showing:

1. Location of the Uncompahgre Project area, the Grand Valley, and selected water-sampling

sites on major streams..................................................................................................................................... 4

2. Selected features in the Uncompahgre Project area........................................................................................ 7

3. Selected features in the Grand Valley............................................................................................................. 8

4. Generalized surficial geology of the Uncompahgre Project........................................................................... 9

IV CONTENTS
5. Grand Valley geology: (A) surficial geology, (B) generalized extent of the cobble aquifer, and

(Q generalized geologic section.................................................................................................................... 10

6. Distribution of soil-sampling sites within townships in irrigated areas of the Uncompahgre Project........... 17

7. Graph showing percent of variance in selenium concentrations in soils and alfalfa for different levels

in the sampling design............................................................................................................................................. 20

8. Map showing locations of type areas and locations of wells and springs in the Uncompahgre Project

area where samples were collected for chemical analysis...................................................................................... 22

9. Map showing location of type area and wells in the Grand Valley where samples were collected for

chemical analysis..................................................................................................................................................... 23

10. Schematic showing hydrogeologic setting and ground-water flow in (A) Reed Wash type area and

(B) Dry Creek type area.......................................................................................................................................... 26

11. Graph showing water-level altitude and specific conductance of water in well 143NO, Grand Valley,

March 1992-March 1993........................................................................................................................................ 27

12. Graphs showing historical water-level altitudes and dissolved-solids concentrations in water from

wells 125Q5 and 208KO located in the Grand Valley............................................................................................. 29

13. Trilinear diagram showing percentages of major ions in water from selected wells during the irrigation

season in the Uncompahgre Project area and in the Grand Valley.......................................................................... 30

14. Graphs showing oxygen and hydrogen stable isotope values in water from selected wells, streams, and

rivers in the Uncompahgre Project area and in the Grand Valley........................................................................... 33

15. Stability-field diagram for selenium species in natural waters and data points calculated from seleniumspeciation data collected for this study................................................................................................................... 34

16. Schematic showing conceptual model of the occurrence of selenium in Mancos Shale, shale residuum,

alluvium, and ground water in the Uncompahgre Project area and in the Grand Valley........................................ 38

17. Graph showing platinum-electrode potential and concentrations of dissolved selenium in water from

wells in Mancos Shale residuum............................................................................................................................. 40

18. Graph showing dissolved selenium in water from wells in shale residuum and alluvium during the

irrigation season and nonirrigation season.............................................................................................................. 40

19. Graphs showing correlation of dissolved solids and dissolved selenium in water from wells in alluvium

and shale residuum.................................................................................................................................................. 42

20. Scanning-electron photomicrograph of framboidal pyrite collected on 0.45-micrometer water-filter

paper from a well located outside the irrigated Grand Valley area, October 1991................................................. 43

21. Graphs showing correlations of dissolved selenium with various dissolved constituents in water from

wells in the Uncompahgre Project area and in the Grand Valley............................................................................ 46

22. Graph showing dissolved-selenium concentrations and the LOWESS smoothed curves of selenium 

concentrations for the Gunnison River at Whitewater and the Colorado River near the Colorado-Utah 

State line, 1975-92.................................................................................................................................................. 50

23. Schematic diagram showing the lower Gunnison River Basin and relative locations of selected surfacewater-sampling sites................................................................................................................................................ 52

24. Graph showing selenium loads in the Gunnison River at Whitewater and the Colorado River near the

Colorado-Utah State line, July 1991-August 1992, and the residual loads............................................................ 54

25. Schematic diagram showing the southern part of the Uncompahgre Project, relative locations of surfacewater-sampling sites, and selenium concentrations in samples collected during synoptic surveys, 

January-March 1992............................................................................................................................................... 56

26. Schematic diagram showing the northern part of the Uncompahgre Project, relative locations of surfacewater-sampling sites, and selenium concentrations in samples collected during synoptic surveys, 

January-March 1992............................................................................................................................................... 57

27-29. Graphs showing:

27. Selenium concentrations in samples from Dry Creek at site DRY1, Cedar Creek at site GDI, 

Loutsenhizer Arroyo at sites LZAM and LZA1, and an unnamed drainage at site RD1, 

July 1991-August 1992.................................................................................................................................. 60

28. Selenium concentrations in the Uncompahgre River at Delta, 1987-92........................................................ 62

29. Selenium concentrations in the Uncompahgre River, February 19-20,1992................................................ 63

CONTENTS V
30. Schematic diagram showing the western part of the Grand Valley, relative locations of surfacewater-sampling sites, and selenium concentrations in samples collected during synoptic surveys, 

February-March 1992............................................................................................................................................. 65

31. Schematic diagram showing the eastern part of the Grand Valley, relative locations of surfacewater-sampling sites, and selenium concentrations in samples collected during synoptic surveys, 

February-March 1992............................................................................................................................................. 66

32-43. Graphs showing:

32. Mean selenium concentrations in irrigation-drainage samples collected in March 1991 and in

February and March 1992 at the 20 reconnaissance sites in the Grand Valley............................................... 67

33. Selenium concentrations in samples from Salt Creek at site SC, Reed Wash at site RW1, and

Leach Creek at site LC1, March 1991-August 1992...................................................................................... 69

34. Selenium loads in the Uncompahgre River, February 19-20,1992............................................................... 71

35. Seasonally weighted mean selenium loads for 12 sites in the Uncompahgre Project, 1991-92.................... 72

36. Mean selenium loads of three sets of samples collected in 1991-92 for 20 sites in the Grand Valley........... 75

37. Selenium loads in Salt Creek at site SC, Reed Wash at site RW1, Big Salt Wash at site BSW1,

and Leach Creek at site LC1, March 1991-August 1992............................................................................... 76

38. Relation of seasonally weighted mean selenium load to drainage-basin area within the irrigated

area for 20 basins in the Grand Valley............................................................................................................ 78

39. Relations of selenium concentration to specific conductance for Loutsenhizer Arroyo and

Reed Wash....................................................................................................................................................... 81

40. Relations of selenium load to dissolved-solids load for Loutsenhizer Arroyo (site LZA1) and

Reed Wash (site RW1).................................................................................................................................... 83

41. Relation of estimated selenium yield based on sampling during 1991-92 to estimates of

dissolved-solids yield by the Bureau of Reclamation for 15 basins in the Grand Valley............................... 84

42. Selenium and chloride concentrations in irrigation-drainage samples from the Grand Valley,

November 1991-March 1992, and the estimated evaporative-concentration line.......................................... 84

43. Relations of selenium to nitrite-plus-nitrate concentrations in water samples collected during the 

nonirrigation season from streams and ditches on the eastern side of the Uncompahgre Project 

and in the Grand Valley north of the Colorado River, 1991-92..................................................................... 88

44-47. Maps showing:

44. Location of 1991 biota sampling sites on tributary streams in the Uncompahgre Project............................. 96

45. Location of 1991 biota sampling sites adjacent to the Uncompahgre Project................................................ 99

46. Location of 1991 biota sampling sites on tributary streams in the Grand Valley........................................... 101

47. Location of sampling sites on major rivers where fish samples were collected in 1992................................ 105

48-51. Graphs showing:

48. Mean selenium concentrations in four fish-trophic groups in the Colorado, North Fork of the

Gunnison, Gunnison, and Uncompahgre Rivers............................................................................................. 106

49. Mean selenium concentrations in three fish-trophic groups collected from four sites on the

Colorado River................................................................................................................................................ 108

50. Mean selenium concentrations in three fish-trophic groups collected from four sites in the

Gunnison River Basin..................................................................................................................................... 109

51. Mean selenium concentrations in three fish-trophic groups collected from three sites on the

Uncompahgre River........................................................................................................................................ 110

52. Map showing location of sampling sites at wetlands in the Uncompahgre Project area......................................... 116

53. Map showing location of sampling sites at wetlands in the Grand Valley.............................................................. 119

TABLES

1. Distribution of soil-sampling sites by geologic units in townships of the Uncompahgre Project area.................. 18

2. Geometric mean and observed range of total- and water-extractable selenium concentrations in soil 

and total-selenium concentrations in alfalfa for all samples from the Uncompahgre Project area, 

by geologic unit....................................................................................................................................................... 19

VI CONTENTS
3. Geometric mean concentrations of arsenic, molybdenum, uranium, and zinc in soils from alluvium

overlying Mancos Shale in the Uncompahgre Project area and in soils from the Western United States ............. 21

4. Hydrogeologic settings of the ground-water-flow systems affected by irrigation in the

Uncompahgre Project area and in the Grand Valley............................................................................................... 25

5. Results of thermodynamic speciation calculations for water from wells in the Uncompahgre Project area

and from the Grand Valley...................................................................................................................................... 32

6. Evaporite mineral species and salt efflorescence associated with the Mancos Shale and shale residuum in

the Grand Valley..................................................................................................................................................... 33

7. Semiquantitative clay and bulk mineralogy of Mancos Shale bedrock core, shale residuum, alluvium, and

ground-water-filter residuum.................................................................................................................................. 36

8. Element and organic-carbon concentrations in soil and rock samples from the Uncompahgre Project area

and from the Grand Valley...................................................................................................................................... 37

9. Geochemical environments of the ground-water-flow systems, hydrogeologic settings, and ranges of 

dissolved-selenium concentrations in Mancos Shale residuum and alluvium in the irrigated Uncompahgre 

Project area and in the Grand Valley...................................................................................................................... 38

10. Concentrations of dissolved redox-active constituents, water properties, and hydrogeologic setting 

of water from wells in alluvium and shale residuum in the Uncompahgre Project area and in the 

Grand Valley........................................................................................................................................................... 39

11. Speciation of dissolved selenium in selected ground-water samples from the Uncompahgre Project area

and from the Grand Valley...................................................................................................................................... 41

12. Speciation of extractable selenium in alluvial-aquifer-sediment samples from the Grand Valley......................... 42

13. Energies of reaction for oxidation of selenium by oxygen and nitrate................................................................... 44

14. Sulfur-isotope data for water from selected wells in shale residuum and alluvium............................................... 45

15. Summary of selenium concentrations in the Gunnison and Colorado Rivers and Plateau Creek and the

number of samples exceeding aquatic-life criteria of the U.S. Environmental Protection Agency....................... 49

16. Summary of selenium concentrations in irrigated areas of the Uncompahgre Project in 1991-92 and the

number of samples that exceeded aquatic-life criteria of the U.S. Environmental Protection Agency................. 55

17. Summary of selenium concentrations in samples collected during the synoptic surveys of the Uncompahgre

Project, January through March 1992..................................................................................................................... 58

18. Summary of selenium concentrations in irrigated areas in the Grand Valley and the number of samples

that exceeded aquatic-life criteria of the U.S. Environmental Protection Agency................................................. 64

19. Summary of selenium concentrations in samples collected during the synoptic surveys of the Grand Valley,

February and March 1992....................................................................................................................................... 68

20. Concentrations of dissolved selenium, selenate, and selenite in water samples collected from five streams

in the Grand Valley, March 24, 1992...................................................................................................................... 70

21. Pearson correlation coefficients of selenium concentration to specific conductance and to dissolved-solids,

major-ion, boron, and uranium concentrations for the Uncompahgre Project and the Grand Valley.................... 80

22. Summary of dissolved trace-constituent concentrations in surface-water samples from the Uncompahgre

Project area and from the Grand Valley, 1991-92.................................................................................................. 89

23. Concentrations of selenium, uranium, total carbon, and organic carbon in bottom-sediment, salt-crust,

and soil samples...................................................................................................................................................... 91

24. Concentrations of total, soluble, and adsorbed selenium and the percent selenite in bottom-sediment

samples from eight streams, March 1992............................................................................................................... 92

25. Statistical summary of selenium concentrations in selected biota samples collected in 1991 in the

Uncompahgre Project area...................................................................................................................................... 97

26. Number of whole-body fish samples collected in the Uncompahgre Project area that exceeded the 

1984 National Contaminant Biomonitoring Program 85th-percentile concentrations for seven trace 

constituents............................................................................................................................................................. 97

27. Statistical summary of selenium concentrations in selected biota samples collected during 1991 from

the Grand Valley..................................................................................................................................................... 102

28. Number of whole-body fish samples collected in the Grand Valley that exceeded the 1984 National

Contaminant Biomonitoring Program 85th-percentile concentrations for seven trace constituents...................... 103

CONTENTS VII
29. Number of whole-body fish samples collected in 1992 from the Colorado, North Fork of the Gunnison, 

Gunnison, and Uncompahgre Rivers that exceeded the 1984 National Contaminant Biomonitoring 

Program 85th-percentile concentrations for seven trace constituents..................................................................... 107

30. Concentrations of selenate and selenite selenium causing acute toxicity to Colorado squawfish and

razorback suckers.................................................................................................................................................... 110

31. Hazard assessment of selenate and selenite selenium in water samples collected in March 1992 

from five streams in the Grand Valley to three life stages of Colorado squawfish and razorback 

suckers..................................................................................................................................................................... Ill

32. Concentrations of boron, uranium, vanadium, and zinc causing acute toxicity to Colorado squawfish

and razorback suckers............................................................................................................................................. 113

33. Hazard assessment of boron and uranium in water samples collected from five streams in the

Grand Valley to three life stages of Colorado squawfish and razorback suckers.................................................... 114

34. Hazard assessment of vanadium and zinc in water samples collected from five streams in the

Grand Valley to three life stages of Colorado squawfish and razorback suckers.................................................... 114

35. Geometric mean and maximum concentrations of the 1984 National Contaminant Biomonitoring

Program for selected organochlorine pesticides and polychlorinated biphenyls.................................................... 115

36. Statistical summary of selenium concentrations in selected bird-food and bird-tissue samples 

collected during 1991-92 from wetlands in the Uncompahgre Project, wetlands adjacent to the 

Uncompahgre Project, and wetlands in the Grand Valley ...................................................................................... 117

37. Geometric mean selenium concentrations in water, bottom-sediment, and biota samples from 

wetlands in the Uncompahgre Project, wetlands adjacent to the Uncompahgre Project, and 

wetlands in the Grand Valley.................................................................................................................................. 124

38. Selenium concentrations in fish-fillet and bird-breast samples from the Uncompahgre Project 

area and the Grand Valley, and the quantity of each sample that could be consumed without 

exceeding the maximum recommended daily quantity for an adult human........................................................... 125

VIII CONTENTS
CONVERSION FACTORS AND VERTICAL DATUM

Multiply

acre

acre

centimeter (cm)

cubic foot per second (ft3/s)

foot (ft)

foot per day (ft/d)

foot per second (ft/s)

gallon per minute (gal/min)

inch (in.)

inch per year (in/yr)

kilogram (kg)

liter (L)

meter (m)

microgram per day (|ig/d)

micrometer (jim)

mile (mi)

milliliter (mL)

millimeter (mm)

ounce (oz)

pound per day (Ib/d)

pound per day per 100 acres [(lb/d)/100 acres]

ton per day (ton/d)

ton per year (ton/yr)

By

4,047

0.4047

0.3937

0.02832

0.3048

0.3048

0.3048

0.06309

25.4

25.4

2.205

0.2642

3.281

0.0000000353

0.00003937

1.609

0.0610

0.03937

28.35

0.4536

0.4536

0.9067

0.9067

To obtain

square meter

hectare

inch

cubic meter per second

meter

meter per day

meter per second

liter per second

millimeter

millimeter per year

pound

gallon

foot

ounce per day

inch

kilometer

cubic inch

inch

gram

kilogram per day

kilogram per day per 100 acres

metric ton per day

metric ton per year

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

°F = 9/5°C + 32

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

°C = 5/9(°F-32)

The following terms and abbreviations also are used in this report:

microgram per gram (|lg/g)

microgram per kilogram (|lg/kg)

microgram per liter (|lg/L)

microgram per liter per year [(|0.g/L)/yr]

millivolt (mV)

milligram per kilogram (mg/kg)

milligram per liter (mg/L)

microsiemens per centimeter at 25 degrees Celsius (jiS/cm)

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

For those who wish to convert dry-weight concentrations to wet-weight concentrations for biological 

samples, the equation is:

wet weight = dry weight [1 - (percent moisture/100)].

CONTENTS IX
Detailed Study of Selenium and Other Constituents 

in Water, Bottom Sediment, Soil, Alfalfa, and 

Biota Associated with Irrigation Drainage in the 

Uncompahgre Project Area and in the Grand Valley, 

West-Central Colorado, 1991-93

£y David L. Butler, Winfield G. Wright, Kathleen C. Stewart, Barbara Campbell Osmundson, 

Richard P. Krueger, ancfDan\e\ W. Crabtree

Abstract

In 1985, the U.S. Department of the Interior 

began a program to study the effects of irrigation 

drainage in the Western United States. These 

studies were done to determine whether irrigation 

drainage was causing problems related to human 

health, water quality, and fish and wildlife 

resources. Results of a study in 1991-93 of 

irrigation drainage associated with the 

Uncompahgre Project area, located in the lower 

Gunnison River Basin, and of the Grand Valley, 

located along the Colorado River, are described in 

this report. The focus of the report is on the 

sources, distribution, movement, and fate of 

selenium in the hydrologic and biological systems 

and the effects on biota. Generally, other traceconstituent concentrations in water and biota were 

not elevated or were not at levels of concern.

Soils in the Uncompahgre Project area 

that primarily were derived from Mancos 

Shale contained the highest concentrations of 

total and water-extractable selenium. Only 5 of 

128 alfalfa samples had selenium concentrations 

that exceeded a recommended dietary limit for 

livestock. Selenium data for soil and alfalfa 

indicate that irrigation might be mobilizing and 

redistributing selenium in the Uncompahgre 

Project area.

Distribution of dissolved selenium in 

ground water is affected by the aqueous geochemical environment of the shallow ground-water

system. Selenium concentrations were as high as 

1,300 micrograms per liter in water from shallow 

wells. The highest concentrations of dissolved 

selenium were in water from wells completed in 

alluvium overlying the Mancos Shale of 

Cretaceous age; selenium concentrations were 

lower in water from wells completed in Mancos 

Shale residuum. Selenium in the study area could 

be mobilized by oxidation of reduced selenium, 

desorption from aquifer sediments, ion exchange, 

and dissolution. Infiltration of irrigation water 

and, perhaps nitrate, provide oxidizing conditions 

for mobilization of selenium from alluvium and 

shale residuum and for transport to streams and 

irrigation drains that are tributary to the 

Gunnison, Uncompahgre, and Colorado Rivers.

Selenium concentrations in about 

64 percent of water samples collected from 

the lower Gunnison River and about 50 percent 

of samples from the Colorado River near the 

Colorado-Utah State line exceeded the 

U.S. Environmental Protection Agency criterion 

of 5 micrograms per liter for protection of aquatic 

life. Almost all selenium concentrations in 

samples collected during the nonirrigation 

season from Mancos Shale areas exceeded the 

aquatic-life criterion. The maximum selenium 

concentrations in surface-water samples were 

600 micrograms per liter in the Uncompahgre 

Project area and 380 micrograms per liter in the 

Grand Valley.

Abstract 1
Irrigation drainage from the Uncompahgre 

Project and the Grand Valley might account for as 

much as 75 percent of the selenium load in the 

Colorado River near the Colorado-Utah State line. 

The primary source areas of selenium were the 

eastern side of the Uncompahgre Project and the 

western one-half of the Grand Valley, where there 

is extensive irrigation on soils derived from 

Mancos Shale. The largest mean selenium loads 

from tributary drainages were 14.0 pounds per 

day from Loutsenhizer Arroyo in the 

Uncompahgre Project and 12.8 pounds per day 

from Reed Wash in the Grand Valley. Positive 

correlations between selenium loads and 

dissolved-solids loads could indicate that salinitycontrol projects designed to decrease dissolvedsolids loads also could decrease selenium loads 

from the irrigated areas. Selenium concentrations 

in irrigation drainage in the Grand Valley were 

much higher than concentrations predicted by 

simple evaporative concentration of irrigation 

source water. Selenium probably is removed 

from pond water by chemical and biological 

processes and incorporated into bottom 

sediment. The maximum selenium concentration 

in bottom sediment was 47 micrograms per gram 

from a pond on the eastern side of the 

Uncompahgre Project.

Selenium concentrations in some aquatic 

plants and aquatic invertebrates exceeded a 

dietary guideline of 3 micrograms per gram 

dry weight for protection of fish and wildlife 

resources. More than 75 percent of wholebody fish samples collected from rivers and 

tributaries throughout the study area had 

selenium concentrations that exceeded the 

1984 National Contaminant Biomonitoring 

Program 85th-percentile concentration of 

0.73 microgram per gram wet weight. 

Whole-body fish samples from the Gunnison 

River had significantly higher geometric 

mean selenium concentrations than fish samples 

from the Uncompahgre and Colorado Rivers.

Based on a risk assessment using selenium 

data for five Grand Valley streams, the selenate 

form of selenium could be a moderate risk to the

swim-up life stage (less than 99 days old) of the 

endangered Colorado squawfish and to three 

tested life stages of the endangered razorback 

sucker. Zinc concentrations in Adobe Creek, 

Leach Creek, and Indian Wash were in the highrisk category for the swim-up life stage of 

Colorado squawfish.

Selenium concentrations in food items of 

migratory birds collected at wetlands in the 

Uncompahgre Project area exceeded the dietary 

guideline for protection of consumer wildlife. 

Fathead minnows from ponds had high selenium 

concentrations, and the maximum concentration 

was 110 micrograms per gram dry weight. 

Selenium concentrations in bird eggs and livers 

generally were in the range of uncertainty 

regarding biological risk; however, some samples 

from the Uncompahgre Project area had selenium 

concentrations in the high-risk category. Of 

65 bird eggs incubated, only one had any visual 

deformity. Five recently hatched birds and two 

embryos that were found dead at two ponds in the 

Uncompahgre Project area had selenium concentrations ranging from 19 to 43 micrograms per 

gram dry weight, but no abnormalities or deformities were observed.

Toxicity tests on irrigation-drainage water 

from five Grand Valley streams indicated no 

significant toxicity differences between control 

and test samples, except for the test on fathead 

minnows exposed to water from Leach Creek. 

Bioaccumulation of selenium from water to biota 

was significant in the study area, and the highest 

bioaccumulation factor was about 20,900 from 

water to bird livers. Selenium accumulates in fish 

and birds in the study area to levels that would 

limit human consumption if eaten on a daily basis. 

It is unlikely that fish and birds taken in the study 

area are consumed by humans on a daily basis.

INTRODUCTION

High concentrations of selenium have been 

detected in irrigation drainage in the San Joaquin 

Valley in California. In 1983, incidences of mortality, 

abnormalities, and reproductive failure in waterfowl

Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
were discovered by the U.S. Fish and Wildlife Service 

(FWS) at the Kesterson National Wildlife Refuge in 

the western San Joaquin Valley, California, where 

irrigation-drainage water was impounded. Because of 

the concern that problems related to selenium or other 

trace inorganic or organic constituents in irrigation 

drainage might not be limited to the Kesterson 

Reservoir area, the U.S. Department of the Interior 

(DOI) began a program in 1985 to determine whether 

irrigation-related problems existed at other DOI 

constructed or managed irrigation projects, national 

wildlife refuges, or other wetland areas for which the 

DOI has responsibility. About 600 irrigation projects 

and major wildlife resource areas have been 

constructed or are managed in 17 Western States by 

DOI bureaus. The program evolved into five phases 

(Deason, 1986): (1) Site identification, (2) reconnaissance investigations, (3) detailed studies, (4) planning, 

and (5) remediation. Activities in the first three phases 

are conducted by study teams consisting of scientists 

from the U.S. Geological Survey (USGS), the FWS, 

the Bureau of Reclamation (BOR), and the Bureau of 

Indian Affairs. A USGS scientist heads each study 

team. Phase 4 and phase 5 of the program are headed 

by the BOR.

The reconnaissance investigations are designed 

to determine whether irrigation drainage (1) has 

caused or has the potential to cause harmful effects on 

human health or on fish and wildlife or (2) might limit 

the suitability of water for beneficial uses. The 

duration of the reconnaissance investigations was 

about 2 years. Detailed studies were initiated if the 

reconnaissance investigations indicated that 

potentially serious water-quality problems were 

related to irrigation drainage. The purpose of detailed 

studies is to gather sufficient information to provide 

the scientific understanding needed for development 

of reasonable alternatives to mitigate or resolve 

identified problems. The purpose of planning is to 

develop coordinated action with appropriate Federal, 

State, and local agencies and water users for remediation of problem areas. The final phase involves 

implementation of remedial actions developed during 

planning.

A reconnaissance investigation of the Gunnison 

and Uncompahgre River Basins and of Sweitzer 

Lake (fig. 1) documented elevated concentrations of 

selenium in some water, bottom-sediment, and biota 

samples collected in 1987-88 (Butler and others, 

1991). The reconnaissance report indicated that

irrigation drainage from the Uncompahgre Project, a 

BOR irrigation project, might be a primary source of 

selenium, dissolved solids, and other constituents to 

the Gunnison and Uncompahgre Rivers and to 

Sweitzer Lake. The extent of the irrigated area served 

by the Uncompahgre Project is shown in figure 1. The 

Uncompahgre Project area is defined as the irrigated 

area served by the Uncompahgre Project plus adjacent 

areas in the lower Gunnison River Basin, such as the 

Smith Fork, North Fork, and Tongue Creek Basins 

(fig. 1). Because of the limited scope of the reconnaissance studies, source areas and distribution of 

selenium in the Uncompahgre Project area were not 

determined, nor were effects to biota documented. 

Also, trace-element data for water and biota were 

not collected in other irrigated areas in the lower 

Gunnison River Basin. Because of these factors, a 

detailed study of the Uncompahgre Project area was 

initiated in 1991.

The Grand Valley was included with the 

Uncompahgre Project area in the detailed study. There 

are similarities of climate, physiography, agricultural 

crops, and geology between the Grand Valley and the 

Uncompahgre Project area. Also, the BOR identified 

irrigation drainage in the Grand Valley (fig. 1) as a 

large source of dissolved solids (salinity) to the upper 

Colorado River (Bureau of Reclamation, 1978). A 

marine shale, the Mancos Shale, was identified as the 

primary source of the salinity, and selenium problems 

have been documented in other Western States where 

marine shales are irrigated (See and others, 1992; 

Stephens and others, 1992). Historical selenium 

data for the Colorado and Gunnison Rivers also 

indicated that the Grand Valley might be a source 

area of selenium.

Purpose and Scope

This report describes results of the detailed 

study of the Uncompahgre Project area and of 

the Grand Valley during 1991-93. The general 

objectives of the detailed study are (1) to determine 

the sources, distribution, movement, and fate of 

contaminants in the hydrologic and biologic systems; 

and (2) to measure contaminant concentrations, 

determine exposure pathways, and document effects 

on biota.

INTRODUCTION
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Specific objectives of this report are to:

1. Describe concentrations and distribution of

selenium and other trace constituents in soil and 

in alfalfa in irrigated areas of the Uncompahgre 

Project.

2. Describe the concentrations and distribution of 

selenium and other trace constituents in ground 

water in the Uncompahgre Project area and in 

the Grand Valley and the hydrochemical 

processes affecting selenium release and 

mobilization in ground water in irrigated areas 

on Mancos Shale terrain.

3. Discuss selenium concentrations and loads in the 

Gunnison and Colorado Rivers.

4. Describe distribution and variability of selenium 

concentrations in surface water and in bottom 

sediment in irrigated areas in the Uncompahgre 

Project area and in the Grand Valley.

5. Discuss selenium loads in streams, washes, and 

drains in the Uncompahgre Project area and in 

the Grand Valley and identify areas that are 

major sources of selenium to the Gunnison, 

Uncompahgre, and Colorado Rivers.

6. Describe concentrations and distribution of 

dissolved solids, major constituents, and 

selected trace constituents in surface water in 

the Uncompahgre Project area and in the Grand 

Valley and the relations of selenium to 

dissolved solids and selected constituents.

7. Describe concentrations and distribution of 

selenium and other trace constituents in fish, 

aquatic plants, and aquatic invertebrates in 

tributary streams and washes draining the 

Uncompahgre Project area and the Grand 

Valley.

8. Describe concentrations and distribution of 

selenium and other trace constituents in fish 

from the Gunnison, Uncompahgre, and 

Colorado Rivers and the potential effects of 

selenium on threatened and endangered fish.

9. Describe concentrations and distribution of 

selenium and other trace constituents in 

waterfowl and their effects on waterfowl 

reproduction in the Uncompahgre Project 

area and in the Grand Valley.

10. Discuss food-chain bioaccumulation of selenium.

11. Discuss implications of selenium concentrations 

to human health.

Soil, alfalfa, ground-water, mineralogic, 

surface-water, bottom-sediment, and biota samples 

were collected from March 1991 to August 1992 in the 

Uncompahgre Project area and in the Grand Valley. 

Selenium was the trace constituent of primary 

concern. Some water, bottom-sediment, and aquifersediment samples were analyzed for selenate and 

selenite selenium. Other trace elements were analyzed 

in water and biota samples collected in 1991. Also, 

streams and reservoirs outside the irrigated land 

served by the Uncompahgre Project (fig. 1) were 

sampled to determine if selenium and other traceconstituent concentrations were of concern in other 

irrigated areas in the lower Gunnison River Basin.

Soil and alfalfa samples were collected in 

irrigated areas of the Uncompahgre Project in 

June 1991. Soil samples were analyzed for total and 

extractable selenium, and trace constituents and alfalfa 

samples were analyzed for total selenium. Groundwater samples were collected in February and 

March 1992 to define ground-water quality during the 

nonirrigation season. Some ground-water samples 

were analyzed for hydrogen, oxygen, and sulfur 

isotopes. Samples of bedrock core, shale residuum, 

alluvium, and ground-water filtrate were collected for 

semiquantitative analyses of clay and bulk mineralogy.

Reconnaissance-level sampling of streams was 

done in 1991 in the Uncompahgre Project area and in 

the Grand Valley. Detailed synoptic sampling of 

streams, washes, and ditches draining irrigated areas 

in the Uncompahgre Project and in the Grand Valley 

was done during January through March 1992. 

Constituents analyzed in synoptic samples were 

selenium, boron, nitrite plus nitrate, dissolved solids, 

and major constituents. Selenium data were collected 

at selected surface-water sites in 1993 and were used 

for analyses of selenium distribution and loads in the 

study area. Bottom-sediment samples were collected 

from selected streams and ponds in 1992.

Biota samples collected in 1991 included 

aquatic plants and invertebrates, fish, birds, and 

bird eggs. Sample collection from March to 

September 1991 was designed to determine if other 

trace constituents or pesticides were of concern in 

the Uncompahgre Project and in the Grand Valley. 

Fish samples were collected from the Gunnison, 

Uncompahgre, and Colorado Rivers during March and 

April 1992 for selenium and selected trace-constituent 

analyses. Wetland sites were monitored for bird usage 

and nesting from April through July 1992. Birds and

INTRODUCTION
bird-food items were collected at wetland sites. Bird 

eggs were collected and incubated. Bioassay samples 

were collected in March 1992 from five of the major 

streams in the Grand Valley and analyzed by the 

U.S. Environmental Protection Agency.

Water-quality, bottom-sediment, hydrogeologic, mineralogic, and biota data collected for 

the detailed study are listed in a report by Butler 

and others (1994). Selenium and other traceconstituent data for the soil and alfalfa samples 

collected in the Uncompahgre Project area are in a 

report by Crock and others (1994). Water-extractable 

data for selenium and other constituents for soil 

samples are discussed in Stewart and others (1993).

Description of Study Area

The Uncompahgre Project area and the Grand 

Valley are located in west-central Colorado (fig. 1). 

The Uncompahgre Project is located along the 

Uncompahgre River in the lower Gunnison River 

Basin. The Uncompahgre Project extends about 34 mi 

between the towns of Colona and Delta (fig. 2). The 

irrigated part of the Grand Valley is located along the 

Colorado River, centered around Grand Junction 

(fig. 3). The confluence of the Gunnison and Colorado 

Rivers is at Grand Junction (fig. 3). Other communities in the Grand Valley include Palisade and Clifton, 

east of Grand Junction, and Fruita and Loma, west of 

Grand Junction.

Physiography, Soils, Ciimate, and Land Uses

The Uncompahgre River Valley principally 

is an agricultural area where irrigation water is 

supplied by diversion from the Gunnison River 

through the Gunnison Tunnel (fig. 1) and by water 

from the Uncompahgre River and its tributaries. 

About two-thirds of the 86,000 irrigated acres of 

the Uncompahgre Project is located west of the 

Uncompahgre River on a series of terraces separated 

by small valleys that were eroded by tributaries of the 

Uncompahgre River. The terraces consist of stream 

deposits and outwash remnants that have relatively 

low soluble salt content (Bureau of Reclamation, 

1984). Irrigated areas east of the Uncompahgre River 

compose about 34 percent of the project area and 

consist of soil formed primarily from weathered 

Mancos Shale. Soils derived from Mancos Shale 

commonly are referred to as adobe soils and are high

in salt content. Alluvial deposits derived primarily 

from Mancos Shale compose parts of the Cedar Creek 

and Loutsenhizer Arroyo Basins (fig. 2). Elevation of 

irrigated areas in the Uncompahgre Project ranges 

from about 4,900 ft at Delta to about 5,900 ft near 

Colona.

The approximately 70,000 acres of irrigated 

land in the Grand Valley are served by Federal and 

by private irrigation systems. The Federal system 

serves about 38,000 acres, and the remainder of the 

irrigated acreage is served by private systems. About 

30,000 acres served by the Federal system is located 

on soils derived from Mancos Shale or from alluvium 

overlying Mancos Shale between the Colorado River 

and the Government Highline Canal (fig. 3). The 

remainder of the Federal system serves irrigated land 

on Orchard Mesa, a low alluvial terrace located in the 

southeast part of the Grand Valley (fig. 3). The Grand 

Valley is relatively flat and gradually slopes upward to 

the northeast to the Book Cliffs, which are about 

1,500 ft above the valley floor. Elevation of irrigated 

areas in the Grand Valley ranges from about 4,400 to 

4,800 ft. Numerous small streams and washes dissect 

the valley and discharge into the Colorado River.

The Uncompahgre Project area and the 

Grand Valley have an arid to semiarid climate 

characterized by cool to occasionally cold winters 

and hot summers. Annual precipitation ranges from 

about 8 in. at Grand Junction and Delta to 13 in. at 

Colona. Annual evaporation in both areas is about 

50 to 70 in. In most years, snowfall is less than 

30 in.

The length of the growing season is quite 

variable in the study area because of local climate 

variability. The growing season ranges from about 

112 to 148 days in the Uncompahgre Project area 

and from about 150 to 190 days in the Grand Valley. 

Major crops produced are alfalfa, feed corn, hay, 

small grains, potatoes, beans, onions, fruit, sweet 

corn, melons, and soybeans. Cattle are wintered 

at many farms, and there are numerous small cattlefeeding operations and dairies.

Urban and residential areas in the Uncompahgre 

Project area are centered around Montrose, Olathe, 

and Delta. Considerable urban and residential areas 

are in the Grand Valley in the vicinity of Grand 

Junction and Clifton. Some agricultural land in both 

areas has been sold for rural-residential and commercial development. The conversion of agricultural land 

to nonagricultural uses has been increasing in the 

study area.

Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and In the Grand Velley, West-Central Colorado, 1991-93
108°15' 108°00' 107°45'

38°45' -

38°30' -

38°15'

GENERALIZED EXTENT 

OF IRRIGATED AREA

0 10 MILES

I ] I

0 5 10 KILOMETERS

Figure 2. Selected features in the Uncompahgre Project area.

Geologic Setting

Rocks exposed in the study area range in 

age from Precambrian to Upper Cretaceous. The 

Mancos Shale, the terrace deposits on the western 

side of the Uncompahgre Project, the valley-fill 

deposits in the eastern end of the Grand Valley 

(the cobble aquifer), and the alluvium overlying the 

Mancos Shale are the primary geologic units affected 

by irrigation in the study area Other rocks in the

study area primarily are exposed on the outer edges 

of the irrigated valleys. The following descriptions 

of the geology of the study area were derived from 

Meeks (1950), Williams (1964), Lohman (1965), 

Craig (1971), Schneider (1975), Tweto and others 

(1976), Schwochow (1978), Cashion (1973), Bureau 

of Reclamation (1986a), and Steven and Hail (1989). 

Geologic maps of the irrigated parts of the study area 

are shown in figures 4 and 5.

INTRODUCTION
& s.

3ff |lft

3 "

s

0 5 MILES

,_____I

0 5 KILOMETERS

EXPLANATION

GENERAUZED EXTENT OF 

IRRIGATED AREA

Figure 3. Selected features in the Grand Valley.
108°15' 108°00' 107°45'

38°45' 

38°30' -

38°15'

DESCRIPTION OF MAP UNITS

QUATERNARY ALLUVIUM

QUATERNARY TERRACE DEPOSITS

MANCOS SHALE OF UPPER CRETACEOUS AGE

ROCKS OF LOWER CRETACEOUS AND JURASSIC AGE

GENERALIZED EXTENT 

OF IRRIGATED AREA

0 10 MILES

0 5 10 KILOMETERS

Figure 4. Generalized surficial geology of the Uncompahgre Project

The Mancos Shale of Upper Cretaceous 

age underlies almost all of the Grand Valley, 

underlies the eastern one-half and part of the 

western one-half of the Uncompahgre Project area 

(figs. 4, 5A), and outcrops in many areas throughout 

western Colorado and Utah. The formation ranges 

in thickness from 2,000 to 3,000 ft and consists 

of massive, fossiliferous marine shale that contains 

interbedded sandstone, siltstone, and volcanic-ash 

layers. The Mancos Shale is the lateral equivalent

of the Niobrara Formation, Cody Shale, and Pierre 

Shale in Montana, Nebraska, South Dakota, and 

Wyoming.

The Mancos Shale is a regressive prodeltaplume complex genetically related to the deltaic 

systems active along the western interior seaway 

of Cretaceous age and is divided into the lower 

shale member, the Perron Sandstone Member 

(or the Juana Lopez Member), the Mancos B 

Member, and the Buck Tongue Member (Cashion, 

1973; Gill and Hail, 1975; Cole and Young, 1991).

INTRODUCTION 9
I

11

ifli

ioi f

CD

39°15' -

5 KILOMETERS

EXPLANATION

DESCRIPTION OF MAP UNITS

ALLUVIUM OF QUATERNARY AGECLAY, SILT, AND GRAVELS

MANGOS SHALE OF UPPER 39°00' 

CRETACEOUS AGE

ROCKS OF LOWER CRETACEOUS 

AND JURASSIC AGE

108° 15'

GENERALIZED EXTENT 

OF IRRIGATED AREA

Figure 5. Grand Valley geology: (A) Surficial geology, (ff> generalized extent of the cobble aquifer, and (Q generalized geologic section.
B

109°00' 108°45' 108-301 108°15'

4,780 4,780

4,480 VERTICAL EXAGGERATION 53X

3 MILES

0 1 2 3 KILOMETERS

Figure 5. Grand Valley geology: (A) Surficial geology, (B) generalized extent of the cobble aquifer, and 

(C) generalized geologic section Continued.

4,680

4,580

4,480

INTRODUCTION 11
The lower shale member contains massive and 

laminated silty-mudstone and sandy-siltstone 

parasequences that contain disseminated pyrite, 

lenticular kerogen and coal, and pyrite minerals 

associated with ash beds. Little information is 

available on the Perron Sandstone Member. The 

Mancos B Member (called the Prairie Canyon 

Member by Cole and Young, 1991) is characterized 

by lenticular, cyclic, upward parasequences that range 

in thickness from 6 to 90 ft and consists of five main 

lithofacies: silty claystone, sandstone-claystone, 

sandy siltstone, bioturbated muddy sandstone, and 

sandy dolomite (as beds and concretions). Volcanicash layers occur as interbeds throughout the Mancos 

Shale and range in thickness from less than 1 in. to 

2 ft. The ash layers in the study area could have 

originated from the Sevier Orogeny in central Utah. 

Volcanic ash could be the source of selenium and other 

trace constituents in the Mancos Shale.

The Mesaverde Formation of Upper Cretaceous 

age overlies the Mancos Shale (fig. 5) and forms the 

caprock for the Book Cliffs that border the northern 

edge of the Grand Valley. The formation consists of 

massive sandstone, gray shale, and coal, and ranges in 

thickness from 60 to 100 ft near the Book Cliffs, but is 

about 2,300 ft thick in areas north of the Grand Valley. 

The Mesaverde Formation has been eroded away and 

is not present in the Uncompahgre Project area.

Terrace deposits on the western side of the 

Uncompahgre Project area (fig. 4) consist of glacial 

outwash derived from the San Juan Mountains. 

These deposits range in thickness from 5 to 60 ft 

and are poorly sorted materials consisting of fine 

rock flour, sand, gravel, cobbles, and boulders. 

The parent material of terrace deposits generally 

consists of granite, rhyolite, and welded tuff mixed 

with weathered and transported detritus from the 

Dakota Sandstone and the Mancos Shale.

Valley-fill deposits in an ancient valley of the 

Colorado River are in the central and eastern parts 

of the Grand Valley (fig. 5B). The deposits were 

named the "cobble aquifer" (Bureau of Reclamation, 

1986a), although the ground-water quality is poor 

and unacceptable for domestic, agricultural, and 

industrial uses. The cobble aquifer is late Holocene 

to Pleistocene in age, ranges in thickness from 10 to 

65 ft, and consists of well-sorted pebbles, cobbles, and 

boulders in a sandy matrix. Alluvium and eolian sand 

may locally cover the gravels (part of the Quaternary 

clay, silt, and gravels shown in fig. 5A). Mudflows of

Mancos Shale mixed with detritus from the Mesaverde 

Formation overlies the cobble aquifer in some 

locations (part of the Quaternary clay, silt, and gravels 

shown in figure 5A).

Alluvium overlying the Mancos Shale consists 

of clay, silt, and gravel deposits of Holocene age. 

Alluvial deposits commonly are a combination of 

fluvial sediments and mudflows of Mancos Shale 

residuum. In the Grand Valley, the alluvium consists 

of Mancos Shale residuum mixed with detritus 

weathered from the Mesaverde Formation that forms 

the caprock for the Book Cliffs (fig. 5). In the 

Uncompahgre Project area, alluvium overlying the 

Mancos Shale in the eastern and southeastern parts of 

the area consists of Mancos Shale residuum mixed 

with detritus from the Dakota Sandstone, weathered 

ash-flow tuff, and andesite from the mountains east of 

the Uncompahgre Project area.

Mineral resources are rich in the areas 

surrounding the Uncompahgre Project area and the 

Grand Valley; however, little mineral resource 

development has occurred in the irrigated areas, 

except for limited natural gas exploration. Uranium 

and vanadium have been mined from the Morrison 

Formation west of the Uncompahgre Plateau; oil shale 

has been developed in the Green River Formation 

north and northeast of the Grand Valley; coal has been 

mined in the Mesaverde Formation and Dakota 

Sandstone; isolated deposits of copper, gold, 

molybdenum, beryllium, and titanium have been 

mined from granitic rocks near Unaweep Canyon 

south of the Grand Valley; bentonitic clays have been 

mined from the Morrison Formation; sulfur has been 

extracted from the Mancos Shale and alunite from the 

Morrison Formation in the upper Gunnison River 

Basin; and gypsum, halite, and potash have been 

mined in the Paradox and Sinbad Valleys along the 

southwestern edge of the Uncompahgre Plateau. 

Selenium occurs with many of these mineral deposits; 

however, the mineral resources development has not 

affected the irrigated areas on a large scale.

Irrigation Projects

Uncompahgre Project

Irrigation began in the 1880's in the 

Uncompahgre Valley, and by 1890 about 

30,000 acres of land were irrigated by private 

systems that used water from five small diversion

12 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
dams along the Uncompahgre River. In 1901, the 

State of Colorado began building the 5.8-mi-long 

Gunnison Tunnel from the Gunnison River to 

supply more water to the Uncompahgre Valley. 

Because of insufficient funding, the State could not 

complete the tunnel, and the BOR completed 

construction of the Gunnison Tunnel in 1909 (fig. 1) 

and a diversion dam on the Gunnison River in 1912. 

The Uncompahgre Project, as it was called, 

included existing and new canals and laterals. 

Taylor Park Reservoir (fig. 1) was built in 1937 to 

supply the project with a dependable water supply. 

Operation and maintenance of the Uncompahgre 

Project was transferred to the Uncompahgre Valley 

Water Users Association in 1932.

The Uncompahgre Project supplies water for 

irrigation of about 86,000 acres in the lower Gunnison 

River Basin (figs. 1 and 2). About two-thirds of the 

irrigated land is west of the Uncompahgre River, 

where soils are more conducive to agriculture. Historically, diversion of water from the Gunnison River 

accounted for about two-thirds of the irrigation water 

for the project, and water from the Uncompahgre 

River accounted for the remainder of the irrigation 

water. The Gunnison Tunnel discharges into the 

South Canal near Highway 50 east of Montrose (fig. 

1). Some water from the South Canal is used to 

irrigate land southeast of Montrose, but most of the 

water discharges into the Uncompahgre River to 

augment flows for downstream canal diversions. 

The irrigation season normally is from early April 

through October. Historically, water was diverted 

from the Uncompahgre River into canals during 

winter (hereinafter referred to as winter water) to 

provide water for livestock.

A part of the subsurface-irrigation drainage and 

surface-return flow discharges into the Uncompahgre 

River upstream from Delta. Much of the water 

diverted by canals downstream from Montrose 

consists of return flows (subsurface and surface) from 

upstream irrigated areas. Between Peach Valley 

Arroyo and Roubideau Creek (fig. 2), irrigation 

drainage and return flows from Uncompahgre Project 

land discharge into the Gunnison River through 

natural drainage pathways or manmade ditches. There 

is considerable irrigated land in the lower Gunnison 

River Basin upstream from Delta, such as in the Smith 

Fork, North Fork, and Tongue Creek Basins (fig. 1).

Grand Valley

Settlement began in the Grand Valley in 1881, 

and the private Grand Valley Canal (fig. 3) was 

completed prior to 1910. The Grand Valley Canal 

irrigates approximately the southern one-half of the 

Grand Valley that is north of the Colorado River 

between Palisade and Loma (fig. 3). The Government 

Highline Canal (the Grand Valley Project) was 

completed in 1917. The Federal system provides 

water to areas between the Government Highline 

Canal and the Grand Valley Canal, to Orchard Mesa, 

and to some areas between Clifton and Palisade. The 

Grand Valley Water Users Association operates and 

maintains the Federal system north of the Colorado 

River, which serves about 30,000 acres. The 

remainder of the valley north of the Colorado River 

is served by the private Grand«Valley Irrigation 

Company. Orchard Mesa is served by the Orchard 

Mesa Irrigation District, and the Redlands area (fig. 3) 

is served by the private Redlands Water and Power 

Company.

The Colorado River is the source of all irrigation water for the Grand Valley, except for the 

Redlands area, which uses water from the Gunnison 

River that is diverted about 2 mi upstream from the 

mouth of the Gunnison River. The Government 

Highline Canal diverts water from the Colorado River 

immediately upstream from the confluence with 

Plateau Creek (fig. 3), and the Grand Valley Canal 

diverts water near Palisade. Much of the subsurfaceirrigation drainage and surface-return flow discharges 

into natural streams and washes that dissect the Grand 

Valley (fig. 3) and discharge into the Colorado River. 

In addition to natural drains, there is about 600 mi of 

man-made surface drains, many of which are in the 

private irrigation system.

Salinity-Control Projects

In 1974, the Colorado River Basin Salinity 

Control Act (Public Law 93-320) was passed by the 

U.S. Congress. That law authorized various investigations and construction of projects with the objective of 

decreasing the salt (dissolved-solids) loading to the 

Colorado River. The Uncompahgre Project and the 

Grand Valley have been included in the salinitycontrol program.

The Lower Gunnison Basin Unit of BOR's 

Colorado River Water Quality Improvement 

Program includes the Uncompahgre Project and

INTRODUCTION 13
other irrigated areas in the lower Gunnison River 

Basin. The BOR (1982,1984) estimated that about 

360,000 tons/yr of salt from irrigation-induced 

sources in the Uncompahgre Project is conveyed to 

the Uncompahgre and Gunnison Rivers, and the salt 

ultimately is discharged to the Colorado River. The 

salt loading is attributed to conveyance-system losses, 

on-farm seepage, and deep percolation of irrigation 

water into weathered and fractured shale of the 

Mancos Shale and into alluvium derived from shale. 

As this irrigation-induced ground water flows through 

the shale, salt is picked up and eventually is 

discharged to drains and rivers and, thus, into the 

Colorado River system.

The Winter Water Replacement Program of the 

Lower Gunnison Basin Unit (Bureau of Reclamation, 

1987) provides a replacement winter-water supply for 

livestock operators who depended on Uncompahgre 

Project canals and laterals for their livestock watering 

needs during the winter. Winter canal flows were 

terminated, and replacement water supplies were 

provided by expanding the existing rural domesticwater systems. The replacement program was 

expected to decrease salt loading to the Colorado 

River by 74,000 tons/yr. As of October 1994, the 

replacement program was 95 percent complete.

The East Side Lateral Program of the Lower 

Gunnison Basin Unit is planned to replace about 

195 mi of earthen irrigation laterals and about 7 mi 

of small canals east of the Uncompahgre River with 

underground pipeline. That program is planned to 

decrease salt loading from the Uncompahgre Project 

to the Colorado River by about 65,000 tons/yr. 

Implementation of the program was scheduled to 

begin in fiscal year 1995; however, as of early 1996, 

this project has not been started.

The Grand Valley Unit is a salinity-control 

project that consists of the Colorado River Valley 

between Palisade and the Colorado-Utah State line. 

The Grand Valley Unit contributes an estimated 

580,000 tons/yr of salt to the Colorado River through 

deep percolation of water from canals, laterals, and 

on-farm irrigation (Bureau of Reclamation, 1983, 

1985). Between 1980 and 1982, the BOR lined 7 mi 

of canal and placed 34 mi of earthen laterals in 

underground pipeline in the Reed Wash Basin (fig. 3) 

for a test program known as Stage One (Bureau of 

Reclamation, 1978). Post-construction monitoring 

indicated that Stage One improvements decreased the

annual salt loading to the Colorado River by 

21,900 tons. The Stage Two program of the Grand 

Valley Unit presently (1996) is under construction 

and completion is scheduled in 1998 (Bureau of 

Reclamation, 1985,1986b). Stage Two is planned 

to replace about 320 mi of laterals with underground 

pipeline and to line about 40 mi of canals, decreasing 

the salt loading to the Colorado River by an estimated 

151,000 tons/yr.

The Natural Resources Conservation Service 

(NRCS), formerly the Soil Conservation Service of 

the U.S. Department of Agriculture, is conducting 

the on-farm parts of the salinity-control program in 

the Lower Gunnison Basin Unit and in the Grand 

Valley Unit. The total irrigated area in the Lower 

Gunnison Basin Unit that was studied by NRCS 

was 171,000 acres, which includes the Uncompahgre 

Project. The estimated total salt loading from irrigation sources in the Lower Gunnison Basin Unit (the 

171,000 irrigated acres) was about 640,000 tons/yr 

(Bureau of Reclamation, 1984). The NRCS is 

improving the private on-farm irrigation water 

delivery and application systems through a volunteer 

cost-share program. Those improvements include 

leveling of irrigation land, underground and gated 

pipelines, sprinkler and drip-application systems, 

concrete-lined ditches, and associated irrigation watermanagement practices. The NRCS estimates that the 

total salt decrease attributed to on-farm improvements 

to be about 166,000 tons/yr in the Lower Gunnison 

Basin Unit and about 132,000 tons/yr in the Grand 

Valley Unit (Hedlund, 1994).

Previous Studies

Much of the water-quality data collected in the 

study area were for salinity studies. As a result, traceconstituent data often were not collected; however, a 

few studies provide selenium concentrations in water, 

soil, plants, and biota in the Uncompahgre Project 

area and the Grand Valley. Selenium data were 

collected as early as 1934-36 in the Uncompahgre 

Project area and in the Grand Valley (Anderson and 

others, 1961). These data were for the Gunnison and 

Colorado Rivers in the Grand Junction area, for the 

Uncompahgre River, for a few drains in irrigated 

areas of the Uncompahgre Valley and Grand Valley, 

and for soil and salt-crust samples. Anderson and 

others (1961) reported a selenium concentration of

14 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
2,680 jig/L in a new (when the sample was collected 

in the 1930's) drain near Mack in the Grand Valley. 

The USGS has collected selenium and other traceconstituent data since 1975 for the Gunnison and 

Colorado Rivers. These selenium data are 

summarized later in this report in the "Selenium 

Concentrations and Loads in the Gunnison and 

Colorado Rivers" section. The U.S. Department 

of Energy (1986) collected trace-constituent data, 

including selenium, from some wells near a uranium 

mill-tailings pile in Grand Junction.

Because of a suspected problem concerning 

human consumption of fish that had high levels of 

selenium, the Colorado Division of Wildlife collected 

selenium data for fish from Sweitzer Lake near Delta 

in 1974 and 1977 (Colorado Division of Wildlife, 

written commun., 1987). A summary of the data is in 

Butler and others (1991). Because of results of the 

fish sampling, a sign was posted in 1977 at Sweitzer 

Lake that warned about high selenium concentrations 

in fish in the lake.

A reconnaissance investigation of the Gunnison 

and Uncompahgre Rivers and Sweitzer Lake (Butler 

and others, 1991) was done in 1988-89 to determine 

if irrigation drainage from the Uncompahgre Project 

was affecting water, fish, and wildlife resources. 

Butler and others (1991) reported high selenium 

concentrations in some water, bottom-sediment, 

and biota samples and that the Uncompahgre 

Project was a significant source of selenium, 

dissolved solids, and sulfate to the Uncompahgre 

River. The maximum selenium concentration in 

water samples was 320 jig/L from the Garnet 

Canal diversion into Sweitzer Lake. A bottomsediment sample from Sweitzer Lake had 41 jig/g 

of selenium. Selenium concentrations in wholebody fish samples from Sweitzer Lake ranged 

from 15.2 to 50.0 jig/g dry weight. There also 

were large selenium concentrations in some of 

the bird samples collected from Sweitzer Lake 

and from the Escalante State Wildlife Area, which 

is along the Gunnison River downstream from Delta 

(fig. 1). However, the reconnaissance investigation 

did not document adverse effects of selenium on fish 

and birds.

Selenium concentrations in soil, water, 

vegetables, and field crops in irrigated areas of the 

Uncompahgre Project and the Grand Valley were 

reported in Follett and others (1990). Sampling was 

limited to a few sites in each area. Follett and others

(1990) reported low selenium concentrations in 

vegetables (broccoli and sweet com) and in field 

crops (alfalfa and wheat).

Considerable dissolved-solids and majorconstituent data were collected in the study area 

for salinity studies and other studies related to 

agriculture. A study of irrigation-return-flow water 

quality in the Grand Valley was done by Duke and 

others (1976). Salinity data collected for a salinitycontrol demonstration project in the Grand Valley 

were reported by Binder and others (1978). The 

BOR has done several investigations concerning 

salinity studies in the Uncompahgre Project (Bureau 

of Reclamation, 1984,1987) and in the Grand Valley 

(Bureau of Reclamation, 1978,1983,1985). The 

report by the BOR (1983) described a detailed water 

budget for the Grand Valley. The USGS collected 

dissolved-solids and major-ion data for several 

streams and washes in the Grand Valley between 1973 

and 1983 for the salinity investigations. Whittig and 

others (1983) did a salinity investigation in the West 

Salt Creek Basin, located west of irrigated areas in the 

Grand Valley.

An early survey of soils in the Uncompahgre 

Valley was done in 1910 by the U.S. Department of 

Agriculture (Nelson and Kolbe, 1912). A more recent 

soil survey of the Uncompahgre Project area was done 

by Cline and others (1967). A study of soil in saline 

areas of the Grand Valley was done by Fireman and 

others (1951). Spears and Kleven (1978) did a soil 

survey of Mesa County, which includes the Grand 

Valley. Lusby (1978) described results of a longterm study of grazing effects on sediment runoff in 

a nonirrigated area in the western end of the Grand 

Valley.

Meeks (1950) investigated geology and ground 

water in the irrigated areas of the Uncompahgre 

Valley. Another investigation of ground water in the 

Uncompahgre Valley was done by Craig (1971). A 

reconnaissance study of ground-water resources in the 

lower Gunnison River Basin was described by Brooks 

and Ackerman (1985). Lohman (1965) described the 

geology and hydrology of artesian aquifers in the 

Grand Valley. Uranium and selenium-indicator plants 

that were used for uranium prospecting in the 

Colorado Plateau are described by Cannon (1954).

The BOR did several water-resource and 

hydrogeologic investigations of the Uncompahgre 

Project and the Grand Valley as part of the salinitycontrol projects in each area. A report by the BOR

INTRODUCTION 15
(1977) contains ground-water reconnaissance and 

well-inventory information for the Uncompahgre 

Project (Lower Gunnison Basin Unit). The report 

also has some water-quality data, including data for 

arsenic and boron concentrations in ground water. 

Information about land and water resources, the 

irrigation system, water quality, and hydrogeology 

for the Uncompahgre Project is in two BOR reports 

(1982,1984). Hydrogeologic investigations related 

to irrigation in the Grand Valley were reported in three 

BOR reports (1978,1986a, b). The report from 1978 

describes a detailed hydrogeologic investigation of 

the Reed Wash Basin for the Stage One salinity 

project. Geologic logs and salt-loading information 

for the cobble aquifer in the Grand Valley are in a 

BOR report (1986a).

Studies have been done since the early 1980's 

concerning threatened and endangered fish species in 

the Upper Colorado River Basin, and part of that work 

was done in the Grand Valley area. Most of the 

studies in the Grand Valley area concerned the 

Colorado River squawfish and the razorback sucker. 

Behnke and Benson (1983) and Archer and others 

(1985) described population, habitat, and reservoir 

effects on endangered fish in the Upper Colorado 

River Basin. A study of bonytail chub was described 

in Kaeding and others (1986), and a study of Colorado 

squawfish and razorback suckers in a reach of the 

Colorado River in the Grand Valley was described by 

Osmundson and Kaeding (1989). Concentrations of 

trace constituents in Colorado squawfish were 

described by Krueger (1988). Concentrations of 

organochlorine pesticides and trace constituents in 

endangered fish were described in Kurey (1985a, b).

Ac knowledg ments

The authors thank the numerous property 

owners for allowing access to their property for collection of samples. We also thank Jack Cunningham and 

Kenneth Weston (BOR) for compilation of historical 

ground-water and aquifer-test data, Roger Fujii 

(USGS) for selenium-speciation analysis and for 

his insights concerning selenium geochemistry, 

David Wolny for his interpretation and valuable 

information on the local geology, and Jeffery Foster 

(USGS) for assistance with collection of surfacewater and ground-water data. The authors 

also thank Larry Fukui of RUST Geotech, Inc.,

in Grand Junction, Colo., for mineralogic analysis, 

and Dorothy Hahn and Carol Burns for assistance with 

biota sampling and data compilation. Bioassay tests 

on water and bottom sediment were done with the help 

of the following employees of the U.S. Environmental 

Protection Agency: Loys Parrish and Bill Schroeder 

in Denver, Colo., and Mark Smith in Cincinnati, Ohio.

SOILS AND ALFALFA INVESTIGATIONS

An unbalanced, four-level, stratified random 

sampling design was used to define sources, distribution, and variability of selenium and other traceconstituent concentrations in the soils and associated 

alfalfa among and within geologic units in the 

Uncompahgre Project area. As part of the soilsampling scheme, geologic units were mapped and 

described based on earlier geologic maps and soilsurvey maps of the project area (Marshall, 1959; 

Williams, 1964; Cline and others, 1967; Tweto and 

others, 1976; Steven and Hail, 1989). Soils derived 

from five geologic units were sampled: Upper 

Cretaceous Dakota Sandstone (Kd); Upper 

Cretaceous Mancos Shale (Km); Quaternary 

deposits in ancient terraces and alluvial fans (Qt); 

Quaternary (Holocene) alluvium overlying Mancos 

Shale (Qm); and Quaternary (recent) flood-plain 

alluvium deposited by the Uncompahgre River and 

other streams (Qr). In the Uncompahgre Project 

area, most of the soils of unit Qm were derived from 

Mancos Shale. One site was sampled on Quaternary 

alluvium derived from Dakota Sandstone (Qd).

Fifteen townships were included in the study 

area, and the distribution of soil-sampling sites is 

shown in figure 6. All soil samples were collected in 

irrigated areas. Random sections were selected in 

each township until all geologic units in a township 

had been selected, in triplicate, for soil and alfalfa 

sampling (fig. 6). The sample distributions by 

geologic unit from the randomly selected sections are 

listed in table 1. Township 4 had four sites for the 

geologic unit Km; geologic unit Qd was sampled only 

in township 13 (table 1). If a preselected site was 

unacceptable in the field because of irrigation or 

absence of alfalfa, a new site was selected in 

an adjacent section as close as possible to the 

original site. To assess variability within a section, 

two samples about 100 m apart were collected in 

14 randomly selected sections.

16 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
R96W R95W

R10W R9W R8W

\ 

10

10

I

I

15 KILOMETERS

15 MILES

I

Figure 6. Distribution of soil-sampling sites within townships in irrigated areas of the 

Uncompahgre Project.

Soil cores of 1 m were removed using a

9-cm bucket auger, and the entire core was homogenized by mixing in a plastic container. A 1-kg 

portion of the 1-m core mixture was placed in a paper 

bag for transport to the laboratory. The top 20 cm 

of several alfalfa plants (Medicago saliva L.) at

10-percent bloom stage were sampled within a 

few meters of the auger bore, composited, placed 

in paper bags, and air dried. At several sites, 

alfalfa could not be obtained, and only the soil 

cores were taken.

Sample Preparation and 

Analytical Methods

Soil samples were air dried in the laboratory at 

ambient room temperature (about 20°C), disaggregated using a mechanical mortar and pestle, and 

sieved to pass a 2-mm screen (10 mesh). A split of 

this material was ground to less than 100 mesh using a 

ceramic-plate grinder. The split was used for total 

analysis; the unground, 2-mm fraction was used for 

extractions. Field-dried alfalfa samples were washed

SOILS AND ALFALFA INVESTIGATIONS 17
Table 1. Distribution of soil-sampling sites by geologic units in townships of the Uncompahgre Project area

[Numbers under geologic unit indicate the number of sites in each unit within each township; geologic units are defined as follows: 

Kd, Upper Cretaceous Dakota Sandstone; Km, Upper Cretaceous Mancos Shale; Qt, Quaternary terrace and alluvial-fan deposits; 

Qm, Quaternary (Holocene) alluvium overlying Mancos Shale; Qr, Quaternary (recent) flood-plain alluvium deposited by the 

Uncompahgre River and other streams; Qd, Quaternary alluvium derived from Dakota Sandstone; , no samples]

Township 

number

(fig. 6)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Township

15 south

15 south

15 south

51 north

51 north

51 north

50 north

50 north

50 north

49 north

49 north

49 north

48 north

48 north

48 north

Range

(all west) Kd

96

95

94

11

10

09

11 3

10

09

10 3

09

08

10 3

09

08

Geologic unit

Km

3

3

3

U

3

3

~

3

3

3

3

3

3

3

Qt

3

3

3

3

3

3

3

3

«

-

--

Qm

3

3

3

3

-

-

3

3

3

3

3

~

3

3

Qr

3

3

~

3

~

3

~

3

3

~

3

--

Qd

~

-

~

~

~

-

-

2 1

~

--

'Analytical data for all sites included for calculation of geometric means and ranges, but only first three sites used for analysis of 

variance (ANOVA).

Analytical data included for calculation of geometric means and ranges, but not for ANOVA.

using tap water, rinsed several times in demineralized 

water, redried at ambient room temperature, and 

ground to pass a 2-mm screen in a Wiley mill. Each 

ground sample was mixed and split, and a part was 

ashed at 450°C in a muffle furnace for 24 hours.

Soils and plant ash were decomposed using a 

low-temperature mixed acid digestion (Crock and 

others, 1983). Solutions were analyzed simultaneously for 40 constituents using inductively coupled 

plasma atomic-emission spectroscopy (ICP-AES) 

(Crock and others, 1983). Soils and dried plant 

material for arsenic and selenium determinations were 

decomposed by mixed acids and hydrogen peroxide 

(Crock and others, 1994). Determinations were made 

by hydride-generation atomic absorption spectroscopy 

(HGAAS) (Crock and Lichte, 1982; Sanzolone and 

Chao, 1987).

Extractions on soils were done using deionized 

water (one part soil to five parts water, by weight) by 

agitation for 16 hours on a horizontal reciprocating 

shaker (Stewart and others, 1990). Solutions were 

clarified by centrifugation and filtration (0.45-flm

filter). Conductivity, pH, and anion concentrations 

were determined in solution by conductivity meter, 

standard combination pH electrode, and ion chromatograph (Wilson and others, 1987; Stewart and others, 

1990). A part of each solution was predigested to 

oxidize organic matter, according to a modification 

developed by Presser and Barnes (1985), prior to 

arsenic and selenium determination by HGAAS.

Selenium Concentrations 

in Soils and Alfalfa

The geometric means and ranges for total 

selenium in all soil samples from the Uncompahgre 

Project area and in soil samples from each geologic 

unit (Crock and others, 1994) are listed in table 2. 

Total-selenium concentrations in the 142 soil samples 

ranged from 0.1 to 8.6 \lg/g (table 2). The geometric 

mean selenium concentration for all soil samples was 

1.4 Mg/g, compared to a geometric mean of 0.23 pg/g 

for Western soils (Shacklette and Boerngen, 1984).

18 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Residual soils derived from Mancos Shale (Km) and 

alluvium overlying Mancos Shale (Qm) contained the 

highest geometric mean selenium concentrations of 

2.2 and 2.9 jlg/g (table 2). These values are 10 to 

13 times the geometric mean for Western soils. 

The geometric mean total-selenium concentrations 

in soils derived from the Quaternary terrace deposits 

(Qt), recent flood-plain alluvium (Qr), and Dakota 

Sandstone (Kd) were lower than the geometric mean 

concentrations in soils derived from Mancos Shale 

(Km), but exceeded the geometric mean for Western 

soils.

The geometric mean concentrations of waterextractable selenium for the soil samples, grouped by 

geologic unit, are listed in table 2. Concentrations 

ranged from 0.005 to 1.8 Jlg/g dry weight. As with 

the results of total-selenium analysis, the geometric 

mean concentration of water-extractable selenium was 

higher in alluvium overlying Mancos Shale (Qm) than 

in the residual soils derived from Mancos Shale (Km). 

The percent of total selenium that was water extractable was uniformly low for all geologic units and 

ranged from 1.5 to 2.7 percent.

Total-selenium concentrations in 128 alfalfa 

samples from the Uncompahgre Project area ranged 

from less than 0.03 to 9.5 jlg/g dry weight (table 2).

Data for individual samples are listed in Crock and 

others (1994). The geometric mean selenium concentration of 0.33 jlg/g (table 2) for all alfalfa samples 

exceeded the recommended minimum for cattle feed 

of 0.1 |lg/g (Fisher and others, 1987). Only five alfalfa 

samples exceeded the recommended dietary tolerance 

limit of 4 jig/g (Byers, 1935). The alfalfa data from 

the Uncompahgre Project area indicates lower 

selenium concentrations compared to the Kendrick 

Reclamation Project area in Wyoming, where 

15 percent of alfalfa samples from irrigated fields 

exceeded the tolerance limit (See and others, 1992). 

As with total and water-extractable selenium in soil, 

the highest geometric mean selenium concentrations 

in alfalfa were in plants growing on alluvium 

overlying Mancos Shale (Qm).

Selenium concentrations in alfalfa do not 

show the same trend as selenium concentrations in 

soil when the geometric means for the geologic units 

are compared. Residual soils derived from Mancos 

Shale (Km) and Quaternary terrace deposits (Qt) 

contained higher total-selenium concentrations in 

soil than did recent flood-plain alluvium (Qr), but 

selenium concentrations in alfalfa from units Km and 

Qt were significantly lower (significance level 0.05) 

than the concentrations in alfalfa from unit Qr.

Table 2. Geometric mean and observed range of total- and water-extractable selenium concentrations in soil and totalselenium concentrations in alfalfa for all samples from the Uncompahgre Project area, by geologic unit

[Selenium concentrations in soil and alfalfa, in micrograms per gram; geologic units are defined as follows: Kd, Upper Cretaceous Dakota Sandstone; 

Km, Upper Cretaceous Mancos Shale; Qt, Quaternary terrace and alluvial-fan deposits; Qm, Quaternary (Holocene) alluvium overlying Mancos Shale; 

Qr, Quaternary (recent) flood-plain alluvium deposited by the Uncompahgre River and other streams; <, less than; analytical duplicates not included; 

data for all samples includes one sample for geologic unit Qd, Quaternary alluvium overlying Dakota Sandstone]

Statistic

Detection ratio

Geometric mean

Observed range 

Detection ratio

Geometric mean

Observed range 

Detection ratio

Geometric mean

Observed range

All samples

142:142

1.4

.1-8.6 

128:128

.027

.005-1.8 

118:128

.33

<.03-9.5

Kd Km Qt

SOIL, TOTAL SELENIUM1

11:11 43:43 28:28

.5 2.2 1.0

.3-.6 .3-7.5 .3-3.4 

SOIL, WATER-EXTRACTABLE SELENIUM2

9:9 40:40 24:24

.009 .043 .013

.005-.019 .005-1.8 .005-.062 

ALFALFA, TOTAL SELENIUM1

10:11 31:35 25:26

.12

<.03-.69

.28 .25

<.03-1.6 <.03-1.8

Qm

36:36

2.9

.9-8.6

33:33

.048

.008-.86 

30:34

.56

<.03-6.3

Qr

23:23

.6

.1-2.9 

21:21

.018

.005-1.0 

21:22

.48

<.03-9.5

'Data from Crock and others (1994). Site duplicates were considered independent samples. 

Data from Stewart and others (1993). Site duplicates were averaged to obtain one value for each site.

SOILS AND ALFALFA INVESTIGATIONS 19
Selenium in alfalfa from unit Qr was not significantly 

different from selenium concentrations in alfalfa 

from alluvium overlying Mancos Shale (Qm), 

although the flood-plain alluvial soils contained 

only about 21 percent of total selenium and about 

38 percent of the water-extractable selenium as did 

the soils on alluvium overlying Mancos Shale. In 

addition, the correlation of total selenium in alfalfa 

with water-extractable selenium in soils on unit Qr is 

0.77, which is much higher than the same correlation 

for all samples of 0.47 (Stewart and others, 1993). 

Results from analysis of variance (ANOVA) 

indicate that selenium distribution in the 

Uncompahgre Project is not uniform among geologic 

units or within geologic units. The percent of the 

variance for total and water-extractable selenium in 

soil and total selenium in alfalfa that are accounted for 

by each of the four levels in the ANOVA sampling

design (Stewart and others, 1993; Crock and others, 

1994) are shown in figure 7. The ANOVA results in 

figure 7 show the variance among geologic units and 

the variance within geologic unit at the township, 

section, and within section scale. Fifty-seven percent 

of the variance in total selenium in soil was accounted 

for by geologic units. By contrast, only 30 percent of 

the total variance in water-extractable selenium in soil 

is accounted for by geologic units; differences among 

sections in the same geologic unit accounted for more 

of the total variance in water-extractable selenium 

than did differences among geologic units.

Ranges of selenium concentration were much 

greater in alfalfa from the alluvial units Qm (soils 

overlying Mancos Shale) and Qr (soils on recent 

flood-plain alluvium) than the ranges for the other 

geologic units Kd (soils derived from Dakota 

Sandstone), Km (soils derived from Mancos Shale,

60

55

O 5°

|

ID

Z 40

Z 

ID

ID 

W

Z

ID 

O

45

35

30

25

= 20

15

ID 10 

O 

EC 

ID

TOTAL SELENIUM. SOIL 

WATER-EXTRACTABLE SELENIUM, SOIL 

TOTAL SELENIUM, ALFALFA

AMONG GEOLOGIC 

UNITS

AMONG 

TOWNSHIPS

AMONG 

SECTIONS

Figure 7. Percent of variance in selenium concentrations in soils and alfalfa for different levels in the 

sampling design.

20 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and In the Grand Valley, West-Central Colorado, 1991-93
and Qt (soils derived from terrace and alluvial-fan 

deposits) (table 2). Only 13 percent of the total 

variation in selenium concentration in alfalfa was 

attributed to differences among geologic units. 

About one-third of the total variation resulted from 

concentration differences within sections, indicating 

high variability between samples collected in close 

proximity within the same geologic unit. Similar 

large variations in selenium concentrations in alfalfa 

collected in close proximity on the same parent 

geologic unit were reported for alfalfa in the Kendrick 

Reclamation Project area in Wyoming (See and others, 

1992) and in native vegetation from seleniferous soils 

in South Dakota (Moxon and others, 1938; Olson and 

others, 1942).

The results indicate that selenium is mobilized 

from Mancos Shale and redistributed downslope in the 

Uncompahgre Project area by irrigation. The ANOVA 

results and the high correlations of selenium in alfalfa 

with water-extractable soil selenium, especially in 

alluvial soils, indicate that the forms of selenium have 

changed from immobile selenides incorporated in 

sulfide minerals and selenites sorbed to other iron 

minerals in the residual shale (Lakin, 1961; Elrashidi 

and others, 1989) to a higher proportion of the more 

mobile selenate in the alluvium (Geering and others, 

1968; Balistrieri and Chao, 1987). This change in 

chemical species might occur because of increased 

exposure to ground water and of changing redox 

conditions in geographically lower elevations and 

could be expected to occur under the alkaline soil 

conditions that are present in the project area. 

Displacement of selenium downslope from a source 

area and an increase in content of dissolved species 

in contact with ground water was reported for the 

San Joaquin Valley in California (Tidball and others, 

1989) and for the Kendrick Reclamation Project area 

in Wyoming (See and others, 1992). Because the 

Uncompahgre Project area generally is well drained, 

selenium does not accumulate to the levels that were 

reported in soils and alfalfa from the Kendrick 

Reclamation Project area (See and others, 1992), 

where irrigation drainage was evaporated in a closed 

basin.

Other trace constituents of environmental 

interest that had higher concentrations in soils of 

the Uncompahgre Project area than in soils in the 

Western United States were arsenic, molybdenum, 

uranium, and zinc. Concentrations of these trace 

constituents were highest in soils from alluvium 

overlying Mancos Shale (unit Qm), and the

geometric means for unit Qm are listed in table 3. 

Alfalfa from three of the five geologic units had 

mean copper to molybdenum ratios of less than 2:1. 

A minimum ratio of 2:1 in livestock feed is considered 

necessary to prevent symptoms of molybdenuminduced copper deficiency in cattle (Miltmore and 

Mason, 1971; Erdman and others, 1978).

Table 3. Geometric mean concentrations of arsenic, 

molybdenum, uranium, and zinc in soils from alluvium 

overlying Mancos Shale in the Uncompahgre Project 

area and in soils from the Western United States

[Concentrations, in micrograms per gram; data for Western United States 

soils from Shacklette and Boerngen (1984)]

Area Arsenic Molybdenum Uranium Zinc

Uncompahgre Project 

Alluvium from 

Mancos Shale 

(unit Qm)

Western United States .9

110

55

GROUND-WATER INVESTIGATIONS

The objectives of the ground-water investigations were to describe concentrations and distribution 

of selenium and other trace constituents in ground 

water in the Uncompahgre Project area and in the 

Grand Valley and to describe the hydrochemical 

processes affecting selenium release and mobilization 

in ground water in irrigated areas on Mancos Shale 

terrain. The approach was collection of geologic 

and ground-water-quality data from type areas that 

were representative of the hydrogeology of the whole 

area. Selection of type areas was based on the 

availability of historical ground-water data, existing 

wells, and geologic information. The type areas 

include:

1. S weitzer Lake area in the eastern part of the 

Uncompahgre Project area representing 

terrane on Mancos Shale residuum (fig. 8);

2. Dry Creek Basin in the western part of the 

Uncompahgre Project area representing 

terrane on Quaternary terrace deposits 

(fig. 8); and,

3. Reed Wash Basin in the Grand Valley 

representing terrane on Mancos Shale 

residuum and alluvium overlying 

Mancos Shale (fig. 9).

Hydrogeologic characteristics of the type areas are 

described in the "Ground-Water Hydrology" section.

GROUND-WATER INVESTIGATIONS 21
108°15' 108°00' 107°45'

38°45'

38°30'

38°15'

SL1 

SLSW1

SAMPLE WELL AND NUMBER 

SAMPLE SPRING AND NUMBER

GENERALIZED EXTENT 

OF IRRIGATED AREA

OUTLINE OF TYPE AREA

10 MILES

0 5 10 KILOMETERS

Figure 8. Locations of type areas and locations of wells and springs in the Uncompahgre Project area where 

samples were collected for chemical analysis.

In the type areas, existing wells were inventoried and historical ground-water data were compiled, 

wells were installed where needed to supplement the 

existing wells, sediment-core samples were collected 

from boreholes, geologic information was compiled, 

and a data logger was installed in one well. In 

addition to the type-area investigations, wells were 

inventoried and ground-water samples were collected 

over a wide area to describe the areal distribution of 

selenium and other trace constituents. Deep wells

(privately owned) were inventoried and sampled to 

describe the boundary conditions for possible traceconstituent migration into the study area from deep 

aquifers. Where available, wells were inventoried and 

sampled outside the irrigated areas to describe natural 

or nonirrigated conditions. Samples of Mancos Shale 

bedrock were collected from cores archived at the 

USGS Core Research Center in Denver, Colo. 

Historical ground-water data, aquifer-test results, 

and digital-map data were compiled by the BOR.

22 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Q.

(0 (0

2

0)

 o

I 

o

0)

£

c

 o

c5

0) 

CL

O)

O)

GROUND-WATER INVESTIGATIONS 23
Sample Collection and 

Analytical Methods

Many of the shallow wells used for the groundwater investigation were installed by the BOR as part 

of the salinity-control program in the Uncompahgre 

Project area and in the Grand Valley (Bureau of 

Reclamation, 1986a, b). Five shallow monitoring 

wells were installed by the USGS for this study using 

the hollow-stem auger method three wells were 

installed in alluvium (Reed Wash type area) and two 

wells were installed in shale residuum (Sweitzer Lake 

type area). Aquifer-sediment samples were collected 

at 5-ft intervals using a split-spoon sampler. 

Sediment-core samples for trace-element analysis 

were air dried, and samples for selenium-speciation 

analysis were frozen. After installation of 2-in. 

polyvinyl chloride (PVC) casing with 5-ft screens, 

the wells were developed by purging about 30 well 

volumes of water from the wells. Depths of wells in 

the study area (BOR, USGS, and private) ranged from 

8 to 1,000 ft below land surface.

Borehole samples were collected during installation of the shallow wells, topical samples were 

collected (outcrop of ash layers), and Mancos Shale 

bedrock-core samples were obtained from cores 

archived at the USGS Core Research Center in 

Denver, Colo. Selected sediment-core, bedrock-core, 

and topical samples were analyzed for (1) major and 

trace constituents by ICP-AES (Crock and others, 

1983); (2) selenium and arsenic by continuous-flow 

HGAAS (Crock and Lichte, 1982; Sanzolone and 

Chao, 1987); (3) uranium and thorium by delayed 

neutron-activation counting (McKown and Mallard, 

1987); and (4) total organic carbon by combustioninfrared detection (Jackson and others, 1987). 

Sequential extractions were performed on selected 

sediment-core samples (Fio and Fujii, 1990), and 

selenium speciation of the extraction fluids was 

performed by column chromatography and HGAAS 

(Makita and Fujii, 1992). X-ray diffraction (XRD) 

analyses were performed on bedrock, shale residuum, 

alluvium, and ground-water-filter residuum samples 

by L.M. Fukui (Senior Petrologist, Chem-RUST 

Geotech, Grand Junction, Colo.). Bedrock, shale 

residuum, alluvium, and ground-water-filter 

residuum samples were analyzed by scanning 

electron microscopy (SEM).

Water samples from wells and one spring 

were analyzed for (1) major ions, (2) selected trace 

constituents (including uranium), (3) nitrogen species,

(4) dissolved organic carbon, (5) stable isotopes of 

oxygen and hydrogen, (6) nitrogen isotopes of 

dissolved nitrate, (7) sulfur isotopes of dissolved 

sulfate, and (8) selenium species. Three to six 

well volumes of water were purged from the wells 

prior to sample collection, using a centrifugal 

pump. Samples were collected using a rinsed 

Teflon bailer and were carefully decanted into 

clean 1-L polyethylene bottles, using a bottomemptying device on the bailer to prevent sample 

agitation and loss of volatile components (Barcelona 

and others, 1984). Dissolved-oxygen measurements 

for ground water were performed within the borehole 

by lowering the oxygen probe (Yellow Springs 

Instruments, Inc., dissolved-oxygen probe) on a 

cable and moving the probe vertically about 1 ft/s 

in the middle of the screened interval; the dissolvedoxygen meter was calibrated according to atmospheric 

pressure and the ambient ground-water temperature, 

and the meter was set to zero using a solution devoid 

of oxygen. Sample filtration and preservation, 

field-alkalinity titration, and measurement of water 

properties (pH, Eh, and specific conductance) were 

performed in a field vehicle. Oxidation-reduction 

potential (Eh, or redox potential) was measured 

using a platinum electrode, which was checked 

against ZoBell's solution at the ambient groundwater temperature. Water samples were filtered 

through a 0.45-p,m filter for alkalinity titrations, 

dissolved major-ion, nitrogen species, and traceconstituent analyses. Samples were filtered through 

a 0.20-p,m filter for sulfur-isotope and seleniumspeciation analyses. Samples collected for dissolvedorganic-carbon analyses were filtered through a 

0.45-jim silver filter. Samples collected for uranium 

and oxygen- and hydrogen-isotope analyses were 

unfiltered. Water samples collected for cation 

analyses were acidified using nitric acid to pH < 2; 

samples collected for nitrogen species analysis were 

preserved using liquid mercuric chloride; samples 

collected for mercury analyses were preserved using 

potassium dichromate; and samples collected for 

oxygen and hydrogen stable isotopes were preserved 

using mercuric chloride tablets. Water-sample filters 

were saved and dried for SEM and XRD analyses to 

describe the paniculate phases that might affect 

ground-water quality.

A data logger was installed in one well in 

the Grand Valley for continuous recording of the 

water level and specific conductance of the ground

24 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
water. Two wells were sampled monthly one 

in Mancos Shale residuum and one in alluvium 

overlying Mancos Shale to describe the seasonal 

effects of irrigation on trace-element concentrations 

in ground water.

Historical ground-water-quality and groundwater-level data were compiled by the BOR to 

describe the long-term effects of irrigation on ground 

water in the study area. Hydraulic data from historical 

aquifer tests performed by the BOR also were 

compiled by the BOR.

Ground-Water Hydrology

Ground water in the study area affected by 

irrigation occurs in shallow systems consisting of 

(1) Mancos Shale residuum, (2) alluvium overlying 

Mancos Shale, (3) the cobble aquifer in eastern

Grand Valley, and (4) terrace deposits in the 

western Uncompahgre Project area. The hydrogeologic settings, general lithologic descriptions, 

and general hydrogeologic characteristics of these 

ground-water-flow systems are described in 

table 4. Although the shallow systems might not 

have contained appreciable quantities of water in 

their natural (preirrigation) state, irrigation now 

provides substantial quantities of water for the 

existence of a water-table system in the shallow 

unconsolidated materials. The shallow groundwater systems generally discharge to local drains, 

streams, and rivers (fig. 10). Hydrogeologic 

characteristics differ greatly over short distances 

in the shallow system (table 4) because of the 

differences in composition of alluvium (which 

include paleochannels consisting of gravels and 

cobbles, fig. 10) and the differences in weathering 

and fracturing of the Mancos Shale.

Table 4. Hydrogeologic settings of the ground-water-flow systems affected by irrigation in the Uncompahgre Project area and 

in the Grand Valley

[Fm, Formation; ft, feet; n, number of data points; <, less than; ft/d, feet per day; hydraulic-conductivity data were provided by the Bureau of Reclamation; 

DC, Dry Creek type area; RW, Reed Wash type area; SL, Sweitzer Lake type area; , hydrogeologic setting not studied as a type area]

Hydrogeologic 

setting

Type 

area

Source 

rock(s)

General lithologic description 

and hydrogeologic characteristics

Alluvium on 

Mancos Shale

RW Mancos Shale, Mesaverde Fm (Grand 

Valley); Mancos Shale, Dakota Fm, 

San Juan Tuff, and gneisses of the 

Black Canyon of the Gunnison and 

Cimmaron Ridge areas (Uncompahgre 

Valley)

Cobble aquifer 

(Grand Valley)

Terrace deposits 

(western 

part of the 

Uncompahgre 

Valley)

Mancos Shale 

residuum

Grand Mesa basalts, Mesaverde Fm, 

Green River Fm, Wasatch Fm, 

Mancos Shale

DC Glacial outwash and alluvium from 

the San Juan Mountains and the 

Uncompahgre Plateau

SL Mancos Shale

Clay, silt, sand, and gravel deposits of Holocene age. Grand Valley: 

Erosional detritus of clay, sand, and silt; mixture of Mesaverde Fm 

(indicated by Inoceramus and Pychnodantes fossils) and shalederived clays (smectite and illite) that might be derived from 

mudflows. Thickness ranges from 5 to 40 ft. Eastern part of 

Uncompahgre Valley: Erosional detritus from Precambrian 

gneisses, Oligocene tuffs, and the Dakota Fm of Cretaceous age. 

Thickness ranges from 5 to 25 ft. Hydraulic conductivities range 

from 8 to 140 ft/d and average 74 ft/d (n=3).

Valley-fill deposits in an ancient valley of the Colorado River. 

Well-sorted pebbles, cobbles, and boulders in sandy matrix; 

alluvium and eolian sand might locally cover gravels. Late 

Holocene to Pleistocene age. Thickness ranges from 10 to 65 ft. 

Hydraulic conductivities range from 38 to 510 ft/d and average 

190 ft/d (n=26).

Poorly sorted materials ranging from fine rock flour through sand, 

gravel, cobbles, and boulders (Meeks, 1950). Glacial outwash 

and alluvium of Pleistocene age locally covered by recent alluvium. 

Thicknesses range from 5 to 60 ft. Hydraulic conductivity from 

one hydraulic test was 650 ft/d.

Clayey, silty residuum consisting of smectite, illite, and kaolinite 

clays. Abundant secondary gypsum occurs in shale partings 

and fractures; pyrite concretions altered to limonite; ash layers 

weathered to kaolinite and quartz containing some jarosite. 

Thickness ranges from <1 to 100 ft. Hydraulic conductivities 

range from 4.5 to 300 ft/d and average 88 ft/d (n=17).

GROUND-WATER INVESTIGATIONS 25
A REED WASH TYPE AREA

4,490 -

C/3

LLJ

O 

00

<

H 4,470-

LLJ 

LU

Mancos Shale Residuum

r 4,510

4,450 VERTICAL SCALE EXAGGERATED

B

5,600 -|

5,500

5,400

C/3

> 5,300- 

O

CQ

LU 5,200

LU '

5,100

0 100 200 FEET

___I_____i

0 30 60 METERS

DRY CREEK TYPE AREA

5,000 VERT| CAL SCALE EXAGGERATED

1 MILE

- 4,490

-4,470

4,450

r 5,600

- 5,500

5,400

- 5,300

5,200

- 5,100

5,000

0 1 KILOMETER

EXPLANATION

INFERRED GROUND -WATER FLOW DIRECTION

Figure 10. Schematic showing hydrogeologic setting and ground-water flow in (A) Reed Wash type area and 

(£) Dry Creek type area.

26 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Deep artesian ground-water-flow systems occur 

in the Uncompahgre Project area (fig. 10) and in the 

Grand Valley. Upward ground-water flow from these 

artesian systems into the shallow systems is not 

certain, except through wells that penetrate the deep 

aquifers. Detailed descriptions of the deep groundwater-flow systems are reported in Meeks (1950), 

Lohman (1965), and Craig (1971).

Recharge to the shallow ground-water-flow 

system occurs through the seasonal application of 

irrigation water, from snowmelt during spring, and 

from infrequent thunderstorms during summer. 

Water-level fluctuations in well 143NO (located in the 

Grand Valley, fig. 9) and the fluctuations of specific 

conductance of ground water are shown in figure 11. 

Both respective traces of transducer and specificconductance data show the effects of irrigation on 

ground-water recharge. Water-level altitude in 

well 143NO increased during the irrigation season 

(April-September 1992) (fig. 11), and the water-level 

altitude slowly declined during the winter of 1992-93. 

The increase indicates storage of ground water in the 

shallow aquifers of the irrigated area surrounding 

well 143NO. Specific conductance of ground water

in well 143NO decreased during late summer and early 

fall (August-October 1992) (fig. 11) following the 

irrigation season, indicating a delayed dilution of the 

ground-water-quality constituents in response to 

irrigation recharge. Irrigation recharge occurs through 

leakage of unlined canals and irrigation ditches and 

through flood-irrigation practices in the study area. 

Calculations of recharge are difficult because 

techniques frequently used for estimating recharge 

(such as hydrograph separation) might not accurately 

represent ground-water recharge in the irrigated area 

where streams fluctuate greatly. However, a spring in 

the Sweitzer Lake type area seemed to constantly 

discharge about 0.25 gal/min. Assuming that the 

discharge from the spring is constant throughout the 

year, a rough estimate of recharge in the 160-acre 

irrigated basin is about 0.03 in/yr. Snowmelt from 

mesas and mountains adjacent to the Uncompahgre 

Project area and the Grand Valley increases streamflows during early spring and recharges the natural 

ground-water system near intermittent streams; 

however, a determination of natural recharge to the 

ground-water system from intermittent streams was 

not made.

LU

LU

LU

LU 

_J

DC 

UJ

I

4,526

LU 

CO

> 4,525 

O

CD

LU 4,524

LL

4,523

4,522

4,521 M

WATER LEVEL

SPECIFIC CONDUCTANCE

M J J A S O N

1992 

DATE, AT 1800 HOURS

J F M 

1993

4,600

4,400

4,200

4,000

3,800

3,600

3,400

3,200

3,000

DC LU 0_

co

0° 

rr & 

o m

ii LU 

S CC

* & 

£= UJ

occ

D UJ

OP

to

O 

LU 

D_ 

CO

Figure 11. Water-level altitude and specific conductance of water in well 143NO, Grand Valley, 

March 1992-March 1993.

GROUND-WATER INVESTIGATIONS 27
Historical Ground-Water Quality

As part of the salinity-control program, 

ground-water-quality data were collected by BOR 

for about 75 shallow monitoring wells installed in 

the Uncompahgre Project area and about 270 shallow 

monitoring wells installed in the Grand Valley. 

Major-ion data were collected annually, quarterly, 

and sometimes monthly during 1974-84. The 

BOR wells were essential for the collection of 

trace-element data published as part of this report.

In general, the historical ground-water-quality 

data (Jack Cunningham, Bureau of Reclamation, 

written commun., 1992) indicate high calcium, 

sodium, sulfate, and dissolved-solids concentrations 

in ground water (sources of the salinity problems). 

The historical ground-water-quality data indicate 

that high dissolved-solids concentrations are ubiquitous in areas underlain by Mancos Shale. The historical data also indicate a relation between water level 

in wells (reflecting the seasonal application of irrigation water) and dissolved-solids concentrations in 

ground water (fig. 12). When water levels rise 

because of irrigation, dissolved-solids concentrations 

in ground water decrease; when water levels fall 

because of cessation of irrigation in winter, dissolvedsolids concentrations in ground water increase 

(fig. 12). This cyclic flushing of the ground-water 

system has transported many soluble constituents 

from shallow aquifers to the streams and rivers, 

which could result in an eventual decrease in 

dissolved-solids concentrations and an eventual 

decline in the salinity problem; however, this effect 

is not evident in the 12 years of data presented for 

well 125Q5 (fig. 12). Ground-water-quality data 

are not available for the beginning of irrigation 

(in the 1880's in the Grand Valley); therefore, the 

amount of decrease in dissolved-solids concentrations 

in ground water since the beginning of irrigation is 

unknown.

Major-Ion Chemistry

Factors affecting concentrations of major ions in 

ground water of the Uncompahgre Project area and of 

the Grand Valley include: (1) The source of irrigation 

water; (2) dissolution, oxidation, and mobilization 

of major constituents in Mancos Shale, alluvium 

overlying Mancos Shale, and terrace deposits; 

(3) oxidation-reduction state of the ground water;

and (4) evaporative concentration of ground water. 

Because of the widespread presence of the Mancos 

Shale, the general quality of ground water in the study 

area is poor because of the calcium, sodium, and 

magnesium salts weathered from Mancos Shale. 

However, one area in the southwestern part of the 

Uncompahgre Project area has relatively good quality 

ground water because of the absence of Mancos Shale 

detritus in the terrace deposits. Few wells are used 

for drinking-water supply, and only those wells 

located on terrace deposits in the western part of 

the Uncompahgre Project area are useful for lawn 

irrigation. A characteristic of the poor groundwater quality is the presence of salt crust (or 

efflorescent minerals) evident in ground-waterdischarge zones.

The quality of the source of irrigation water 

differs between the two parts of the study area. 

Dissolved-chloride concentrations are much higher 

in the Colorado River, which receives sodium chloride 

water from hot springs, than in the Uncompahgre 

and Gunnison Rivers. The mean chloride concentration in ground water of the Uncompahgre Project 

area is 24 mg/L (13 wells) and in the Grand Valley 

is 262 mg/L (10 wells). The difference in mean 

chloride concentrations is caused by the different 

source water for irrigation.

Observations of the general characteristics 

of ground-water quality in the study area can be 

described using a water-analysis diagram (fig. 13). 

This diagram, sometimes referred to as a trilinear 

diagram or a Piper diagram, shows the relations 

among concentrations of cations and anions. The 

values are expressed as percentages of the total 

milliequivalents per liter of cations (lower left 

triangle) or of anions (lower right triangle) and are not 

shown as actual concentrations. The central quadrilinear graph shows the combined cationic and anionic 

chemical quality of the water by a third point, which is 

at the intersection of the rays projected from the points 

on the cation and anion plots. Ground water in 

Mancos Shale residuum and in alluvium overlying 

Mancos Shale predominantly is a calcium magnesium 

sodium sulfate water type, and water from some of 

the wells predominantly is a sodium sulfate water 

type. Water from Mancos Shale residuum tends to 

fall into a group different from water from the 

alluvium because of the difference in percentages 

of sodium (the mean sodium concentration in 

Mancos Shale residuum is 630 mg/L, and the mean 

in alluvium overlying Mancos Shale is 350 mg/L).

28 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
4,670

4,665

4,660

4,655

_

LU

y 4,650

UJ 

CO

g 4,645

O 

co

UJ 4,640

~iiiir 

WATER LEVEL WELL 125Q5

DISSOLVED SOLIDS

i i i i

6,000

5,000

4,000

cc

UJ

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986

3,000 S

Q_ 

CO

<

2,000 g

d

z

1,000 zo

LU 

O

ID

F 4,605 -j!

i

UJ

S 4,600 

CC

UJ

< 4,595

4,590

4,585

4,580

A C-IC.

\\\\\\\\

'. \ WATER LEVEL

' \ A \A A vi/VVj

^

: t A f\ /! t 1 \J\ / \ / 1 /I

7

\ / \ / i /

\ i w 4 / 

V 1 VJk

: DISSOLVED SOLIDS

i i i i i i i i

i i i i

WELL 208KO

(COBBLE AQUIFER) AA :

\/

M

J v

.

i i i i

u_ 

UJ

12,000 z

O

O

CO

10,000 § 

0

CO

6 

8,000 >

0

CO

CO

6,000 0

4,000

2,000

n

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986

Figure 12. Historical water-level altitudes and dissolved-solids concentrations in water from wells 125Q5 and 

208KO located in the Grand Valley (Jack Cunningham, Bureau of Reclamation, written commun., 1992).

GROUND-WATER INVESTIGATIONS 29
CALCIUM PERCENT CHLORIDE

EXPLANATION

WATER FROM WELL IN MANGOS SHALE RESIDUUM

MEAN DISSOLVED-SOLIDS CONCENTRATION = 5,270 MILLIGRAMS PER LITER 

MEAN pH = 6.93

A WATER FROM WELL IN ALLUVIUM OVERLYING MANGOS SHALE

MEAN DISSOLVED-SOLIDS CONCENTRATION = 4,730 MILLIGRAMS PER LITER 

MEAN pH = 6.94

O WATER FROM WELL IN TERRACE DEPOSITS, WESTERN PART OF THE 

UNCOMPAHGRE PROJECT AREA

MEAN DISSOLVED-SOLIDS CONCENTRATION =810 MILLIGRAMS PER LITER 

MEANpH = 7.15 Ca u u Cl 

CATIONS ANIONS 

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

Figure 13. Trilinear diagram showing percentages of major ions in water from selected wells during the irrigation 

season in the Uncompahgre Project area and in the Grand Valley.

30 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Water from wells in terrace deposits in the western 

Uncompahgre Project area was distinctly different, 

being predominantly a calcium sulfate tending toward 

a calcium bicarbonate water type. The mean 

dissolved-solids concentrations additionally describe 

differences in water quality between the different 

hydrogeologic settings. Water in Mancos Shale 

residuum and in alluvium overlying Mancos Shale had 

a mean dissolved-solids concentration of 5,270 mg/L 

and 4,730 mg/L, respectively. Water in terrace 

deposits on the western part of the Uncompahgre 

Project area had a mean dissolved-solids concentration 

of 810 mg/L.

The thermodynamic equilibrium speciation of 

major dissolved constituents in ground water of the 

study area was determined using the geochemical 

speciation program WATEQ4F (Ball and Nordstrom, 

1991). Saturation indices are listed for different 

mineral compounds that were in the aqeuous phase 

(table 5); saturation indices greater than zero indicated 

oversaturation (and possible precipitation of that 

mineral), and values less than zero indicated undersaturation (where that mineral could continue to dissolve). 

Ground water in Mancos Shale residuum and alluvium 

overlying Mancos Shale is near saturation or oversaturated with calcite, aragonite, dolomite, gypsum, 

ferrihydrite, and quartz. Ground water in terrace 

deposits is near saturation or oversaturated only with 

calcite, ferrihydrite, and quartz. Mineral species in 

ground water of the study area identified by the model 

are similar to the evaporite mineral species identified 

by Whittig and others (1982) in salt efflorescence 

samples collected from the Grand Valley (table 6).

Evapotranspiration can affect ground-water 

quality by concentrating major ions and trace 

constituents called evaporative concentration. 

Effects of evapotranspiration on ground-water 

recharge could be great in the Uncompahgre Project 

area and in the Grand Valley because annual precipitation averages about 8 to 10 in/yr, and estimates 

of pan evaporation range from 40 to 50 in/yr. 

However, the effects of evaporative concentration 

in the Uncompahgre Project area and in the Grand 

Valley are not great, based on data for oxygen and 

hydrogen stable-isotope values (fig. 14). When 618O 

[or the O-18/O-16 stable-isotope ratio referenced to the 

Vienna Standard Mean Ocean Water (VSMOW)] and 

52H (or the H-2/H-1 stable-isotope ratio referenced to 

the VSMOW) from surface and ground water are 

plotted on one graph for the Uncompahgre Project area 

and the Grand Valley, the data plot near and parallel to

the meteoric water line (Gat, 1981) and do not indicate 

an evaporation trend. When plotted separately by 

area, a slight evaporation trend is indicated by the 

data; however, outliers from one well in the Grand 

Valley seem to skew the results (fig. 14). The meteoric 

water line is described by 62H = 8 618O + 10 (Gat, 

1981). The evaporation line for the Uncompahgre 

Project area can be defined by 62H = 6.7 S18O - 12.4 

(r2 = 0.89); the evaporation line for the Grand Valley 

can be defined by S^H = 4.2 618O - 53.9 (r2 = 0.71). 

The differences in the groupings of data points for the 

two areas might be related to precipitation used for 

irrigation water falling and being stored at different 

altitudes of the Rocky Mountains and being affected 

by reservoir evaporation, particularly in the Gunnison 

River Basin (data for the Uncompahgre Project area). 

Effects of evaporative concentration on ground water 

are evident in water from a few wells inside and in 

water from a well located outside the irrigated area; 

however, in general, evaporative concentration is not a 

major factor in ground-water quality in the irrigated 

areas. The effects of evaporation might not be great in 

the study area because flood-irrigation practices keep 

the system well flushed, and the relatively steep terrain 

provides rapid drainage of the shallow ground-water 

system compared to other irrigation-drainage study 

areas, such as the broad sedimentary basin of the 

San Joaquin Valley, Calif. (Deverel and others, 1984; 

Dubrovsky and others, 1990).

Review of Selenium Geochemistry

Selenium is known to have volcanic, 

magmatic, and hydrothermal sources (Trelease 

and Beath, 1949; Coleman and Delevaux, 1957; 

Sindeeva, 1964; Howard, 1977) and is present in 

relation to hydrothermal and roll-type uranium 

deposits (Sindeeva, 1964; Fishman and others, 1985; 

Fishman and Reynolds, 1986). Selenium enrichment 

of geologic materials occurred during the Cretaceous 

age and was related to volcanic emanations of 

element-rich gases and ashes. Volcanic detritus was 

ultimately deposited in the Cretaceous sea that 

covered much of the interior of North America. The 

organic-rich reducing environment of the Cretaceous 

sea bottom was capable of precipitating sulfide 

minerals, such as pyrite (FeS2) and chalcopyrite 

(CuFeS2). Because selenium is isomorphous with 

sulfur, selenium substituted for sulfur in the crystalline 

structure of sulfide minerals in the Cretaceous shales.

GROUND-WATER INVESTIGATIONS 31
2,

Table 5. Results of thermodynamic speciation calculations for water from wells in the Uncompahgre Project area and from the Grand 

[ , no data; speciation calculations performed using WATEQ4F (Ball and Nordstrom, 1991); spg, spring]

,* 

3 2.

0 C

fi

1 2 at s

fl

3 o

 D §

|i

>i

at ^

a if

|I

» 5

|l

of 8.

If

8 w

£-

3 =

Ii

ll

Site name 

or

number 

(figs. 8

and 9)

Sampling 

date

Saturation indices

Log Calcite Aragonlte Dolomite Gypsum Anhydrite Epsom ite Mirabilite Siderite Ferrihydrite Jar

MANCOS SHALE RESIDUUM

SL1

SL2

SLSW1 (spg)

108U01

Schmalz

09-11-91

03-02-92

09-11-91

03-02-92

03-05-92

10-04-91

07-02-91

03-23-92

-1.62

-1.60

-1.47

-1.63

-2.44

-2.15

-1.66

-1.69

0.01

-.03

.02

-.02

.66

1.1

-.10

.0

-0.14

-.19

-.13

-.17

.50

.96

-.25

-.14

0.0

-.19

.04

-.25

1.2

2.8

-.66

-.44

-0.05 -0.30 -2.3

.0 -.25 -2.3

-.04 -.29 -2.4

.01 -.24 -2.3

-.02 -.26 -2.1

-.28 -.53 -2.1

TERRACE DEPOSITS

-.76 -1.0 -3.5

-.84 -1.1 -3.6

-3.4

-3.3

-3.7

-3.3

-2.1

-2.2

-6.2

-6.2

-

-0.40 1.8

-2.3 3.7

-.55 -3.3

-3.8 1.7

-4.2 2.3

-

-1

-

-

ALLUVIUM OVERLYING MANCOS SHALE

RWG1

RWG2

RWG3

08-28-91

02-20-92

08-28-91

02-20-92

08-28-91

02-18-92

-1.55

-1.52

-1.48

-1.33

-1.38

-1.29

.02

.07

.09

-.13

-.06

-.15

-.13

-.07

-.06

-.28

-.22

-.31

.56

.62

.36

-.16

-.14

-.37

.05 -.19 -1.7

.04 -.21 -1.7

.0 -.25 -2.1

-.01 -.28 -2.2

-.02 -.27 -2.3

-.02 -.26 -2.3

-2.1

-2.3

-3.3

-3.6

-4.3

-4.3

_.

..

-2.8 1.5

-

1 Well located outside of the irrigated area.
Selenium, therefore, might be directly related to 

volcanic-ash layers in Cretaceous shales. In contrast, 

Sindeeva (1964, p. 290-291) indicated that selenium 

was present in sizable amounts in recent sea-bottom 

sediments and could be precipitated directly from river 

water entering the oceans, but these recent selenium 

concentrations in ocean sediment also could be attributed to the burning of fossil fuels (particularly coal), 

which puts trace concentrations of selenium into the 

atmosphere, lakes, rivers, and oceans.

Table 6. Evaporite mineral species and salt efflorescence 

associated with the Mancos Shale and shale residuum in 

the Grand Valley (from Whittig and others, 1982)

[X, mineral species identified; --, mineral species not identified]

Evaporite 

species

Gypsum (CaSO4 2H2O)

Epsomite (MgSO4 10H2O)

Hexahydrite (MgSO4 6H2O)

Pentahydrite (MgSO4 5H2O)

Starkeyite (MgSO4 4H2O)

Kieserite (MgSO4 H2O)

Mirabilite (Na2SO4 10H2O)

Thenardite (Na2SO4)

Loewite [Na4Mg2(SO4)4 5H2O]

Bloedite [Na2Mg(CO4)2 4H2O]

Occurring 

in 

Mancos 

Shale

X

--

-

--

--

--

--

--

--

--

Occurring 

in 

efflorescence

X

X

X

X

X

X

X

X

X

X

Selenium is present as a trace constituent in 

pyrite in the Mancos Shale within the study area; 

concentrations range from 0.003 to 1.6 percent 

(Coleman and Delevaux, 1957). When the Mancos 

Shale is exposed to weathering, selenium weathers 

(or is oxidized) from pyrite (where the selenium has 

a valence state of -2) to further transformations in 

valence state (-2 -> 0 -> +4 -> +6) that become 

increasingly more soluble with increasing valence: 

elemental selenium (Se°, selenium valence of 0), 

selenite (SeIVO32~, selenium valence of+4), biselenite 

(HSeIVO3~, selenium valence of +4), and selenate 

(SeVIO42~, selenium valence of +6). The oxidation 

of selenium compounds can be represented by the 

following half reactions:

Se° + 3H20 o Se1^2- + 6H+ + 4e~ E° =-0.875 mV

Se° + 3H2O o HSeIVO3~+5H+ + 4e~ E° =-0.778 mV

Se1^2-+ H2O o Se^O^- + 2H+ + 2e~ E° =-0.880 mV

HSeIVO3~ + H2O o Se^Ot2- + 3H+ + 2e~ E° = -1.075 mV

-102

-110

-120

-128

: UNCOMPAHGRE PROJECT 

AREA

METEORIC WATER LINE /./

-18 -16 -14

-102

CC

CL -110

X

CM 

CO

-120

-128

GRAND VALLEY

METEORIC WATER LINE

-18 -16 -14

818O, IN PER MIL (VSMOW)

EXPLANATION

 METEORIC WATER LINE

- EVAPORATION LINE 

 GROUND WATER 

A STREAMS AND DRAINS

+ UNCOMPAHGRE RIVER NEAR MONTROSE 

AND COLORADO RIVER NEAR CAMEO

VSMOW VIENNA STANDARD MEAN OCEAN WATER

Figure 14. Oxygen and hydrogen stable isotope values in 

water from selected wells, streams, and rivers in the 

Uncompahgre Project area and in the Grand Valley.

The species that are present in solution after 

oxidation of reduced selenium species primarily 

are a function of the pH and the oxidation-reduction 

potential (Eh-pH) of the geochemical environment 

(fig. 15). Selenite (SeIVO32~) and biselenite 

(1186^03") readily adsorb to aquifer sediments or 

coprecipitate with ferric oxyhydroxide complexes,

GROUND-WATER INVESTIGATIONS 33
VT *? and selenate (Se 0)4 ) is very soluble and is easily 

transported in an oxidized hydrologic system 

(Howard, 1977; Electric Power Research Institute, 

1980; Balistrieri and Chao, 1987; Bar-Yosef and 

Meek, 1987). Selenate can adsorb to aquifer 

sediments, but selenate forms an outer-layer complex 

(more easily desorbed) with iron oxides, whereas 

selenite forms an inner-layer complex (less easily 

desorbed) with iron oxides (Hayes and others, 1987). 

These adsorption processes are completely reversible 

(Electric Power Research Institute, 1980; Balistrieri 

and Chao, 1987), indicating that selenium could be 

desorbed from clay and mineral surfaces and put back 

into solution.

Previous studies have been published about the 

distribution of selenium in the San Joaquin Valley, 

California. Sources of dissolved selenium in ground 

and surface water in the San Joaquin Valley were 

hypothesized to be sulfides in shale (the Moreno 

Formation of Cretaceous age) of the Coast Range of

California (Presser and others, 1990) and sediments 

of the west-central San Joaquin Valley derived from 

Coast Range rocks (Deverel and others, 1984; 

Gilliom, 1989; Izbicki, 1989). Studies by Deverel 

and Millard (1988) and by Fujii and Deverel (1989) 

concluded that high concentrations of selenium in 

ground and surface water were a result of evaporative 

concentration of ground water, which was indicated by 

isotopic enrichment of oxygen and hydrogen stable 

isotopes and by correlation of elevated selenium 

concentrations with ground-water salinity. The 

hydrogeochemical processes involved in the immobilization of selenium from the aqueous phase has been 

the focus of several studies (Dubrovsky and others, 

1990; Masscheleyn and others, 1991; Sposito and 

others, 1991; White and others, 1991). The wellaccepted conclusions of these studies are that 

dissolved selenium primarily occurs in the selenate 

form and is transported to deeper zones (10 to 50 ft) of 

reducing geochemical environments where selenium

1,200

e

o 1,000-

800

600

400

200

0

-200

-400

-600

LU

5

Q_

o

LU 

CC

O

§

Seu (10'b)

 WATER FROM NITRATE OXIDATION BATCH EXPERIMENTS

+ WATER FROM WELLS IN THE UNCOMPAHGRE PROJECT AREA

A WATER FROM WELLS IN THE GRAND VALLEY

D WATER FROM STREAMS IN THE GRAND VALLEY

O STREAMBED SEDIMENTS AFFECTED BY IRRIGATION

6 7 

pH

8 10

Figure 15. Stability-field diagram for selenium species in natural waters and 

data points calculated from selenium-speciation data collected for this study. 

Thermodynamic data are from Garrels and Christ (1965) and from P.P. Bell and 

D.R. Parker (University of California, Riverside, written commun., 1991).

34 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
is reduced to the less mobile selenite and elemental 

selenium species. Hypotheses for mobilization and 

transport of selenium by oxidation-reduction and 

adsorption/desorption processes were described, and 

the platinum-electrode potential was observed to be an 

important parameter for recognizing geochemical 

conditions suitable for the reduction of selenium.

Selenium can be absorbed from some soils by 

food and forage plants in sufficient amounts to make 

them highly poisonous to animals. Several native 

range plants of the Western United States require 

selenium and can uptake selenium in concentrations 

as high as 15,000 parts per million (Trelease and 

Beath, 1949, p. 85); however, the concentrations of 

selenium in the plants vary according to seasonal 

growth and development (Beath and others, 1937). 

Several of these native plants (for example, Astragalus 

bisulcatus, Stanleya bipinnata, Xylorhiza parryi, and 

Oonopsis condensatd) are called primary indicators 

for the presence of highly seleniferous soils. Other 

native plants tolerate selenium and are called 

secondary indicators. The presence of seleniumindicator plants is significant enough to allow the 

mapping of selenium-bearing geological formations 

from the recognition of these plants. The uptake of 

selenium by farm crops differs by species, soil pH, soil 

texture, and soil organic matter; however, certain 

grains and hay grown on seleniferous soils can uptake 

selenium in concentrations high enough to be deleterious to animal health. Selenium accumulation in 

farm crops generally ranges from greatest to least in 

root and bulb crops, field crops, leafy vegetables, seed 

vegetables, vegetable fruits, and tree fruits (Mikkelson 

and others, 1989, p. 74).

According to Trelease and Beath (1949), native 

seleniferous vegetation is widely distributed in 

Colorado. Most of the seleniferous plants have been 

collected on formations of Cretaceous age. The 

Dakota, Mesaverde, and McDermott Formations 

(sandstones) have plants with the lowest selenium 

concentrations. The Mancos Shale, Greenhorn, and 

Benton Formations bear plants with medium concentrations of selenium, and the Pierre Shale and Niobrara 

Formation produce highly seleniferous vegetation. 

Seleniferous plants are capable of converting 

inorganic selenium (probably as Se°, SeIVO32~, 

or SeVIO42~) into organic selenium (such as 

selenomethionine, selenocysteine, and selenocystine), 

transforming the selenium into a much more toxic 

form (Thompson-Eagle and Frankenberger, 1992).

Microbiological processes might be important 

for transformation of selenium species. Bacteria and 

fungi can volatilize Se°, SeC>32~, SeO42~, and 

organoselenides into mobile methylated derivatives, 

such as dimethylselenide and dimethyl diselenide 

(Frankenberger and Karlson, 1992; Thompson-Eagle 

and Frankenberger, 1992). Selenium reduction 

and volatilization have been studied as possible 

mitigative measures to ameliorate seleniumcontaminated agricultural drainwater. Oxidized 

forms of selenium (such as mobile aqueous SeO4 ) 

can be reduced to less mobile forms (Se° and SeO32~) 

by anaerobic bacteria (Oremland, Hollibaugh, and 

others, 1989; Oremland and others, 1990; Steinberg 

and Oremland, 1990). Reduced selenium species 

can be oxidized by autotrophic bacteria (Lipman 

and Waksman, 1922; Sarathchandra and Watkinson, 

1981); however, little work has been performed to 

investigate the mechanisms involved in selenium 

oxidation (Sarathchandra and Watkinson, 1981; 

Thompson-Eagle and Frankenberger, 1992).

In summary, at least four processes oxidationreduction, adsorption/desorption, vegetative uptake 

and transformation, and microbial mediation could 

affect selenium speciation and concentrations in 

natural water and in the environment. Reduction 

originally played a role in precipitation of sulfide 

minerals (incorporating trace concentrations of 

selenium) possibly related to volcanic-ash-layer 

deposits in Cretaceous geological formations. Uplift 

and emergence exposed the shale to natural 

weathering that oxidized the sulfides and released 

selenium into the environment. Irrigation is speeding 

up the weathering process in which selenium is 

mobilized in the hydrologic environment.

Sources of Selenium and 

Other Trace Constituents

Core samples of Mancos Shale, Mancos Shale 

residuum, and alluvium were collected from the study 

area and were analyzed to determine the concentrations of selenium and other trace constituents in the 

solid-phase samples (table 7). Selenium is isomorphous with sulfur and substitutes in the crystalline 

structure of sulfide minerals (Coleman and Delevaux, 

1957); therefore, core samples were collected to 

describe the relation of sulfide mineralization (and the 

source of selenium) to depositional environment in the 

Mancos Shale. Disseminated sulfides in the Mancos

GROUND-WATER INVESTIGATIONS 35
Shale core range in size from 0.01 to 0.05 mm viewed 

under 20X microscope. The disseminated sulfides 

almost always are associated with darker layers 

(organic-rich lenses) in shale bedrock. Sulfide 

euhedrons (0.1 to 0.5 mm) occur immediately adjacent 

to the bottom side of ash layers that were examined in 

core. XRD and SEM analyses of sulfide-mineral 

euhedrons adjacent to ash layers indicate that the 

sulfides in this part of the Mancos Shale are pyrite 

(table 7).

The selenium concentration in a Mancos 

Shale bedrock sample (representing disseminated 

pyrite) was 0.8 (ig/g, selenium concentration in one 

ash-layer sample was less than the reporting limit 

(<0.1 (ig/g), and selenium concentration in one 

pyrite sample adjacent to the bottom side of an 

ash layer was 19 |4,g/g (table 8). Coleman and

Delevaux (1957) tabulated selenium concentrations 

from sulfides in the Mancos Shale ranging from 

0.002 to 1.6 percent (2 to 1,600 ^ig/g).

Analyses of trace-element concentrations in 

shale residuum and weathered ash indicate that more 

selenium and uranium were contained in the 

weathered ash sample than in shale residuum and 

bedrock samples. XRD analyses of weathered ash 

indicate that the dominant clay minerals are kaolinite 

and smectite (table 7). Bar-Yosef and Meek (1987) 

and Balistrieri and Chao (1987) indicated that 

selenium is readily adsorbed by kaolinite and smectite. 

Kaolinite is present in shale residuum and alluvium 

(table 7), and smectite is present in weathered ash. 

These clay minerals adsorb trace constituents and 

could be sources for the leaching of selenium and 

other trace constituents.

Table 7. Semiquantitative clay and bulk mineralogy of Mancos Shale bedrock core, shale residuum, alluvium, and 

ground-water-filter residuum (L.M. Fukui, Chem-Nuclear Geotech, written commun., 1992)

[Ill/Smec, illite/smectite; nd, not detected; tr, less than 7 percent of intensity of dominant phase; min, 7 to 30 percent of intensity of dominant 

phase; mod, 30 to 70 percent of intensity of dominant phase; dom, predominant mineral in X-ray pattern; sdom, 70 to 99 percent of intensity 

of dominant phase; cdom, predominant minerals have same relative intensities; P, plagioclase; K, potassium; ft BLS, feet below land surface]

Sample 

description

Mancos Shale bedrock

Sulfide euhedron from core

Ash layer from core

Weathered ash layer

Residuum, SL1, 5 ft BLS

Residuum, SL1, 15.5 ft BLS

Alluvium, RWG1, 6 ft BLS

Alluvium, RWG1, 24.5 ft BLS

Sample 

description

Mancos Shale bedrock

Sulfide euhedron from core

Ash layer from core

Weathered ash layer

Residuum, SL 1,5 ft BLS

Residuum, SL1, 15.5 ft BLS

Alluvium, RWG 1,6 ft BLS

Alluvium, RWG1, 24.5 ft BLS

Alluvium, RWG1, 27.5 ft BLS

Ground-water-filter residuum1

Illite

nd

nd

nd

nd

cdom

cdom

cdom

cdom

Quartz

dom

nd

dom

mod

sdom

dom

dom

dom

sdom

dom

Ill/Smec

tr

nd

min

nd

min

tr

mod

mod

Feldspar

P K

tr nd

nd nd

nd nd

nd nd

tr tr

tr tr

min tr

tr nd

tr tr

min-tr nd

Kaolinite Smectite Chlorite

min

nd

min

dom

cdom

cdom

cdom

cdom

- Calcite

min

nd

tr?

nd

min

min-mod

min

mod

dom

min-tr

min

nd

mod

sdom

min

min

tr

tr

Dolomite

nd

nd

nd

tr

min

min

min

min-mod

min

min-tr

Gypsum

nd

nd

nd

nd

dom

nd

nd

tr

nd

nd

tr?

nd

min?

nd

tr

tr

tr

tr

Pyrite

nd

dom

nd

nd

nd

nd

nd

nd

tr

tr

*Well 108UO, which is completed in shale residuum and located outside the irrigated area.

36 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 8. Element and organic-carbon concentrations in soil and rock samples from the Uncompahgre Project area and from 

the Grand Valley

[Se, selenium; U, uranium; V, vanadium; Fe, iron; ug/g, micrograms per gram, which is equivalent to parts per million; BLS, below land surface; ft BLS, 

feet below land surface; , no data]

Sample Se 

description 0*9/9)

U

(119/9)

V 

fag/9)

Fe 

(percent)

Organic 

carbon 

(percent)

MANGOS SHALE BEDROCK

Mancos Shale, bedrock sample from core, 2,317 ft BLS 0.8

Ash layer, bedrock sample from core, 2,271 ft BLS <. 1

Pyrite and shale, bedrock sample adjacent to ash layer, 2,297 ft BLS 1 .6

Pyrite euhedron, isolated from bedrock core, adjacent to ash layer 19

7.4

13.9

11.2

~

50

4

13

-

1.7

1.8

2

~

1.36

.12

1.55

~

MANCOS SHALE RESIDUUM

Weathered ash, outcrop of ash layer, upper Mancos Shale 3.9

Shale residuum, sediment-core sample from well SL2, 2 ft BLS 1.1

Shale residuum, sediment-core sample from well SL2, 7 ft BLS .8

Shale residuum, sediment-core sample from well SL2, 9 ft BLS .8

Gypsum, from bedding-plane parting, well SL1 <1

8.38

4.77

4.63

4.16

~

40

130

160

160

~

2.3

2.1

2.4

2.5

~

.39

.44

.56

.51

-

ALLUVIUM OVERLYING MANCOS SHALE

Alluvium, sediment-core sample from well RWG1, 6 ft BLS 3.6

Alluvium, sediment-core sample from well RWG1, 8 ft BLS 2.8

Alluvium, sediment-core sample from well RWG1, 27 ft BLS 1.7

4.55

4.48

3.79

150

140

100

2.2

2.1

2

.38

.32

.32

Selenium concentrations in alluvium samples 

were more than 4 times greater than in shale bedrock 

(table 8), indicating a concentrating effect from 

adsorption of selenium onto clay minerals and 

possibly reduction of selenium to a less mobile 

form. The concentrations of selenium, uranium, 

and vanadium indicate an upward gradient, possibly 

indicating transport toward the ground surface by 

capillary action and a concentrating effect from 

evaporation.

Occurrence of Selenium 

in Ground Water

Distribution

The conceptual model for the occurrence, 

mobilization, and redistribution of solid-phase and 

dissolved selenium in the Mancos Shale, shale 

residuum, and alluvium includes: (1) Oxidation of 

pyrite containing trace concentrations of selenium; 

(2) mobilization of selenium by ground water under 

oxidizing conditions; (3) immobilization and redistribution of selenium by biogeochemical reduction,

coprecipitation, or adsorption, or all three; or 

(4) transport of oxidized selenium to streams and 

drains. Aqueous selenium complexes could migrate 

from oxidized zones into zones of geochemically 

reducing environments where dissolved selenium 

would be converted to less mobile species (or 

redistributed) by biogeochemical reduction, adsorption, or coprecipitation. Adsorbed selenium could 

be mobilized by desorption or oxidation, or both. 

A conceptualized model of the occurrence of selenium 

in Mancos Shale, shale residuum, and alluvium in the 

irrigated areas of the Uncompahgre Project area and 

the Grand Valley is shown in figure 16.

In the Uncompahgre Project area and in the 

Grand Valley, distribution and concentrations of 

dissolved selenium differ according to geochemical 

environment and hydrogeologic setting (table 9). 

In general, the highest concentrations of dissolved 

selenium as high as 1,300 jig/L were in water 

from wells completed in alluvium overlying the 

Mancos Shale and where nitrate concentrations 

were high. The lowest concentrations of 

dissolved selenium were in water from wells 

completed in terrace deposits on the western side

GROUND-WATER INVESTIGATIONS 37
Shale residuum

(1) SeO32-, SeO42-, Se°

(2) SeO42- and SeO32-

(3)-150<Eh<450mV

Irrigation recharge (O2, NO3*>

T V T T T V

Alluvium

(1)SeO32-andSeO42- 

adsorbed

(2) SeO42- (summer); SeO32 

(winter)

(3) 350 < Eh < 500 mV

Shale bedrock

(1) FeS2 with Se incorporated

(2) Se not in solution

(3) Eh < 0 mV (1) Form of Se in solid phase

(2) Form of Se in aqueous phase

(3) Ranges of aqueous redox potential

Groundwater-flow 

direction

10m

Figure 16. Conceptual model of the occurrence of selenium (Se) in Mancos Shale, shale residuum, 

alluvium, and ground water in the Uncompahgre Project area and in the Grand Valley.

of the Uncompahgre Valley (the terrace deposits 

are of glacial origin and do not contain detritus from 

weathered Mancos Shale), in water from wells 

completed in Mancos Shale residuum, and where iron 

concentrations were high.

The distribution of selenium and other elements 

(such as nitrogen, iron, and manganese) are affected 

by the oxidation potential of the geochemical environment. Where table 9 lists the general classifications 

of the geochemical environments pertinent to 

selenium occurrence and distribution, table 10 lists 

ground-water data for common indicators of the 

oxidation-reduction characteristics of the subsurface 

geochemical environment. Where geochemical 

conditions are anoxic, dissolved iron, manganese, 

and ammonia concentrations are relatively high, and 

dissolved oxygen, nitrate, and selenium concentrations 

are low (table 10). In contrast, where geochemical 

conditions are oxic, dissolved iron, manganese, and 

ammonia concentrations are relatively low, and 

dissolved oxygen, nitrate, and selenium concentrations 

are high (table 10). Many factors can affect these 

conditions, such as rainfall, snowmelt, irrigation, and 

bacteria that can mediate chemical reactions.

Table 9. Geochemical environments of the ground-waterflow systems, hydrogeologic settings, and ranges of 

dissolved-selenium concentrations in Mancos Shale 

residuum and alluvium in the irrigated Uncompahgre Project 

area and in the Grand Valley

[Oxic/anoxic environment, ground water with dissolved-oxygen concentrations ranging from 0.5 to 2 mg/L (milligrams per liter); anoxic, < 0.5 mg/L; 

u,g/L, micrograms per liter; Mn, manganese; Fe, iron; NO3~, nitrate]

Geochemical Hydrogeologic 

environment setting

Ranges of dissolvedselenium 

concentrations in 

ground water

Oxic/anoxic

Oxic/anoxic 

Mnrich

Oxic/anoxic 

Ferich

Anoxic 

Mnrich

Anoxic 

Ferich

Anoxic 

NO3~ rich

Alluvium on 

Mancos 

Shale

Alluvium, 

shale 

residuum

Mancos Shale 

residuum

Mancos Shale 

residuum

Mancos Shale 

residuum

Alluvium on 

Mancos 

Shale

Moderate 43 to 80 Hg/L

Low to high <1 to 170 \igfL

Low <1 to 35 ng/L

Low to moderate <1 to 65 ng/L

Very low <1 ng/L

Moderate to high 25 to 

l,300ng/L

38 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Centrai Colorado, 1991-93
Table 10. Concentrations of dissolved redox-active constituents, water properties, and hydrogeologic setting of water from wells in a

the Uncompahgre Project area and in the Grand Valley

[mV, millivolts; mg/L, milligrams per liter; ug/L, micrograms per liter; NO3~, dissolved nitrate as N; NH^, dissolved ammonia as N; Se, dissolved selenium as Se; 

Fe, dissolved iron as Fe; <, less than]

OS to 

wile

number Hydrogeoiogic 

(figs. 8 setting 

and 9)

SL1 Shale residuum

SL2 Shale residuum

130L3 Shale residuum

RWG1 Alluvium

O

0 RWG2 Alluvium

c

z

p

^ B254334 Shale residuum/alluvium

^

m

5 108UO Shale residuum (outside

Z irrigation area)

1

§

§

0

_ Platinum-electrode 

sampling 4411 u 

._. potential pn

aate (mV)

09-11-91

12-05-91

01-10-92

09-11-91

03-02-92

09-03-91

02-19-92

08-28-91

12-10-91

01-13-92

02-20-92

08-28-91

02-20-92

06-19-91

03-04-92

10-04-91

03-20-92

360

460

472

370

247

335

310

434

460

430

447

405

414

500

390

-120

150

7.04

6.96

6.83

6.94

7.02

6.97

6.57

7.12

7.10

7.10

7.09

6.96

6.84

6.39

6.86

8.01

7.73

Dissolved 

oxygen 

(mg/L)

0.50

.30

.30

1.60

.50

.80

.80

.60

.00

.00

.03

.60

.10

.20

.10

.03

.00

N03- 

(mg/L)

3.10

3.50

3.70

1.80

.96

4.10

3.81

210

190

180

190

1.20

1.00

9.82

3.25

.005

<.05

NH4* 

(mg/L)

0.07

.02

.02

.08

.04

1.40

1.10

.07

.04

.02

.03

.10

.01

.09

.01

2.80

2.50

(u

1

1

1
The oxidation-reduction potential (or redox 

potential, expressed in millivolts) can be approximated in the field by a platinum electrode. Platinumelectrode potential is an important indicator of 

geochemical conditions low platinum-electrode 

potential (generally less than about 400 mV) indicates 

a geochemically reducing environment and high 

platinum-electrode potential (generally greater than 

about 400 mV) indicates an oxidizing geochemical 

environment. In the Uncompahgre Project area and in 

the Grand Valley, dissolved selenium occurs in lower 

concentrations where the platinum-electrode potential 

is lower and the hydrogeologic system is more 

reducing, such as in the Mancos Shale residuum 

(fig. 17). These conditions indicate that selenium is 

more mobile in oxygenated environments (onsite 

platinum-electrode potential greater than about 

300 mV), and selenium is immobilized in reducing 

environments (onsite platinum-electrode potential less 

than about 300 mV). In the reduction sequence of 

aqueous complexes called the redox ladder (Berner, 

1981), reduction of selenate follows oxygen and 

nitrate, which then are followed by the reduction of 

manganese, iron, and sulfate.

Seasonal Effects

Dissolved selenium concentrations in ground 

water of the study area generally tend to increase 

during summer (irrigation season) when irrigation 

water is applied. Selenium concentrations generally 

decrease in winter (nonirrigation season) when 

irrigation water is turned off and reducing geochemical conditions remove selenium from the aqueous 

phase (fig. 18). Of the 16 wells sampled seasonally, 

selenium concentrations were lower in winter than 

in summer in water from 9 of 16 wells. Primarily, 

water from wells in shale residuum showed these 

seasonal effects; water from wells in alluvium 

might not indicate such seasonal effects because 

oxidizing conditions could be maintained in the 

alluvial aquifer.

During the nonirrigation season, dissolved 

selenium can be reduced to less mobile species 

where geochemical conditions are favorable for 

selenium reduction. Selenium-speciation data 

for dissolved selenium in selected groundwater samples collected during the nonirrigation

co 5

0 <

HI OC

CO ft co y

Q 5

z

SHALE RESIDUUM

-200 200 400 600

ONSITE PLATINUM-ELECTRODE POTENTIAL, 

IN MILLIVOLTS

Figure 17. Platinum-electrode potential and concentrations of dissolved selenium in water from wells in 

Mancos Shale residuum. (Hand-fitted correlation 

shown.)

of

UJ rr 

I- UJ

Z hD 0-

Z CO

co

Q 

UJ

O 5

CO 7 CO ?

1,000 F 

500

200

100

50

EACH POINT 

REPRESENTS 

ONE WELL THAT 

WAS SAMPLED 

TWICE: SUMMER 

AND WINTER

SELENIUM

CONCENTRATIONS - 

TEND TO INCREASE:

DURING :

SUMMER

10 100

DISSOLVED SELENIUM, SUMMER, 

IN MICROGRAMS PER LITER

1,000

Figure 18. Dissolved selenium in water from wells in shale 

residuum and alluvium during the irrigation season (summer) 

and nonirrigation season (winter).

season are listed in table 11. The percent of SeIVC>32~ 

(selenite) in two samples was 28.3 and 50.5 percent 

of the total dissolved-selenium concentration in the 

samples, indicating that reducing conditions could 

have been converting SeVIO42~ (selenate) to SeTVO32~ 

and tending to remove dissolved selenium from the 

aqueous phase.

40 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage In the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 11. Speciation of dissolved selenium in selected ground-water samples from the Uncompahgre Project area and from 

the Grand Valley (analyses courtesy of Roger Fujii and AT. Chalmers, U.S. Geological Survey, written commun., 1992)

[ug/L, micrograms per liter; mg/L, milligrams per liter; Se, selenium; SeVIO42~, selenate; Se^C^2", selenite]

Site 

number 

(figs. 8 

and 9)

RWG1

RWG2

SL1

Schmalz

Hydrogeologic 

setting

Alluvium in Mancos Shale setting, Grand Valley, 

NO3~ = 200 mg/L

Alluvium overlying Mancos Shale, Grand Valley

Mancos Shale residuum, Uncompahgre Valley

Terrace deposits, Uncompahgre Valley

Sampling 

date

03-19-92

03-19-92

03-23-92

03-23-92

Dissolved 

Se 

(ug/L)

1,056.2

1.9

50.1

6.9

Sevl042-

(ng/D

1,055.4

1.4

49

3.4

Se'v032-

(ug/L)

0.8

.5

1.1

3.5

Percent 

Se'v032-

0.08

28.3

2.3

50.5

Mobilization and Transport 

of Selenium

Selenium in the Uncompahgre Project area and 

in the Grand Valley can be mobilized by oxidation of 

reduced selenium species, by desorption of selenate 

and selenite salts from aquifer sediments, and by 

dissolution of selenium. Dissolved selenium can be 

immobilized by natural reducing conditions. During 

summer, selenium could be mobilized and transported 

more readily by irrigation water; during winter, the 

water table lowers and geochemical conditions 

become more reducing by the action of naturally 

occurring microorganisms, which tend to remove 

selenium from solution in the ground-water system. 

Selenium mobilization in the study area is affected by 

salinity, desorption or ion exchange, and oxidationreduction processes, depending on the hydrogeologic 

system that controls the geochemical environment, 

similar to hypotheses presented by See and others 

(1992).

Selenium can be mobilized by oxidation of 

reduced selenium species. Oxidation of selenium 

occurs by the chemical actions of dissolvedoxygen and nitrate in ground water. Nitrate can 

act as an oxidant to mobilize selenium (Wright, 

1994; Wright and McMahon, 1994; Wright, 

1995), and nitrate can inhibit selenium reduction 

(Oremland and others, 1989). More discussion 

on this topic is provided in the "Selenium Oxidation." 

section.

Dissolved selenium can be mobilized by desorption of selenate and selenite salts that are adsorbed 

to aquifer sediments. Sequential extractions were

performed on aquifer-sediment samples to estimate 

the water-extractable selenium (selenate salts) 

and the surface-adsorbed selenium (selenite salts) 

(Roger Fujii and A.T. Chalmers, U.S. Geological 

Survey, written commun., 1992). An estimate of 

the water-extractable selenium was obtained by 

using a 0.25M KC1 (potassium chloride) extraction 

solution that indicates the potentially water-soluble 

selenium in the sediment sample (Fujii and others, 

1988). An estimate of the surface-adsorbed selenium 

was obtained by using a 0.1M K2HPO4 (potassium 

phosphate) extraction solution adjusted to pH 8 

with KOH (potassium hydroxide) (Fujii and 

others, 1988). Selenium-speciation analyses were 

performed on the extractions (Makita and Fujii, 

1992). Speciation of sequential extractions from 

two alluvial-aquifer-sediment samples indicates 

that the extractions estimating the water-soluble 

phases consisted mostly of selenate (SeVIO42~), 

and the extractions estimating the surface-adsorbed 

phases consisted mostly of selenite (SeIVO32~) 

(table 12). Selenate forms a weakly bonded, 

outer-sphere complex; selenite adsorbs much more 

strongly than selenate and forms a stronger bonded, 

inner-sphere complex (Electric Power Research 

Institute, 1980; Balistrieri and Chao, 1987; Hayes 

and others, 1987).

Kaolinite and smectite (dominant clay minerals 

in Mancos Shale residuum, weathered ash layers, 

and alluvium) are reported to readily adsorb selenium 

and affect selenium concentrations in soil solutions 

(Electric Power Research Institute, 1980; Bar-Yosef 

and Meek, 1987); and dissolved selenium can 

coprecipitate with ferric hydroxide (Electric Power

GROUND-WATER INVESTIGATIONS 41
Table 12. Speciation of extractable selenium in alluvial-aquifer-sediment samples from the Grand Valley (analyses 

courtesy of Roger Fujii and AT. Chalmers, U.S. Geological Survey, written commun., 1992)

[Total KCI extractable Se is the potentially water-soluble selenium in sediment sample; total K2HPO4 extractable Se is an estimate of the surfaceadsorbed selenium; ug/g, micrograms per gram; ft BLS, feet below land surface]

Site 

number

(fig- 9)

RWG1

RWG1

Hydrogeologic

Alluvium on Mancos Shale,

Grand Valley, 13 ft BLS

Alluvium on Mancos Shale, 

Grand Valley, 24 ft BLS

Sampling 

date

08-25-91

08-25-91

(ng/g)

0.16

.020

SPofIn sample

1.5

4

Total K2HPO4 

extractable Se

(ng/g)

0.12

.084

Percent Selv032- 

in sample

96

76

Research Institute, 1980), adsorb to goethite (Hayes 

and others, 1987), or precipitate as iron selenite after 

fluid migration to reducing zones (Elrashidi and 

others, 1987). These adsorption processes are 

completely reversible (Balistrieri and Chao, 1987), 

indicating that desorption could mobilize selenium. 

In addition, high salinity concentrations could 

change the electrical nature of the outer layer of 

clay minerals (the salinity effect), whereby the 

hydroxyl ion could displace selenium oxyanions 

from outer-layer adsorption sites (Balistrieri and 

Chao, 1987), thereby mobilizing selenium.

Selenium can be mobilized by dissolution. 

Dissolved selenium in ground water of the 

Uncompahgre Project area and of the Grand Valley 

is positively correlated with dissolved-solids 

concentrations in water from wells completed in 

alluvium, but dissolved selenium is negatively 

correlated with dissolved-solids concentrations 

in water from wells completed in shale residuum 

(fig. 19). The negative correlation for shale 

residuum could be related to dissolved-solids 

concentrations that increase during the nonirrigation 

season (a concentrating effect), and biogeochemical 

reduction of dissolved selenium occurs concurrently 

with the concentrating effect (Wright and Butler, 

1993). Consequently, dissolved selenium is more 

mobile in alluvium than in shale residuum, which 

also is reflected by selenium concentrations in 

shale residuum.

Selenium can be immobilized by natural 

reducing conditions. Framboidal pyrite (fig. 20) 

was collected on filter paper from filtered ground 

water from a well (100 ft deep in Mancos Shale 

residuum) located outside the irrigated area in the

Grand Valley. The geochemical conditions were 

sufficiently reducing (Eh = -120 mV) to precipitate 

pyrite from available dissolved iron and abundant 

sulfate in the ground water; likewise, selenium 

would be reduced and immobilized and possibly 

incorporated into the crystalline structure of the 

pyrite. Similar reducing conditions might occur 

during winter within the irrigated areas that could 

immobilize dissolved selenium. If not for irrigation, 

geochemical conditions within the study area could 

be reducing; however, irrigation infiltration provides 

oxidizing conditions suitable for mobilization and 

transport of selenium.

10,000

£ 1,000

100

w^ 10

LUOC

og

ALLUVIUM

10,000

1,000

100

10

SHALE RESIDUUM

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

DISSOLVED SOLIDS, 

IN MILLIGRAMS PER LITER

10,000

Figure 19. Correlation of dissolved solids and dissolved 

selenium in water from wells in alluvium (r = +0.85) and 

shale residuum (r = -0.55) (from Wright and Butler, 1993).

42 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Figure 20. Scanning-electron photomicrograph of framboidal pyrite collected on 

0.45-micrometer water-filter paper from a well located outside the irrigated Grand 

Valley area, October 1991. Platinum-electrode potential was 120 millivolts, and 

was +6.4 per mil. Scale bar in lower left indicates 10 micrometers.

Selenium Oxidation

Reduced selenium complexes occurring in the 

Mancos Shale residuum and alluvium might be 

mobilized by oxidants, such as dissolved oxygen 

and nitrate, in ground water. In ground-water 

systems that can become depleted of oxygen, such 

as organic-rich systems in the Mancos Shale and 

Mancos Shale residuum, NO3~ might act as an 

oxidant for mobilization of selenium. NO3~ 

concentrations in ground-water samples range from 

< 0.05 to 210 mg/L as N in the study area (table 5). 

Thermodynamic relations for selenium oxidation 

indicate that elemental selenium could be oxidized 

to selenite (SeIVO3 2~) and selenate (SeVIO42~) by 

nitrate and nitrite, and theoretical energies for some 

of the reactions almost equal the energy available 

for oxidation of selenium by oxygen (table 13). 

Several investigators have indicated that NO3~ 

inhibits the reduction of SeVIO42~. Oremland and 

others (1989), in anaerobic culture experiments, 

indicated that the inhibition of SeVIO42~ reduction 

by NO3~ was caused by either action of NO3~ as a 

competitive electron acceptor or that NO3~ is able

to reoxidize reduced, precipitated selenium from 

sediment. Weres and others (1989) postulated that 

poised the pE value too high for significant

Se O42~ reduction. Because selenium reduction 

was the focus of these investigations, the oxidation 

of selenium by NO3~ was not elaborated upon 

further. Korom (1992), in a review of denitrification processes, presented a reaction for oxidation of 

FeS2 by NO3~, which yields N2 and SO42~ and also 

indicates that the process might be mediated by 

bacteria.

Laboratory-batch experiments were 

performed for this study to test the hypothesis 

that selenium could be oxidized by nitrate. Samples 

of Mancos Shale residuum were placed in different 

concentrations of nitrate. All preparations were 

made in an argon-filled glovebox to prevent 

the introduction of atmospheric oxygen into the 

samples. Preliminary results indicate that 

selenium can be oxidized by nitrate under laboratory conditions of elevated temperature (25°C) 

and nitrate concentrations of 5, 50, 100, and 

1,000 mg/L (Wright and McMahon, 1994; Wright, 

1995); however, extrapolation of these rates to field

GROUND-WATER INVESTIGATIONS 43
conditions have not been done. The oxidation rate 

depends on the nitrate concentration. Additional 

research is needed to document if these reactions 

actually are occurring and if the reaction rate is high 

enough to be of significance in ground water associated with irrigation in the study area.

Table 13. Energies of reaction for oxidation of selenium by 

oxygen and nitrate

[0, IV, and VI indicate the oxidation state of the selenium; AGR°, Gibbs 

free energy term; kJ/mol, kiloJoules per mole; thermodynamic data are 

from Garrels and Christ (1965)]

Selenium 

reactant

Se°

HSe^O-f

SeIV032-

FeHSeIV032+

Oxidant

02

N03~

02

NO3"

NO2~

02

NO3~

02

N03~

N03~

Selenium 

product

Se^O,2-

ServO32-

Se^2'

Se^2'

SeVIO42"

SeVI042-

SeVI042-

SeVI042-

HSeIV03~

SeVI042-

Theoretical 

energy 

for reaction 

(AGR°, kJ/mol)

-724

-525

-264

+46

-464

-270

+8.7

-300

-27

+94

Indicators of Microbial Activity

Selenium is sensitive to oxidation-reduction 

processes in a ground-water system. Many of these 

processes are mediated by microbes (bacteria), 

and a carbon source (such as kerogen in Mancos 

Shale and Mancos Shale residuum) is necessary 

for bacterial metabolism. This microbial activity 

leaves imprints in geochemical characteristics of 

the water. Sulfur isotopes of dissolved sulfate 

(83 SO4) can be such an indicator of microbial 

activity.

The sulfur-34/sulfur-32 isotope (834S) values 

of pyrite in the Mancos Shale range from -34 to 

+22 per mil CDT (Canyon Diablo troilite) (G. Breit, 

U.S. Geological Survey, oral commun., 1992). The 

S34SO4 values for ground water in the study area 

range from -26 to +6.4 per mil CDT; however, in 

the irrigated areas, 834SO4 ranges from -26.4

to -8.9 per mil CDT (table 14). When 834SO4 

in summer (irrigation-season samples) to winter 

(nonirrigation-season samples) are compared, some 

of the wells show a depletion in 834SO4, and some 

of the wells show an enrichment in 834SO4. Wells 

in Mancos Shale residuum all showed a depletion in 

834S04 values.

The effects of irrigation water and the presence 

of anthropogenic nitrate could keep ground water 

in Mancos Shale residuum in a transitional redox 

state. The winter dissolved-SO42~ pool could 

partly be reduced to intermediate oxidation states 

(such as sulfite or thiosulfate) with no isotope effect 

(Kemp and Thode, 1968) and then reoxidized by 

oxidizing bacteria that could impart a fractionation 

showing a depletion in 834SO4 (Fry and others, 

1988). These dynamic changes in 834SO4 indicate 

an active microbial system that could affect selenium 

mobility.

Relation of Selenium to 

Other Constituents

Dissolved selenium in ground water in the 

Uncompahgre Project area and in the Grand 

Valley primarily is determined by the oxidizing 

conditions present in the ground water. Oxidizing 

conditions suitable for selenium mobilization are

indicated by the relation of dissolved selenium * /

to other dissolved constituents (fig. 21). Presser 

and others (1990) reported that selenium precipitates with magnesium-based evaporite complexes 

(such as thenardite) and does not precipitate 

with calcium-based evaporite complexes (such 

as gypsum), indicating that selenium prefers to 

complex with magnesium-based compounds in 

solution, which might be related to the solubilities 

of the different calcium- and magnesium-based 

evaporites and the saturation states of the evaporite 

complexes that weather from marine shales. A 

positive correlation between dissolved magnesium 

and dissolved selenium is shown in figure 21; 

however, this trend does not conclusively prove 

any geochemical significance regarding selenium 

mobilization in the presence of dissolved 

magnesium.

44 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 14. Sulfur-isotope data for water from selected wells in shale residuum and alluvium

[Sampling date is significant for irrigation season April through October; 834SO4 values, in per mil 

referenced to the CDT (Canyon Diablo troilite) standard; , no previous data for comparison]

Site 

number 

(figs. 8 

and 9)

130L3

130L3

125Q5

125Q5

SL1

SL1

B254334

108U01

Sampling 

date

8MS04 

(per mil) Comments

MANCOS SHALE RESIDUUM

September 1991 -14.8

February 1992

September 1991

February 1992

September 1991

February 1992

June 1991

October 1991

-16.0

-8.9

-19.7

-20.9

-22.0

-26.4

+6.4

Depleted 1.2 per mil from September 1991

-

Depleted 10.8 per mil from September 1991

--

Depleted 1.1 per mil from September 1991

Framboidal pyrite on water-filter paper

ALLUVIUM OVERLYING MANCOS SHALE

RWG1

RWG1

RWG2

RWG2

RWG3

RWG3

143NO

August 1991

February 1992

August 1991

February 1992

August 1991

February 1992

February 1992

-21.0

-20.8

-20.3

-20.6

-19.6

-18.5

-17.0

-

Enriched 0.2 per mil from August 1991

~

Depleted 0.3 per mil from August 1991

-

Enriched 1.1 per mil from August 1991

~

Well located outside the irrigated area.

Selenium is positively correlated with nitrate 

(fig. 21). When conditions in the ground water are 

reduced to the point where ammonia is the dominant 

nitrogen species, selenium is reduced out of the 

dissolved phase; hence, selenium and ammonia are 

negatively correlated (fig. 21). Iron goes into solution 

when oxidation-reduction conditions reach a point 

where ferrous iron (Fe2*, the more soluble iron 

species) becomes the dominant iron species; oxidized 

selenium species become immobilized in such 

reducing conditions (fig. 21). The negative correlations of these oxidation-reduction-sensitive species 

with dissolved selenium in ground water indicates a 

high level of bacterial activity in the Mancos Shale 

that is capable of reducing selenium out of the 

dissolved phase. If not for irrigation, oxidationreduction conditions in the Mancos Shale would be 

sufficiently reducing to prevent the mobilization and 

transport of dissolved selenium.

The trace elements uranium and vanadium 

react similarly to oxidation-reduction conditions 

as does selenium. Selenium and uranium and 

selenium and vanadium, therefore, are positively 

correlated in water from wells in the Uncompahgre 

Project area and in the Grand Valley (fig. 21). 

Selenium and sulfate are positively correlated in 

water from wells in the Uncompahgre Project area and 

in the Grand Valley. Selenium is isomorphous with 

sulfur and substitutes in the crystalline structure of 

sulfide minerals; therefore, when pyrite in the Mancos 

Shale is oxidized by infiltrating irrigation water 

(putting sulfate into solution), selenium also is 

oxidized and brought into solution. Selenium precipitates with magnesium sulfate evaporite complexes; 

therefore, when evaporite complexes are dissolved by 

infiltrating irrigation water, selenium is put into 

solution.

GROUND-WATER INVESTIGATIONS 45
200

o a

D> gg

J!

28

kl « 5-

i

200 400 600

MAGNESIUM

100 1.000 10,000

IRON

200

150

100

50

200

150 -

100 -

5 10

NITRATE

15

1,000

MILLIGRAMS PER LITER

200

150

100

50

200

150

100

20 40 60

URANIUM

80 10 100 1,000

VANADIUM

MICROGRAMS PER LITER

0.001 

200

150 -

100 -

MI

Figure 21. Correlations of dissolved selenium with various dissolved constituents in water from wells in the Uncompahgre Proj

Grand Valley.
SURFACE-WATER INVESTIGATIONS

Selenium data for surface-water sites were 

used to determine area! and seasonal variations of 

selenium concentrations and loads in the irrigated 

areas. Selenium loads were used to delineate drainage 

basins that were major source areas of selenium 

discharging into the Gunnison and Colorado Rivers. 

Dissolved-solids and major-ion data were collected to 

describe general water quality and to determine if 

there were relations between concentrations of 

selenium and dissolved solids and major constituents 

in the study area. Data also were collected for other 

trace constituents and pesticides that could be 

potentially harmful to biota.

Sample collection in 1991 was essentially an 

expanded reconnaissance of the Uncompahgre Project 

area and a first-time reconnaissance of the Grand 

Valley to collect selenium and other trace-constituent 

data for streams, washes, and ditches draining 

irrigated areas. Selenium and other trace-constituent 

data for surface water in the study area were scarce, 

except from a few sites on the Gunnison and Colorado 

Rivers and from Sweitzer Lake. Also, there was a lack 

of information about selenium in irrigated areas 

outside the Uncompahgre Project in the lower 

Gunnison River Basin where irrigation drainage could 

be a source of selenium. During July 1991, samples 

were collected at 4 sites each on the Gunnison and 

Uncompahgre Rivers, at 17 sites on major streams 

or canals draining the Uncompahgre Project, and at 

5 sites on tributaries of the Gunnison River that 

drain irrigated areas adjacent to or upstream from the 

Uncompahgre Project. For a reconnaissance of the 

Grand Valley, 20 sites were sampled in March and in 

late August-early September in 1991. Major tributary 

streams and washes of the Colorado River and 

selected smaller drainages were sampled to provide a 

general geographical distribution of sites in the Grand 

Valley. Samples for analysis of various insecticides 

were collected in the Grand Valley at 11 of the 

reconnaissance sites and at 2 sites on the Colorado 

River in the summer of 1991. Water samples also 

were collected in June 1991 from three ponds in or 

adjacent to the Uncompahgre Project and from one 

pond in the Grand Valley.

In 1992, samples were analyzed for fewer 

constituents, and sampling focused on the irrigated 

areas in the Uncompahgre Project and in the Grand 

Valley. Data were collected for synoptic surveys 

during the winter of 1992 to describe the distribution

of selenium and other constituent concentrations and 

loads in all major drainage basins and in selected 

drainage ditches in the irrigated areas. The synoptic 

data also were used to determine water-quality differences between and within basins. Samples for 

synoptic surveys for individual drainage basins were 

collected in 2 days, although sampling for all the 

surveys was done over about 7 weeks from late 

January to mid-March. In February 1992, streamdischarge data and water-quality samples were 

collected at 16 sites on the Uncompahgre River, 

including sites at all diversions for winter water, and at 

major inflows to estimate selenium and other constituent loading to the river during low and stable stream 

discharge. Monthly samples were collected at four 

tributary sites and from the Uncompahgre River in 

water year 1992 to describe seasonal variations in 

constituent concentrations and loads. Seleniumspeciation data were obtained for five streams in the 

Grand Valley in March 1992 for use in a risk assessment of selenate and selenite to endangered fish. 

Water samples were collected from two ponds in the 

Uncompahgre Project area and two ponds in the Grand 

Valley in June 1992.

Sample Collection and 

Analytical Methods

Onsite measurements at surface-water sites 

included stream discharge, specific conductance, 

pH, water temperature, and dissolved oxygen. 

Water-quality measurements were made using 

methods similar to methods described by Knapton 

(1985). Stream discharge for sites at streamflowgaging stations was determined from the stage record 

and the stage-discharge rating table. Otherwise, 

stream discharge was measured using methods 

described by Rantz and others (1982).

Water samples were collected using depthintegrating samplers and equal-width methods 

(Ward and Hair, 1990) when possible. At sites 

where the water depth was too shallow, samples 

were collected from the centroid of flow or at several 

verticals in a cross section using 1-L plastic or glass 

bottles. Samples for selected organophosphate and 

carbamate insecticide analyses were collected 

directly in bottles supplied by the USGS National 

Water Quality Laboratory (NWQL) in Arvada, Colo.

SURFACE-WATER INVESTIGATIONS 47
Samples collected for analyses of dissolvedinorganic constituents and uranium were filtered in the 

field through a 0.45-p,m cellulose filter using a plastic 

filter unit. Samples for analyses of inorganic constituents, uranium, and insecticides were processed and 

preserved for shipment to the NWQL according to 

standard procedures of the USGS. Samples for 

selenium-speciation analyses were acidified with 

hydrochloric acid and chilled with ice.

Quality-assurance sampling at surface-water 

sites included collection of three deionized water 

blanks that were filtered and processed as regular 

water samples, five sequential duplicate samples, 

and five split samples. Selenium was not detected 

in the blanks (less than 1 |J.g/L). For the duplicate 

and split samples in which selenium concentrations 

exceeded 10 |J.g/L, the difference between concentrations in the original sample and in the qualityassurance sample ranged from 3 to 12 percent. Except 

for uranium, the differences in other trace-constituent 

concentrations between duplicate or split samples 

generally were less than 10 percent.

The analyses of water samples for major 

constituents, nitrogen and phosphorus species, 

and trace constituents (except uranium) were 

done at the NWQL. Analytical methods described 

by Fishman and Friedman (1989) and laboratory 

quality-assurance methods described by Jones 

(1987) were used. Uranium was analyzed by a 

laboratory contracted by NWQL using a method 

described by Thatcher and others (1977). Insecticides 

in water were analyzed using methods described by 

Wershaw and others (1987). Stable isotopes of 

hydrogen and oxygen were analyzed using methods 

described by Epstein and Mayeda (1953) and Kendall 

and Coplen (1985). Selenium-speciation analyses 

of water samples were done by HGAAS (Fio and 

Fujii, 1990). The selenium-speciation analyses 

were done by the USGS in Sacramento, Calif. 

(R.F. Fujii, U.S. Geological Survey, written 

commun., 1992).

Selenium Concentrations and Loads 

in the Gunnison and Colorado Rivers

Selenium data have been collected since 

August 1975 at USGS streamflow-gaging stations 

on the Gunnison River at Whitewater (site GUN6)

and on the Colorado River near the Colorado-Utah 

State line (site COLS) (fig. 1) for the USGS's 

National Stream Quality Accounting Network. 

Selenium data for the Gunnison River downstream 

from the Gunnison Tunnel (site GUN1), at Delta 

(site GUN4), and upstream from Escalante Creek 

(site GUN5) (fig. 1) were collected for the reconnaissance investigation of the Gunnison and Uncompahgre 

River Basins (Butler and others, 1991). Most of 

the selenium data for site GUN4 during water 

years 1991-92 were collected for another waterquality program of the USGS. Selenium data also 

were collected for the detailed study at selected 

gaging stations on the Gunnison and Colorado 

Rivers during 1991-92 to supplement ongoing 

data-collection activities.

Selenium Concentrations

Selenium concentrations for sites on the 

Gunnison and Colorado Rivers and for Plateau 

Creek (fig. 1) are summarized in table 15. Median 

selenium concentrations in the Gunnison and 

Colorado Rivers upstream from irrigated areas were 

less than 1 (ig/L (table 15). That concentration also 

is representative of the selenium concentration in most 

of the irrigation-supply water for both areas, except 

for the Redlands area (fig. 3) in the Grand Valley, 

which is irrigated with water diverted from the 

Gunnison River downstream from site GUN6 (fig. 1). 

Selenium concentrations increase in the Gunnison and 

Colorado Rivers downstream from irrigated areas. 

The median concentrations at the two outflow sites 

for each river (sites GUN6 and COLS) are greater 

than at the sites upstream from the irrigated areas 

(table 15). Selenium concentrations in about 

64 percent of the samples from site GUN6 and 

almost 50 percent of the samples from site COLS 

exceeded the chronic aquatic-life criterion of 

5 ng/L established by the U.S. Environmental 

Protection Agency (1987). The chronic criterion 

is a recommended guideline for protection of aquatic 

life from long-term exposure to selenium that could 

result in reproduction problems or mortality in fish 

and wildlife through food-chain bioaccumulation. 

The selenium concentrations in the lower Gunnison 

River and in the Colorado River in the Grand 

Valley area might be of concern in the recovery 

of endangered fish in the study area. Presently (1996),

48 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
the FWS and BOR are studying locations along the 

Gunnison River downstream from Delta and along 

the Colorado River in the Grand Valley as possible 

recovery sites for endangered fish.

Analysis of the seasonal distribution of 

selenium concentrations (1975-92) at sites GUN6 

and COLS indicated maximum selenium concentrations occurred during August through October, 

which is the last one-half of the irrigation season. 

The selenium concentrations were slightly higher 

during the late irrigation season than during winter, 

although stream discharge was not necessarily 

lower in late summer than during winter at the 

two sites, particularly for the Colorado River. 

Minimum selenium concentrations occurred during 

May and June, coincident with the peak snowmelt 

runoff.

Time-series plots of selenium concentrations 

for the Gunnison River at Whitewater (site GUN6) 

and Colorado River near the Colorado-Utah State line 

(site COLS) are shown in figure 22. A smooth curve 

was fitted through the data points for each site using a 

LOWESS (LOcally WEighted Scatterplot Smoothing) 

procedure, which uses residual weighting functions 

to decrease the effect of outliers (Schertz and 

others, 1991). Generally, the smoothed curves 

indicate decreasing concentrations to about 1980, 

a minimum in the mid-1980's, followed by a slight

increase, and then relatively no change since about 

1987. The low concentrations in the 1980's can be 

accounted for by dilution caused by high stream 

discharge in the Colorado River Basin from 1983 

to 1986.

A computer program called ESTREND 

(Estimate TREND) (Schertz and others, 1991) 

was used to test the selenium data for monotonic 

time trends. The program uses a nonparametric test, 

the seasonal Kendall test, to examine water-quality 

data for monotonic trends. The seasonal Kendall 

test decreases the effect that seasonal differences 

in constituent concentrations could have on trend 

detection by comparing data from the same seasons 

of the year. For trend tests on selenium data, the 

year was divided into the nonirrigation season 

(November through March) and the irrigation season 

(April through October); the irrigation season was 

further divided into the runoff season (April through 

June) and the post-runoff season (July through 

October). The variability in stream discharge could 

mask or overwhelm trends in selenium concentrations. 

The ESTREND program computes trend statistics 

on the actual concentrations and on the flow- 

(stream discharge) adjusted concentrations. Trend 

tests on flow-adjusted concentrations increase the 

chance of detecting a trend in concentrations that 

was the result of some effect other than a change in 

stream discharge.

Table 15. Summary of selenium concentrations in the Gunnison and Colorado Rivers and Plateau Creek and the number of 

samples exceeding aquatic-life criteria of the U.S. Environmental Protection Agency

[Concentrations in micrograms per liter; chronic aquatic-life criterion is 5 micrograms per liter, and the acute criterion is 20 micrograms per liter 

(U.S. Environmental Protection Agency, 1987); <, less than; a single value is listed for the range for one sample; --, no value]

Site 

number

GUN1

GUN3

GUN4

GUNS

GUN6

COL2

COLS

COLS

PLT

Site 

name

Gunnison River downstream from the Gunnison Tunnel

Gunnison River at Austin

Gunnison River at Delta

Gunnison River upstream from Escalante Creek

Gunnison River at Whitewater

Colorado River near Cameo

Colorado River upstream from Gunnison River

Colorado River near the Colorado-Utah State line

Plateau Creek near mouth

Period 

of 

record

1987-91

1991

1987-92

1987-91

1975-92

1980-92

1991

1975-92

1991-92

Number 

of 

samples

5

1

13

5

73

1 22

1

75

7

Median

<1

-

4

5

7

1

-

5

<1

Range

<!-<!

1

<1-13

4-10

<l-25

<l-2

2

<1-30

<l-2

Number of samples 

exceeding criteria

Chronic

0

0

2

2

47

0

0

37

0

Acute

0

0

0

0

1

0

0

1

0

Includes one sample collected in January 1992 at Palisade because of ice conditions at site COL2.

SURFACE-WATER INVESTIGATIONS 49
o 8.

B> Q. B'"<

B> O

<n =i

CD (/>

= £

5 2.

CD c

11

I*

* 9 fi> S

ff

CD O

7§

o (o

8

s 

- 5 ** CD

> 3 

CD 5T

n» _.

o M

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If w

0 § 

3 O

I 3

2r a

cF 3 «< o "«*

3g

ii

i?» 2. 5

O Q.

| 5

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(O (0

r§ s °

1Q.

o«:

30

28 

EC 

LU

^~

Zj ^

EC

Q. 24

!5

< 22

DC

O 20

DC

O

2 18

Z

g 16

0

t 14

DC

Z 12

LU 

O y

O

o

^ 8

Z

a 6

LU

4

2

0

l l I I I I I I I l I I I l 

O 13I IKIMIQOM QIV/CD 1 OVA/CCC Ol IDV/C dUlNlMIOwIN niVCrl L.vWCOO 1/UrfVC

........ COLORADO RIVER-LOWESS CURVE

4- GUNNISON RIVER AT WHITEWATER (SITE GUN6)

O COLORADO RIVER NEAR THE COLORADO-UTAH STATE L

+

4-

- + e-

 o

4- +

- O + +

O O O

 ""^^Qj- O O 4- O Of- +

- + '^^-<t o 4- o 4- + 

 > 4- "^^^^_ + 4- O 4- + 

"^^r-BD ++O 4- + > 

O 4- ""^^ 6 4^L_<3 + O 4^ QD 

O ""---... O ^ --Ok- _Q£X- -^ ^+0- 

O Of 0 O ©" -0 ........+ Q .........-H---"" O 

4- 4- 4- 6 +O 0 OOO O O

OOO+ OOOO0 O 

O © 4- 4- 4- 

++4- 4- O 

i i i i i i i i i i i i i i 

1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 

YEAR

Figure 22. Dissolved-selenium concentrations and the LOWESS smoothed curves of selenium concentrations for the Gunni

Scita f2l INft\ anH thn <"V>lr»raHr> Qiwor noar tha rVilnrarln.l Itah Qtato lino (c\ia HOI A\ 1Q7^_Q9
The ESTREND program was used to 

analyze the selenium data for 1975-92 for the 

Gunnison River (site GUN6) and Colorado River 

(site COLS) for monotonic time trends. A trend 

was considered significant if the significance level 

(p) of the statistical test was 0.05 or less. For the 

Gunnison River, neither the trend in selenium concentrations of-0.13 (|ig/L)/yr was significant (p = 0.28) 

nor was the trend in flow-adjusted concentrations of

-0.18 (|ig/L)/yr significant (p = 0.065). For comparative purposes, sulfate concentrations for 1975-92 also 

were analyzed for trends. Sulfate concentrations 

could be affected by irrigation drainage in the study 

area. For 1975-92, the concentration and flowadjusted concentration trends for sulfate were not 

significant for the Gunnison River, which is the same 

result as for the selenium data.

For the Colorado River, the trend in selenium 

concentrations of-0.20 (|ig/L)/yr (p = 0.003) 

and the trend in flow-adjusted concentrations of

-0.15 (|ig/L)/yr (p = 0.005) were significant for 

1975-92. Conversely, neither the concentration 

nor the flow-adjusted concentration trends for 

sulfate in the Colorado River for 1975-92 were 

significant. Because of the decrease in selenium 

concentrations in the Colorado River during the 

1970's (fig. 22), trend analysis was done on the 

Colorado River selenium data for 1978-92, 1979-92, 

and 1980-92. For these three periods, the seleniumconcentration trends were not significant, but the flowadjusted concentration trends were still significant 

[-0.14 to -0.16 (|lg/L)/yr with p values of 0.002 

to 0.006]. The trend results seem to indicate a 

decreasing trend in flow-adjusted selenium concentrations for the Colorado River downstream from the 

Grand Valley.

Selenium Loads

Selenium loads in the Gunnison River, 

Uncompahgre River, and tributaries of the Gunnison 

River were used to estimate the proportion of selenium 

load in the Gunnison River at Whitewater (site GUN6, 

the outflow site for the Gunnison River Basin) that 

is discharged from the Uncompahgre Project. To 

obtain such an estimate, the selenium loads in 

the Gunnison River at Delta (site GUN4), the 

Uncompahgre River at Delta (site UC16), and the

Gunnison River at Whitewater (fig. 23) were 

estimated. Estimates of selenium loading from 

the Uncompahgre Project into the Gunnison River 

between Peach Valley Arroyo and site GUN4 and 

between the Uncompahgre River and Roubideau 

Creek (fig. 23) also were made. Selenium-load 

estimates were considered approximate because 

of the limited quantity of selenium-load data for 

the Gunnison River and for tributaries.

Results of the sampling in July 1991 indicate 

that the selenium load in the Gunnison River increased 

from 5.6 to 23.1 Ib/d between sites GUN3 and GUN4 

(fig. 23). About 53 percent of the load in the Gunnison 

River at site GUN4 in July 1991 was accounted for 

in Peach Valley Arroyo (site PVA1), an unnamed 

drainage near Read (site RD1), and the Bonafide 

Ditch (site BFD) (fig. 23). Most of the selenium 

load from the three drainages was at sites RD1 

and BFD. For three sets of samples collected at 

sites RD1, BFD, and GUN4 in 1992, the selenium 

loads at sites RD1 and BFD accounted for 29 to 

45 percent of the selenium load at site GUN4. 

Unmeasured selenium loading into the Gunnison 

River between Peach Valley Arroyo and site GUN4 

is from other ditches and drainages that discharge 

relatively small selenium loads from irrigated 

areas of the Uncompahgre Project and selenium 

loads from nonproject irrigated areas north of the 

river. Selenium data were collected for the detailed 

study in 1993 for tributaries of the Gunnsion River 

that drain the Uncompahgre Project (unpublished 

data, U.S. Geological Survey). For three sets of 

samples collected in the summer of 1993, about 

53 percent of the selenium load at site GUN4 was 

accounted for by discharge from the Uncompahgre 

Project upstream from site GUN4. Based on selenium 

data collected during 1991-93, 50 percent of the 

selenium load at site GUN4 was assumed to be from 

the Uncompahgre Project.

Selenium loads for sites GUN4 and GUN6 

and for site UC16 were estimated from daily selenium 

loads. Daily selenium loads were computed using 

predicted selenium concentrations from regression 

relations of selenium concentration to stream 

discharge and from the daily mean stream discharge 

for water years 1991-92. Regression relations had 

coefficient of determinations ranging from 0.52 to 

0.82 (all significance levels were less than 0.05).

SURFACE-WATER INVESTIGATIONS 51
i

RD1

EXPLANATION

SAMPLE SITE AND NUMBER 

DIRECTION OF FLOW

Figure 23. Schematic diagram showing the lower Gunnison River Basin and relative locations of selected surface-water-sam
Fifty percent of the difference in selenium load 

between sites GUN4 and GUN6, minus the load 

for site UC16, was assumed to be selenium loading 

from the Uncompahgre Project into the Gunnison 

River between the Uncompahgre River and Roubideau 

Creek (fig. 23). That estimate is derived from 

selenium data collected in 1991-92 (Butler and others, 

1994) and from data collected in 1993 (unpublished 

data, U.S. Geological Survey). Selenium loads for the 

Uncompahgre River (discussed in the "Selenium 

Loads" section) indicate that about 95 percent of the 

selenium load in the river at site UC16 was from the 

Uncompahgre Project. The fraction of the selenium 

load at site GUN6 attributable to the Uncompahgre 

Project was computed using: [(0.5 times the selenium 

load at site GUN4) plus (0.95 times the selenium load 

at site UC16) plus (0.5 times the difference in 

selenium load between sites GUN6 minus GUN4 

and UC16)] divided by the selenium load at 

site GUN6. Using the periodic selenium and daily 

stream-discharge data for water years 1991-92, 

the Uncompahgre Project accounted for about 

68 percent of the selenium load in the Gunnison 

River at Whitewater (site GUN6). That estimate 

is considered a general approximation because of 

the limited selenium-loading data for streams, washes, 

and ditches that discharge into the Gunnison River 

between sites GUN3 and Roubideau Creek.

Selenium loads in the Gunnison River at 

Whitewater (site GUN6) and in the Colorado River 

near the Colorado-Utah State line (site COLS) in water 

years 1991-92 for samples collected within 2 days 

of each other are shown in figure 24. Site COLS is 

the outflow site for the entire study area and is 

downstream from all irrigation drainage from the 

Gunnison River Basin and from the Grand Valley. The 

residual selenium load shown in figure 24 is equal to 

the selenium load of the Colorado River at site COLS 

minus the load for the Gunnison River at site GUN6. 

The residual load represents the selenium load in the 

Colorado River upstream from the Grand Valley plus 

the selenium load from the Grand Valley. The 

selenium load in the Colorado River upstream from 

the Grand Valley is essentially equal to the selenium 

load in the Colorado River at site COL2 (fig. 1), plus 

the small load from Plateau Creek (site PLT). Because 

ungaged surface- and ground-water dissolved-solids 

loads from natural sources into the Grand Valley are 

negligible (Bureau of Reclamation, 1983), selenium 

loads from natural sources in the Grand Valley also

are assumed to be negligible. Therefore, selenium 

loading from the Grand Valley was assumed to be 

from irrigation-induced sources.

A regression of selenium concentration to 

stream discharge using data for 1991-92 was 

determined for the Colorado River at site COLS 

and then used to estimate daily mean selenium loads 

for that site. The stream discharge for the Colorado 

River upstream from the Grand Valley was equal to 

the combined stream discharge of the Colorado River 

at site COL2 and Plateau Creek at site PLT (fig. 1). 

About 75 percent of the selenium concentrations 

were reported as less than 1 |lg/L in samples 

collected in 1991-92 at sites COL2 and PLT. An 

arbitrary mean selenium concentration of 0.7 |lg/L 

was used to compute the daily mean selenium loads 

in the Colorado River upstream from the Grand Valley. 

The selenium-load estimates for water years 1991-92 

indicate that the Colorado River upstream from the 

Grand Valley accounted for about 10 percent of the 

selenium load at site COLS, the Gunnison River 

(site GUN6) accounted for about 45 percent of the 

load, and the Grand Valley accounted for the 

remaining 45 percent of the selenium load in the 

Colorado River.

These selenium-load estimates for the Gunnison 

and Colorado Rivers are considered approximations, 

but might be useful for comparing relative contributions of the Uncompahgre Project and the Grand 

Valley with the selenium load in the Colorado 

River. About two-thirds of the selenium load in the 

Gunnison River at site GUN6 was attributed to the 

Uncompahgre Project; therefore, the Uncompahgre 

Project accounts for about 30 percent of the selenium 

load in the Colorado River at site COLS. The 

Grand Valley accounts for about 45 percent of the 

selenium load in the Colorado River at site COLS. 

Although the uncertainty associated with these 

estimates was not determined, the seleniumloading estimates indicate that the Uncompahgre 

Project and the Grand Valley are major sources of 

selenium to the Colorado River.

Longer term selenium loads in the Gunnison 

River and from the Grand Valley were estimated 

by using the selenium data collected during water 

years 1976-92 for sites GUN6 and COLS (fig. 22). 

The same methods used for estimating selenium 

loads for the Colorado River upstream from the 

Grand Valley and for sites GUN6 and COLS for 

water years 1991-92 were used to estimate selenium

SURFACE-WATER INVESTIGATIONS 53
140

GUNNISON RIVER AT WHITEWATER 

(SITE GUN6)

RESIDUAL

COLORADO RIVER NEAR THE 

COLORADO-UTAH STATE LINE 

(SITE COLS)

JULY 91 SEPT 91 NOV91 DEC 91 JAN 92 FEB92 MAR 92 JUNE 92 AUG92

Figure 24. Selenium loads in the Gunnison River at Whitewater (site GUN6) and the Colorado River near the ColoradoUtah State line (site COLS), July 1991-August 1992, and the residual loads. (Only samples collected at both sites in the 

same week are shown. Residual is the selenium load at site COLS minus the selenium load at site GUN6.)

loads for water years 1976-92. The selenium load 

in the Colorado River upstream from the Grand Valley 

accounted for about 9 percent, the Gunnison River 

about 43 percent, and the selenium load from the 

Grand Valley about 48 percent of the selenium load in 

the Colorado River at site COLS. The results for the 

Gunnison River and Grand Valley selenium loads for 

water years 1976-92 are comparable to the estimates 

for water years 1991-92.

Distribution, Magnitude, and Variation 

of Selenium Concentrations

Distribution, magnitude, and variation of 

selenium concentrations are described for irrigated 

areas in the Uncompahgre Project and in the Grand 

Valley and in areas adjacent to the Uncompahgre 

Project. Selenium concentrations are compared 

to aquatic-life criteria established by the 

U.S. Environmental Protection Agency (1987).

Irrigation drainage is considered the groundwater component of water in streams, washes, canals, 

and ditches within the irrigated areas. Surface-return 

flow is defined as the irrigation water from field runoff 

and the excess water in the canals and laterals that was 

discharged into streams and washes during the irrigation season. Winter water is water that was diverted 

from streams into canals and laterals during the 

nonirrigation season (November-March) for livestock 

water supplies. Unused winter water also was 

discharged into the streams and washes and essentially 

is equivalent to surface-return flow.

Uncompahgre Project Area

Tributaries, Canals, and Ditches

All water samples collected during the nonirrigation season from the east side of the Uncompahgre 

Project exceeded the chronic aquatic-life criterion of 

5 |Xg/L and all but one sample exceeded the acute

54 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
criterion of 20 H-g/L (table 16). Sampling sites are 

shown in schematic diagrams for the Uncompahgre 

Project in figures 25 and 26. The schematic diagrams 

show major streams, washes, canals, and miscellaneous ditches. Eight of the 11 irrigation-season 

samples from the eastern side that exceeded the acute 

criterion were monthly samples collected from 

Loutsenhizer Arroyo at site LZA1 (fig. 26). Selenium 

concentrations in about one-half the nonirrigationseason water samples from the western side exceeded 

5-jlg/L chronic criterion, but only three concentrations 

exceeded the 20 ^ig/L acute criterion. Although 

surface-return flow diluted selenium concentrations 

during the irrigation season, almost all selenium 

concentrations in irrigation-season samples from the 

eastern side exceeded the chronic aquatic-life criterion 

(table 16). The three samples from the eastern side 

that had selenium concentrations that did not exceed 

the chronic aquatic-life criterion were collected from 

Gretts, Markley, and Brozina Ponds (sites GTP, MKP, 

and BZP in fig. 26). The source water for these ponds 

is irrigation-supply water, surface-return flow, and 

irrigation drainage. Chemical and biological 

processes might remove some selenium from water in 

the ponds and immobilize the selenium in bottom 

sediment (Lemly and Smith, 1987).

Selenium concentrations were much higher 

in surface water on the eastern side of the 

Uncompahgre Project (east of the Uncompahgre 

River) than on the western side (table 16). The 

larger selenium concentrations on the eastern side 

of the Uncompahgre Project primarily are the result 

of geologic differences between the eastern and

western sides (fig. 4). About 85 percent of the 

irrigated land on the eastern side is on soils primarily 

derived from Mancos Shale (adobe soils), but only 

about 4 percent of the irrigated land on the western 

side is on adobe soils (Bureau of Reclamation, 1982). 

Most of the irrigated land on the western side is on 

soils derived from terrace deposits and the Dakota 

Sandstone that probably contain none or small 

amounts of shale detritus. Geometric mean concentrations of total and water-extractable selenium were 

about 2 to 5 times greater for soils derived from the 

Mancos Shale than for soils derived from the Dakota 

Sandstone or terrace deposits (table 2).

An analysis of the distribution of selenium 

concentrations in the Uncompahgre Project during 

the nonirrigation season was done by examination 

of the data collected for the synoptic surveys in 1992. 

Selenium concentrations in samples collected for 

the synoptic surveys were much higher on the eastern 

side of the project than on the western side (table 17). 

Selenium concentrations in each sample collected 

during synoptic surveys are shown on the schematic 

diagrams in figures 25 and 26. Major drainage basins, 

selected ditches, and canals that had winter water 

were sampled during synoptic surveys. Excluding 

winter-water samples (coded WW in table 17), 

median selenium concentrations for synoptic-survey 

samples collected in the major drainage basins and 

geographical areas on the eastern side of the 

Uncompahgre Project ranged from 59 to 170 jlg/L, 

compared to 1 to 16 ^ig/L for the western side of 

the project.

Table 16. Summary of selenium concentrations in irrigated areas of the Uncompahgre Project in 1991-92 and the number 

of samples that exceeded aquatic-life criteria of the U.S. Environmental Protection Agency

[Concentrations in micrograms per liter; west and east irrigated areas separated by the Uncompahgre River; irrigation-season samples were collected in 

July and October 1991 and April through August 1992; nonirrigation-season samples were collected from November 1991 through March 1992; chronic 

aquatic-life criterion is 5 micrograms per liter, and the acute criterion is 20 micrograms per liter (U.S. Environmental Protection Agency, 1987); <, less 

than]

Area

West

West

East

East

Season

Irrigation

Nonirrigation

Irrigation

Nonirrigation

Number 

of 

samples

14

37

36

36

Median

1.5

5

11

105

Range

<l-9

<l-55

2-56

19-260

Number of samples 

exceeding criteria

Chronic

4

18

33

36

Acute

0

3

11

35

SURFACE-WATER INVESTIGATIONS 55
UC4

EXPLANATION

SAMPLE SITE AND NUMBER

(90) SELENIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

DIRECTION OF FLOW

UC1

COLONA

Figure 25. Schematic diagram showing the southern part of the Uncompahgre Project, relative locations of 

surface-water-sampling sites, and selenium concentrations in samples collected during synoptic surveys, 

January-March, 1992.

56 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
CC2

EXPLANATION

SAMPLE SITE AND NUMBER

(34) SELENIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

DIRECTION OF FLOW

Figure 26. Schematic diagram showing the northern part of the Uncompahgre Project, relative locations of surface-watersampling sites, and selenium concentrations in samples collected during synoptic surveys, January-March 1992.

SURFACE-WATER INVESTIGATIONS 57
Table 17. Summary of selenium concentrations in samples collected during the synoptic surveys of the Uncompahgre Project, 

January through March 1992

[Concentrations in micrograms per liter; (WW), winter-water samples; <, less than]

Drainage basin 

or canal

Number 

of sites

Sites included 

(figs. 25 and 26) Median Range

AREAS WEST OF THE UNCOMPAHGRE RIVER

West Canal/Montrose and Delta Canal (WW)

Ironstone Canal (WW)

Horsefly and Happy Canyon Creek Basins

Spring Creek Basin and Mexican Gulch

Dry Creek Basin

Miscellaneous ditches

Cummings Gulch and Seep Creek Basins

Roubideau Creek Basin

4 WST, UC3, MDC1, MDC2

3 UC9, IRC1, IRC2

4 HFC, HCC1, HCC2, DOL

5 SP1-SP4, MXG

7 DRY1-DRY5, CC1, CC2

5 UNBL, C AL, ASH, UND1, UND2

5 CMG1,CMG2,BXG,SEP1,SEP2

5 RB1-RB3,BMC,WSC

AREAS EAST OF THE UNCOMPAHGRE RIVER

Selig Canal (WW)

Dry Cedar and Cedar Creek Basins

Loutsenhizer Arroyo Basin

Peach Valley Arroyo and unnamed drainage

Garnet Canal and Bonafide Ditch

3 UC7, SGC1, SGC2

9 DCC1, DCC2, CD1-CD4, MA1-MA3

8 LZA1-LZA5, WLZ1-WLZ3

5 PVA1,PVA2,RD1-RD3

7 GAR1-GAR4, UNGR, RFD, BFD

2.5 

11.5

1

2

7

9.5

16

4

9

59

170

88

160

2-3

8-16 

<l-2

1-3 

<1-10

6-55 

10-23

5-12

32-95

100-230

19-200

23-260

Perhaps one reason for lower selenium concentrations on the western side of the Uncompahgre 

Project during the nonirrigation season was dilution 

of subsurface-irrigation drainage by winter water. 

Winter water was discharged into all of the major 

tributaries on the western side of the Uncompahgre 

Project during collection of samples for the synoptic 

surveys in early 1992. The sole source of winter water 

in canals was water diverted from the Uncompahgre 

River. During the synoptic sampling in 1992, winter 

water was diverted only into the Selig Canal on the 

eastern side and into all three canals that serve the 

western side of the project (figs. 25 and 26). Analysis 

of the selenium concentrations in streams and in 

winter water (table 17) and of selenium concentrations 

in ground-water samples from the Dry and Spring 

Creek Basins does not indicate that dilution by winter 

water is the only cause of lower selenium concentrations on the western side during the nonirrigation 

season. The similarity of selenium concentrations 

between winter water and major streams on the 

western side (figs. 25 and 26; table 17) indicates 

that either most of the water in the streams consisted 

of winter water or that selenium concentrations in 

irrigation drainage that discharged into western-side 

streams and ditches were not markedly different than

concentrations in winter water. For example, a 

comparison of selenium concentrations in Dry Creek 

with concentrations in winter water in the Ironstone 

Canal and in the Montrose and Delta Canal was made 

using figures 25 and 26. The samples at sites IRC1, 

MDC1, and MDC2 were collected during the same 

2 days as the samples from Dry Creek. Water samples 

from Dry Creek represented mixtures of irrigation 

drainage, winter water, and small quantities of natural 

flow. The selenium concentrations in Dry Creek at 

sites DRY1, DRY2, and DRY3 ranged from 8 to 

11 Jlg/L in February (fig. 26), and these sites are 

downstream from the Ironstone Canal and its laterals. 

At site DRY4, which could only receive winter water 

from the Montrose and Delta Canal, the selenium 

concentration was 3 jlg/L. The relatively high 

selenium concentrations (for the western side) 

in the Cummings Gulch and Seep Creek Basins 

(table 17) could partly be the result of transport of 

winter water into those basins by laterals from the 

Ironstone Canal.

The water samples from sites ASH, UNBL, and 

SEP2 (fig. 26) were the only samples from the western 

side in which selenium concentrations (23 to 55 jig/L) 

were substantially greater than concentrations in 

winter water. Most of the water sampled at these

58 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
three sites was assumed to be irrigation drainage 

because of the small stream discharges (less than 

0.5 ft /s) and because dissolved-solids concentrations 

were much higher in these drainages than in winter 

water diverted from the Uncompahgre River. Those 

three sites are near or in areas where soils could 

contain detritus from the Mancos Shale. Although 

irrigation drainage from Mancos Shale areas on 

the western side can contain substantial selenium 

concentrations, areas on Mancos Shale represent 

only about 4 percent of the irrigated area west of 

the Uncompahgre River.

The highest selenium concentrations during 

the nonirrigation season on the eastern side of 

the Uncompahgre Project generally were in the 

Loutsenhizer Arroyo Basin, in an unnamed drainage 

near Read, and in the Garnet Canal area (fig. 26; 

table 17). The small amount of winter water in the 

Selig Canal (7.5 ft3/s in March 1992) probably did 

not have a major effect on selenium concentrations 

in streams on the eastern side of the project. The 

synoptic-survey data (figs. 25 and 26) indicate 

that selenium concentrations on the eastern side 

of the Uncompahgre Project had large variability 

among sites and within drainage basins during the 

nonirrigation season. For example, the maximum 

selenium concentration in surface-water samples 

collected in 1991-92 in the Uncompahgre Project 

area was 260 |Lig/L for site RFD, and a sample 

collected from a nearby natural drainage at site UNGR 

had 200 }lg/L of selenium (fig. 26). By contrast, the 

synoptic-survey sample collected from the Garnet 

Canal at site GARS (fig. 26), about 2 mi southwest 

of site RFD, had only 23 |Lig/L of selenium. The 

synoptic-survey samples were collected within a 

2-day period in March 1992 at sites RFD, UNGR, 

and GARS, and the sole water source at those sites 

was subsurface-irrigation drainage. There also 

were major differences in selenium concentrations 

in the Cedar Creek and Loutsenhizer Arroyo Basins 

(figs. 25 and 26).

Water from the Garnet Canal is used to keep 

Sweitzer Lake full during the year. During the irrigation season, water from the Uncompahgre River and 

Loutsenhizer Arroyo is used to fill the Garnet Canal. 

Historically, there were periods of insufficient flow in 

the Uncompahgre River at the Garnet diversion to 

fill the canal during the irrigation season. Therefore, 

a diversion from Loutsenhizer Arroyo is used (at 

site LZAM in fig. 26) to divert some of the water in

the arroyo into the Garnet Canal to supplement the 

river diversion. During the nonirrigation season, the 

river diversion is closed, and a small quantity of water 

from Loutsenhizer Arroyo usually flows into the canal 

and is discharged into Sweitzer Lake. As a result, 

irrigation drainage having some of the highest 

selenium concentrations in the Uncompahgre Project 

area discharges into Sweitzer Lake.

Selenium concentrations seem to be further 

increased in the Garnet Canal upstream from Sweitzer 

Lake by the discharge of irrigation drainage that 

contains high selenium concentrations into the canal 

between site GAR1 and the diversion into Sweitzer 

Lake at site GAR2. The water sample collected in 

March 1992 at site GAR1 consisted solely of water 

diverted from Loutsenhizer Arroyo. The selenium 

concentration increased from 160 to 240 jug/L between 

sites GAR1 and GAR2 (fig. 26), and the discharge in 

the canal increased by about 0.4 ft3/s. In March 1993, 

two drainage ditches located about 0.4 mi south of 

Sweitzer Lake at sites DR1 and DR2 (fig. 26) were 

sampled. Site DR1 is on a small ditch draining a 

seep area near the canal, and site DR2 is on a drainage 

ditch that extends to the east and southeast of the 

Garnet Canal. Both ditches discharge directly into the 

canal. On March 2,1993, a sample from site DR2 had 

a selenium concentration of 600 |Lig/L. On March 24, 

a sample from site DR1 had a selenium concentration 

of 73 |Lig/L, and a sample from site DR2 had a concentration of 520 flg/L. The selenium concentrations 

for site DR2 were much higher than the maximum 

selenium concentration of 260 |Llg/L (at site RFD) 

for the surface-water samples collected in 1991-92 

in the Uncompahgre Project area. Stream discharge 

at site DR1 was about 0.01 ft3/s and at site DR2 

about 0.10 ft3/s. Visual observations of flow in the 

Garnet Canal seem to indicate that various reaches in 

the canal upstream and downstream from sites DR1 

and DR2 were gaining or losing water. Apparently, 

discharge of irrigation drainage from the drainage 

ditch at site DR2 and from nonpoint seepage into 

the canal caused the higher selenium concentrations 

in the Garnet Canal at the Sweitzer Lake diversion 

(site GAR2) than in the canal at site GAR1.

Selenium concentrations in eastern-side 

drains were much lower during the irrigation season 

compared to the nonirrigation season (sites CD1, 

LZA1, and RD1 in fig. 27). Large quantities of 

surface irrigation-return flow dilute selenium concentrations in eastern-side drains during the irrigation

SURFACE-WATER INVESTIGATIONS 59
season. Stream discharges measured at the time 

of sampling were about 5 to 15 times greater during 

the irrigation season than during the nonirrigation 

season for the samples shown in figure 27. The ranges 

of instantaneous stream discharges measured in 

eastern-side streams were 15.4 to 215 ft3/s for Cedar 

Creek (11 measurements at site GDI), 13.3 to 109 ft3/s 

for Loutsenhizer Arroyo (13 measurements at 

site LZA1), and 4.9 to 76 ft3/s for the unnamed 

drainage near Read (6 measurements at site RD1). 

Despite the dilution effect by surface-return flows 

during the irrigation season, selenium concentrations 

in all water samples from Loutsenhizer Arroyo at 

site LZA1 exceeded the acute aquatic-life criterion 

of 20 jlg/L (U.S. Environmental Protection Agency, 

1987) (fig. 27).

Selenium concentrations had much less seasonal 

variation in western-side streams than in easternside streams, as indicated by the small range of 

concentrations (4 to 10 jlg/L) in Dry Creek at 

site DRY1 (fig. 27). The range in stream discharge 

for 11 instantaneous stream-discharge measurements 

at site DRY1 (July 1991 to June 1992) was 51.9 to 

200 ft3/s. Selenium concentrations were only slightly 

lower in the samples collected in July 1991 compared 

to samples collected in the winter of 1992 at the 

outflow sites on Horsefly Creek (site HFC), Happy 

Canyon Creek (site HCC1), Spring Creek (site SP1), 

Cummings Gulch (site CMG1), and Roubideau Creek 

(site RB1), although stream discharge averaged about 

five times greater for samples collected in July 1991 

than for samples from the winter of 1992.

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A UNNAMED DRAINAGE (SITE RD1)

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D

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A D D

ODD

D

A

o o o

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JULY AUG SEPT OCT NOV DEC | JAN FEB MAR APR MAY JUNE JULY AUG 

1991 1992

Figure 27. Selenium concentrations in samples from Dry Creek at site DRY1, Cedar Creek at site CD1, Loutsenhizer 

Arroyo at sites LZAM and LZA1, and an unnamed drainage at site RD1, July 1991-August 1992.

60 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Uncompahgre River

Samples were collected approximately monthly 

from May 1991 to August 1992 at the outflow site for 

the Uncompahgre River at Delta (site UC16 in fig. 26), 

and all selenium data collected at site UC16 since 

1987 are shown in figure 28. The median selenium 

concentration in the Uncompahgre River at site UC16 

was 15 |ig/L. Concentrations generally were higher 

during the nonirrigation season (November through 

March) than during the irrigation season. Selenium 

concentrations throughout the Uncompahgre Project 

would have been diluted during the irrigation season 

because of the import of about 1,000 ft3/s of water 

from the Gunnison River that had less than 1 |ig/L 

of selenium. Except for two samples, selenium 

concentrations in water samples collected at 

site UC16 exceeded the chronic criterion for aquatic 

life (5 |J.g/L), and seven of the concentrations 

exceeded the acute criterion of 20 |4.g/L (fig. 28). 

By comparison, the median selenium concentration 

for 14 samples collected during 1987-92 from 

the Uncompahgre River at Colona (site UC1 in 

fig. 25), which is upstream from irrigated areas of 

the Uncompahgre Project, was 1 |ig/L, and the 

maximum concentration was 2 |J,g/L.

Results of a gain-loss study of the Uncompahgre 

River on February 19-20,1992, indicated that 

selenium concentrations increased noticeably in only 

3 of the 15 sampled reaches between sites UC1 and 

UC16 (fig. 29). Stream discharge was measured and 

water samples were collected at 16 sites on the 

Uncompahgre River (fig. 29). Relative locations are 

shown in figures 25 and 26. Winter-water diversions 

from the Uncompahgre River were measured and 

sampled at site WST (West Canal at Highway 550), 

site UC3 (Montrose and Delta Canal), site UC7 

(Selig Canal), and site UC9 (Ironstone Canal). The 

three reaches of the river that had conspicuous 

increases in selenium concentrations coincided with 

inflow from Dry Cedar Creek (reach UC5-UC6), 

Cedar Creek (reach UC7-UC8), and Loutsenhizer 

Arroyo (reach UC12-UC13) (figs. 25, 26, and 29). 

Inflow from western-side streams either caused no 

change in selenium concentrations or diluted the 

concentrations in the Uncompahgre River. The large 

decrease in concentration between sites UC14 and 

UC15 was caused by Dry Creek, which almost 

doubled the stream discharge of the Uncompahgre 

River. Data collected for the gain-loss study indicated

that the largest selenium concentrations in the 

Uncompahgre River occurred in the reach between 

Loutsenhizer Arroyo and Dry Creek and not at the 

outflow site in Delta (site UC16).

Adjacent Areas in the 

Lower Gunnison River Basin

Selenium data were collected once, in 

July 1991, in selected drainage basins in the lower 

Gunnison River Basin to document selenium concentrations in irrigated areas adjacent to the Uncompahgre 

Project. Samples were collected from five streams 

draining irrigated areas in the lower Gunnison 

River Basin: Smith Fork (site SMF), North Fork 

(site NFK1), Currant Creek (site CRC), Alfalfa Run 

(site APR), and Tongue Creek (site TGC) (fig. 23). 

Since December 1990, the USGS has collected 

periodic selenium data at a gaging station on lower 

Leroux Creek (fig. 23), which drains irrigated areas 

in the lower North Fork Basin. Selenium was not 

detected in water samples from the Smith Fork nor 

in Alfalfa Run (fig. 23). Selenium was detected in 

samples from the North Fork (4 |ig/L), Currant Creek 

(19 Hg/L), and Tongue Creek (6 ^g/L). The range 

of selenium concentrations in nine samples from 

Leroux Creek was 7 to 21 |j,g/L. Although many 

of the samples were collected in summer, the samples 

collected at the latter four sites were at relatively 

low stream discharge. Upstream diversions for 

irrigation depleted water in the lower North Fork, 

Leroux, Currant, and Tongue Creek Basins during 

summer.

Grand Valley

There was a large range of selenium concentrations in irrigated areas of the Grand Valley (table 18). 

The samples summarized in table 18 include all 

samples collected from streams, washes, drainage 

ditches, and ponds located in irrigated areas of the 

Grand Valley, excluding the Colorado River. 

Sampling sites are shown on the schematic diagrams 

in figures 30 and 31. Selenium concentrations in 

samples collected during the nonirrigation season in 

the Grand Valley exceeded the chronic aquatic-life 

criterion of 5 |ig/L (U.S. Environmental Protection 

Agency, 1987) in 73 of 76 samples, and selenium 

concentrations exceeded the acute criterion of 

20 }J.g/L in about 82 percent of the samples (table 18).

SURFACE-WATER INVESTIGATIONS 61
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62 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
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UC1 UC2 UC3 UC4 UC5 UC6 UC7 UC8 UC9 UC10 UC11 UC12 UC13 UC14 UC15 UC16

SITE NUMBER 

Figure 29. Selenium concentrations in the Uncompahgre River, February 19-20,1992.

SURFACE-WATER INVESTIGATIONS 63
Selenium concentrations in about 84 percent of 

the samples collected during the irrigation season 

exceeded the chronic criterion. Surface-water samples 

(excluding ponds) collected in the Grand Valley 

during the nonirrigation season probably are representative of irrigation-drainage discharge. Three water 

samples collected in June 1991 and 1992 from ponds 

RDP and MWP (fig. 30) had selenium concentrations 

equal to the chronic criterion of 5 \lgiL.

Selenium concentrations were much higher in 

irrigation drainage than in the irrigation-supply water 

from the Colorado River or in water at reference sites, 

which are upstream from the irrigated area. The 

median selenium concentration was 1 ^g/L in water 

samples from site COL2 on the Colorado River 

(table 15) upstream from the Grand Valley. Selenium 

concentrations ranged from 1 to 6 ^g/L in five samples 

from reference sites on West Salt Creek (site WSC2), 

East Salt Creek (site ESC3), and Big Salt Wash 

(site BSW3) (site locations in fig. 30).

Table 18. Summary of selenium concentrations in irrigated 

areas in the Grand Valley and the number of samples that 

exceeded aquatic-life criteria of the U.S. Environmental 

Protection Agency

[Concentrations in micrograms per liter; irrigation-season samples 

collected in August through October 1991 and April through August 1992; 

nonirrigation-season samples collected in March 1991 and from 

November 1991 through March 1992; chronic aquatic-life criterion is 

5 micrograms per liter, and the acute criterion is 20 micrograms per liter 

(U.S. Environmental Protection Agency, 1987); <, less than]

Season

Irrigation 

Nonirrigation

Number of 

samples

44 

76

Median

10 

50

Range

<l-58 

1-380

Number of 

samples 

exceeding 

criteria

Chronic Acute

37 7 

73 62

There are large differences in selenium concentrations between adjacent drainage basins in the Grand 

Valley. A comparison of the mean selenium concentrations in samples collected in March 1991 and in 

February and March 1992 for the 20 reconnaissance 

sites is shown in figure 32. Relative locations of the 

20 sites are shown in figures 30 and 31. Some of the 

highest selenium concentrations were at outflow 

sites in some of the larger drainage basins within 

the irrigated area, such as sites SC (Salt Creek), 

site RW1 (Reed Wash), and site LC1 (Leach Creek). 

Larger streams also tended to have the largest 

stream discharges for the 20 reconnaissance sites.

The median concentration of the samples collected 

in March 1991 at the 20 sites was 28.5 ^g/L, but the 

discharge-weighted mean concentration was 77 ^g/L; 

for the samples collected in February and March 1992, 

the median concentration was 36.5 ^g/L, but the 

discharge-weighted mean concentration was 74 u,g/L.

The selenium concentrations in samples 

collected for synoptic surveys in February and 

March 1992 indicate a highly variable distribution of 

selenium concentrations in irrigation drainage in the 

Grand Valley. For the synoptic surveys of the Grand 

Valley in 1992,44 sites were sampled, including the 

20 reconnaissance sites that were sampled in 1991. 

The synoptic-survey data are shown in figures 30 and 

31 and are summarized by geographical areas in 

table 19. The synoptic data are assumed to represent 

selenium concentrations in irrigation drainage in the 

Grand Valley. The synoptic samples were collected 

when natural runoff was nonexistent or minimal, and 

there were no diversions for winter water. On an areal 

basis, the highest median selenium concentrations in 

the Grand Valley were in the Salt Creek and Reed 

Wash Basins, which drain the western part of the 

irrigated area (fig. 30, table 19). The lowest median 

selenium concentrations were for the Redlands and 

Orchard Mesa areas (table 19). The Redlands area and 

Orchard Mesa are on terrace deposits overlying 

Mancos Shale, and there would be less selenium in the 

terrace deposits.

The synoptic-survey data indicated variable 

selenium concentrations within some of the major 

tributary basins. In the Salt Creek Basin, selenium 

concentrations in East Salt Creek (site ESC1) were 

more than twice as high as in West Salt Creek 

(site WSC1) or in Mack Wash (site MW1) (fig. 30). 

Water samples from the outflow site on Reed Wash 

(site RW1) represented mixtures of irrigation drainage 

in the Reed Wash Basin that had variable selenium 

concentrations. The selenium concentration of 

260 iig/L for the East Branch of Reed Wash 

(site RWEB) was much higher than concentrations 

in Peck and Beede Wash (site RWPB) or in an 

unnamed tributary at site RWTR (fig. 30). The 

maximum selenium concentration reported in a 

Grand Valley surface-water sample was 380 ^g/L 

in the sample collected in the nonirrigation season at 

site RW4 on upper Reed Wash. Site RW4 also had the 

largest selenium concentration of irrigation-season 

samples. However, the stream discharge at site RW4 

was less than 1 percent of the stream discharge in 

Reed Wash at site RW1.

64 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
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SAMPLE SITE AND NUMBER

(24) SELENIUM CONCENTRATION, 

IN MICROGRAMS PER LITER

DIRECTION OF FLOW

Figure 30. Schematic diagram showing the western part of the Grand Valley, relative locations of surface-water-sampling site

in samples collected during synoptic surveys, February-March 1992.
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Figure 31. Schematic diagram showing the eastern part of the Grand Valley, relative locations of surface-water-sa

concentrations in samples collected during synoptic surveys, February-March 1992.
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ACUTE AQUATIC-LIFE CRITERION

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Figure 32. Mean selenium concentrations in irrigation-drainage samples collected in March 1991 and in February and 

20 reconnaissance sites in the Grand Valley.
Table 19. Summary of selenium concentrations in samples collected during the synoptic surveys of the Grand Valley, 

February and March 1992

[Concentrations in micrograms per liter; reference sites not included]

Drainage basin 

or area

Salt Creek and Reed Wash Basins

Big Salt and Little Salt Wash Basins 

and miscellaneous ditches

Area west of Grand Junction and 

east of Fruita

Redlands area

Area east of Grand Junction to Clifton area

Orchard Mesa area

Number 

of sites

12

6

12

2

7

5

Sites Included 

(figs. 30 and 31)

WSC1, ESC1, ESC2, SC, MW1, RW1-RW4, RWTR, 

RWPB, RWEB

FR1, BSW1, BSW2, BSEB, LSW1, LSW2

AC1, AC2, HW1, HW2, PRW, PSW1, PSW2, AD, 

LC1, LC2, LCTR, IRD

RED, LKG

IW, GJ1-GJ3, LW, CF1, CF2

OMD, OM1-OM4

Median

115

31.5

50

26.5

44

11

Range

21-380

24-48

23-160

16-37

1-110

5-30

Another area of apparently higher selenium 

concentrations in the Grand Valley is drained by 

upper Leach Creek (site LC2) and by an unnamed 

wash (fig. 30) adjacent to the Leach Creek Basin. 

The unnamed wash becomes the Independent 

Ranchmens Ditch where the wash intersects the 

Grand Valley Canal, and the ditch discharges into 

Leach Creek at site IRD (fig. 30). Much of the eastern 

part of the Leach Creek Basin and the area drained 

by the unnamed wash are residential and commercial 

areas. Samples also were collected at multiple 

sites in several other drainage basins in the western 

part of the Grand Valley, but large differences 

among selenium concentrations were not apparent 

(fig. 30).

Selenium concentrations were variable in the 

sampled washes and ditches between Grand Junction 

and the Clifton area (fig. 31). Selenium concentrations 

in Indian Wash (site IW) were considerably higher 

than the concentrations in three unnamed drainage 

ditches (sites GJ1, GJ2, and GJ3 in fig. 31). Site CF2, 

on an unnamed drainage ditch east of Clifton, had 

the lowest selenium concentrations in surfacewater samples from irrigated areas in the Grand 

Valley; however, selenium concentrations were 

much higher in another unnamed drainage ditch at 

site CF1, which is located about 2 mi southwest of 

site CF2 (fig. 31).

Selenium concentrations were much lower 

in irrigation-season samples compared to the 

nonirrigation-season samples in the Grand Valley; 

these seasonal differences were similar to the seasonal 

differences in selenium concentrations for surface 

water on the eastern side of the Uncompahgre Project. 

Seasonal distribution of selenium concentrations are 

shown for three frequently sampled sites in the Grand 

Valley in figure 33. Large quantities of surface-return 

flow from canals, laterals, ditches, and field runoff are 

discharged into streams and washes during the irrigation season, which dilute selenium concentrations in 

irrigation-drainage water. Historical stream-discharge 

data for discontinued gaging stations on eight streams 

and washes in the Grand Valley (data collected in 

water years 1973-83) indicated that 86 to 95 percent 

of the annual stream discharge occurred during the 

irrigation season (April through October). The 

median selenium concentration for the irrigationseason samples collected from August 27 to 

September 4,1991, at the 20 reconnaissance sites was 

8.5 Jlg/L, compared to 28.5 jlg/L for nonirrigationseason samples collected in March 1991 and 

36.5 Jig/L for nonirrigation-season samples collected 

in February 1992 at the same 20 sites. Despite 

dilution during the irrigation season, selenium concentrations in most surface-water samples collected 

during the irrigation season in the Grand Valley 

exceeded the chronic aquatic-life criterion (table 18). 

Four of the eight samples collected at site RW1 on

68 Detailed Study of Selenium and Other Constituents In Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Reed Wash (fig. 33) during the irrigation season 

exceeded the acute aquatic-life criterion of 20 jig/L 

(U.S. Environmental Protection Agency, 1987).

At least 96 percent of the dissolved selenium 

in samples collected on March 24,1992, from five 

streams in the Grand Valley was selenate (table 20). 

Selenate is the most soluble form of selenium and 

is stable in alkaline and oxygenated conditions 

(Presser and others, 1990). Field measurements 

of pH were greater than 7.5, and dissolved oxygen 

usually was near saturation in all water samples 

from streams, washes, and ditches in the Grand Valley. 

The selenium-speciation results infer that once 

selenium is discharged by ground water (irrigation 

drainage) into the oxygenated environment of the 

streams, selenium remains in solution as selenate

and is transported out of the irrigated areas into the 

Colorado River. The selenate and selenite data for 

the five Grand Valley tributaries were used for riskassessment analysis for endangered fish (described 

later in the report in the "Risk Assessment for 

Endangered Fish from Selenate and Selenite" section).

Magnitude and Variation of 

Selenium loads

The magnitude and variation of selenium loads 

in irrigated areas of the Uncompahgre Project and in 

the Grand Valley are discussed in this section. 

Selenium-load data are used to describe major source 

areas of selenium from the irrigated areas.

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U.S. ENVIRONMENTAL PROTECTION AGENCY 

ACUTE AQUATIC-LIFE CRITERION

O

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A 

O

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MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC | JAN FEB MAR APR MAY JUNE JULY AUG

1991 1992

Figure 33. Selenium concentrations in samples from Salt Creek at site SC, Reed Wash at site RW1, and Leach Creek at 

site LC1, March 1991-August 1992.

SURFACE-WATER INVESTIGATIONS 69
Table 20. Concentrations of dissolved selenium, selenate, and selenite in water samples collected from five streams in 

the Grand Valley, March 24,1992

[Concentrations in micrograms per liter; data from Roger Fujii (U.S. Geological Survey, written commun., 1992); sites shown in figs. 30 and 31]

Site 

number

SC

RW1

AC1

LC1

IW

Stream 

name

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

Dissolved 

selenium

72.8

126.2

74.8

85.6

86.1

Selenate 

selenium

71.7

126.2

72.5

83.4

82.8

Selenite 

selenium

1.1

0

2.3

2.2

3.3

Percent 

selenate

98.5

100.0

96.9

97.4

96.2

Uncompahgre Project Area

The gain-loss study of the Uncompahgre River 

on February 19-20,1992, indicated that the selenium 

load increased 13.4 Ib/d between Colona (site UC1) 

and Delta (site UC16), which was 96 percent of the 

load at Delta (fig. 34). Relative locations of the 

sampling sites on the Uncompahgre River and on 

tributaries are shown in figures 25 and 26. The actual 

gain in selenium load probably was greater than 

13.4 Ib/d because some of the selenium load in winter 

water that was diverted from the Uncompahgre River 

was not measured at site UC16. Winter water is lost 

by consumptive use, canal and lateral seepage, and 

part of the unused winter water was discharged into 

the Gunnison River between Cummings Gulch and 

Roubideau Creek (fig. 26). Winter-water diversions 

were made upstream from site UC2 (West Canal) 

and immediately downstream from site UC3 

(Montrose and Delta Canal), site UC7 (Selig Canal), 

and site UC9 (Ironstone Canal).

Much of the increase in selenium load in the 

Uncompahgre River between Colona and Delta 

occurred in 4 of the 15 sampled reaches. Much 

of the gain in selenium load in reach UC5-UC6 

(fig. 34) is from Dry Cedar Creek (site DCC1 in 

fig. 25), which accounted for about 50 percent 

of the load at UC6. Cedar Creek (site GDI) 

accounted for about 93 percent of the increase in 

selenium load between sites UC7 and UC8 and for 

about 74 percent of the selenium load at site UC8. 

The largest increase in selenium load occurred in the 

reach between sites UC12 and UC13. Loutsenhizer 

Arroyo (site LZAM in fig. 26) discharges into the

Uncompahgre River between sites UC12 and UC13, 

and the load at site LZAM was 1.7 Ib/d greater 

than the difference in loads between sites UC13 

and UC12. That difference could result from errors 

in discharge measurements or in the selenium 

analyses. Loutsenhizer Arroyo accounted for about 

90 percent of the gain in dissolved-solids load in 

reach UC12-UC13; a reasonable assumption is 

that at least 90 percent of the gain in selenium 

load in reach UC12-UC13 was from Loutsenhizer 

Arroyo. The selenium load from Dry Creek 

(site DRY1) accounts for much of the gain in 

reach UC14-UC15. Some of the selenium load in 

Dry Creek in February 1992 was from return flow of 

winter water that was discharged from the Ironstone 

Canal and its laterals.

Selenium loading into other reaches of the 

Uncompahgre River were relatively small. Stream 

reaches that had a decrease in selenium load (fig. 34) 

either lost water to alluvium or had winter-water 

diversions. Based on mass-balance calculations 

for reaches where selenium loads increased, 

unaccounted inflow of water containing higher 

selenium concentrations might be discharging into 

reaches UC8-UC9, UC10-UC11, and UC13-UC14. 

The selenium concentrations computed by massbalance differences ranged from 23 to 52 (Xg/L in 

unaccounted inflows for these three reaches. 

Those concentrations were higher than selenium 

concentrations in winter water, which infers that 

part of the gain of selenium load in the three reaches 

was from ground-water discharge into the 

Uncompahgre River and not from return flow of 

winter water.

70 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage In the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
16

14

Q

DC

111

Q_

CO 10

Q

Z

O

Q.

Q 

<

O

111

111 

CO

2 -

UC1 UC2 UC3 UC4 UC5 UC6 UC7 UC8 UC9 

SITE

UC10 UC11 UC12 UC13 UC14 UC15 UC16

Figure 34. Selenium loads in the Uncompahgre River, February 19-20,1992.

The mean selenium load in the Uncompahgre 

River at site UC16 for 20 samples collected in 

water years 1991-92 was 18.7 Ib/d. A seasonally 

weighted mean was computed by weighting the 

mean selenium load for irrigation-season samples 

(April through October) and the mean load for the 

nonirrigation-season samples (November through 

March) according to the fraction of the year that 

each season composes (0.59 for irrigation, 0.41 

for nonirrigation). The seasonally weighted mean 

selenium load was equal to the mean selenium 

load for the Uncompahgre River at site UC16 

(water years 1991-92), which indicates that there 

were no large differences in selenium loads between 

the irrigation season and nonirrigation season in the 

Uncompahgre River.

The largest selenium loads in tributary 

streams are on the eastern side of the Uncompahgre 

Project, and the largest seasonally weighted 

mean selenium load was 14.0 Ib/d for Loutsenhizer 

Arroyo at site LZA1. The seasonally weighted 

mean selenium loads for all samples collected at 

outflow sites on 12 tributary streams or canals 

draining the Uncompahgre Project are shown in 

figure 35. Because of canal diversions on the 

Uncompahgre River, part of the selenium load 

discharging from the Dry Cedar and Cedar 

Creek Basins on the eastern side of the project 

is rediverted into canals near or downstream 

from Montrose (figs. 25 and 26). The rediversion 

of considerable quantities of surface-return flow and 

irrigation drainage from the Uncompahgre River for

SURFACE-WATER INVESTIGATIONS 71
irrigation-supply water complicates the determination 

of the total-selenium load from the Uncompahgre 

Project. Perhaps a more accurate measure of selenium 

load from the project would be to determine selenium 

loads in all streams, canals, and ditches that discharge 

into the southern side of the Gunnison River between 

Peach Valley Arroyo and Roubideau Creek.

Water from Loutsenhizer Arroyo is used to 

supplement the water supply for the Garnet Canal; 

therefore, some of the selenium load from the arroyo 

is redistributed by the Garnet Canal system into 

areas east of Delta. The diversion structure on the 

arroyo is between site LZA1 and the Uncompahgre 

River. The fraction of the selenium load in the Garnet 

Canal that came from Loutsenhizer Arroyo was not 

determined. A small fraction of the annual flow of 

the Garnet Canal is diverted into Sweitzer Lake; 

therefore, a small fraction of the selenium load from

Loutsenhizer Arroyo might eventually be incorporated 

into Sweitzer Lake bottom sediment through chemical 

and biological processes. Tail water from the Garnet 

Canal discharges into the Relief Canal at site GAR4 

(fig. 26), which discharges into the Bonafide Ditch. 

The Bonafide Ditch discharges into the Gunnison 

River immediately upstream from site GUN4 (figs. 23 

and 26).

Much of the selenium load from the western 

side of the Uncompahgre Project apparently 

discharges from the northern part of the area, primarily 

from Dry Creek, Cummings Gulch, Seep Creek, and 

Roubideau Creek (fig. 26). Mass-balance calculations 

and synoptic-survey data indicate an unknown fraction 

of the selenium loads in those streams during the 

nonirrigation season of 1991-92 probably came from 

winter water transported by the Ironstone Canal and 

its laterals into these basins. It also is likely that

16

14

O 

DC 

LLJ 

Q.

W 

O"Z.

O Q.

9

3

LLJ 

UJ

10

4 -

2 -

WEST OF UNCOMPAHGRE RIVER

6 NUMBER OF SAMPLES

EAST OF UNCOMPAHGRE RIVER

13

MFC HCC1 SP1 DRY1 CMG1 DCC1 CD1 LZA1 PVA1 RD1 BFD

Figure 35. Seasonally weighted mean selenium loads for 12 sites in the Uncompahgre Project, 1991-92.

72 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
irrigation-supply water from the Ironstone Canal is 

a source of selenium loading to the western side of the 

project during the irrigation season.

Selenium loads within major drainage basins 

on the eastern side of the Uncompahgre Project were 

variable based on the synoptic-survey data collected 

in 1992 and on data collected in water year 1993. 

For four sets of samples, about 35 to 42 percent of 

the selenium load discharging from Cedar Creek 

(at site GDI) was from Montrose Arroyo (fig. 25), 

although Montrose Arroyo accounted for less than 

20 percent of the stream discharge in Cedar Creek. 

The unnamed west tributary of Loutsenhizer 

Arroyo (at site WLZ1 in fig. 26) accounted for about 

40 percent of the stream discharge, but only about 

20 percent of the selenium load in Loutsenhizer 

Arroyo at site LZA1 in March 1992. About 90 percent 

of the selenium load in the unnamed drainage near 

Read at site RD1 in March 1992 was discharged into 

the reach between sites RD2 and RD3 (fig. 26).

Selenium data collected during 1991-93 in 

the Cedar Creek and Loutsenhizer Arroyo Basins 

(figs. 25 and 26) indicate that some of the selenium 

load measured at outflow sites GDI and LZA1 

originated outside the irrigated area of the 

Uncompahgre Project. There are some areas that 

are irrigated with non-project water in both drainage 

basins that are upstream from the Uncompahgre 

Project. There are extensive areas of Mancos Shale 

in the upper Cedar Creek and Loutsenhizer Arroyo 

Basins. Irrigation drainage from the nonproject areas 

might contribute a small selenium load to Cedar Creek 

and Loutsenhizer Arroyo throughout the year. Natural 

runoff in the Cedar Creek and Loutsenhizer Arroyo 

Basins also might contribute to the selenium load in 

these streams. Natural runoff in Loutsenhizer Arroyo 

probably is limited to brief periods of snowmelt 

runoff in spring and runoff associated with heavy 

rainstorms. In addition to natural runoff associated 

with snowmelt and rain, there seems to be a small 

natural base-flow discharge into upper Cedar Creek. 

Three sets of samples were collected in March 1992 

and March 1993 during periods of snowmelt runoff 

from Mancos Shale areas in the upper Cedar Creek 

and Loutsenhizer Arroyo Basins. For those samples, 

about 24 to 45 percent of the selenium load in Cedar 

Creek and about 13 to 36 percent of the selenium 

load in Loutsenhizer Arroyo were accounted for at

the most upstream sampling site. Apparently, some 

of the selenium load measured at outflow sites on both 

streams was caused by snowmelt in the drainage 

basins upstream from the Uncompahgre Project. 

The selenium load resulting from snowmelt on 

Mancos Shale would be highly variable from year 

to year, depending on weather conditions. There 

was an accumulation of snow at low elevations in the 

Cedar Creek and Loutsenhizer Arroyo Basins in the 

winter of 1992. In some winters, snow accumulation 

at lower elevations in the Uncompahgre Project area is 

minimal, and the resulting snowmelt runoff from the 

Mancos Shale areas would be minimal.

The fraction of the selenium load in Cedar 

Creek and Loutsenhizer Arroyo from nonproject 

sources might be less in the irrigation season than 

in the noninigation season. Nonproject sources of 

selenium accounted for less than 5 percent of the 

selenium load in Loutsenhizer Arroyo and about 10 to 

15 percent of the selenium load in Cedar Creek based 

on samples collected during the irrigation seasons in 

1991 and 1993. During periods when the irrigationseason samples were collected, irrigation drainage 

from nonproject land probably was the sole source 

of selenium load to upper Loutsenhizer Arroyo, and 

non-project irrigation drainage and perhaps a small 

quantity of natural ground-water discharge were 

potential selenium sources to upper Cedar Creek.

The synoptic-survey results for March 1992 

indicated that the Garnet Canal gained 1.1 Ib/d of 

selenium between sites GAR1 and GAR2 (fig. 26). 

Most of the gain in selenium probably was from the 

drainage ditches at sites DR1 and DR2 (fig. 26) and 

from diffuse seepage of irrigation drainage into the 

canal. Essentially all the increase in selenium load in 

the Garnet Canal downstream from Sweitzer Lake 

(between sites GAR2 and GAR4) was from irrigationdrainage discharge from an unnamed natural drainage 

downstream from site UNGR (fig. 26). The selenium 

load discharged from the Garnet Canal (site GAR4 in 

fig. 26) accounted for almost all the selenium load in 

the Bonafide Ditch at site BFD in the samples 

collected on March 12, 1992.

Analysis of selenium loads for the more 

frequently sampled surface-water sites (the sites 

shown in fig. 35) indicated variable seasonal differences in mean selenium loads between irrigation 

and nonirrigation seasons. The mean selenium

SURFACE-WATER INVESTIGATIONS 73
load for seven irrigation-season samples for Cedar 

Creek at site CD1 was 7.3 Ib/d, compared to the 

mean load of 4.1 Ib/d for three nonirrigation-season 

samples. The mean selenium load of 14.7 Ib/d for 

eight irrigation-season samples from Loutsenhizer 

Arroyo at site LZA1 is only slightly greater than the 

mean load of 13.0 Ib/d for five nonirrigation-season 

samples. The seasonal differences between selenium 

loads for an unnamed drainage near Read at site RD1 

and Dry Creek at site DRY1 were not large.

In July 1991, water samples were collected from 

the Gunnison River at sites GUN3 and GUN4 and 

from four tributaries draining irrigated areas adjacent 

to or upstream from the Uncompahgre Project 

(fig. 23). The selenium load in the Gunnison River at 

site GUN3, which is upstream from all irrigation 

drainage from the Uncompahgre Project, was 5.6 Ib/d. 

About 50 percent of the load at site GUN3 was 

accounted for by the North Fork of the Gunnison 

River (site NFK1 in fig. 23). The sum of the selenium 

loads from the four other tributaries that drain 

nonproject irrigated areas (fig. 23) the Smith Fork 

(site SMF), Currant Creek (site CRC), Alfalfa Run 

(site APR), and Tongue Creek (site TGC) was only 

0.2 Ib/d in July 1991. When compared to selenium 

loading from streams draining the irrigated areas of 

the Uncompahgre Project, selenium loading from 

other drainage basins in the lower Gunnison River 

Basin were small. However, the basins outside the 

project were sampled only once, and the selenium 

loads might not be representative of the selenium 

loads throughout the year.

Grand Valley

The largest mean selenium loads in tributary 

streams draining the Grand Valley were for Salt Creek 

(site SC), Reed Wash (site RW1), Big Salt Wash 

(site BSW1), and Leach Creek (site LC1). Mean 

selenium loads for the 20 reconnaissance sites in the 

Grand Valley are compared in figure 36. The loads 

shown in figure 36 are arithmetic means of the 

selenium loads for the three sets of samples collected 

in March 1991, in late summer of 1991, and in 

February or March 1992. The greatest mean selenium 

load shown in figure 36 is 11.2 Ib/d for site RW1; 

however, the seasonally weighted mean selenium load 

for site RW1, based on the 15 samples collected 

during 1991-92, was 12.8 Ib/d. The selenium loads 

for the 20 sites might represent much of the selenium

load discharging from tributary streams into the 

Colorado River. However, the selenium loads from 

numerous, small washes and drainage ditches were not 

measured. There probably is discharge of irrigation 

drainage directly into the Colorado River in ground 

water that is not intercepted by tributary streams, 

washes, and drainage ditches. Ground-water studies 

by BOR (1986a) indicate that the cobble aquifer 

discharges directly into the Colorado River in the 

Fruita area. The cobble aquifer probably is of 

sufficient depth in many areas in the Grand Valley 

so that surface streams and drainage ditches might 

not intercept appreciable quantities of water from 

the cobble aquifer

Most of the irrigation-induced selenium load 

from the Grand Valley is from irrigated areas west 

of the Gunnison River and north of the Colorado 

River. Ninety-three percent of the sum (33.3 Ib/d) 

of the mean selenium loads for the 20 sites shown in 

figure 36 was accounted for in the drainage basins in 

the western part of the Grand Valley between Salt 

Creek and Leach Creek (fig. 3). About 70 percent 

of the irrigated land in the Grand Valley is west of 

Grand Junction and north of the Colorado River 

(Bureau of Reclamation, 1986b). Most of the irrigated 

land in the western part of the Grand Valley is on soils 

derived from alluvium on Mancos Shale or shale 

residuum.

Selenium loads for sites on the Redlands 

area, the area east of the Gunnison River and north 

of the Colorado River, and on Orchard Mesa generally 

were smaller than loads in the streams and washes 

west of Grand Junction (figs. 30, 31, and 36). Except 

for Indian Wash and an unnamed drainage near Clifton 

at site CF1, selenium loads in washes and ditches 

in those areas generally ranged from 0.01 to 0.20 Ib/d 

(fig. 36). One reason for smaller selenium loads in 

these areas is that the washes and drainage ditches 

have small drainage-basin areas, and subsequently 

the stream discharges are smaller compared to 

stream discharges in larger basins. The Redlands 

and Orchard Mesa areas primarily are on terrace 

deposits. Soils derived from terrace deposits 

should contain less material derived from the 

Mancos Shale, and consequently, irrigation drainage 

from terrace deposits generally had lower selenium 

concentrations compared to other areas in the Grand 

Valley (table 19). The Redlands area accounts for 

about 6 percent, the area east of Grand Junction to

74 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
I I I I I I I I I I I I \ I

EAST OF GUNNISON RIVER AND 

NORTH OF COLORADO RIVER

WEST OF GUNNISON RIVER AND 

NORTH OF COLORADO RIVER

ORCHARD MESA 

(EAST OF GUNNISON

RIVER, SOUTH OF 

COLORADO RIVER)

SC RW1 FR1 BSW1 LSW1 AC1 HW1 PRW PSW1 AD LC1 RED IW GJ1 LW CF1 CF2 OMD OM2 OM3

SITE

Figure 36. Mean selenium loads of three sets of samples collected in 1991-92 for 20 sites in the Grand Valley.

the east end of the valley (north of the Colorado 

River) accounts for about 16 percent, and Orchard 

Mesa about 11 percent of the irrigated area in 

the Grand Valley. Large sections of the Redlands, 

the area between Grand Junction and Clifton, and 

western Orchard Mesa are residential and commercial 

areas.

The synoptic data collected during the 

nonirrigation season in early 1992 were used to 

examine selenium-loading differences within the 

major streams that drain the Grand Valley. In 

Reed Wash in March 1992, about one-half the 

selenium load at the outflow site (RW1 in fig. 30) 

was from the East Branch (site RWEB). Surveys 

on Big Salt Wash, Little Salt Wash, Adobe Creek, 

Hunter Wash, and Persigo Wash (fig. 30) indicated 

that 67 to 81 percent of the selenium load at the 

outflow sites was gained in the lower reaches of 

the streams. Much of the increase in selenium loads

in those five streams was caused by increases in 

stream discharge rather than by downstream increases 

in selenium concentrations.

Selenium loads in some streams and drains in 

the Grand Valley were higher in the irrigation season 

(April through October) than in the nonirrigation 

season (November through March). For the 

20 reconnaissance sites (fig. 36), the sum of selenium 

loads for the samples collected March 20-22, 1991, 

was 18.0 Ib/d, compared to 60.0 Ib/d for samples 

collected August 27-September 4,1991, at the 

same sites. Four sites were sampled at least five 

times, and the selenium loads for each sample are 

shown in figure 37. For Reed Wash at site RW1, 

the mean selenium load for irrigation-season 

samples (16.6 Ib/d) is more than twice the mean 

load of nonirrigation-season samples (7.5 Ib/d). 

Greater irrigation-season selenium loads also were 

indicated for Salt Creek (site SC) and Big Salt Wash

SURFACE-WATER INVESTIGATIONS 75
(site BSW1) (fig. 37). By contrast, the mean selenium 

load for irrigation-season samples for Leach Creek 

at site LC1 was about 25 percent less than the 

mean load for the nonirrigation-season samples. 

During the nonirrigation season, irrigation drainage 

from the Independent Ranchmens Ditch is discharged 

into Leach Creek at site IRD. During the irrigation 

season, most of the water in the Independent 

Ranchmens Ditch flows in a pipe over Leach Creek 

to irrigate fields west of Leach Creek. Therefore, most 

of the selenium load in the Independent Ranchmens 

Ditch was not discharged into Leach Creek during the 

irrigation season.

Although the discharges in Grand Valley 

streams and washes are much higher in the irrigation 

season than in the nonirrigation season, the increase in

discharge does not account for all of the increase in 

selenium loads during the irrigation season. The range 

of stream-discharge measurements for the 15 water 

samples collected from Reed Wash at site RW1 was 

7.0 to 168 ft3/s, and the range for 8 samples from Salt 

Creek at site SC was 7.1 to 203 ft3/s. In the Grand 

Valley, a large quantity of unused irrigation water is 

discharged from canals and laterals into the washes 

during the irrigation season. However, selenium 

concentrations in the irrigation-supply water are too 

small to be the sole cause of higher selenium loads 

in streams and washes during the irrigation season. 

In drainage basins, such as Reed Wash, either groundwater (irrigation-drainage) discharge had to increase 

or selenium concentrations in ground water had to 

increase to account for the higher selenium loads

24

22

20

Q 

CC

ff 16

CO 

Q 

Z ,4

o

Q_ 

2 12

Q" 

O 10

8 

Z 

LU

LLJ - 

CO b

4 -

2 -

O SALT CREEK (SITE SC) +

-(- REED WASH (SITE RW1)

A BIG SALT WASH (SITE BSW1)

D LEACH CREEK (SITE LC1)

O

D

D

A

D

O

D D

+ 

O

A

D

O

O

O

D

O

A

D

D D D

MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC | JAN FEB MAR APR MAY JUNE JULY AUG

1991 1992

Figure 37. Selenium loads in Salt Creek at site SC, Reed Wash at site RW1, Big Salt Wash at site BSW1, and Leach Creek 

at site LC1, March 1991-August 1992.

76 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
at site RW1 during the irrigation season. Water levels 

in shallow wells in the Grand Valley are higher during 

the irrigation season than during the nonirrigation 

season, which indicates the potential for increased 

ground-water discharge into tributary streams, washes, 

and drainage ditches that dissect the valley. Higher 

selenium concentrations could occur during the irrigation season because of more oxidizing conditions in 

ground water or by the flushing of soluble salts that 

contain selenium. Water samples collected from 

Reed Wash during this study were not affected by 

surface runoff from precipitation or snowmelt; 

therefore, seasonal differences in selenium loads 

were not caused by natural runoff.

There is a statistically significant (significance 

level less than 0.001) relation of seasonally weighted 

mean selenium load to drainage-basin area within 

the irrigated area for the 20 reconnaissance sites 

in the Grand Valley (fig. 38). The coefficient of 

determination of the relation shown in figure 38 is 

0.74. The four streams with the largest selenium 

loads in the Grand Valley also have the largest 

drainage areas within the irrigated area of the 

Grand Valley. That relation is not surprising 

because the larger drainage basins would tend to 

have more miles of canals, laterals, and ditches 

and more irrigated acreage than smaller basins; 

therefore, the large basins would have greater 

recharge to shallow ground water from distributionsystem losses and deep percolation. The selenium 

yield, in pounds per day per 100 acres of drainage 

area, was determined for the 20 sites in figure 38 

by dividing the seasonally weighted mean selenium 

load of all the samples collected at a site by the 

drainage-basin area within the irrigated boundaries 

(bounded by the Government Highline Canal) 

and multiplying by 100. The largest selenium yields 

were 0.115 (lb/d)/100 acres for the drain near Clifton 

at site CF1 and 0.088 (lb/d)/100 acres for Reed Wash 

at site RW1. Most basins had selenium yields between 

0.020 and 0.060 (lb/d)/100 acres. The selenium yields 

were less than 0.01 (lb/d)/100 acres for Appleton 

Drain (site AD in fig. 30), Lewis Wash (site LW 

in fig. 31), an unnamed drainage east of Clifton at 

site CF2, and Orchard Mesa Drain (site OMD).

General surficial geology does not seem 

to explain differences in selenium loads in the 

Grand Valley. For example, the selenium load 

for Reed Wash at site RW1 is about twice the 

selenium load for the Salt Creek Basin at site SC 

(fig. 38), although the drainage-basin areas 

within the irrigated area are about equal for both 

basins. General geologic differences between 

the basins are not readily apparent because most 

of the irrigated soil in both basins is derived either 

from alluvium on Mancos Shale or Mancos Shale 

residuum (fig. 5A). However, based solely on 

field observations, a higher percentage of the 

Reed Wash Basin is irrigated compared to the 

Salt Creek Basin, which could explain some of 

the difference in selenium loads between the basins. 

Land-use factors and small-scale geologic differences might explain some of the differences in 

selenium loads among tributary streams in the 

Grand Valley.

Dissolved Solids and Major Constituents 

and Relations to Selenium

The irrigated areas in the Uncompahgre 

Project and in the Grand Valley have been 

identified in salinity studies by the BOR (1978, 

1982) as major sources of salt (dissolved 

solids) to the upper Colorado River. These 

studies indicated that the Uncompahgre Project 

contributes about 360,000 tons/yr and the Grand 

Valley about 580,000 tons/yr of salt to the 

Colorado River. Because of previous, ongoing, 

and proposed salinity-control projects in the 

Uncompahgre Project and in the Grand Valley, 

there is an interest in determining if there are 

relations of selenium to dissolved solids and if 

projects designed to decrease dissolved-solids 

loading could affect selenium loading. Relations 

of selenium concentrations and loads to dissolvedsolids concentrations and loads were analyzed 

to determine if source areas of selenium also are 

source areas of dissolved solids. The major ions 

that compose dissolved solids and their relations 

to selenium also are discussed.

SURFACE-WATER INVESTIGATIONS 77
GEOGRAPHICAL AREA IN 

THE GRAND VALLEY

WEST VALLEY 

REDLANDS 

EAST VALLEY 

ORCHARD MESA 

RW1 SITE CODE

REGRESSION LINE (COEFFICIENT OF DETERMINATION 0.74)

0.01 * 

0.005

100 200 500 1,000 2,000 5,000 10,000 20,000

DRAINAGE-BASIN AREA WITHIN THE IRRIGATED AREA, IN ACRES

Figure 38. Relation of seasonally weighted mean selenium load to drainage-basin area within the irrigated area for 

20 basins in the Grand Valley.

78 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and In the Grand Valley, West-Central Colorado, 1991-93
Dissolved Solids

Irrigation water from the Gunnison River 

(through the Gunnison Tunnel) into the Uncompahgre 

Project had dissolved-solids concentrations less 

than 150 mg/L and was a calcium bicarbonate 

water. Dissolved-solids concentrations in the 

Uncompahgre River at Colona during 1991-92 

ranged from about 250 to 450 mg/L and was a calcium 

sulfate bicarbonate water. Both rivers are regulated 

by reservoirs, and dissolved-solids concentrations in 

the Gunnison and Uncompahgre Rivers upstream from 

the Uncompahgre Project had less seasonal variation 

than is typical of streams in western Colorado. There 

is considerable variability in the quality of irrigation 

water delivered within the Uncompahgre Project. 

Water into the South and West Canals (fig. 25) is 

directly from the Gunnison Tunnel, and this irrigation 

water had low dissolved-solids and major-ion concentrations. Water diverted from the Uncompahgre River 

into the six canals downstream from the South Canal 

(figs. 25 and 26) has increasing dissolved-solids and 

major-ion concentrations in the downstream direction 

compared to the original irrigation water because of 

the effects of surface-return flow, irrigation drainage, 

and consumptive losses.

The median dissolved-solids concentration for 

nonirrigation-season samples from the western side 

of the Uncompahgre Project was 1,160 mg/L, and 

concentrations ranged from 648 to 4,840 mg/L. 

The median dissolved-solids concentration for 

nonirrigation-season samples from the eastern side 

was 4,110 mg/L, and concentrations ranged from 

1,470 to 6,380 mg/L. Water composition generally 

was a mixed-cation sulfate water. Sulfate composed 

more than 70 percent of the anion concentration in 

most nonirrigation-season samples, and sulfate was 

greater than 80 percent of the anion composition in 

most nonirrigation-season samples from the eastern 

side.

Water samples collected from the Colorado 

River near Cameo at site COL2 (fig. 31) are representative of water quality of the irrigation-supply water 

for all irrigated areas in the Grand Valley, except 

the Redlands area, which is irrigated by water from 

the Gunnison River. During 1991-92, dissolvedsolids concentrations in discrete samples collected 

at site COL2 ranged from 196 to 654 mg/L during 

April through October. Water composition at

site COL2 varied during the irrigation season and 

had higher ion percentages of calcium bicarbonate 

during snowmelt runoff and higher percentages 

of sodium chloride during low flow. Water samples 

from the Gunnison River at site GUN6, about 

7 mi upstream from the irrigation diversion for the 

Redlands area, had slightly higher dissolved-solids 

concentrations than samples from site COL2 and 

was a calcium sulfate water.

The median dissolved-solids concentration was 

3,600 mg/L for nonirrigation-season samples in the 

Grand Valley, and concentrations ranged from 1,760 to 

6,170 mg/L. There were no winter-water diversions in 

the Grand Valley when nonirrigation-season samples 

were collected; therefore, nonirrigation-season 

samples from streams probably consisted of shallow 

ground-water discharge from irrigated areas. The 

nonirrigation-season samples were a mixed-cation 

sulfate water, and sulfate accounted for more than 

70 percent of the anion equivalent concentration. 

The nonirrigation-season samples often had higher 

percentages of magnesium and sulfate and lower 

percentages of sodium, chloride, and bicarbonate 

compared to irrigation water from the Colorado 

River.

The Pearson correlation coefficients of selenium 

concentration to specific conductance, dissolvedsolids, major-ion, boron, and uranium concentrations 

in surface-water samples collected within the irrigated 

areas, separated by irrigation and nonirrigation 

seasons, are listed in table 21. The Uncompahgre 

Project data were separated into the western and 

the eastern sides of the Uncompahgre River, and 

the Grand Valley data were separated into areas north 

and south of the Colorado River. Irrigation on the 

western side of the Uncompahgre Project and in the 

Grand Valley south of the Colorado River primarily is 

on terrace deposits; irrigation on the eastern side of the 

Uncompahgre Project and in the Grand Valley north of 

the Colorado River primarily is on Mancos Shale or on 

alluvium containing substantial amounts of shale.

Many of the correlation coefficients of selenium 

to the variables listed in table 21 were less than 

0.50 for samples collected in Mancos Shale areas 

(eastern side of Uncompahgre Project; north of 

Colorado River). In some instances, relatively large 

sample sizes (more than 30 samples) probably 

improved the chance that the correlations of selenium

SURFACE-WATER INVESTIGATIONS 79
with other water-quality constituents would be statistically significant. Most of the correlations for the 

irrigation-season samples from the eastern side of 

the Uncompahgre Project and for the nonirrigationseason samples from the Grand Valley north of the 

Colorado River were not statistically significant 

(at a significance level of 0.05). Specific conductance 

usually is highly correlated with dissolved-solids 

concentrations, and because it can be measured in 

the field, specific conductance can be used to monitor 

dissolved-solids concentrations. Selenium concentrations seemed to have greater variability than specific 

conductance in irrigated areas on Mancos Shale, 

which results in the relatively low correlations of 

selenium and specific conductance (table 21). For 

that reason, specific conductance probably is not 

an accurate indicator of selenium concentration 

for irrigated areas on Mancos Shale for broad 

geographical areas.

If data for specific sites are used, relations 

of selenium to dissolved-solids concentrations and 

specific conductance improved markedly because 

the variance in selenium concentrations between 

sites or drainage basins was removed. Some of 

the variance between drainage basins could be

caused by differences in geology and land use. 

Relations of selenium concentration to specific 

conductance for samples collected during 1991-92 

from Loutsenhizer Arroyo (sites LZA1 and LZAM) 

and Reed Wash (site RW1) are shown in figure 39. 

Specific conductance was used instead of dissolvedsolids concentrations because there were more 

data for specific conductance, but regressions 

using dissolved-solids concentrations would have 

essentially the same relations. The coefficients 

of determination were 0.97 for the Loutsenhizer 

Arroyo relation and 0.94 for the Reed Wash relation. 

Coefficients of determination were greater than 0.89 

for relations of selenium to specific conductance for 

Cedar Creek (site CD1), Salt Creek (site SC), and 

Leach Creek (site LC1). For Dry Creek (site DRY1), 

which does not drain Mancos Shale lands to a great 

extent, the coefficient of determination was only 0.65.

The salinity-control projects, which are 

designed to decrease dissolved-solids loads, could 

have an effect on selenium loads in the irrigated areas 

of the Uncompahgre Project area and the Grand 

Valley. The salinity-control projects are designed to 

decrease distribution-system losses and deep percolation of applied water, which would result in less 

recharge to shallow aquifers in the Mancos Shale.

Table 21 . Pearson correlation coefficients of selenium concentration to specific conductance and to dissolved-solids, 

major-ion, boron, and uranium concentrations for the Uncompahgre Project and the Grand Valley

[Areas of the Uncompahgre Project are west or east of the Uncompahgre River; areas of the Grand Valley are north or south of the Colorado River; 

irr, irrigation season; non, nonirrigation season; *, correlation not significant at 0.05 probability level; , correlation not reported for sample sizes less 

than 6; samples from mainstem rivers and reference sites were not included]

Area

West

West

East

East

Season

Irr

Non

Irr

Non

Specific 

conductance

0.77

.84

.20*

.50

Dissolved 

solids

0.78

.85

.19*

.45

Calcium

0.75

.69

.49

.47

Ma9- Sodne- . ium sium

Sulfate

UNCOMPAHGRE PROJECT

0.69 0.57 0.79

.74 .86

-.12* -.22*

.15* .48

.86

-.11*

.42

Alkalinity

-0.03*

.41

.36*

-.07*

Chloride

-0.11*

.27*

-.04*

.27*

Nitrate 

plus 

nitrite

0.91

.57

.82

.71

Boron

0.23*

.88

-.04*

.43

Uranium

0.42*

--

-.17*

~

GRAND VALLEY

North

North

South

South

Irr

Non

Irr

Non

.71

.13*

_

-.02*

.77

.15*

_

-.16*

.85

.44

_

-.36*

.68 .25*

.17* -.08*

_

.36* .39*

.75

.14*

_

-.24*

.44

-.08*

_

.43*

.17*

-.09*

_

-.51*

.80

.61

_

.93

.50

.08*

_

.46*

.54

.65

_

-

80 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
200

180

160

140

CC 

HI 

Q_

CO

§ 12° 

g

o

z- 100

o

\-

Q 80

O 

O

HI 60

_l 

HI 

CO

40

20

O

LOUTSENHIZER ARROYO REGRESSION LINE 

(COEFFICIENT OF DETERMINATION 0.97)

REED WASH REGRESSION LINE 

(COEFFICIENT OF DETERMINATION 0.94)

LOUTSENHIZER ARROYO (SITES LZA1 AND LZAM) 

REED WASH (SITE RW1)

o -

1,000 2,000 3,000 4,000 5,000

SPECIFIC CONDUCTANCE, IN MICRO-SIEMENS 

PER CENTIMETER AT 25 DEGREES CELSIUS

Figure 39. Relations of selenium concentration to specific conductance for Loutsenhizer Arroyo and 

Reed Wash.

SURFACE-WATER INVESTIGATIONS 81
Less recharge to aquifers in the Mancos Shale would 

decrease the quantity of irrigation-drainage discharge, 

which should decrease dissolved-solids loads. Linearregression relations of selenium loads to dissolvedsolids loads were determined for major tributary 

streams in the Uncompahgre Project and the Grand 

Valley. The relations of selenium and dissolved-solids 

loads among drainage basins are highly variable, and 

coefficients of determination range from about 0.32 

to 0.90. For some streams, additional variables 

apparently are needed to relate selenium load 

to dissolved-solids load. The load relations for the 

two tributary streams that had the largest selenium 

loads in the study area, Loutsenhizer Arroyo and Reed 

Wash, are shown in figure 40. The load relation for 

Loutsenhizer Arroyo (coefficient of determination 

0.36) was much weaker than the relation for Reed 

Wash (coefficient of determination 0.82). Reasons 

why the relation is less well defined for Loutsenhizer 

Arroyo are not known. Loutsenhizer Arroyo and 

Reed Wash drain irrigated areas that are predominately on soils derived from shale, and the major land 

use in both drainage basins is irrigated agriculture. 

Neither basin had been substantially affected by 

urbanization.

The relations of selenium and dissolved-solids 

loads for tributary streams had positive slopes, which 

might indicate that a decrease in dissolved-solids 

load in a particular drainage basin could be accompanied by a decrease in selenium load. Such relations 

could imply that the salinity-control projects, which 

are designed to decrease dissolved-solids loads from 

irrigated areas, also could decrease selenium loads. 

Potential decreases in dissolved-solids loads and 

selenium loads resulting from salinity-control projects 

assume that the projects decrease the quantity of 

ground-water discharge (irrigation drainage) and that 

the concentrations of dissolved solids and selenium in 

ground water are relatively unchanged. Actual effects 

of salinity-control projects on selenium loads have not 

been measured in field studies in the Uncompahgre 

Project or in the Grand Valley.

Gains and losses of dissolved-solids loads in 

the Uncompahgre River in February 1992 had a 

similar pattern as selenium loads. Dry Creek 

discharged the largest dissolved-solids load into the 

river, and Loutsenhizer Arroyo contributed the second 

largest load to the river. Dry Creek had about four 

times greater stream discharge than Loutsenhizer

Arroyo (at site LZAM downstream from the Garnet 

Canal diversion). Part of the dissolved-solids load 

in Dry Creek in February 1992 probably was from 

winter water that was diverted from the Uncompahgre 

River.

For the 20 reconnaissance sites in the Grand 

Valley, streams that had the largest selenium loads 

(fig. 36) also had the largest dissolved-solids loads. 

The largest seasonally weighted mean dissolvedsolids loads for 1991-92 were 318 tons/d for Reed 

Wash at site RW1 (13 samples) and 271 tons/d for 

Salt Creek at site SC (6 samples). Generally, the 

relative differences in dissolved-solids loads among 

the 20 sites were less pronounced than were differences in selenium loads. The dissolved-solids load 

per unit drainage-basin area is the dissolved-solids 

yield. Data for 15 of the basins in the Grand Valley 

shown in figure 36 have dissolved-solids yield data 

listed in a BOR report (1978), and a plot of the relation 

of estimated selenium yield to estimated dissolvedsolids yield for these 15 basins is shown in figure 41. 

The selenium and dissolved-solids yields are based on 

the total drainage-basin area within the irrigated area 

and not on irrigated acreage within the basin. The 

highly significant coefficient of determination of 0.81 

(significance level less than 0.001) for the relation of 

selenium yield to dissolved-solids yield for the 

15 basins shown in figure 41 implies that drainage 

basins in the Grand Valley that have high dissolvedsolids yields have high selenium yields.

Chloride and Evaporative Concentration

By assuming chloride is a conservative ion, 

a relation of selenium and chloride concentrations 

was used to analyze possible effects of evaporative 

concentration on selenium concentrations in the 

Grand Valley. Mass-balance methods were used to 

estimate a chloride budget for the Grand Valley for 

water years 1976-92. The flow-weighted chloride 

concentration of the Colorado River downstream from 

the Grand Valley (site COLS) was 54 mg/L, compared 

to 51 mg/L for the combined inflow of the Colorado 

and Gunnison Rivers into the Grand Valley. The 

small increase in chloride concentration could result 

from consumptive water loss in the Grand Valley by 

evaporative concentration of the irrigation water or 

from computational errors.

82 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
LOUTSENHIZER ARROYO

REGRESSION LINE (COEFFICIENT 

OF DETERMINATION 0.36)

24

22

20

£ 18 

Q.

CO

D

o

Q_

16

Z 14

Q* 

Q 12

LJU

LJU 

CO

10

6 -

150 200

REED WASH

REGRESSION LINE (COEFFICIENT 

OF DETERMINATION 0.82)

250 300 350 400

100 200 300 400 500

DISSOLVED-SOLIDS LOAD, IN TONS PER DAY

600 700

Figure 40.

(site RW1).

Relations of selenium load to dissolved-solids load for Loutsenhizer Arroyo (site LZA1) and Reed Wash

SURFACE-WATER INVESTIGATIONS 83
CO 

LJJ 

CC

0.13 

0.12- 

0.11 -

DATA POINT FOR INDIVIDUAL BASIN

REGRESSION LINE (COEFFICIENT 

OF DETERMINATION 0.81)

^^

O 0.09

£ 0.08- 

CO

2 4 6 8 10 12 14 16 18 20 22 24 

DISSOLVED-SOLIDS YIELD, IN TONS PER ACRE

Figure 41. Relation of estimated selenium yield based on 

sampling during 1991-92 to estimates of dissolved-solids 

yield by the Bureau of Reclamation for 15 basins in the Grand 

Valley.

Selenium and chloride concentrations in irrigation drainage from the Grand Valley are plotted with 

the evaporative-concentration line in figure 42. The 

evaporative-concentration line is constructed by 

projecting mean irrigation-season concentrations 

of 1 ja,g/L for selenium and 107 mg/L for chloride for 

irrigation water from the Colorado River along a 

1:1 line. Because the chloride concentrations 

predicted from the evaporative-concentration line are 

in the same range as the chloride concentrations of the 

samples (fig. 42), the chloride concentrations in irrigation drainage could be caused by simple evaporation. 

The selenium concentrations of the samples are much 

greater than the selenium concentrations predicted 

from the evaporative-concentration line; therefore, 

simple evaporation of irrigation water does not 

account for the selenium concentrations in the Grand 

Valley (fig. 42). Many selenium concentrations were 

about 5 to 100 times greater than the concentrations 

predicted by evaporative concentration, implying that 

the Grand Valley is a source area for selenium.

Other Major Constituents

Much of the gain in dissolved solids in 

irrigation water in the Uncompahgre Project 

and in the Grand Valley was from gain in calcium, 

magnesium, sodium, and sulfate ions. Weathering 

of shale detritus or of evaporite sulfate minerals 

would contribute calcium, magnesium, sodium, 

and sulfate. Dissolution of carbonate minerals 

by irrigation drainage could result in a gain in 

calcium, magnesium, and sodium. Cation 

exchange on clay minerals could substitute 

calcium and magnesium ions for sodium ions, or 

vice versa. Dissolution of gypsum and oxidation 

of sulfides, such as pyrite, would produce sulfate. 

Only a small part of the increase in dissolved solids 

was the result of increases in alkalinity concentrations (almost all the alkalinity was bicarbonate ions 

based on the pH of samples).

500

400

300

200

100

70

CC 

LJJ

13

DC 

LJJ 

Q.

CO

<

O 50

O 40

CC

O 30

2

Z 20

O

g 10

cc ^

UJ 5

8 4

Q 3

LJJ

LJJ 

CO

0.7 

0.5

0 O OO

o 00°

CD8°0o

O IRRIGATION-DRAINAGE 

SAMPLE

 EVAPORATIVECONCENTRATION 

LINE

50 100 150 200 250 300 350 400 450 500

CHLORIDE CONCENTRATION,

IN MILLIGRAMS PER LITER

Figure 42. Selenium and chloride concentrations in irrigationdrainage samples from the Grand Valley, November 1991- 

March 1992, and the estimated evaporative-concentration line.

84 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Nonirrigation-season samples from the 

Uncompahgre Project and the Grand Valley were 

supersaturated with the carbonate minerals calcite 

and dolomite. The computer program WATEQ4F 

(Ball and Nordstrom, 1991) was used to compute 

ion speciation, mineral saturation, and mineralsaturation indices for selected samples collected 

from tributary streams and ditches during February 

and March 1992. Alkalinity concentrations 

were limited because equilibrium solubilities 

controlled the carbonate and bicarbonate 

concentrations. The irrigation water coming into 

the Uncompahgre Project and into the Grand Valley 

was undersaturated with gypsum (calcium sulfate). 

More than 50 percent of the samples from the 

Mancos Shale areas (the eastern side of the 

Uncompahgre Project and most of the Grand 

Valley) were approximately at equilibrium with 

gypsum, and the remainder of the samples was 

slightly undersaturated with gypsum. The change 

in saturation indices for gypsum from undersaturated 

in the irrigation-supply water to saturation or near 

saturation in irrigation drainage from Mancos Shale 

areas could imply that dissolution of gypsum is 

occurring in the study area. Dissolution of other 

evaporite minerals formed by weathering of Mancos 

Shale, such as epsomite and thenardite, would produce 

magnesium, sodium, and sulfate.

Sulfate concentrations were much higher in 

water samples from the irrigated areas compared 

to sulfate concentrations in irrigation-supply 

water for the Uncompahgre Project and in the 

Grand Valley. Sulfate concentrations were 

about 20 mg/L in irrigation water diverted through 

the Gunnison Tunnel, based on six samples 

collected during 1987-92 from the Gunnison River 

downstream from the tunnel diversion (site GUN1 

in fig. 23) and from the South Canal. Sulfate 

concentrations were about 150 to 250 mg/L in 

samples collected during the irrigation season 

from the Uncompahgre River upstream from the 

Uncompahgre Project (site UC1 in fig. 25). For 

samples collected during the nonirrigation season, 

sulfate concentrations ranged from 200 to 

3,100 mg/L for the western side of the Uncompahgre 

Project and from 790 to 4,200 mg/L for the eastern 

side of the project. Sulfate concentrations in 

irrigation-supply water from the Colorado River 

upstream from the Grand Valley (site COL2)

generally ranged from about 50 to 175 mg/L. 

Sulfate concentrations in non-irrigation-season 

samples collected in the Grand Valley ranged 

from 760 to 3,900 mg/L.

Differences in the distribution of selenium 

and sulfate might indicate differences in source areas, 

mobility, deposition, and transport of these constituents (Izbicki, 1989). Ratios of the concentrations 

of sulfate to selenium were computed for water 

samples from the study area. Lower ratios of sulfate 

to selenium concentrations in water samples could 

imply that selenium was enhanced in the water 

compared to sulfate and might indicate the most 

important selenium-source areas. In samples from 

the Uncompahgre Project, sulfate to selenium ratios 

generally were greater than 100,000 for samples 

from reference sites (sites outside the irrigated area) 

and for samples of irrigation-supply water from the 

Gunnison and Uncompahgre Rivers. Ratios also 

were greater than 100,000 for irrigated areas that 

had low selenium concentrations on the western 

side of the project, such as in the Happy Canyon 

Creek, Spring Creek, and Roubideau Creek Basins 

(figs. 25 and 26). Sulfate to selenium ratios were 

about 40,000 to 70,000 in samples from lower Dry 

Creek, Cummings Gulch, and Seep Creek, and these 

lower ratios might partly be the result of seleniumenhanced water from the Uncompahgre River being 

transported into these basins by the Ironstone 

Canal. Sulfate to selenium ratios were lower for 

samples collected in the nonirrigation season from 

the eastern side of the project and generally ranged 

from 10,000 to 30,000. The lowest ratios were for 

samples from areas having some of the higher 

selenium concentrations, such as the Loutsenhizer 

Arroyo Basin, an unnamed drainage near Read 

(upstream from site RD1), and from sites GAR2 

and UNGR in the Garnet Canal area.

The irrigation-supply water from the Colorado 

River upstream from the Grand Valley had sulfate to 

selenium ratios greater than 100,000. Most irrigationdrainage samples from the Grand Valley had ratios 

between 10,000 and 60,000, and the lowest ratios 

were for water samples from areas that had the higher 

selenium concentrations, such as East Salt Creek, 

upper Reed Wash, the East Branch of Reed Wash, 

sites IRD and LC2 in the Leach Creek Basin, and 

Indian Wash (figs. 30 and 31). The only samples 

collected in irrigated areas of the Grand Valley that

SURFACE-WATER INVESTIGATIONS 85
had sulfate to selenium ratios greater than 100,000 

were samples from areas of lower selenium concentrations, such as the ditches and washes represented by 

sites GJ1 and CF2 and sites on Orchard Mesa. The 

sulfate to selenium ratios for four of the five samples 

from reference sites on West Salt Creek, East Salt 

Creek, and Big Salt Wash were greater than 1,000,000, 

which implies that, in nonirrigated areas on Mancos 

Shale, sulfate was easily mobilized, but selenium was 

not readily mobilized.

The ratios of sulfate to selenium for pondwater samples were greater than the ratios for 

stream samples from Mancos Shale areas in the 

Uncompahgre Project area and in the Grand Valley. 

Six water samples from ponds in the Uncompahgre 

Project area had ratios between 110,000 and 400,000. 

The ratios of sulfate to selenium were 118,000 

and 74,000 for samples from Reid Pond (site RDP 

in fig. 30) in the Grand Valley. The ponds are all 

located in Mancos Shale areas and probably receive 

some inflow of subsurface-irrigation drainage. 

Higher ratios for pond-water samples could 

indicate that chemical and biological processes 

were removing some of the selenium from 

ponds, but were not appreciably affecting sulfate 

concentrations.

Selenium and nitrite-plus-nitrate concentrations 

were significantly correlated (table 21) in the 

Uncompahgre Project and in the Grand Valley. Water 

samples from irrigated areas had considerably higher 

nitrite-plus-nitrate concentrations than irrigationsupply water, which is not unusual for agricultural 

areas where fertilizers are used on fields and where 

there are numerous small diary and cattle-feeding 

operations. Nitrite-plus-nitrate concentrations in 

irrigation-supply water and in samples from reference 

sites usually were less than 0.2 mg/L. The median 

nitrite-plus-nitrate concentrations for nonirrigationseason samples collected in 1991-92 were 2.9 mg/L 

for the western side of the Uncompahgre Project, 

7.6 mg/L for the eastern side, and 5.2 mg/L for the 

Grand Valley. Median concentrations do not include 

selenium data for reference sites, ponds, or mainsiem 

rivers. The maximum nitrite-plus-nitrate concentrations for 1991-92 for the Uncompahgre Project were 

14 mg/L (selenium concentration 23 |ig/L) for the 

western side at site SEP2 (fig. 26) and 21 mg/L

(selenium concentration 240 pg/L) for the eastern 

side at site GAR2. However, a sample from a 

drainage ditch at site DR2 (fig. 26) had a nitrite-plusnitrate concentration of 73 mg/L in March 1993. That 

sample also had a selenium concentration of 600 pg/L, 

which was the maximum selenium concentration in 

surface-water samples from the study area. The 

maximum nitrite-plus-nitrate concentration for 

1991-92 in the Grand Valley was 46 mg/L (selenium 

concentration 100 pg/L) for a drainage ditch near 

Clifton at site CF1 (fig. 31). However, site CF1 is near 

sewage-treatment ponds, and perhaps much of the 

nitrogen at site CF1 was from nonagricultural sources. 

Excluding site CF1, the maximum nitrite-plus-nitrate 

concentration in Grand Valley surface water was 

19 mg/L at site RW4 on upper Reed Wash (fig. 30). 

The selenium concentration of 380 |ig/L associated 

with that sample from site RW4 was the maximum 

selenium concentration in surface-water samples 

from the Grand Valley.

Because laboratory experiments have indicated 

that nitrate can oxidize selenium from Mancos Shale 

samples (Wright and McMahon, 1994; Wright, 1995), 

relations of selenium and nitrite-plus-nitrate concentrations in surface water in irrigated areas on Mancos 

Shale were examined. In the oxidized environment 

of surface water (excluding ponds), almost all of the 

nitrite-plus-nitrate determination represents nitrate. In 

surface-water samples from Mancos Shale areas, high 

selenium concentrations often were associated with 

high nitrite-plus-nitrate concentrations, although there 

were some samples with high selenium concentrations 

that did not contain unusually high nitrite-plus-nitrate 

concentrations. Also, a number of the samples from 

Mancos Shale areas that had some of the lower 

selenium concentrations were associated with some 

of the lower nitrite-plus-nitrate concentrations. The 

linear relations between selenium and nitrite-plusnitrate concentrations were examined using samples 

collected from tributary streams and ditches on the 

eastern side of the Uncompahgre Project and in the 

Grand Valley north of the Colorado River. Irrigation 

in these areas primarily is on soils derived from 

alluvium that contain variable quantities of shale 

detritus or is on soils derived from shale residuum. 

Samples collected only during the nonirrigation 

season (November through March) were used in

86 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
the regression analysis, and most of those samples 

were representative of subsurface-irrigation drainage 

from shale areas. The relations shown in figure 43 

are statistically significant (significance level 0.05), 

although the coefficients of determination for both 

relations do not exceed 0.50. The relations in 

figure 43 indicate that higher selenium concentrations 

generally are accompanied by higher nitrite-plusnitrate concentrations. Statistical correlations between 

selenium and nitrite-plus-nitrate concentrations does 

not prove that a cause-and-effect mechanism actually 

exists in the field (that nitrate oxidizes selenium). If 

there are significant sources of selenium and nitrogen 

available, oxidizing conditions could cause high 

concentrations of both constituents, which could result 

in positive correlations between constituent concentrations in the ground water that is discharged into 

streams, washes, and ditches. However, there seems 

to be an association between high selenium concentrations and high nitrite-plus-nitrate concentrations in 

some surface-water samples collected in Mancos 

Shale areas in the Uncompahgre Project and in the 

Grand Valley.

Other Trace-Constituent Concentrations

Many concentrations of trace constituents 

potentially toxic to biota, such as arsenic, cadmium, 

copper, lead, mercury, and zinc, were about equal to or 

less than analytical reporting limits (table 22). In 

addition to selenium, Colorado has aquatic-life criteria 

(Colorado Department of Health, 1989) for arsenic, 

cadmium, chromium, copper, lead, mercury, uranium, 

and zinc. Except for mercury, concentrations of these 

trace constituents in surface-water samples were less 

than the chronic criteria established by the State. 

The criterion for mercury is 0.01 flg/L, but the 

analytical reporting limit was 0.1 fXg/L; therefore, 

it is unknown if mercury concentrations exceeded 

the aquatic-life criterion. Cadmium was detected in 

only two samples, and mercury was detected in only 

one sample (table 22).

There was a substantial increase in boron 

concentrations from the applied irrigation water 

to irrigation-drainage water in the study area. 

Boron concentrations were about 10 to 50 flg/L 

in the irrigation-supply water for the Uncompahgre 

Project and the Grand Valley. In the Uncompahgre

Project area, the maximum boron concentration was 

1,600 |lg/L (table 22) in a drainage ditch at site ASH 

(fig. 26). In the Grand Valley, the maximum boron 

concentration was 790 flg/L in a drainage ditch at 

site GJ2 (fig. 31). In the Grand Valley, some of the 

highest boron concentrations were in smaller basins 

or drainage ditches. Boron concentrations generally 

ranged from about 250 to 600 flg/L in nonirrigationseason samples from the Uncompahgre Project and 

the Grand Valley.

Uranium was detected in all surface-water 

samples collected in the study area (table 22). 

The median uranium concentrations are lower for 

the Uncompahgre Project area than for the Grand 

Valley. All the samples from the Uncompahgre 

Project area that had uranium analysis were 

collected during the irrigation season, whereas 

almost one-half of the uranium data for the Grand 

Valley were obtained during the nonirrigation 

season. Many constituent concentrations are lower 

in the irrigation season compared to the nonirrigation 

season because of dilution by surface return flows. 

Median uranium concentrations of samples collected 

at the 20 reconnaissance sites in the Grand Valley were 

much higher for March 1991 (31 Hg/L) than for 

August and September 1991 (8.8 |lg/L). The 

two highest uranium concentrations from the 

Uncompahgre Project area were in samples from 

ponds 62 flg/L at Ferrier Pond near Austin 

(site FRP in fig. 23) and 16 |lg/L from Markley Pond 

(site MKP in fig. 26). Ferrier Pond was a reference 

site located outside of the irrigated area served by 

the Uncompahgre Project; the pond is located in 

Mancos Shale terrain and might receive irrigation 

drainage from mesas north of the pond. In the Grand 

Valley, the maximum uranium concentration was 

83 |ig/L (table 22) in a sample collected at site BSW3 

(fig. 30), a reference site located on Big Salt Wash 

upstream from the Government Highline Canal. 

Surface-water samples that had some of the highest 

uranium concentrations in the Grand Valley were from 

sites that had high selenium concentrations, such as 

upper Reed Wash (site RW4) and Indian Wash 

(site IW). However, there also were high uranium 

concentrations in samples that had some of the lower 

selenium concentrations, such as samples from 

Orchard Mesa Drain (site OMD in fig. 31).

SURFACE-WATER INVESTIGATIONS 87
300

250

200

150

DC 100

or iii 

o_

CO

or 

o 

o 

or 

o

50

REGRESSION LINE (COEFFICEINT 

OF DETERMINATION 0.50)

EASTERN SIDE OF 

UNCOMPAHGRE PROJECT

10 12 14 16 18 20 22

~ 400 |

Z

o

0? 350

111 

O

O

o

300

111

HI 

CO

250

200

150

100

REGRESSION LINE (COEFFICIENT 

OF DETERMINATION 0.38)

GRAND VALLEY NORTH 

OF COLORADO RIVER

NITRITE-PLUS-NITRATE CONCENTRATION, 

AS NITROGEN, IN MILLIGRAMS PER LITER

Figure 43. Relations of selenium to nitrite-plus-nitrate concentrations in water samples collected 

during the nonirrigation season from streams and ditches on the eastern side of the Uncompahgre 

Project and in the Grand Valley north of the Colorado River, 1991-92.

88 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 22. Summary of dissolved trace-constituent concentrations in surface-water samples from the Uncompahgre 

Project area and from the Grand Valley, 1991-92

[Concentrations in micrograms per liter; number detected is the number of samples reported as equal to or greater than the reporting limit; 

<, less than; includes samples collected from irrigated areas, reference sites, and mainstem rivers]

Trace 

constituent

Reporting 

llmlt(s)

Number 

of 

samples

Number 

detected Median Maximum Minimum

UNCOMPAHGRE PROJECT AREA

Arsenic

Boron

Cadmium

Chromium

Copper

Lead

Mercury

Molybdenum

Selenium

Uranium

Vanadium

Zinc

Arsenic

Boron

Cadmium

Chromium

Copper

Lead

Mercury

Molybdenum

Selenium

Uranium

Vanadium

Zinc

1

10

1

1

1

1

.1

1

1

.6

1.0

3,10

1

10

1,2

1

1

1

.1

1

1

.6

1.0

3,10

37

168

61

37

61

61

61

38

197

37

38

61

45

122

45

45

45

45

45

54

125

54

54

45

30

168

1

7

56

6

0

30

177

37

37

45

GRAND VALLEY

19

122

1

16

43

0

1

54

124

54

54

33

1

180

<1

<1

2

<1

<.l

3

9

6.2

2.7

6

<1

280

<1

<1

1

<1

<.l

7.5

24

15.5

4.5

9

3

1,600

1

3

6

4

<.l

10

260

62

7.7

59

2

790

2

3

4

<1

.2

70

380

83

15

30

<1

10

<1

<1

<1

<1

<.l

<1

<1

1.5

<1.0

<3

<1

20

<1

<1

<1

<1

<.l

1

<1

3.5

2.4

<10

Insecticides in the Grand Valley

Pesticide data were collected in 1988 for the 

reconnaissance investigation of the Uncompahgre 

Project (Butler and others, 1991). Insecticide samples 

were collected at 11 outflow sites on tributaries 

and at two sites on the Colorado River in the 

Grand Valley in the summer of 1991. Samples were 

analyzed for 15 organophosphate and 11 carbamate 

insecticides. Analytical results indicated that most 

compounds were not detected or were detected at low 

concentrations. Nine organophosphate concentrations 

equaled or exceeded the reporting limit of 0.01

Chlorpyrifos was detected in two samples at concentrations of 0.06 and 0.01 ^ig/L. Diazinon was detected 

in five samples at concentrations of 0.01 to 0.12 \ig/L, 

and the maximum concentration was in the sample 

from Hunter Wash (site HW1 in fig. 30). Parathion 

was detected in two water samples at concentrations of 

0.01 iig/L in Reed Wash (site RW1) and 0.09 |ig/L in 

Persigo Wash (site PSW1). The aquatic-life criterion 

for parathion is 0.04 \ig/L (U.S. Environmental 

Protection Agency, 1986). Butler and others (1991) 

reported parathion concentrations greater than 

0.04 |Xg/L in the Gunnison and Uncompahgre Rivers

SURFACE-WATER INVESTIGATIONS 89
in samples collected in July 1988. The only carbamate 

insecticide detected in water samples from the Grand 

Valley was a concentration of 0.8 |ig/L (reporting limit 

0.5 |ig/L) of 3-hydroxycarbofuran in a sample from 

Hunter Wash.

The insecticide data were collected in 1991 as 

a one-time reconnaissance of some of the pesticides 

used in the Grand Valley. A wide variety of crops 

are grown in the Grand Valley, and pesticide usage 

can vary depending on the type of crops grown and 

on the weeds and insects associated with the crops. 

Pesticides are applied at different times of the year. 

A much more comprehensive sampling program than 

was possible for the detailed study would be needed to 

more adequately assess pesticide concentrations in 

surface water in the study area.

SELENIUM AND OTHER CONSTITUENTS 

IN BOTTOM SEDIMENT

Bottom-sediment samples were collected from 

eight streams and washes and from four ponds at 

wetland sites in the study area for analysis of total 

selenium. Samples from streams were collected in 

March 1992, and samples from ponds were collected 

in June 1992. A sample was collected from a salt crust 

near an avocet nesting area adjacent to Brozina Pond 

in the northeast part of the Uncompahgre Project. A 

soil sample was collected on a hillside adjacent to 

Reid Pond in the Grand Valley where prince's plume 

(Stanleya pinnata), a selenium-indicator plant, was 

growing. Soluble and adsorbed selenium concentrations and the percent selenite were determined for the 

eight bottom-sediment samples collected from streams 

to provide additional information about selenium in 

bottom sediment.

Sample Collection and 

Analytical Methods

A BMH-53 sampler (Ward and Harr, 1990) 

was used to collect bottom-sediment samples from 

pool and backwater areas in streams where sediment 

deposits were sufficient for collection of samples. 

A minimum of six to eight cores were collected 

and composited in a stainless-steel bucket using a 

stainless-steel spoon. The salt-crust sample was a

surficial sample collected using a spoon, and the soil 

sample was collected using a spoon and shovel at the 

base of prince's plume plants at shallow depths (less 

than 6 in.). Bottom-sediment and soil samples were 

analyzed by the Geologic Division of the USGS in 

Lakewood, Colo. For bottom sediment, the lessthan 0.0625-mm size fraction was analyzed for 

selenium, uranium, 40 trace constituents, total 

carbon, and organic carbon by methods described 

by Harms and others (1990). The samples for totalselenium analysis were digested in a mixture of 

nitric, perchloric, and hydrofluoric acids; dried; and 

redissolved in hydrochloric acid (Harms and others, 

1990). The salt-crust and soil samples were analyzed 

for the same constituents on the less-than 2-mm size 

fraction.

A subsample of the bottom-sediment samples 

collected from streams was used for the analysis of 

soluble and adsorbed selenium and selenite. The 

subsamples were placed in glass jars, were frozen, 

and were shipped in dry ice. The soluble and 

adsorbed selenium were determined by sequential 

extractions on the bottom-sediment samples using 

potassium chloride (KC1) and potassium phosphate 

(K2HPO4) on the same sample (Fujii and others, 

1988). The KC1 extract was considered an approximation of soluble selenium, and the K2HPO4 extract an 

approximation of the adsorbed selenium on the 

sediment. Selenite concentrations were determined 

in each extract by methods described by Fio and Fujii 

(1990). Most of the selenate selenium probably was 

extracted by KC1; therefore, the nonselenite fraction 

in the KC1 extract was assumed to be selenate. The 

analyses of soluble and adsorbed selenium and 

selenite for bottom-sediment samples were done by 

the U.S. Geological Survey in Sacramento, Calif. 

(Roger Fujii, U.S. Geological Survey, written 

commun., 1992).

Selenium Concentrations

In general, selenium concentrations were greater 

in bottom-sediment samples from ponds than in 

samples from streams (table 23). All selenium 

concentrations listed in table 23 exceeded the upper 

base-line concentration of 1.4 |ig/g for soils in the 

Western United States (Shacklette and Boerngen, 

1984). Selenium concentrations greater than or equal

90 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
to 4 |tig/g in sediment are a level of concern for fish 

and wildlife because of food-chain bioaccumulation 

(Lemly and Smith, 1987). Selenium concentrations 

in bottom-sediment samples from the four ponds and 

the samples from Loutsenhizer Arroyo (site LZA1), 

Reed Wash (site RW1), Leach Creek (site LC1), and 

Indian Wash (site IW) exceeded 4 |4,g/g (table 23). 

The selenium concentration of 47 |J,g/g in the bottomsediment sample from Brozina Pond (site BZP) 

was similar to the selenium concentration of 

41 |Lig/g reported for a bottom-sediment sample 

from Sweitzer Lake (Butler and others, 1991). 

Brozina Pond is about 3 mi southeast of Sweitzer 

Lake in Mancos Shale terrain. Selenium is enriched 

in bottom sediment compared to water, but more so in 

ponds than in streams. The ratio of selenium concentrations in bottom-sediment samples to concentrations 

in water samples collected concurrently at each site 

ranged from 29 to 250 for streams and from 2,670 to 

23,500 for ponds. The largest ratio of selenium 

concentrations was for Brozina Pond. The selenium 

concentrations in the salt crust collected adjacent 

to Brozina Pond and in the soil sample collected

among selenium-indicator plants near Reid Pond 

were similar to selenium concentrations in bottom 

sediment from streams (table 23).

Total-selenium concentrations in bottom 

sediment from the three streams in the Uncompahgre 

Project (table 23) were slightly higher than totalselenium concentrations in soil samples (data in Crock 

and others, 1994) that were collected from irrigated 

fields in the same drainage basin. The median totalselenium concentration for 11 soil samples collected in 

the Cedar Creek Basin was 2.9 |J.g/g, and the median 

for 15 soil samples collected in the Loutsenhizer 

Arroyo Basin was 3.5 jag/g. The bottom-sediment 

samples collected at the outflow sites for these basins 

(sites GDI and LZA1 in table 23) had slightly higher 

selenium concentrations than median concentrations 

in soil. Most of the irrigated land in Cedar Creek and 

Loutsenhizer Arroyo Basins consists of soils derived 

from residual Mancos Shale or of alluvium derived 

primarily from shale. The total-selenium concentration in the bottom-sediment sample from Dry Creek is 

about twice the median concentration in irrigated soils 

in the Dry Creek Basin. Irrigated soils in Dry Creek

Table 23. Concentrations of selenium, uranium, total carbon, and organic carbon in bottom-sediment, 

salt-crust, and soil samples

[Sample types are: hot, bottom sediment; crust, sample from a salt crust adjacent to Brozina Pond; soil, a shallow soil sample from a 

hillside near Reid Pond where prince's plume, a selenium-indicator plant, was present; ug/g, micrograms per gram; bottom-sediment 

samples from streams were collected in March 1992 and samples from ponds in June 1992]

Site 

number

Stream 

or pond

Sample 

type

Selenium

(ug/g)

Uranium

0*9/9)

Total 

carbon 

(percent)

Organic 

carbon 

(percent)

UNCOMPAHGRE PROJECT AREA (sites shown in figs. 25 and 26)

DRY1

GDI

LZA1

MKP

BZP

BZP

Dry Creek

Cedar Creek

Loutsenhizer Arroyo

Markley Pond

Brozina Pond

Brozina Pond

Bot

Bot

Bot

Bot

Bot

Crust

2.0

3.5

6.9

16

47

3.4

4.46

5.18

5.09

12.5

12.1

5.62

2.32

2.96

3.34

5.48

9.30

1.56

0.67

.88

.60

1.73

2.72

.56

GRAND VALLEY (sites shown in figs. 30 and 31)

SC

MWP

RDP

RDP

RW1

AC1

LC1

IW

Salt Creek

Pond in Mack Wash

Reid Pond

Reid Pond

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

Bot

Bot

Bot

Soil

Bot

Bot

Bot

Bot

1.9

21

15

4.1

6.3

3.9

5.6

16

4.55

6.28

7.39

4.90

5.62

6.45

5.45

6.69

2.32

5.62

3.79

3.48

3.44

3.37

3.35

3.17

.51

2.21

1.95

.94

.62

.92

.93

1.25

SELENIUM AND OTHER CONSTITUENTS IN BOTTOM SEDIMENT 91
Basin primarily are derived from terrace deposits and 

the Dakota Sandstone. Bottom sediment in tributary 

streams does not seem to be accumulating appreciable 

quantities of selenium when compared to the selenium 

content of the parent soils.

Selenium can be removed from water and 

immobilized in sediment through chemical and 

bacterial reduction, adsorption, coprecipitation, and 

deposition of plant and animal tissue (Lemly and 

Smith, 1987). In reducing environments, selenide and 

elemental selenium are very insoluble in water 

(Elrashidi and others, 1989), and selenium can become 

incorporated into the bottom sediment. Reducing 

conditions are likely near the bottom of most ponds 

and lakes; therefore, selenium enrichment in bottom 

sediment in ponds is not unexpected. The ponds, 

lakes, and wetlands in the Uncompahgre Project area 

and the Grand Valley probably function as sinks for 

selenium. Because selenium concentrations and loads 

in water discharging into and out of ponds and lakes 

were not measured, the magnitude of selenium 

removal in ponds and lakes in the study area was not 

determined. However, probably much more selenium 

is discharged to streams, washes, and ditches and 

transported out of the study area by the Gunnison and 

Colorado Rivers compared to the amount of selenium 

that is immobilized in pond and lake sediment.

Total-selenium concentrations are compared 

to the soluble and adsorbed selenium for bottomsediment samples from the eight streams in table 24. 

The soluble selenium (the KC1 extract) accounted for 

about 1 percent or less of the total-selenium concentration in all samples (table 24). The surface-adsorbed 

selenium (the ^HPC^ extract) accounted for about 

2.5 to 16.9 percent of the total selenium. Except 

for site LC1, the soluble and adsorbed selenium 

extracted by KC1 and K2HPO4 accounted for 

less than 10 percent of the total selenium in the 

eight bottom-sediment samples (table 24). The 

unextracted selenium could be chemically bound in 

oxides by precipitation or coprecipitation; could be 

present in insoluble, reduced chemical forms; or might 

be bound in organic matter. Fujii and others (1988) 

reported that the proportion of adsorbed selenium to 

total selenium in soils in the San Joaquin Valley in 

California ranged from 1 to 11 percent, which is 

similar to the results for bottom sediment from streams 

in the study area.

The percent selenite in both selenium extracts 

was 25 percent or less, except for the sample from 

Salt Creek (site SC) in the Grand Valley (table 24). 

Fujii and others (1988) reported that most of the 

surface-adsorbed selenium in soils was selenite, 

which, except for the sample from site SC, was not

Table 24. Concentrations of total, soluble, and adsorbed selenium and the percent selenite in bottom-sediment 

samples from eight streams, March 1992

[Concentrations in micrograms per gram; KC1- (potassium chloride-) extractable selenium approximates the soluble selenium, and K2HPO4- 

(potassium phosphate-) extractable selenium approximates the adsorbed selenium; selenium-extract data from Roger Fujii (U.S. Geological 

Survey, written commun., 1992)]

Site 

number

Stream 

name

Total 

selenium

KCI-extractable 

(soluble) selenium

ConcentrationPercent

selenite

K2HPO4-extractable 

(adsorbed) selenium

Concentration

Percent

selenite

UNCOMPAHGRE PROJECT AREA (sites shown in figs. 25 and 26)

DRY1

GDI

LZA1

Dry Creek

Cedar Creek

Loutsenhizer Arroyo

2.0

3.5

6.9

0.010

.007

.038

0

1.5

10

0.054

.181

.430

25

18

19

GRAND VALLEY (sites shown in figs. 30 and 31)

SC

RW1

AC1

LC1

IW

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

1.9

6.3

3.9

5.6

16

.027

.057

.048

.071

.076

26

15

12

5.0

14

.128

.156

.291

.944

.813

90

13

19

11

10

92 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
true for the bottom-sediment samples collected from 

the eight streams in the study area. Geochemical 

conditions in soils from the San Joaquin Valley might 

be different than geochemical conditions for bottom 

sediment in streams in the Uncompahgre Project 

and the Grand Valley.

For the 12 bottom-sediment samples collected 

from streams and ponds (table 23), there were 

significant (significance level 0.05) positive correlations between selenium concentrations and concentrations of total carbon and organic carbon. The 

correlation coefficient for selenium and organic 

carbon was 0.95. Selenium was positively correlated 

with organic carbon in mining overburden in 

Wyoming (Naftz and Rice, 1989). Singh and others 

(1981) reported that organic carbon had a positive 

effect on selenate and selenite adsorption in semiarid 

soils. The sample from Brozina Pond that had the 

maximum selenium concentration of 47 (Ag/g in 

bottom sediment also had the maximum organic 

carbon concentration of 2.72 percent. Perhaps 

the high organic-carbon content of the bottomsediment sample from Indian Wash (site IW in 

table 23) might partly account for the high selenium 

concentration in Indian Wash bottom sediment 

compared to the other seven streams where bottom 

sediment was sampled.

Other Trace-Constituent 

Concentrations

Most other trace-constituent concentrations 

in bottom sediment (Butler and others, 1994) were 

less than the upper base lines for soil in the Western 

United States (Shacklette and Boerngen, 1984). 

Uranium concentrations in 9 of the 14 samples 

listed in table 23 exceeded the upper base line for 

uranium of 5.3 (Xg/g (Shacklette and Boerngen, 1984). 

The differences in uranium concentrations between 

pond bottom sediment and stream bottom sediment 

were considerably less than were the differences in 

selenium concentrations, especially in the Grand 

Valley (table 23). Molybdenum concentrations in 

three of the four bottom-sediment samples from ponds 

were slightly greater than the upper soils base line of 

4 jig/g. The salt-crust sample from Brozina Pond has 

unusually low concentrations of some trace constituents, such as barium, chromium, vanadium, and zinc, 

compared to bottom-sediment samples.

BIOLOGICAL INVESTIGATIONS

Sample Collection

Biological samples were collected by the FWS 

using standard equipment and techniques (U.S. Fish 

and Wildlife Service, 1986,1990). Fish were 

collected using electroshocking equipment and seine 

or gill nets. Fish were rinsed, weighed, measured for 

length, and immediately frozen on dry ice until stored 

in a freezer. Whole-body fish samples from tributaries 

were composited by species into groups of three or 

more fish as directed by the sampling protocol. Fish 

samples from rivers were processed as individual 

samples for data-analysis purposes. Fish-fillet and 

-egg samples were taken from individual fish and were 

not composited. Fish samples for analyses of 

inorganic contaminants were frozen in plastic bags. 

Fish samples for analyses of organic compounds were 

wrapped in aluminum foil and placed in plastic bags.

Vascular plants and algae were collected by 

handpicking. These samples were placed in 

chemically cleansed jars, weighed, and frozen. Algae 

samples probably contained green algae (Chlorophyta) 

and blue-green algae (Cyanophyta). Stream invertebrates were collected using a kick screen. Several 

easily identifiable invertebrate groups were combined 

to obtain sufficient biomass for analysis. Crayfish 

were collected when present.

Birds were shot using steel shot, and livers were 

removed using stainless-steel dissecting equipment. 

Bird liver was considered to be the best organ for 

a general trace-element scan (Ohlendorf, 1993), 

although other tissues might be better indicators for 

specific elements (such as kidney for cadmium and 

bone for lead). Dissecting equipment was cleansed 

using water and soap and rinsed using distilled water 

and benzene prior to the removal of each liver. Bird 

livers and muscle tissue were placed in chemically 

cleansed jars, weighed, and frozen. Livers from each 

bird group were sometimes composited, which 

resulted in two or three livers constituting one sample.

After locating nests, bird eggs were removed, 

the egg volume was determined (by water displacement), and eggs were opened to examine embryos for 

possible developmental abnormalities. After examination, eggs were placed in chemically cleansed jars, 

weighed, and frozen. Small eggs, such as blackbird 

eggs, were composited to provide sufficient biomass 

for analysis.

BIOLOGICAL INVESTIGATIONS 93
The sampling period depended on availability 

of bird eggs and prefledgling birds. Because 

prefledglings generally are confined to a local area 

until they fledge, trace elements and pesticides in 

their tissues are obtained from food and water in the 

area where the birds are reared. An attempt was made 

to collect prefledglings immediately before fledging 

because older birds would have been exposed to 

any contaminants present in the area for a longer 

time period than younger birds. Such collections 

were not always possible because of time limitations 

in the sampling effort and because of considerable 

predation on young birds. Although developmental 

abnormalities among embryos in bird eggs cannot 

be detected before the eggs have reached one-half 

term (Ohlendorf and others, 1986), eggs were 

collected as soon as they were discovered because 

of the high risk of predation. Therefore, early collection to ensure that representative egg samples were 

available for contaminant analysis often outweighed 

the loss of pathological information related to field 

observations.

Because of high predation rates on bird eggs and 

hatchlings, incubation studies were conducted. For 

the incubation study, eggs were collected in the field 

and placed into egg cartons with Kleenex tissues to 

keep the pointed end down (Stromberg, 1975; Brown, 

1979). The egg cartons were kept cool during 

transportation to the laboratory. Usually, the eggs 

were transferred from egg cartons to trays with the 

small end down and incubated at a temperature of 

99°F and at a relative humidity of 55 percent 

(Stromberg, 1975; Brown, 1979). Eggs were 

mechanically turned at 4-hour intervals. Eggs 

were candled and a hatching date was projected 

by using a field guide for incubation lengths for each 

species (Harrison, 1979). Newly hatched young were 

examined for external deformities. Eggs that failed to 

hatch within a few days of their hatching date were 

opened and examined. All incubated specimens were 

frozen and sent to the laboratory for trace-element 

analysis.

Analytical Methods

The quantitative analyses of trace constituents in biological samples were performed by 

the Environmental Trace Substances Research 

Center in Columbia, Mo. This laboratory was 

contracted by the FWS Patuxent Analytical Control

Facility in Laurel, Md. Biological samples 

were analyzed for selenium and selected trace 

constituents.

Analyses for most trace constituents in biota 

samples were performed using inductively coupled 

argon-plasma atomic-absorption spectrometry after 

complete digestion of the sample by strong acids. 

Analyses for selenium and arsenic in biota samples 

were performed using HGAAS, and analysis for 

mercury was performed using flameless cold-vapor 

atomic-absorption spectrometry.

Organochlorine pesticides were analyzed 

at Mississippi State University in Mississippi 

State, Miss. Methods consisted of solvent extraction 

and electron-capture gas chromatography. Polycyclic 

aromatic hydrocarbons (PAH's) were analyzed at 

Texas A&M University in College Station, Tex. 

The quantitative analyses were performed by using 

a mass spectrometer detector in the selected ionmonitoring mode.

Data Analysis

Statistical analyses were conducted using a 

statistical software program called Statistical Analysis 

System (SAS Institute, 1982). Geometric mean, 

geometric deviation, maximum, and minimum 

concentrations were determined.

Fish data for the Gunnison, Uncompahgre, and 

Colorado Rivers were analyzed by using a General 

Linear Model (GLM) for unbalanced data. A two-way 

ANOVA and the Tukey-Kramer Honestly Significant 

Difference (HSD) (SAS Institute, 1982) were used to 

compare selenium residues in fish among the three 

rivers, among individual sites on the three rivers, and 

among three fish trophic groups.

Data Interpretation

Many chemical, physical, and biological factors 

affect the toxicity of environmental contaminants to 

biological organisms. Chemical and physical factors 

include contaminant type, chemical species or form, 

pH, water temperature, dissolved oxygen, hardness, 

salinity, and multiple-chemical exposure (antagonism 

and synergism). Duration of exposure, quantity 

of contaminant, and exposure pathways from the 

environment to the organism also affect toxicity. 

Some trace constituents are beneficial to organisms

94 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
at small concentrations, but can be toxic at larger 

concentrations. Biological and physiological factors 

affecting toxicity are species, age, sex, and health 

of the organism. Interpretation of contaminant 

concentrations in biota is difficult and complex and 

often might not be possible, using the data collected 

in field studies. One of the best methods for 

interpreting contaminant data is by comparison with 

data collected from other field studies and from 

laboratory studies.

Concentrations of inorganic trace constituents 

in biological samples are extremely variable. Traceconstituent data can be interpreted by comparison 

to available literature to determine if constituent 

concentrations in biota samples exceed concentrations 

that could be harmful to fish and wildlife or exceed 

guidelines for human consumption. A frequently 

used index for interpreting contaminant data for fish 

samples is the National Contaminant Biomonitoring 

Program (NCBP) of the FWS. Schmittand 

Brumbaugh (1990) reported the 85th-percentile 

concentrations for arsenic, cadmium, copper, lead, 

mercury, selenium, and zinc for fish samples collected 

during 1976-84 throughout the United States. The 

85th percentile has been established by NCBP as 

an arbitrary concentration for distinguishing wholebody fish samples that have large concentrations 

for the seven trace constituents relative to a national 

data base. The 85th percentile is not necessarily 

an indicator of potential hazards to fishery resources 

nor can it be used in place of regulatory standards. 

The NCBP means and percentiles were calculated 

using combined fish species, and readers need to 

be aware that there can be substantial differences 

in selenium accumulation between species (Lemly, 

1993). Nevertheless, NCBP data still are useful 

for comparison purposes. Concentrations listed 

in Schmitt and Brumbaugh (1990) are wet-weight 

concentrations; therefore, the dry-weight concentrations for biota samples collected for the detailed 

study (Butler and others, 1994) for the seven 

trace constituents were converted to wet-weight 

concentrations to facilitate comparison to the 

85th-percentile concentrations. The NCBP also has 

collected data for organochlorine pesticides (Schmitt 

and others, 1990).

The NCBP 85th percentile was reported for 

several periods within the sampling period (1976-84) 

(Schmitt and Brumbaugh, 1990). The most recent

compilation was for fish samples collected during 

1984, and the 85th percentiles reported for 1984 are 

used in this report. The previous DOI reconnaissance 

investigations used 85th percentiles based on earlier 

compilation periods.

Selenium Concentrations in Fish Samples 

and Their Food Items in Irrigated Areas

Uncompahgre Project

In general, selenium concentrations were 

higher in biota samples collected east of the 

Uncompahgre River than in samples collected west 

of the Uncompahgre River (Butler and others, 1994, 

tables 23-27). This pattern in selenium data in biota 

was similar to the pattern in surface-water samples. 

Biota-sampling sites within the irrigated area east of 

the Uncompahgre River include sites DCC1, GDI, 

LZA1, and PVA1 (fig. 44), and west of the river 

include sites RFC, HCC1, SP1, SP4, DRY1, DRY5, 

CMG1, and RB1 (fig. 44). Aquatic-plant, aquaticinsect, and crayfish samples collected east of the 

Uncompahgre River from Loutsenhizer Arroyo 

(site LZA1 in fig. 44) contained 4.1, 26, and 

12 jig/g dry weight selenium (Butler and others, 

1994, table 24). An invertebrate sample from Peach 

Valley Arroyo (site PVA1 in fig. 44) contained 

7.1 jig/g dry weight selenium. Lemly and Smith 

(1987) and Lemly (1993) recommended that 3 jig/g 

dry weight selenium be used as a toxic threshold 

guideline for selenium transferred to consumer species 

of fish and wildlife through aquatic food chains. The 

samples from Loutsenhizer and Peach Valley Arroyos 

exceeded this dietary guideline.

Eight plant and algae samples collected 

from the western side of the Uncompahgre River 

had a geometric mean selenium concentration of 

1.0 jig/g dry weight (table 25), which is less than 

the dietary guideline. An aquatic-invertebrate 

sample collected from Horsefly Creek (site HFC 

in fig. 44) on the western side of the Uncompahgre 

River contained 6.1 jig/g dry weight selenium 

and exceeded the 3-jig/g dry-weight dietary guideline. 

A crayfish sample collected from Happy Canyon 

Creek (site HCC1) contained 2.1 jig/g dry weight 

selenium.

BIOLOGICAL INVESTIGATIONS 95
108°15' 108°00' 107°45'

38°45

38°30' 

38°15'

0 10 MILES

0 5 10 KILOMETERS

Figure 44. Location of 1991 biota sampling sites on tributary streams in the Uncompahgre Project.

All eight fish samples collected from streams 

on the eastern side of the Uncompahgre River 

exceeded the 1984 NCBP 85th-percentile selenium 

concentration (table 26) of 0.73 (Ag/g wet weight 

(2.92 (Ag/g dry weight at 75-percent moisture). 

Lemly (1993) advised that a whole-body selenium 

concentration of 4 fxg/g dry weight or greater could 

be of concern for the overall health and reproductive 

vigor of freshwater fish. All fish samples collected 

east of the Uncompahgre River exceeded 4 (Ag/g dry

weight selenium. Two speckled dace samples 

collected east of the Uncompahgre River 

(sites DCC1 and LZA1 in fig. 44) contained 

27 and 28 (Ag/g dry weight selenium (table 25). 

A green sunfish collected from Loutsenhizer 

Arroyo (site LZA1) contained 37 (Ag/g dry 

weight selenium, which is noteworthy because 

centrarchids (the sunfish family) seem to be 

especially sensitive to selenium toxicity (Cumbie 

and Van Horn, 1978; Lemly, 1985).

96 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 25. Statistical summary of selenium concentrations in selected biota samples collected in 1991 in the Uncompahgre 

Project area

[Concentrations in micrograms per gram dry weight; N, number of samples; GM, geometric mean; GD, geometric deviation; ~, no data or not computed; 

data for reference sites on Cedar Creek (CD4) and Roubideau Creek (RB3) were excluded]

Statistic

N 

GM

GD

MAXIMUM

MINIMUM

Aquatic 

plants 

and 

algae

1

4.1

4.1

Aquatic Speckled 

Invertebrates dace

. . Roundta Suckers . . chub

UNCOMPAHGRE 

3250 

PROJECT EAST OF THE UNCOMPAHGRE RIVER

13 27.5 13.9

1.9

26

7.1

1.0

28

27

1.4

22

9.0

Trout

0

-

Green 

sunflsh

1

~

37

37

UNCOMPAHGRE PROJECT WEST OF THE UNCOMPAHGRE RIVER

N

GM

GD

MAXIMUM

MINIMUM

8

1.0

2.3

2.9

.2

3

3.0

1.9

6.1

2.1

3

8.9

1.5

13

5.6

13

2.9

1.6

5.9

1.2

1

-

5.9

5.9

5

5.8

2.3

25

3.4

0

-

-

-

-

SMITH FORK AND NORTH FORK BASINS

N

GM

GD

MAXIMUM

MINIMUM

3

1.6

1.0

1.6

1.6

3

3.1

1.6

4.8

2.0

3

7.3

1.1

7.8

6.9

3

2.7

1.8

3.9

1.4

1

-

6.1

6.1

5

5.8

1.5

9.4

3.6

0

-

-

-

CURRANT CREEK, ALFALFA RUN, AND TONGUE CREEK

N

GM

GD

MAXIMUM

MINIMUM

3

6.6

1.7

12

4.1

3

7.0

1.4

9.4

4.9

2

12.5

1.1

13

12

1

~

-

12

12

2

13.8

1.6

19

10

0

~

~

-

0

-

--

Table 26. Number of whole-body fish samples collected in the Uncompahgre Project area that exceeded the 1984 National 

Contaminant Biomonitoring Program (NCBP) 85th-percentile concentrations for seven trace constituents

[NCBP 85th-percentile concentrations in micrograms per gram wet weight; NCBP data from Schmitt and Brumbaugh (1990); east, samples from eastern 

side of the Uncompahgre Project; west, samples from the western side of the Uncompahgre Project; adjacent 1, samples from the Smith Fork and North 

Fork Basins; adjacent 2, samples from Currant Creek, Alfalfa Run, and Tongue Creek; N, number of samples; numbers in brackets are the number of 

exceedances expressed as a percentage of the number of samples in each group]

Trace 

constituent

Arsenic

Cadmium

Copper 

Lead1

Mercury

Selenium

Zinc

NCBP 

85th 

percentile

0.27

.05

1.0

.22

.17

.73

34.2

East 

(N=8)

0[0]

3 [38]

5 [62] 

2 [25]

0[0]

8 [100]

3 [38]

West 

(N=29)

4 [14]

7 [24]

19 [66] 

9 [31]

4 [14]

22 [76]

5 [17]

Adjacent 1 

(N=15)

1[7]

3 [20]

5 [33] 

0[0]

0[0]

14 [93]

3 [20]

Adjacent 2 

(N=8)

1[12]

0[0]

4 [50] 

0[0]

0[0]

8 [100]

3 [38]

Crawford 

Reservoir 

(N=5)

1[20]

0[0]

2 [20] 

0[0]

0[0]

4 [80]

0[0]

'Some samples had reporting limits greater than the 1984 NCBP 85th percentile.

BIOLOGICAL INVESTIGATIONS 97
Twenty-two of 29 fish samples collected on the 

western side of the Uncompahgre Project exceeded the 

1984 NCBP 85th-percentile selenium concentration 

(table 26). Eight of these 22 samples were collected 

from Dry Creek. One trout sample collected at 

site DRY1 from lower Dry Creek contained 25 jxg/g 

dry weight selenium. Relatively high selenium 

concentrations also were in surface-water samples 

collected from lower Dry Creek compared to the other 

major streams on the western side, and these concentrations are partly attributed to the transport of water 

from the Uncompahgre River by the Ironstone Canal 

into the lower Dry Creek Basin. Two trout liver 

samples collected west of the Uncompahgre River 

(sites HCC1 and SP1 in fig. 44) contained 110 and 

180 jxg/g dry weight selenium. These concentrations 

were much greater than a guideline concentration of 

12 (Xg/g dry weight selenium in fish livers (Lemly, 

1993). Liver tissue often accumulates the greatest 

amount of selenium (Sager and Cofield, 1984), and 

concentrations in liver tissue might be useful in 

assessing recent exposure (Ohlendorf, 1993).

All selenium concentrations in aquatic-plant 

and -invertebrate samples collected in tributaries 

downstream from the confluence of the Uncompahgre 

and Gunnison Rivers (Butler and others, 1994, 

table 27) were less than the dietary guideline of 

3 jxg/g dry weight (Lemly and Smith, 1987; Lemly 

1993). Ten of 13 whole-body fish samples exceeded 

the 1984 NCBP 85th-percentile concentration of 

2.9 jxg/g dry weight (at 75-percent moisture), and 

9 of these samples exceeded the 4-^lg/g selenium 

concentration of concern for reproductive problems in 

whole-body fish (Lemly, 1993). Highest selenium 

concentrations were in fish collected from Cummings 

Gulch (site CMG1 in fig. 44); a trout fillet contained 

9.9 |Xg/g dry weight selenium, a whole-body speckled 

dace sample contained 13 jxg/g dry weight selenium, 

and a whole-body carp sample contained 12 (Xg/g dry 

weight selenium (Butler and others, 1994, table 27).

The geometric mean selenium concentration 

for five sucker samples collected on the eastern side 

of the Uncompahgre Project was 13.9 jxg/g dry weight 

(table 25). Two composite sucker samples from 

site CD4 (fig. 44), a reference site on Cedar Creek, 

contained 6.1 and 4.7 |Xg/g dry weight selenium. 

One composite sucker sample collected at site CD1, 

near the mouth of Cedar Creek, contained almost 

twice as much selenium (12 jxg/g dry weight

selenium) as the sucker samples from site CD4 

(Butler and others, 1994, table 24). The geometric 

mean selenium concentration for 13 sucker samples 

collected west of the Uncompahgre River was only 

2.9 (Xg/g dry weight (table 25). Selenium concentrations in sucker samples from site RB3 (a reference 

site on Roubideau Creek) (fig. 44) contained 1.3 and 

2.9 (Xg/g dry weight selenium. Selenium concentrations downstream at site RB1 on Roubideau Creek 

(fig. 44) were slightly higher than the concentrations 

at site RB3 at 2.2 and 4.6 jxg/g dry weight. Selenium 

concentrations in roundtail chub samples also 

were similar between the sites on Roubideau Creek; 

the samples collected from site RB1 contained 

5.9 (Xg/g dry weight selenium, and those collected 

from site RB3 contained 5.4 jxg/g dry weight 

selenium. Two speckled dace samples collected 

from the eastern side of the project had selenium 

concentrations of 27 and 28 jxg/g dry weight, 

whereas three speckled dace samples collected on 

the western side had a geometric mean of 8.9 (Xg/g 

dry weight selenium.

Areas Adjacent to the Uncompahgre Project

Most biota samples (especially fish) collected 

from areas adjacent to the Uncompahgre Project also 

had high selenium concentrations. Data for adjacent 

sites (fig. 45) were collected in the Smith Fork 

(site SMF) and North Fork (sites NFK2 and NFK3) 

of the Gunnison River, plus Crawford Reservoir 

(site CR) (located on the Smith Fork). Data for 

adjacent sites also include samples collected from 

the Gunnison River at site GUN2 and its tributaries 

between the confluence with the North Fork and the 

confluence with the Uncompahgre River (sites CRC, 

AFR, and TGC in fig. 45). Geometric mean selenium 

concentrations for biota collected from sites adjacent 

to the Uncompahgre Project are listed in table 25.

Almost all whole-body fish samples from the 

Smith Fork, the North Fork of the Gunnison River, and 

Crawford Reservoir had selenium concentrations that 

exceeded the 1984 NCBP 85th-percentile concentration of 0.73 jxg/g wet weight (table 26), and many 

concentrations exceeded the 4-jxg/g dry-weight 

selenium concentration of concern for whole-body 

fish (Lemly, 1993). One whole-body trout sample 

collected from Crawford Reservoir (site CR in 

fig. 45) contained 18 jxg/g dry weight selenium.

98 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
109° 108° 107°

W 

g

GRAND MESA

GRAND VALLEY 

i IRRIGATED AREA

Tay/or Park 

Reservoir

UNCOMPAHGRE PROJECT 

IRRIGATED AREA

Blue Mesa

Morrow Point 

Reservoir

Ridgway Reservoir

40 KILOMETERS

EXPLANATION

SMF SAMPLE SITE 

AND NUMBER

Figure 45. Location of 1991 biota sampling sites adjacent to the Uncompahgre Project.
A whole-body perch sample collected from Crawford 

Reservoir (site CR) contained 11 (ig/g dry weight 

selenium, which is slightly less than the concentration 

of 12 (ig/g dry weight selenium that Lemly and Smith 

(1987) warned could cause reproductive failure in 

warmwater fishes. A fillet sample taken from a yellow 

perch collected from Crawford Reservoir (site CR) 

contained 16 (ig/g dry weight selenium, which 

exceeded the concentration of concern of 8 (ig/g dry 

weight selenium for fish-fillet samples (Lemly, 

1993). Although centrarchids (perch and sunfish) 

seem to be especially sensitive to selenium (Cumbie 

and Van Horn, 1978; Lemly, 1985), the yellow perch 

population in Crawford Reservoir is thriving with 

good reproduction.

Two fish-liver samples taken from a trout 

(site SMF in fig. 45) and a catfish (site CR) either 

equaled or exceeded the concentration of concern for 

fish livers of 12 (ig/g dry weight (Lemly, 1993). An 

egg sample from a trout (site SMF) contained 31 (ig/g 

dry weight selenium, which exceeded the concentration of concern of 10 (ig/g dry weight selenium in fish 

eggs (Lemly, 1993). The high selenium concentrations in biota collected from site SMF are noteworthy 

because selenium was not detected in the surfacewater sample collected in July 1991.

Selenium concentrations in biota samples 

collected from the Gunnison River (site GUN2) and 

the three tributaries (sites CRC, APR, and TGC in 

fig. 45) also were elevated (table 25). Selenium 

concentrations in all six dietary items (aquatic plants 

and invertebrates) from the three tributary sites 

exceeded the dietary guideline of 3 |o,g/g dry weight 

(Lemly and Smith, 1987; Lemly, 1993). All wholebody fish samples collected from the three tributary 

sites exceeded the 1984 NCBP 85th-percentile 

concentration (table 26) and exceeded the 4-(ig/g 

dry-weight concentration of concern for whole-body 

fish (Lemly, 1993). Six whole-body fish samples 

collected from Currant Creek (site CRC), Alfalfa Run 

(site AFR), and the Gunnison River (site GUN2) 

(Butler and others, 1994, table 26) equaled or 

exceeded 12 (ig/g dry weight selenium, which is the 

concentration that Lemly and Smith (1987) warned 

might cause reproductive problems in warmwater fish. 

Selenium concentrations in fathead minnow wholebody samples from three sites (sites GUN2, CRC, 

and TGC) ranged from 7.5 to 20.4 (Xg/g dry weight, 

exceeding known toxicity concentrations for fathead 

minnows. Ogle and Knight (1989) reported that a

whole-body selenium concentration of 6 (Xg/g 

dry weight slowed growth in juvenile and adult 

fathead minnows. Schultz and Hermanutz (1990) 

reported reproductive failure in fish that had a wholebody selenium concentration of 8 (ig/g dry weight. 

Although the selenium concentrations in biota samples 

collected from Alfalfa Run (site AFR) were high, 

selenium was not detected in the single surface-water 

sample collected at site AFR in July 1991.

Grand Valley

Although variable, high selenium concentrations were reported in most Grand Valley biota 

samples (Butler and others, 1994, tables 28-30), 

which is not surprising because most selenium 

concentrations in surface-water samples often 

exceeded the chronic aquatic-life criterion, and many 

selenium concentrations in water samples exceeded 

the acute criterion during the nonirrigation season. 

Biota sampling sites on streams in the Grand Valley 

are shown in figure 46. Statistical summaries of 

selenium concentrations in aquatic plants, crayfish, 

and selected species of whole-body fish are listed in 

table 27. The summary data in table 27 does not 

include composite invertebrate samples nor does it 

include all fish species.

Three aquatic-plant samples were collected in 

Colorado River tributaries in the east Grand Valley, 

upstream from the Gunnison River confluence, and 

had a geometric mean selenium concentration of 

4.1 (ig/g dry weight (table 27). Selenium concentrations in two of the three plant samples from sites IW 

and OM2 (fig. 46) exceeded the toxicity guideline of 

3 jig/g dry weight (Lemly and Smith, 1987; Lemly, 

1993). The geometric mean selenium concentration 

for five invertebrate samples (including crayfish) 

collected in the east Grand Valley was 5.5 (ig/g 

dry weight, and concentrations in three of the five 

invertebrate samples from sites IW and OMD 

(fig. 46) exceeded the guideline of 3 (ig/g dry weight 

selenium. All 15 fish samples collected in the east 

Grand Valley tributaries contained selenium concentrations that exceeded the 1984 NCBP 85th-percentile 

concentration of 0.73 (ig/g wet weight (table 28) and 

exceeded the whole-body concentration of 4 (ig/g 

dry weight that could be of concern for the overall 

health and reproductive vigor of freshwater fish. 

The highest selenium concentration in a fish 

sample was 19 (ig/g dry weight in a red shiner 

collected from Lewis Wash (site LW in fig. 46).

100 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
109°00' 108°45'

00

o

39°15'-

CF1 SAMPLE SITE 

AND NUMBER

GENERALIZED EXTENT OF 

IRRIGATED AREA

Figure 46. Location of 1991 biota sampling sites on tributary streams in the Grand Valley.
The selenium concentrations in fathead minnow 

whole-body samples ranged from 7.9 at site CF1 

to 13 ng/g dry weight at site OMD (table 27) and 

exceeded known toxicity concentrations of 6 (ig/g 

dry weight selenium for fathead minnows (Ogle 

and Knight, 1989; Schultz and Hermanutz, 1990). 

Four aquatic-plant samples collected in the 

middle Grand Valley from tributaries of the Colorado 

River between the Gunnison River confluence and 

the town of Fruita (fig. 46) had a geometric mean 

selenium concentration of 1.8 |ig/g dry weight 

(table 27). The selenium concentration of 7.0 p,g/g 

dry weight from site RED (fig. 46) was the only plant 

sample that had a selenium concentration that 

exceeded the dietary guideline of 3 |ig/g dry weight 

selenium for fish and wildlife (Lemly and Smith,

1987; Lemly 1993). A crayfish sample from Pritchard 

Wash at site PRW (fig. 46) containing 6.2 (ig/g dry 

weight selenium was the only one of three invertebrate 

samples collected from Colorado River tributaries in 

the middle Grand Valley that exceeded the dietary 

guideline of 3 |Xg/g dry weight selenium. All 23 fish 

samples collected in tributary streams in the middle 

Grand Valley contained selenium concentrations that 

exceeded the 1984 NCBP 85th-percentile concentration of 0.73 |Xg/g wet weight (table 28) (Schmitt 

and Brumbaugh, 1990) and all but one sample 

exceeded the whole-body concentration of concern 

of 4 p,g/g dry weight (Lemly and Smith, 1987; Lemly, 

1993). Selenium concentrations in whole-body 

fathead minnows in the middle Grand Valley ranged 

from 8 |0.g/g dry weight (site LC1 in fig. 46) to 22 pg/g

Table 27. Statistical summary of selenium concentrations in selected biota samples collected during 1991 from the 

Grand Valley

[Concentrations in micrograms per gram dry weight; N, number of samples; CM, geometric mean; CD, geometric deviation; , no data];

Statistic

Aquatic pr " **"

algae

Speckled Fathead 

dace minnows Suckers _ Roundtail 

Carp chub

Channel 

catfish

EAST GRAND VALLEY UPSTREAM FROM THE GUNNISON RIVER

N

GM

GD

MAXIMUM

MINIMUM

3

4.1

2.1

7.5

1.8

4

4.6

1.5

7.3

3.0

3 5

7.2 10.1

1.3 1.2

10 13

6.1 7.9

1

-

9.7

9.7

0

-

~

-

2

6.5

2.8

8.5

4.5

0

~

MIDDLE GRAND VALLEY BETWEEN THE GUNNISON RIVER AND FRUITA

N

GM

GD

MAXIMUM

MINIMUM

4

1.8

2.6

7.0

.8

3

4.0

1.5

6.2

2.7

1 5

13.2

1.5

13 22

13 8.0

6

7.8

1.4

10

3.9

0

-

-

-

5

7.6

1.1

8.4

6.8

0

-

--

~

WEST GRAND VALLEY FROM FRUITA TO SALT CREEK (STREAMS)

N

GM

GD

MAXIMUM

MINIMUM

0

-

-

-

2

4.4

1.2

5.0

3.9

3 1

13.4

1.5

19 7.3

8.3 7.3

10

9.2

1.7

19

3.3

0

~

-

-

-

4

8.6

1.4

12

5.8

0

~

-

HIGHLINE RESERVOIR

N

GM

GD

MAXIMUM

MINIMUM

0

-

-

-

-

0

-

-

-

0 0

_.

_.

_-

-

1

-

-

15

15

2

9.2

1.1

10

8.4

0

-

~

-

--

2

5.5

1.2

6.2

4.9

102 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 28. Number of whole-body fish samples collected in the Grand Valley that exceeded the 1984 National Contaminant 

Biomonitoring Program (NCBP) 85th-percentile concentrations for seven trace constituents

[NCBP 8Sth-percentile concentrations in micrograms per gram wet weight; NCBP data from Schmitt and Brumbaugh (1990); east, samples from tributary 

streams located upstream from the Gunnison River; middle, samples from streams located between the Gunnison River and Fruita; west, samples from 

streams located from Fruita to Salt Creek; all, all samples, excluding Highline Reservoir; N, number of samples; numbers in brackets are the number of 

exceedances expressed as a percentage of the number of samples in each group]

Trace 

constituent

Arsenic

Cadmium

Copper

Lead

Mercury

Selenium

Zinc

NCBP 

85th 

percentile

0.27

.05

1.0

.22

.17

.73

34.2

East

(N=15)

0[0]

2 [13]

9 [60]

3 [20]

0[0]

15 [100]

11 [73]

Middle 

(N=23)

1[4]

2 [9]

11 [48]

4 [17]

0[0]

23 [100]

10 [44]

West 

(N=22)

1[4]

8 [36]

11 [50]

7 [32]

0[0]

22 [100]

5 [23]

All 

(N=60)

2 [3]

12 [20]

31 [52]

14 [23]

0[0]

60 [100]

26 [43]

Highline 

Reservoir 

(N=7)

1[20]

2 [29]

3 [43]

2 [29]

0[0]

7 [100]

2 [29]

dry weight (site PSW1) (table 27) and exceeded 

known toxicity concentrations to fathead minnows 

of 6 ^lg/g dry weight (Ogle and Knight, 1989; Schultz 

and Hermanutz, 1990).

No aquatic-plant samples were collected in the 

west Grand Valley from tributary streams of the 

Colorado River between Fruita and Salt Creek. One 

of two invertebrate samples (table 27), a crayfish 

sample containing 5 jig/g dry weight selenium from 

Big Salt Wash (site BSW1 in fig. 46), exceeded the 

dietary guideline of 3 p,g/g dry weight (Lemly and 

Smith, 1987; Lemly, 1993). All of the 22 whole-body 

fish samples collected from tributaries in the west 

Grand Valley, plus 7 whole-body fish samples 

collected from Highline Reservoir (site HR in fig. 46), 

contained selenium concentrations that exceeded the 

1984 NCBP 85th-percentile concentration (table 28) 

and most concentrations also exceeded the concentration of concern of 4 jj,g/g dry weight for whole-body 

fish (Lemly and Smith, 1987; Lemly 1993). The 

highest selenium concentrations in fish collected in 

the west Grand Valley were in sucker and speckled 

dace samples collected from Salt Creek (site SC in 

fig. 46), which contained 19 p,g/g dry weight selenium 

(table 27). Egg and liver tissues were dissected from a 

catfish collected from Highline Reservoir; the egg 

sample contained 11 Jig/g dry weight selenium and the 

liver sample contained 16 p,g/g dry weight selenium 

(Butler and others, 1994, table 28). Lemly (1993) 

stated that 12 jj,g/g dry weight selenium in liver 

tissue and 10 jig/g dry weight selenium in ovary

and egg tissue are biological-effect thresholds for 

the health and reproductive success of freshwater 

anadromous fish. The selenium concentrations in 

the catfish tissue samples from Highline Reservoir 

exceeded these guidelines.

Sample sizes for each biota group were too 

small to use statistical analysis to compare the three 

areas of the Grand Valley, but there was a general 

increase in selenium in roundtail chubs and speckled 

dace samples in a downstream direction (east to west) 

in the Grand Valley (table 27). Only one sucker 

sample was collected in the east Grand Valley, and 

it contained 9.7 p,g/g dry weight selenium. The 

geometric mean selenium concentration for sucker 

samples from the middle Grand Valley was 7.8 p,g/g 

dry weight, and for the west Grand Valley was 

9.2 ^ig/g dry weight (table 27). The geometric 

mean selenium concentration for speckled dace from 

the east Grand Valley was 7.2 p,g/g dry weight; the 

concentration in the one speckled dace sample from 

the middle Grand Valley was 13 jig/g dry weight; 

and the mean concentration for speckled dace from 

the west Grand Valley was 13.4 jig/g dry weight. 

Fish samples collected from Salt Creek and Reed 

Wash (fig. 46), which drain much of the west part 

of the Grand Valley, contained consistently high 

selenium concentrations. The highest median 

selenium concentrations in surface-water samples 

from the Grand Valley also were for the Salt Creek 

and Reed Wash basins.

BIOLOGICAL INVESTIGATIONS 103
Selenium Concentrations in Fish Samples 

from Mainstem Rivers

Beginning in March 1992, before snowmelt 

and irrigation began, fish samples were collected 

from 11 locations on the Colorado, Gunnison, and 

Uncompahgre Rivers (fig. 47). One of the sampling 

sites for the Gunnison River was on the North Fork 

at site NFK2 (fig. 47), the other 10 sites were on the 

three mainstem rivers. Sample locations were selected 

to include river sections that receive major selenium 

loading and could be subject to effects of selenium 

bioaccumulation. The sampling of mainstem rivers 

was designed to collect 10 suckers, 10 speckled 

dace, and 10 roundtail chubs from each of the 

11 sampling sites; however, not all 3 species were 

present at each site. Those three fish species represent 

three trophic levels of species that are abundant in the 

study area.

For all fish species, the 112 fish collected from 

the Gunnison River had an arithmetic mean selenium 

concentration of 7.1 (ig/g dry weight, which was 

significantly higher than the arithmetic mean selenium 

concentrations for the Colorado and Uncompahgre 

Rivers [significance level (p) less than or equal 

to 0.05]. The mean selenium concentration for 

120 fish collected from the Colorado River was 

4.6 (ig/g dry weight, and for 64 fish collected from 

the Uncompahgre River, the mean was 5.1 (ig/g dry 

weight. The mean selenium concentrations for fish 

from the Uncompahgre River and the Colorado River 

are not significantly different (p less than 0.05). Mean 

selenium concentrations for whole-body fish samples 

collected from each of the three river basins exceeded 

the concentration of concern of 4 (ig/g dry weight 

selenium for whole-body fish (Lemly, 1993). The 

mean selenium concentrations for the fish groups 

are shown in figure 48. Mean selenium concentrations 

for roundtail chubs, suckers, and speckled dace 

were highest for the Gunnison River (significantly 

greater for speckled dace; p less than 0.05). Mean 

selenium concentrations in those three fish-trophic 

groups were lowest for the Colorado River 

(significantly so for suckers; p less than 0.05). 

Of the whole-body fish samples from the Gunnison 

River collected in 1992, about 96 percent exceeded 

the 1984 NCBP 85th-percentile concentration for 

selenium (table 29) and about 71 percent exceeded

the selenium concentration of concern of 4 (ig/g 

dry weight for whole-body fish (Lemly, 1993). 

About 80 percent of the whole-body fish samples 

from the Colorado River and the Uncompahgre 

River exceeded the 1984 NCBP 85th percentile. 

Fifty-five percent of Colorado River whole-body 

fish samples and 64 percent of Uncompahgre River 

whole-body fish samples had selenium concentrations 

that exceeded the concentration of concern of 4 (ig/g 

dry weight (Lemly, 1993).

Differences in selenium concentrations among 

fish species were significant. Speckled dace contained 

significantly higher selenium concentrations than the 

other fish species (p-value of 0.05). When data for all 

sites were combined, the mean selenium concentration 

for 105 speckled dace was 8.3 (ig/g dry weight. 

The mean selenium concentration was 5.8 (ig/g dry 

weight in 71 roundtail chubs; 4.7 (ig/g dry weight 

in 10 mottled sculpins; and 3.1 (ig/g dry weight in 

110 suckers. These mean selenium concentrations 

are all significantly different (p-value less than 0.05). 

Species differences accounted for almost 50 percent 

of the variance in the sum of squares using ANOVA 

(p-value equal to 0.0001).

Selenium concentrations in whole-body fish 

were compared between individual sites for each 

of the three rivers. Mean selenium concentrations 

for fish representing three fish trophic groups collected 

from four Colorado River sites are shown in figure 49. 

Mean selenium concentrations for suckers collected 

from the Colorado River indicate a trend of increasing 

concentrations in a downstream direction. The mean 

selenium concentration for suckers from the most 

upstream site (COL1 in fig. 47) was significantly 

less than the mean selenium concentrations of 

suckers collected from the lower three sites (fig. 49). 

In contrast, speckled dace collected from site COL1 

contained a higher mean selenium concentration 

than speckled dace from the lower three sites 

(fig. 49) and was significantly greater than the mean 

selenium concentration in speckled dace collected 

from site COLA Mean selenium concentrations 

in roundtail chubs collected from the Colorado River 

sites were not significantly different (p-value of 0.05), 

but do have a trend of increasing concentration in a 

downstream direction.

104 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
109° 108'

o

GRAND VALLEY 

39°:- IRRIGATED AREA

UNCOMPAHGRE PROJECT 

IRRIGATED AREA

UC16 SAMPLE SITE 

AND NUMBER

Figure 47. Location of sampling sites on major rivers where fish samples were collected in 1992.
14

12

UJ 2 O< 

zee 

OCD 

OD:

-I

UJ CC 

20

<cc

UJO 4

(40)

(42)

(23)

COLORADO RIVER SITES 

GUNNISON RIVER SITES 

UNCOMPAHGRE RIVER SITES" 

(42) SAMPLE SIZE

I STANDARD DEVIATION

(10)

(40)

(40)

(30)

ROUNDTAIL CHUBS SPECKLED DACE SUCKERS MOTTLED 

SCULPINS

Figure 48. Mean selenium concentrations in four fish-trophic groups in the Colorado, North Fork of the Gunnison, Gunnison, 

and Uncompahgre Rivers.

Mean selenium concentrations in fish 

representing three fish-trophic groups collected 

from four sites in the Gunnison River Basin are shown 

in figure 50. The mean selenium concentrations for 

suckers collected from the two lower Gunnison River 

Basin sites (sites GUN4 and GUN6 in fig. 47) were 

significantly higher (p-value less than 0.05) than mean 

selenium concentrations for suckers collected from the 

two upper sites (sites NFK2 and GUN3). Speckled 

dace collected from the outflow site for the Gunnison 

River Basin (site GUN6) contained a significantly 

higher mean selenium concentration than speckled 

dace from the other three sites (fig. 50). There was no 

significant difference among selenium concentrations 

in roundtail chubs for the Gunnison River Basin sites.

Mean selenium concentrations in three fishtrophic groups collected from three Uncompahgre 

River sites are shown in figure 51. There was 

not a significant difference in mean selenium 

concentrations among the three sites for speckled 

dace, although the upper site (site UC2 in fig. 47) 

contained the highest mean selenium concentration. 

The mean selenium concentration for suckers 

collected at the outflow site for the Uncompahgre 

River (site UC16) was significantly higher than 

the concentrations from the upper two sites 

(p-value less than 0.05) (fig. 51). Because only 

one roundtail chub sample was collected from the 

Uncompahgre River, comparisons among sites were 

not possible.

106 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 29. Number of whole-body fish samples collected 

in 1992 from the Colorado, North Fork of the Gunnison, 

Gunnison, and Uncompahgre Rivers that exceeded the 

1984 National Contaminant Biomonitoring Program (NCBP) 

85th-percentile concentrations for seven trace constituents

[NCBP 85th-percentile concentrations in micrograms per gram wet weight; 

NCBP data from Schmitt and Brumbaugh (1990); N, number of samples; 

numbers in brackets are the number of exceedances expressed as a 

percentage of the number of samples in each group]

Trace 

constituent

North Fork

of the

NCBP Colorado Gunnison Uncompahgre 

85th River and River 

percentile (N=120) Gunnison (N=64) 

Rivers 

(N=112)

Arsenic

Cadmium1

Copper

Lead1

Mercury

Selenium

Zinc

0.27

.05

1.0

.22

.17

.73

34.2

8 [6]

4 [3]

18 [15]

23 [19]

19 [16]

95 [79]

36 [30]

9 [8]

3 [3]

42 [38]

15 [13]

13 [12]

108 [96]

28 [25]

6 [9]

18 [28]

15 [23]

17 [27]

0[0]

51 [80]

13 [20]

1 Some samples had reporting limits equal to or greater than the 

1984 NCBP 85th percentile.

A paired t-test was used to test for differences 

between mean selenium concentrations in liver 

and whole-body samples from suckers. As 

expected from other researchers' findings (Sager 

and Cofield, 1984), liver samples contained significantly higher selenium concentrations than wholebody samples. However, selenium concentrations 

in all fish-liver samples were less than the 

concentration of concern of 12 jig/g dry weight 

(Lemly, 1993).

Risk Assessment for Endangered Fish 

from Selenate and Selenite

Acute toxicity tests of the selenium species 

selenate (SeVIO42~) and selenite (SeIVO32~) 

on hatchery-raised Colorado squawfish and 

razorback suckers were conducted during 1993 

(S.J. Hamilton, National Biological Service, written 

commun., 1993). The two endangered fish species 

are endemic to the Colorado River in and downstream 

from the Grand Valley. Selenate and selenite analyses 

were done only on five water samples from the Grand 

Valley; therefore, risk assessment was not done for the

Uncompahgre Project area. Three different life stages 

of each of the two fish species were exposed to the 

inorganic constituents of interest. Concentrations 

representing 96-hour LC50 values (lethal concentration for 50 percent of the test organism during a 

96-hour test) were calculated (table 30). The toxicity 

data in table 30 indicate that selenite selenium is toxic 

to Colorado squawfish and razorback suckers at lower 

concentrations than selenate selenium.

One method used to estimate the hazard of 

chemicals to aquatic life is the margin of safety (or 

margin of uncertainty) between expected environmental concentrations (EEC) of a chemical and 

concentrations of the chemical that cause toxic effects 

in organisms. The 96-hour LC50 concentrations for 

the two endangered fish species were used to 

determine the biologically effective concentration 

(EEC). The EEC values were compared to environmental concentrations of selenate and selenite in water 

samples (the EEC values) collected from five 

drainages in the Grand Valley, which are the 

EEC/EEC ratios. The hazard ratios used to assess 

potential risk are based on dissolved selenium in the 

water. Although most selenium is bioaccumulated 

through the food chain into upper trophic levels of 

aquatic ecosystems (Lemly and Smith, 1987; 

Ohlendorf, 1989; Maier and Knight, 1994), 

waterborne selenium is an important indicator of 

bioaccumulation potential. Based on lethal effects 

observed in acute toxicity tests, hazard ratios 

(EEC/EEC) of less than 100 indicate a high potential 

for hazardous environmental conditions and the 

potential for toxicity (Cairns and others, 1978).

Hazard assessments for three life stages [0- to 

98-day old (swim-up), 99- to 115-day old, and 193- to 

207-day old] of Colorado squawfish and razorback 

suckers are summarized in table 31. The selenate 

concentrations in the five Grand Valley drainages 

could theoretically pose a moderate risk (EEC/EEC 

ratios between 100 and 1,000) to the swim-up life 

stage of the Colorado squawfish and to all tested life 

stages of the razorback sucker (table 31). For all three 

life stages of both species, selenite concentrations 

were in the low risk category or in the range of 

uncertainty (EEC/EEC ratios between 1,000 and 

5,000). Adult Colorado squawfish and razorback 

suckers use backwater habitats in the Colorado River 

during spring prior to spawning.

BIOLOGICAL INVESTIGATIONS 107
12

10

Itree I-Q

otr e ^LU°-

o

(10)

(10)

(10)

Y///A ROUNDTAIL CHUBS 

SPECKLED DACE 

SUCKERS 

(10) SAMPLE SIZE 

I STANDARD DEVIATION

(10)

(10)

COL1 COL4 COL6 COLS

SITE

Figure 49. Mean selenium concentrations in three fish-trophic groups collected from four sites on the Colorado River. 

Sites are in downstream order from left to right.

Other Trace-Constituent Concentrations 

in Fish Tissues and Fish Food Items in 

Irrigated Areas

Uncompahgre Project Area

Other trace constituents in whole-body fish 

samples that exceeded the 1984 NCBP 85th-percentile 

concentrations are listed in table 26. Other than 

selenium, copper most often exceeded the 1984 NCBP 

85th-percentile concentration for whole-body fish. 

Moore and Ramamoorthy (1984) suggested that, in 

unpolluted freshwater, maximum copper concentrations in fish muscle tissue seldom exceed 1 |ig/g wet 

weight (4 |ig/g dry weight at 75-percent moisture). 

All copper concentrations in fillet samples collected 

from the Uncompahgre Project area were less than this 

guideline concentration. Dixon and Sprague (1981)

reported that rainbow trout exposed to 30 |ig/L copper 

for 7 to 21 days accumulated whole-body concentrations of 10 to 11 |ig/g dry weight copper. This concentration range is similar to copper concentrations in 

some whole-body trout samples collected in 

the Uncompahgre Project area (sites HCC1, SP4, 

DRY1, and CR in figs. 44 and 45) (Butler and others, 

1994). Dixon and Sprague (1981) reported that 

mean copper residues in trout prior to copper 

exposure ranged from 3.2 to 4.5 |ig/g dry weight, 

which also are similar to copper concentrations in 

some whole-body trout samples collected in the 

Uncompahgre Project area. Dixon and Sprague 

(1981) reported that rainbow trout were able to 

acclimate to copper, which significantly altered 

their lethal tolerance. Moore and Ramamoorthy 

(1984) indicated that copper concentrations of 10 

to 100 |ig/g dry weight in aquatic plants and 5 to

108 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage In the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
18

16

h- 14

2-2

tree h-Q

otr 

Iff 8

QjCC 

C/)0

58So

 4

2 -

V///A ROUNDTAIL CHUBS 

SPECKLED DACE 

SUCKERS 

(10) SAMPLE SIZE 

I STANDARD DEVIATION (10>

(10)

(10)

(10)

(10) (10)

(10)

(10)

NFK2 GUNS GUN4 GUN6

SITE

Figure 50. Mean selenium concentrations in three fish-trophic groups collected from four sites in the Gunnison River Basin. 

Sites are in downstream order from left to right.

200 }ig/g dry weight in aquatic invertebrates were 

indicative of copper-polluted freshwater systems. 

Aquatic-plant and -invertebrate samples collected 

from the Uncompahgre Project area contained 

copper concentrations in these ranges.

About 20 percent of the cadmium concentrations in whole-body fish collected within and adjacent 

to the Uncompahgre Project exceeded the NCBP 

85th percentile (table 26). Cadmium concentrations 

were less than the 2-}ig/g wet-weight concentration 

that Moore and Ramamoorthy (1984) indicated was 

evidence of probable contamination and less than the 

5-JJg/g wet-weight concentration that Eisler (1985) 

reported was a possible toxic cadmium concentration 

for fish.

Concentrations of arsenic, lead, and mercury in 

a few whole-body fish samples exceeded 1984 NCBP 

85th percentiles (table 26). Moore and Ramamoorthy

(1984) indicated that, in unpolluted or mildly contaminated water, arsenic concentrations in whole-body 

fish should not exceed the range of less than 0.1 

to 0.4 }ig/g wet weight. Fish samples from Dry 

Creek (site DRY1 in fig. 44) (0.55 ng/g wet weight), 

Roubideau Creek (site RB1) (0.63 }ig/g wet weight), 

and Crawford Reservoir (site CR in fig. 45) (2.54 }ig/g 

wet weight) exceeded this guideline. Diminished 

growth and survival have been reported for bluegills 

when arsenic residues in muscle tissue exceeded 

11.3 }ig/g wet weight in immature fish or 5 }ig/g 

wet weight in the adults (Natural Resources Council 

of Canada, 1978). A whole-body yellow perch 

sample collected from Crawford Reservoir (site CR) 

had an arsenic concentration (2.54 jug/g wet weight) 

that was comparable to the toxic arsenic concentrations in adult bluegill muscle. Lead concentrations 

in whole-body fish exceeded the 1984 NCBP

BIOLOGICAL INVESTIGATIONS 109
16

14

<>

cccc 1rt

feS °

OCC 8

o

(3)

ROUNDTAIL CHUBS 

SPECKLED DACE

UUIUIHJ SUCKERS

(10) SAMPLE SIZE 

I STANDARD DEVIATION

(10)

(10)

(10)

0)

(10)

(10)

UC2 UC10

SITE

UC16

Figure 51. Mean selenium concentrations in three fish-trophic groups collected from three sites on the Uncompahgre River. 

Sites are in downstream order from left to right.

85th percentile for two samples collected east of 

the Uncompahgre River and nine samples collected 

west of the Uncompahgre River (table 26). Highest 

lead concentrations were in two bluehead sucker 

samples collected from Dry Creek and Roubideau 

Creek (sites DRY1 and RB1 in fig. 44). Four wholebody fish samples collected from tributaries west of 

the Uncompahgre River contained mercury concentrations that exceeded the 1984 NCBP 85th percentile 

of 0.1 |Lig/g wet weight (table 26) and exceeded 

the dietary guideline of 0.1 p,g/g wet weight for 

the protection of sensitive consumer bird species 

(Eisler, 1987a). The highest mercury concentration of 

0.34 (ig/g wet weight was in a speckled dace sample 

from Spring Creek (site SP4 in fig. 44).

Table 30. Concentrations of selenate and selenite selenium 

causing acute toxicity to Colorado squawfish and razorback 

suckers

[96-hour LC50 concentration is the lethal concentration for 50 percent 

of the test organisms during a 96-hour exposure; data from Hamilton 

(1993)]

Life 

stage 

(age, 

in days)

Less than 99 

(swim-up)

99 to 115 

193 to 207

96-hour LC50 concentrations, 

in micrograms per liter

Colorado 

squawfish

Selenate

67,000

297,000 

320,000

Selenite

14,000

36,000 

31,000

Razorback 

sucker

Selenate

48,000

35,000 

26,000

Selenite

15,000

9,200 

16,000

110 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 31. Hazard assessment of selenate and selenite selenium in water samples collected in March 1992 from five streams 

in the Grand Valley to three life stages of Colorado squawfish and razorback suckers

[Selenate and selenite data from R.F. Fujii (U.S. Geological Survey, written commun., 1992); HgfL, micrograms per liter; hazard assessment is equal to 

BEC/EEC, where BEC is the biologically effective concentration and EEC is the estimated environmental concentration of a contaminant; a hazard assessment greater than 5,000 is low risk, 1,000 to 5,000 is uncertain risk, 100 to 1,000 is moderate risk, and less than 100 is high risk (Cairns and others, 1978); 

--, no risk assessment because contaminant concentration was 0 microgram per liter]

Selenate selenium

Stream Water concentration

(H9/L)

BEC/EEC for the three life stages

Less than 

99 days old 

(swim-up)

99 to 193 to conci 

115 days old 207 days old ft

feter 

miration 

i9/L)

Seienite selenium

BEC/EEC for the three life stages

, Less than 

99 days old 

(swim-up)

99 to 

11 5 days old

193 to 

207 days old

COLORADO SQUAWFISH

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

71.7

126.2

72.5

83.4

82.8

71.7

126.2

72.5

83.4

82.8

934

531

924

803

809

670

380

662

576

580

4,142

2,353

4,097

3,561

3,587

488

277

483

420

423

4,463

2,536

4,414

3,837

3,865

RAZORBACK SUCKER

363

206

359

312

314

1.1

0

2.3

2.2

3.3

1.1

0

2.3

2.2

3.3

12,727

-

6,087

6,364

4,242

13,636

6,522

6,818

4,546

32,727

-

15,652

16,364

10,909

8,364

4,000

4,182

2,788

28,182

--

13,478

14,091

9,394

14,546

6,957

7,273

4,849

About 22 percent of the zinc concentrations 

in whole-body fish samples collected in the 

Uncompahgre Project area exceeded the 1984 NCBP 

85th-percentile concentration (table 26). Schmitt and 

Brumbaugh (1990) reported that the mean zinc 

concentration in common carp (63.4 [ig/g wet weight) 

was significantly higher than the mean for all other 

species combined. The highest zinc concentrations in 

fish collected from the Uncompahgre Project area 

were in carp from sites NFK2, CMG1, and RB1 

(figs. 44 and 45), and these concentrations (Butler and 

others, 1994, tables 25 and 27) were comparable to the 

geometric mean concentrations in carp reported by 

Schmitt and Brumbaugh (1990).

Some samples collected from tributaries west 

of the Uncompahgre River had concentrations that 

exceeded NCBP 85th percentiles for each of the 

seven trace constituents. In other streams in the 

Uncompahgre River Basin, these exceedances 

could be attributed to pollution from mining activities. 

However, no mining is occurring on the Uncompahgre 

Plateau upstream from the irrigated areas, which is 

the source of the natural runoff in tributary streams

on the western side of the project. The trace constituents probably were introduced in the irrigation 

water diverted from the Uncompahgre River, which 

may be affected by runoff from previous mining 

activities.

Grand Valley

As in the Uncompahgre Project area, copper 

was the trace constituent other than selenium that most 

often exceeded the 1984 NCBP 85th-percentile 

concentration for whole-body fish from the Grand 

Valley (table 28). Highest copper concentrations were 

3.0 [Lg/g wet weight (11.1 [Lg/g dry weight) in a red 

shiner sample collected from Lewis Wash (site LW 

in fig. 46) and 3.0 [Lg/g wet weight (10.2 [Lg/g 

dry weight) in the speckled dace sample from Salt 

Creek (site SC) (Butler and others, 1994, tables 28 

and 30). These concentrations are within the range 

of copper concentrations in rainbow trout after 

exposure to 30 [LgfL copper for 7 to 21 days (Dixon 

andSprague, 1981). Most copper concentrations in 

whole-body fish collected from the Grand Valley were

BIOLOGICAL INVESTIGATIONS 111
comparable to the pre-exposure concentrations of 

3.2 to 4.5 jAg/g dry weight reported by Dixon and 

Sprague(1981).

Only two arsenic concentrations in wholebody fish from streams exceeded the 1984 NCBP 

85th-percentile concentration; 0.32 ^ig/g wet weight 

in a bluehead sucker from Leach Creek (site LC1 

in fig. 46) and 0.32 ^ig/g wet weight in a bluehead 

sucker sample from Salt Creek (site SC) (Butler 

and others, 1994, tables 29 and 30). All cadmium 

concentrations were less than the 2-jAg/g wet-weight 

concentration in whole-body vertebrates that Moore 

and Ramamoorthy (1984) suggested was evidence 

of probable cadmium contamination.

Fourteen whole-body fish samples collected 

from Grand Valley tributaries and two samples 

collected from Highline Reservoir (site HR in fig. 46) 

contained lead concentrations that exceeded the 

1984 NCBP 85th-percentile concentration (table 28). 

All mercury concentrations in Grand Valley fish 

were less than the 1984 NCBP 85th-percentile 

concentration (table 28).

Zinc concentrations in fish from the Grand 

Valley were comparable to the concentrations in fish 

collected in the Uncompahgre Project area. Highest 

zinc concentrations in Grand Valley fish (Butler and 

others, 1994, tables 28-30) were in a red shiner 

sample collected from Lewis Wash (site LW in fig. 46) 

in the east Grand Valley and in a speckled dace sample 

collected from Salt Creek (site SC) in the west Grand 

Valley.

Other Trace-Constituent Concentrations 

in Fish Tissues and Fish Food Items 

from Mainstem Rivers

In addition to selenium, copper and zinc 

had the greatest number of exceedances of the 

1984 NCBP 85th-percentile concentrations (Schmitt 

and Brumbaugh, 1990) for whole-body fish collected 

from the three major river basins (table 29). Samples 

from North Fork of the Gunnison and the Gunnison 

River had a higher proportion of copper exceedances 

(38 percent) than samples from the Uncompahgre 

River (23 percent) and the Colorado River 

(15 percent). All of the fish samples from the 

Uncompahgre and Colorado Rivers that exceeded 

the 1984 NCBP 85th percentile for copper contained

between 1 and 2.5 ^ig/g wet weight (Butler and others, 

1994, tables 36 and 37). However, 13 of the Gunnison 

River fish samples had copper concentrations that 

exceeded 2.5 jAg/g wet weight and ranged from 2.68 

to 86.2 jAg/g wet weight. The copper concentrations 

in Gunnison River fish samples indicated considerable 

exposure to a copper-contaminated environment 

(Dixon and Sprague, 1981; Moore and Ramamoorthy, 

1984).

Exceedances of the 1984 NCBP 85th percentile 

for arsenic were fairly evenly distributed among the 

three major river basins (table 29). According to 

Moore and Ramamoorthy (1984), arsenic concentrations of less than 0.1 to 0.4 jAg/g wet weight were 

indicative of fish from environments uncontaminated 

by arsenic. Eight samples from the Gunnison River 

and two samples from the Uncompahgre River 

exceeded the guideline concentration of 0.4 jAg/g 

wet weight. Fish samples collected from the 

Uncompahgre River contained the highest 

proportion of concentrations that exceeded the 

NCBP 85th percentiles for cadmium and lead 

(table 29). All cadmium concentrations were less 

than the guideline concentration of 2 jAg/g wet weight 

that Moore and Ramamoorthy (1984) indicated was 

evidence of probable cadmium contamination. The 

highest lead concentrations were in two flannelmouth 

sucker samples collected from the Gunnison River in 

Delta (site GUN4 in fig. 47), at 4.0 and 6.6 \ig/g dry 

weight (Butler and others, 1994, table 35). Several 

samples had reporting limits for lead greater than 

the 1984 NCBP 85th percentile; therefore, the actual 

number of exceedances is not known.

Mercury concentrations in about 16 percent 

of Colorado River fish samples and about 12 percent 

of Gunnison River fish samples exceeded the 

1984 NCBP 85th-percentile concentration (table 29). 

None of the Uncompahgre River fish samples 

contained mercury concentrations that exceeded the 

1984 NCBP 85th percentile. All mercury concentrations in fish from the Colorado and Gunnison Rivers 

that exceeded the 1984 NCBP 85th percentile also 

exceeded the 0.1-^ig/g wet-weight concentration 

that is recommended for the protection of sensitive 

consumer bird species (Eisler, 1987a), but were 

less than the whole-body concentration of 5 jAg/g 

wet weight for the protection of brook trout 

(Armstrong, 1979).

112 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Fish from the Colorado River had the highest 

proportion of concentrations that exceeded the NCBP 

85th percentile for zinc (30 percent), followed by the 

Gunnison River (25 percent) and the Uncompahgre 

River (20 percent) (table 29). Schmitt and Brumbaugh 

(1990) reported high variability in zinc concentrations 

among fish species. Of all fish collected from 

mainstem rivers during this study, speckled dace often 

had the greatest proportion of zinc concentrations that 

exceeded the NCBP 85th percentile.

Risk Assessment for Endangered Fish 

from Four Trace Constituents

Acute toxicity tests for boron, uranium, 

vanadium, and zinc were conducted (Hamilton, 

1993) on three different life stages of Colorado 

squawfish and razorback suckers by methods 

similar to that described for selenium. Lethal 

concentrations (LCSO's) for each of the three life 

stages of Colorado squawfish and razorback suckers 

are listed in table 32. As was done for selenium, the 

96-hour LC50 concentrations for the two endangered 

fish species (BEC) were compared to actual concentrations of boron, uranium, vanadium, and zinc in water 

samples (EEC) collected from five Grand Valley 

drainages. Hazard assessments (the EEC/EEC ratios) 

for each life stage of the two endangered fish species 

were calculated for the four trace constituents. 

Maximum water concentrations of the four trace 

constituents (Butler and others, 1994, table 5) 

were used to determine the BEC/EEC ratio so that 

the maximum hazard could be estimated. Hazard 

assessments for the three life stages of each 

endangered fish species are listed in table 33 for 

boron and uranium and in table 34 for vanadium and 

zinc.

Most of the hazard estimates for boron, 

vanadium, and zinc (tables 33 and 34) are in the 

moderate range of risk; the BEC/EEC ratio usually 

was between 100 and 1,000. Of most concern are zinc 

concentrations in Adobe Creek, Leach Creek, and 

Indian Wash, which were in the high-risk category for 

the swim-up life stage of Colorado squawfish. Most 

of the hazard ratios for uranium (table 33) were greater 

than 1,000, but less than 5,000, which is in the range 

of uncertain risk.

Table 32. Concentrations of boron, uranium, vanadium, 

and zinc causing acute toxicity to Colorado squawfish and 

razorback suckers

[96-hour LC50 concentration is the lethal concentration for 50 percent of 

the test organisms during a 96-hour exposure; >, greater than; data from 

Hamilton (1993)]

Life stage 

(age, in days)

96-hour LC50 concentration, 

in mlcrograms per liter

Boron Uranium Vanadium Zinc

COLORADO SQUAWFISH

Less than 99 

(swim-up)

99 to 115

193 to 207

279,000

>100,000

511,000

46,000

46,000

46,000

7,800

3,800

4,200

1,700

4,200

12,000

RAZORBACK SUCKERS

Less than 99 

(swim-up)

99 to 115

193 to 207

254,000

279,000

>100,000

46,000

46,000

46,000

8,700

4,000

3,200

4,000

6,400

15,000

Organic Contaminants in Fish Tissues

Four fish samples collected in the Uncompahgre 

Project area and two fish samples collected in the 

Grand Valley area were analyzed for organochlorine 

pesticides and polychlorinated biphenyls (PCB's) 

(Butler and others, 1994, table 38). One flannelmouth 

sucker sample collected from Dry Creek (site DRY1 

in fig. 44) contained concentrations of p,p'-DDE 

(0.63 |ig/g wet weight), p,p'-DDD (0.08 |ig/g 

wet weight), p,p'-DDT (0.14 jig/g wet weight), 

dieldrin (0.09 |ig/g wet weight), and toxaphene 

(0.82 jig/g wet weight) that exceeded the 1984 NCBP 

geometric mean concentrations for these compounds 

(table 35) (Schmitt and others, 1990). A white 

sucker sample collected from Roubideau Creek 

(fig. 44) also contained a p,p'-DDE concentration 

(0.41 jig/g wet weight) that was higher than the 

1984 NCBP geometric mean concentration. The 

organic compound p,p'-DDE is the most persistent 

degradation product of the parent compound 

p,p'-DDT. DDT was banned in the United States 

in 1972. Schmitt and others (1990) reported that 

the percentage of total DDT that consists of DDE 

has increased steadily from 48 percent in 1970 to 

73 percent in 1984.

BIOLOGICAL INVESTIGATIONS 113
Table 33. Hazard assessment of boron and uranium in water samples collected from five streams in the Grand Valley to three 

life stages of Colorado squawfish and razorback suckers

[|ig/L, micrograms per liter; hazard assessment is equal to EEC/EEC, where BEC is the biologically effective concentration and EEC is the estimated 

environmental concentration of a contaminant; a hazard assessment greater than 5,000 is low risk, 1,000 to 5,000 is uncertain risk, 100 to 1,000 is 

moderate risk, and less than 100 is high risk; >, greater than]

Boron

Maximum 

Stream water 

concentration 

(ug/L)

BEC/EEC for the three life stages Maximum

Less than 

99 days old 

(swim-up)

Uranium

BEC/EEC for the three life stages

99 1 -mat water Less than

*i*\? ,j o«-rl 1,1 concentration 99 days old 115 days old 207 days old (wj/L) (sw|m_up)

99 to 

115 days old

193 to 

207 days old

COLORADO SQUAWFISH

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

330

380

330

370

600

330

380

330

370

600

846

734

846

754

465

770

668

770

687

423

>303

>263

>303

>270

>167

846

734

846

754

465

1,549

1,345

1,549

1,381

852

RAZORBACK SUCKER

>303

>263

>303

>270

>167

23

35

17

20

48

23

35

17

20

48

2,000

1,314

2,706

2,300

958

2,000

1,314

2,706

2,300

958

2,000

1,314

2,706

2,300

958

2,000

1,314

2,706

2,300

958

2,000

1,314

2,706

2,300

958

2,000

1,314

2,706

2,300

958

Table 34. Hazard assessment of vanadium and zinc in water samples collected from five streams in the Grand Valley to three 

life stages of Colorado squawfish and razorback suckers

[ug/L, micrograms per liter; hazard assessment is equal to BEC/EEC, where BEC is the biologically effective concentration and EEC is the estimated 

environmental concentration of a contaminant; a hazard assessment greater than 5,000 is low risk, 1,000 to 5,000 is uncertain risk, 100 to 1,000 is 

moderate risk, and less than 100 is high risk]

Vanadium

Maximum 

Stream water 

concentratior 

(ug/L)

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

Salt Creek

Reed Wash

Adobe Creek

Leach Creek

Indian Wash

5

5

5

4

9

5

5

5

4

9

BEC/EEC for the three life stages Maximum

Less than 

i 99 days old 

(swim-up)

1,560

1,560

1,560

1,950

867

1,740

1,740

1,740

2,180

967

water

« K ? t0 , on.1?3 '0 , concentration 11 5 days old 207 days old (ug/L)

760

760

760

950

422

800

800

800

1,000

444

COLORADO SQUAWFISH

840

840

840

1,050

467

RAZORBACK SUCKER

640

640

640

800

356

10

10

20

30

20

10

10

20

30

20

Zinc

BEC/EEC for the three life stages

Less than 

99 days old 

(swim-up)

170

170

85

57

85

400

400

200

133

200

99 to 

11 5 days old

420

420

210

140

210

640

640

320

213

320

193 to 

207 days old

1,200

1,200

600

400

600

1,500

1,500

750

500

750

114 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
Table 35. Geometric mean and maximum concentrations 

of the 1984 National Contaminant Biomonitoring Program 

(NCBP) for selected organochlorine pesticides and 

polychlorinated biphenyls (PCB's)

[Concentrations in micrograms per gram wet weight; NCBP data from 

Schmitt and others (1990)]

Compound

p,p'-DDE 

p,p'-DDD 

p,p'-DDT 

Total DOT

Heptachlor

Dieldrin

Toxaphene 

cis-Chlordane

trans-Chlordane

cis-Nonachlor

trans-Nonachlor

Oxychlordane 

Total PCB's

NCBP 

geometric 

mean 

concentration

0.19 

.06 

.03 

.26

.01

.04

.14 

.03

.02

.02

.03

.01 

.39

NCBP 

maximum 

concentration

4.74 

2.55 

1.79 

9.08

.29

1.39

8.2 

.66

.35

.45

1.0

.29 

6.7

The toxaphene concentration of 0.82 (4,g/g 

wet weight in the flannelmouth sucker from Dry 

Creek (site DRY1) was almost six times greater 

than the 1984 NCBP geometric mean concentration 

(table 35). Mayer and Mehrle (1977) reported that 

toxaphene residues in excess of 0.4 to 0.6 (4,g/g 

wet weight might be hazardous to fish health, and 

Eisler and Jacknow (1985) indicated that such concentrations indicate possible environmental contamination. The toxaphene concentration of 0.82 (4,g/g wet 

weight exceeded these two guidelines. However, the 

toxaphene concentration was less than the U.S. Food 

and Drug Administration's action level of 5.0 (4,g/g 

wet weight in fish that is considered hazardous for 

human consumption (U.S. Environmental Protection 

Agency, 1980).

Bile samples were collected from fish and 

analyzed to provide an indication of polycyclic 

aromatic hydrocarbon (PAH) exposure. Bile is a 

sensitive indicator for assessing the exposure of fish to 

PAH's (McDonald and others, 1991). High concentrations of PAH metabolites in bile indicated actual 

exposure, and high concentrations of PAH's in 

sediment indicated a potential for exposure (Johnston 

andBaumann, 1989).

Ten fish-bile samples collected from stream and 

river sites in and adjacent to the Uncompahgre Project, 

plus one fish-bile sample collected from Crawford 

Reservoir, were analyzed for 24 PAH's. Two fishbile samples collected from streams in the Grand 

Valley area and one fish-bile sample from Highline 

Reservoir also were analyzed for PAH's. All PAH 

data are listed in table 39 in Butler and others (1994). 

One of the compounds included in the PAH analysis 

was a-benzopyrene, a known carcinogen (Eisler, 

1987b). Eisler (1987b) reported that selenium can be 

an effective inhibitor of PAH-induced tumor development. Eisler (1987b) also reported that no criteria or 

standards have been established for PAH's by any 

regulatory agency for the protection of sensitive 

species of aquatic organisms or wildlife. The highest 

concentrations of PAH's, including a-benzopyrene, in 

fish bile collected for this study were in a white sucker 

sample collected from the North Fork of the Gunnison 

River near Paonia (site NFK3 in fig. 45).

Selenium Concentrations in 

Migratory Birds and Their Food Items

Uncompahgre Project Area Wetlands

Migratory birds were sampled from wetlands 

during 1991 and 1992. Wetlands were geographically 

divided into those in the irrigated area of the 

Uncompahgre Project and those adjacent to the 

Uncompahgre Project (fig. 52). Sites in the 

Uncompahgre Project are: Markley Pond (site MKP), 

Gretts Pond (site GTP), Vo-Tech marsh (site VT), 

Sweitzer Lake (site SWL), Brozina Pond (site BZP), 

and a drainage at G Road (site UNGR) (fig. 52). The 

wetland sites adjacent to the Uncompahgre Project 

are: Ferrier Pond (site FRP), Fruitgrowers Reservoir 

(site FGR), St. George Pond (site SGP), and Alkali 

Reservoir (site AKR) (fig. 52).

Selenium concentrations in aquatic-plant and 

algae samples collected from the Uncompahgre 

Project area wetlands ranged from 4.2 (j,g/g dry weight 

at site GTP (fig. 52) to 12 H-g/g dry weight at site BZP 

(Butler and others, 1994, table 32). Fathead minnows 

had selenium concentrations that ranged from 36 |4,g/g 

dry weight at site GTP to 59 (4,g/g dry weight at 

site BZP. An aquatic-invertebrate sample collected 

from site MKP contained 32 (j,g/g dry weight 

selenium. These selenium concentrations greatly 

exceeded 3 (4,g/g dry weight, a dietary selenium

BIOLOGICAL INVESTIGATIONS 115
108°15' 108°00' 107°45'

38°45' 

38°30' 

38°15'

A WETLAND SITES WITHIN

THE UNCOMPAHGRE PROJECT

WETLANDS ADJACENT TO THE 

UNCOMPAHGRE PROJECT

GENERALIZED EXTENT 

OF IRRIGATED AREA

0 10 MILES

0 5 10 KILOMETERS

Figure 52. Location of sampling sites at wetlands in the Uncompahgre Project area.

concentration of concern for protection of fish and bird 

species (Lemly and Smith, 1987; Lemly 1993). The 

selenium concentrations in fathead minnows greatly 

exceeded the concentrations reported in laboratory 

studies in which fathead minnows showed adverse 

effects, especially reproductive failure (Ogle and 

Knight, 1989; Schultz and Hermanutz, 1990).

Selenium concentrations in bird-liver and -egg 

samples (table 36) exceeded normal concentrations 

in some samples and were in the range of potentially

toxic concentrations in other samples. Skorupa 

and Ohlendorf (1991) considered normal selenium 

concentrations for bird eggs to average less than 

or equal to 3 flg/g dry weight and stated that 

there was a high risk of adverse biological effects 

occurring with a population mean of greater than 

or equal to 20 flg/g dry weight selenium in bird eggs. 

Skorupa and Ohlendorf (1991) also advised that a 

population mean selenium concentration between 

3 and 20 flg/g dry weight in bird eggs needed

116 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfatfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
further field studies of reproductive performance 

to assess biological risk. The geometric mean 

selenium concentration for eggs for all bird species 

collected within the Uncompahgre Project was 

14.7 |J,g/g dry weight (geometric deviation 1.8) 

(table 36). The geometric mean selenium concentration in five avocet eggs collected from Brozina Pond 

(site BZP) was 41.9 flg/g dry weight (geometric 

deviation 1.1). Selenium concentrations of that 

magnitude were associated with a 5-percent 

incidence of teratogenic effects (J.P. Skorupa and 

others, U.S. Fish and Wildlife Service, written 

commun., 1993). A liver from an immature avocet 

from site BZP contained 98 flg/g dry weight 

selenium, greatly exceeding 30 flg/g dry weight 

selenium, a concentration associated with a high 

risk of adverse biological effects (J.P. Skorupa and 

others, U.S. Fish and Wildlife Service, written 

commun., 1993). Geometric mean selenium concentrations in coot, killdeer, and blackbird egg samples 

collected in the Uncompahgre Project were in the 

range of concern of 3 to 20 jxg/g dry weight (Skorupa 

and Ohlendorf, 1991).

As in samples collected from wetlands located in 

the irrigated area of the Uncompahgre Project, selenium 

concentrations in bird-livers and -egg samples from 

wetlands adjacent to the project (table 36) exceeded 

normal concentrations in some samples and were in 

the range of adverse health effects and reproductive 

failure in other samples. St. George Pond (site SGP 

in fig. 52) and Ferrier Pond (site FRP) contained 

some of the highest selenium concentrations in 

biota samples collected for this study. One aquaticplant sample collected from Ferrier Pond contained 

68 flg/g dry weight selenium (Butler and others, 

1994, table 33), which is more than 20 times greater 

than the recommended dietary guideline of 3 flg/g 

dry weight selenium for the protection of fish and 

wildlife resources (Lemly and Smith, 1987; Lemly, 

1993). Two fathead minnow samples collected at 

site SGP contained 72 and 110 flg/g dry weight 

selenium (table 36), which not only greatly exceeded 

the dietary guideline for protection of birds, but also 

were much greater than the range of 6 to 8 flg/g 

dry weight associated with adverse health effects 

and reproductive failure in fathead minnows (Ogle 

and Knight, 1989; Schultz and Hermanutz, 1990).

Table 36. Statistical summary of selenium concentrations in selected bird-food and bird-tissue samples collected during 

1991-92 from wetlands in the Uncompahgre Project, wetlands adjacent to the Uncompahgre Project, and wetlands in the 

Grand Valley

[Concentrations in micrograms per gram dry weight; N, number of samples; GM, geometric mean; GD, geometric deviation; , no data]

Statistic

N 

GM

GD

MAXIMUM

MINIMUM

Bird-food samples

Aquatic 

plants

5 

7.3

1.6

12

4.2

Aquatic Fathead 

invertebrates minnows

Green 

sunfish Carp Whole 

body

WITHIN THE UNCOMPAHGRE PROJECT

1 4 1 0 33 

48.7 -- - 13.7

1.2

32 59

32 36

--

55

55 ~

1.7

39

4.2

Bird-tissue samples

Liver

1

98

98

Breast

0

-

Egg

34 

14.7

1.8

47.6

7.0

ADJACENT TO THE UNCOMPAHGRE PROJECT

N

GM

GD

MAXIMUM

MINIMUM

2

23.7

4.4

68

8.3

0 2

91.2

1.3

110

72

0

~

~

--

2

14.6

1.1

15

14

13

18.3

1.5

43

10

14

30.3

1.5

59

17

5

20.4

1.9

47

10

25

13.4

1.6

24

4.1

GRAND VALLEY

N

GM

GD

MAXIMUM

MINIMUM

0

--

-

--

--

0 0

__

._

..

0

-

-

--

--

0

~

-

~

--

10

10.0

1.9

18

2.3

11

23.9

1.5

37

10

0

--

«

-

--

20

6.7

1.5

17

3.9

BIOLOGICAL INVESTIGATIONS 117
The geometric mean selenium concentration for 

25 bird eggs (all species) collected from wetlands 

adjacent to the Uncompahgre Project was 13.4 jig/g 

dry weight (geometric deviation 1.6) (table 36). 

Geometric mean selenium concentrations in coot, 

yellow-headed blackbird, and grebe eggs collected 

in adjacent wetlands also were in the range of concern 

of 3 to 20 |ig/g dry weight (Skorupa and Ohlendorf, 

1991).

Grand Valley Wetlands

Bird samples were collected from six Grand 

Valley wetland sites: Romig Pond (site RMP), Reid 

Pond (site RDP), Thompson Pond (site TMP), Mack 

Wash (site MWP), Reed Wash (site RWEB), and 

19 and M Roads (site 19M) (fig. 53). The geometric 

mean selenium concentration for 20 egg samples of 

all bird species collected in Grand Valley wetlands 

was 6.7 |ig/g dry weight (geometric deviation 1.5) 

(table 36), which is one-half of the geometric mean 

selenium concentrations for eggs collected from the 

Uncompahgre Project area, but the mean is in the 

range of concern. The geometric mean selenium 

concentration in three avocet eggs from Thompson 

Pond (site TMP) was 5.9 |ig/g dry weight (geometric 

deviation 1.2), which was considerably lower than the 

geometric mean selenium concentration for avocet 

eggs from the Uncompahgre Project area (from 

site BZP). The geometric mean selenium concentrations for red-winged blackbird eggs of 7.3 |ig/g dry 

weight (geometric deviation 1.4) and for mallard 

eggs of 6.6 jig/g dry weight (geometric deviation 1.6) 

were in the range of concern (3 to 20 |ig/g dry weight) 

that would need further field reproductive studies to 

determine the biological risk (Skorupa and Ohlendorf, 

1991). The geometric mean selenium concentration 

for seven mallard liver samples collected from Grand 

Valley wetlands was 23.4 |ig/g dry weight (geometric 

deviation 1.5), also in the range of selenium concentrations in bird livers that would need additional 

investigation to assess biological risk.

Other Trace-Constituent Concentrations 

in Birds and Their Food Items

Mercury concentrations in aquatic-plant, 

aquatic-invertebrate, and fathead minnow 

samples collected within the irrigated area of the

Uncompahgre Project at Markley Pond (site MKP), 

Gretts Pond (site GTP), and Brozina Pond (BZP) 

(fig. 52) were less than the 0.1-jig/g wet-weight 

concentration that Eisler (1987a) recommended for 

the protection of sensitive consumer bird species. 

Mercury concentrations in all bird eggs collected from 

the Uncompahgre Project area and in the Grand Valley 

were less than the 0.9-|ig/g wet-weight concentration 

associated with adverse behavioral effects in mallards 

(Heinz, 1979). The highest mercury concentration in a 

bird-egg sample was 1.7 |ig/g dry weight in a piedbilled grebe egg collected from Fruitgrowers 

Reservoir (site FGR in fig. 52).

Mercury concentrations in bird-liver samples 

collected from the Uncompahgre Project area and the 

Grand Valley were less than the 20-jig/g wet-weight 

concentration in bird soft tissues that Finley and others 

(1979) considered to be hazardous to bird health. The 

highest mercury concentration in a bird-liver sample 

was in a pied-billed grebe liver from Ferrier Pond 

(site FRP) that had a concentration of 15.6 jig/g 

dry weight (4.0 |ig/g wet weight). Two western 

grebe samples collected adjacent to the Uncompahgre 

Project from Fruitgrowers Reservoir (site FGR) 

had mercury concentrations of 6.6 |ig/g dry weight 

(2.0 jig/g wet weight) and 1.98 |ig/g dry weight 

(0.49 |ig/g wet weight) (Butler and others, 1994, 

table 33). Eisler (1987a) reported that mercury 

concentrations in birds were highest in species that 

eat fish or other birds. Eisler (1987a) also reported 

that mercury concentrations greater than 1.0 mg/kg 

(|ig/g) fresh weight in any bird sample often were 

associated with proximity to human use of mercury 

and that the effect of mercury residues in birds is not 

fully understood.

Arsenic concentrations in all bird-liver samples 

collected from the Uncompahgre Project area and 

from the Grand Valley were less than the range 2- to 

10-jig/g wet-weight concentration that Eisler (1988) 

indicated as being elevated. Boron concentrations in 

aquatic-plant samples collected from Markley Pond 

(site MKP) and Gretts Pond (site GTP in fig. 52) 

were less than the 30-|ig/g wet-weight concentration 

known to adversely affect waterfowl growth (Eisler, 

1988). Most bird tissue samples collected from the 

Uncompahgre Project area and from the Grand Valley 

contained boron concentrations that were less than the 

analytical reporting limits.

118 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
109°00' 108°45' 108°30' 108° 15'

39°15' -

EXPLANATION

TMP. WETLAND SITES WITHIN 

A THE GRAND VALLEY

GENERALIZED EXTENT OF 

IRRIGATED AREA

39°00'

Figure 53. Location of sampling sites at wetlands in the Grand Valley.
Cadmium concentrations in all bird samples 

collected from the Uncompahgre Project area and 

from the Grand Valley were less than the 2-u.g/g 

wet-weight concentration for whole-body vertebrates, 

or the 10-u.g/g wet-weight concentration for bird livers 

that Eisler (1985) reported was evidence of probable 

cadmium contamination. Eisler (1986) reported that 

chromium concentrations greater than 4 jig/g dry 

weight in organs and tissues of fish and wildlife are 

indicative of chromium contamination. For samples 

collected in the Uncompahgre Project, the chromium 

concentrations of 6.3 jig/g dry weight in a snipe egg 

from Markley Pond (site MKP in fig. 52) and 4.8 u,g/g 

dry weight in a Wilson's phalarope whole-body 

sample from Brozina Pond (site BZP) exceeded the 

guideline for chromium. Chromium concentrations in 

whole-body killdeer samples collected at Sweitzer 

Lake (site SWL) ranged from 3.6 to 3.9 u,g/g dry 

weight and were slightly less than the guideline for 

chromium contamination. Adjacent to the 

Uncompahgre Project, a chromium concentration of 

4.1 u,g/g dry weight in a yellow-headed blackbird 

whole-body sample exceeded the guideline of 4 u,g/g 

dry weight. In the Grand Valley, a widgeon liver 

sample from Reid Pond (site RDP in fig. 53) had a 

chromium concentration of 19.6 Jlg/g dry weight.

Incubation Studies of Bird Eggs

During 1991, an attempt was made to 

monitor nest success at a few wetland sites in the 

Uncompahgre Project area and in the Grand Valley. 

The monitoring had limited success because, at many 

sites, eggs in ground nests were eaten by predators 

before the eggs reached full term. Additionally, nests 

could not be visited on a daily basis, and, because of 

rapid dispersal of the young, visual inspection of 

hatchlings for external deformities or to document 

hatching success was almost impossible. Because of 

these problems, eggs were brought from the field into 

the laboratory and incubated for the 1992 bird study.

Bird eggs were collected from 10 sites in the 

Uncompahgre Project area and from 6 sites in the 

Grand Valley. Bird eggs were either candled or, if 

several eggs were collected from the same nest, one 

egg was cracked open and a hatching date projected. 

The egg contents were saved in an acid-cleansed jar 

and, depending on the species, either kept separate or 

composited with other egg contents from the same 

nest for trace-constituent analysis.

Species collected and incubated included 

yellow-headed and red-winged blackbirds, 

American coots, cinnamon teal, mallards, killdeer, 

American avocets, and common snipe. Because 

of equipment limitations, all eggs collected were 

placed in the same incubator. Temperature and 

humidity were kept constant throughout the 

incubation period, and the eggs were automatically 

turned twice a day.

Hatching success for all eggs placed in the 

incubator was 68 percent. Hatching success for 

eggs incubated from the Uncompahgre Project 

area was 78 percent and for eggs incubated from 

the Grand Valley was 42 percent. Because of the 

large number of variables that can affect hatching 

success, these percentages are not necessarily 

representative of the overall hatching success 

in the study area.

The primary reason for collecting and 

incubating eggs was to inspect visually each hatchling 

for any obvious deformities. Sixty-five eggs were 

incubated during the study period, and only one bird, 

a red-winged blackbird from Mack Wash Pond 

(site MWP), had any visual deformities. This bird had 

fused toes on his left foot, which possibly could be 

attributed to selenium toxicity (Ohlendorf and others, 

1988) or to improper incubator temperature for that 

species (Stromberg, 1975). The whole-body selenium 

concentration in that bird was 8 Jlg/g dry weight, 

which is slightly higher than the geometric mean of 

7.3 u,g/g dry weight for all red-winged blackbirds 

collected at Mack Wash Pond, but is in the range 

of selenium concentrations (3 to 20 jig/g dry weight) 

that indicates further field reproductive studies are 

needed to determine biological risk (Skorupa and 

Ohlendorf, 1991).

Field Observations at Wetland Sites

General field observations were conducted at 

each of the wetland sites on each visit. These observations included recording the number and species of 

birds observed at each of the wetland sites. Most of 

the ponds in the study areas were used extensively 

during spring and fall migrations. Wetland use during 

the nesting season was limited by the pond size. 

Waterfowl that used the ponds displayed aggressive 

behavior, tending to be territorial because of the 

limited amount of suitable nesting habitat.

120 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
At two sites in the Uncompahgre Project area, 

Ferrier Pond and Markley Pond (sites FRP and MKP 

in fig. 52), a detailed study of the hatching success of 

yellow-headed blackbirds was conducted in 1992. 

Initially, American coots were going to be included in 

the hatching success study; however, they rarely 

nested successfully.

On May 20,1992, 118 active yellow-headed 

blackbird nests were located at Ferrier Pond. On 

May 23, 1992, the observers returned to Ferrier 

Pond and marked 25 full clutch (four-egg) nests 

from random locations around the pond for continued 

observation. On June 2, 1992, 10 days after first 

marking the nests, each nest was revisited, and the 

eggs in 23 of 25 nests had hatched. One nest had been 

destroyed, and one nest still contained four eggs. Each 

of the young, which varied in age from 1 to 12 days, 

were inspected in the field for any deformities or 

abnormalities. No deformities were found on any 

yellow-headed blackbirds from Ferrier Pond. After 

inspection, each prefledgling was returned to its nest. 

The site was revisited on June 8,1992, and once again, 

each nest was visited and data recorded. There were 

many fledglings scattered throughout the cattails. The 

birds had fledged completely from 10 of the nests. 

Three nests had three prefledglings still in the nest. 

Six fledglings were collected for trace-constituent 

analysis, including one full-term prefledgling found 

dead in the nest. These birds were analyzed as wholebody samples. The birds were prepared for analysis 

by removing all feathers, legs, and beak. Selenium 

concentrations in these six birds ranged from 10 flg/g 

dry weight in one live bird to 43 p,g/g dry weight in 

the prefledgling found dead in its nest. The appearance of the dead prefledgling indicated it was very 

near fledging. The bird showed no external abnormalities or deformities, but had a selenium concentration 

almost three times higher than the geometric mean 

of 16.1 (Xg/g dry weight selenium (geometric deviation 

1.6) for the other five live fledglings collected from 

this site.

Of the 25 yellow-headed blackbird nests 

monitored at Ferrier Pond, one contained addled 

eggs. Two young were fledged from this nest, but 

two unhatched eggs remained. These two eggs were 

collected and analyzed. The selenium concentration 

of the contents of these two eggs was 13 [ig/g dry 

weight, which is the same as the geometric mean 

selenium concentration of 13 |J,g/g dry weight for the 

other three egg samples (consisting of two eggs each) 

collected from this site. This selenium concentration

was in the range of concern of 3 to 20 fig/g dry 

weight that, according to Skorupa and Ohlendorf 

(1991), would need further field studies to assess 

biological risk.

Adult coots commonly were seen at Ferrier 

Pond. On June 2, 1992, 12 coots were seen on the 

pond. Over the period of the detailed bird-monitoring 

study (May 20, 1992, to July 6, 1992), four coot nests 

were located on the pond. Egg samples were collected 

from three of the nests. The geometric mean selenium 

concentration of the three samples of coot eggs was 

18.6 flg/g dry weight (geometric deviation 1.1), 

which was in the range of concern (Skorupa and 

Ohlendorf, 1991). From two of the coot nests from 

which eggs were collected and selenium concentrations determined, six eggs were incubated to full term. 

No external deformities were detected in these 

hatchlings. The geometric mean selenium concentration for the combined two samples of coot hatchlings 

was 19.8 flg/g dry weight (geometric deviation 1.2). 

No young coots were observed during 1992 on Ferrier 

Pond, even though coots were successfully hatched in 

the laboratory.

A study similar to the one conducted at Ferrier 

Pond also was conducted at Markley Pond (site MKP 

in fig. 52) from May to July 1992. This pond receives 

surface runoff from agricultural lands above the pond 

and from shallow ground water discharging from 

Mancos Shale. Markley Pond was chosen because 

several hunters who leased this pond noticed dead 

birds on or near the pond and attributed mortality to 

causes other than hunting-induced injuries.

On May 19,1992, a prefledgling mallard was 

found dead in a slough that flows into Markley Pond. 

This prefledgling had recently died and appeared 

normal with no visible trauma, deformities, or 

abnormalities. This sample contained a whole-body 

selenium concentration of 26 p,g/g dry weight. A 

second fresh prefledgling mallard was found dead in 

this same general area on May 27, 1992. This bird, 

much like the first bird, had no visible signs of trauma, 

deformities, or abnormalities and was found floating 

among the cattails. This sample contained a wholebody selenium concentration of 27 |J,g/g dry weight.

Red-winged blackbirds also were studied during 

1992 at Markley Pond. However, most red-winged 

blackbird nests were not directly associated with 

the pond and were in remote wetland areas adjoining 

the pond. Selenium concentrations in red-winged 

blackbird eggs and whole-body prefledglings ranged 

from 7.1 to 19 p,g/g dry weight. The geometric mean

BIOLOGICAL INVESTIGATIONS 121
selenium concentration was 13.0 }lg/g dry weight 

(geometric deviation 1.4) for three egg samples and 

10.6 }lg/g dry weight (geometric deviation 1.6) for the 

five prefledglings sampled.

On May 27, 1992, 35 active yellow-headed 

blackbird nests were found scattered throughout the 

cattails on the south and west sides of Markley Pond. 

Twenty nests were flagged using the same method that 

was used at Ferrier Pond. Eight nests were located 

that had full clutches of four eggs, and all other nests 

had zero to three eggs. On June 2,1992, most of the 

nests that previously had zero to three eggs had full 

clutches of four eggs. Four of the nests had new 

hatchlings. By June 9, 1992, all marked nests had 

been pulled down, and eggs and young eaten by 

raccoons.

On June 24,1992, 10 coots were observed on 

Markley Pond, and 8 coot nests were located. Coot 

eggs were collected from four of the nests, and the 

contents analyzed for trace constituents. The 

geometric mean selenium concentration of the four 

egg samples was 24.4 }ig/g dry weight (geometric 

deviation 1.5), which exceeded the concentration 

range of 3 to 20 ^ig/g dry weight associated with 

biological risk (Skorupa and Ohlendorf, 1991). Two 

dead coot hatchlings were found in close proximity to 

the nest where one of the egg samples was collected. 

This egg sample contained a selenium concentration 

of 30.3 }lg/g dry weight. The combined whole-body 

sample of the two dead coot hatchlings had a selenium 

concentration of 19 |lg/g dry weight.

Two additional coot egg samples were brought 

back to the laboratory on June 24, 1992, and placed in 

the incubator. One of the two coot eggs was cheeping 

at the time it was picked up, so was assumed to be in a 

later stage of development. Both eggs were opened on 

June 29,1992, because they had not yet hatched, and 

both embryos were dead. The two embryos had 

reached full term and appeared normal with no 

external deformities. The selenium concentration of 

the two embryos (combined into one sample) was 

39 }lg/g dry weight.

During subsequent visits, no prefledgling 

coots were observed on the pond. Adult coots were 

seen, but only one of the eight nests discovered on 

June 24, 1992, was found on a subsequent visit on 

July 6. This nest contained three eggs, the same 

number of eggs as when discovered on June 24. In 

addition, no other prefledgling waterfowl were seen 

or collected from Markley Pond.

Water and Bottom-Sediment Bioassays

After reviewing the selenium concentrations 

in water from the 1991 sampling, five sampling sites 

on washes in the Grand Valley were selected to 

conduct toxicity tests. The five streams sampled 

had some of the highest selenium concentrations 

in the Grand Valley. The streams were Salt Creek 

(site SC), Reed Wash (site RW1), Adobe Creek 

(site AC1), Leach Creek (site LC1), and Indian 

Wash (site IW) (fig. 46). Water and bottom-sediment 

samples were collected for bioassays in March 1992 

and sent to the U.S. Environmental Protection Agency 

in Denver, Colo., for analysis. Chronic tests on larval 

fathead minnows and daphnia (Ceriodaphnia dubid) 

were conducted according to methods described by 

Weber and others (1989) and according to guidelines 

by the U.S. Environmental Protection Agency 

National Pollutant Discharge Elimination System 

(NPDES) Whole Effluent Toxics Control Program 

of December 1990. Daphnia and 3-day-old fathead 

minnows were exposed to the water samples using 

dilutions that included five selenium concentrations 

(a control, 12.5 percent, 25 percent, 50 percent, and 

100 percent of the original sample concentration). 

Samples from Leach Creek and Indian Wash did 

not have tests at the 12.5-percent concentration 

because of limited sample quantity. Bottomsediment samples were sent to Technology 

Applications, Inc., in Cincinnati, Ohio, for testing 

with amphipods (Hyallela aztecd) and fathead 

minnow embryos.

Water Bioassays

Water bioassays using Ceriodaphnia exposed 

to water samples from the five test sites indicated no 

significant differences between the number of young 

produced in control solutions and in test solutions. 

There were no significant differences in the fathead 

minnow growth in controls compared to the test 

solutions from Salt Creek, Reed Wash, Adobe 

Creek, and Indian Wash. However, growth and 

survival of fathead minnows in bioassays using 

water from Leach Creek indicated potential chronic 

toxicity. When survival of fathead minnows was 

analyzed, the lowest observed effect concentration 

(LOEC) for Leach Creek water was 100 percent 

(the undiluted concentration) and the no-observed 

effect concentration (NOEC) was 50-percent

122 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
concentration. When growth was analyzed, the LOEC 

was 50-percent concentration and the NOEC was 

25-percent concentration.

The U.S. Environmental Protection Agency 

(1987) reported that freshwater aquatic organisms 

would not be adversely affected if the 4-day average 

selenium concentration in the water did not exceed 

5 ng/L more than once every 3 years on the average, 

and if the 1-hour average concentration did not exceed 

20 ng/L more than once every 3 years. All totalselenium concentrations in water samples in the 

five sites greatly exceeded these guideline concentrations (Butler and others, 1994, table 5). Although 

Reed Wash had the largest selenium concentration for 

the samples collected in March 1992 at the five sites 

(table 20), toxicity was reported only for the Leach 

Creek water sample. The presence of one or more 

contaminants, such as other trace constituents, salts, or 

pesticides, in addition to selenium, might have been 

responsible for the harmful environment in the Leach 

Creek sample.

Bottom-Sediment Bioassays

Two different bioassays were conducted on 

bottom-sediment samples. The first test was a 7-day 

growth and survival test using Hyallela. The only 

samples that had results different from the control 

were from Reed Wash and Adobe Creek. The sample 

from Reed Wash had a survival rate of 59 percent, and 

the sample from Adobe Creek had a survival rate of 

43 percent. All other samples had a survival rate of 

greater than 70 percent. None of the mean dry weights 

(the value used to indicate growth) of Hyallela that 

were cultured on bottom-sediments samples were 

statistically different from the mean dry weights of 

Hyallela from the control samples.

The second test on bottom-sediment samples 

was a fathead minnow embryo-larval test, which was 

a first attempt at a new bottom-sediment exposure 

method using embryos. A volume of 25-mL of 

bottom sediment from each of the five tributaries 

was placed in a container and covered by 100 mL of 

distilled water. The 24- to 48-hour-old embryos were 

placed on top of the bottom sediment. Hatch rate and 

survival rate for the control embryos and larvae were 

greater than 90 percent. All embryos exposed to the 

bottom sediments from the five streams were dead in 

less than 24 hours. The cause or causes of the fathead 

minnow embryo deaths are unknown.

Bottom-sediment samples from Salt Creek, 

Reed Wash, Adobe Creek, Leach Creek, and Indian 

Wash had total-selenium concentrations of 1.9, 6.3, 

3.9, 5.6, and 16 jig/g (or mg/kg). Lemly and Smith 

(1987) indicated that a selenium concentration greater 

than or equal to 4 mg/kg was of concern to fish and 

birds because of possibilities of reproductive failure or 

mortality due to food-chain bioconcentration. 

Selenium concentrations in Reed Wash, Leach Creek, 

and Indian Wash exceeded this guideline. As in the 

water samples, other contaminants (including high 

salinity), as well as selenium, could be causing toxic 

effects in bottom sediments.

BIOACCUMULATION OF SELENIUM

Bioaccumulation is the uptake by aquatic 

organisms of chemicals from water directly or 

through consumption of food containing the 

chemicals. Numerous publications have addressed 

the subject of selenium bioaccumulation, including 

Lemly and Smith (1987), Ohlendorf (1989), 

Ohlendorf and others (1993), and Maier and Knight 

(1994). Selenium is bioaccumulated by aquatic 

organisms through two mechanisms: bioconcentration 

and biomagnification. Bioconcentration is the direct 

uptake of dissolved selenium across respiratory and 

epithelial membranes by passive or active processes. 

Biomagnification is the process in which higher 

trophic-level organisms accumulate selenium by 

ingesting and assimilating organo-selenium 

compounds synthesized by lower food-chain 

organisms (Maier and Knight, 1994). Biomagnification is the major pathway of selenium accumulation 

for higher trophic-level biota.

Data summarizing the bioaccumulation of 

selenium in the Uncompahgre Project area and in the 

Grand Valley are listed in table 37. Geometric mean 

selenium concentrations are listed in table 37 for 

water, bottom sediment, and several groups of biota. 

Because bird samples were collected only at wetlands, 

only water, bottom-sediment, and other biological 

samples collected at wetland sites were used to 

describe bioaccumulation in the study area. Selenium 

concentrations are expressed in micrograms per liter 

for water and micrograms per kilogram for bottom 

sediment and biota; both units are equivalent to parts 

per billion.

BIOACCUMULATION OF SELENIUM 123
Table 37. Geometric mean selenium concentrations in 

water, bottom-sediment, and biota samples from wetlands 

in the Uncompahgre Project, wetlands adjacent to the 

Uncompahgre Project, and wetlands in the Grand Valley

[Water concentrations in micrograms per liter; bottom-sediment and biota 

concentrations in micrograms per kilogram (both units are equivalent to 

parts per billion); number in brackets is the number of samples; --, no data]

Sample 

matrix

Water

Bottom sediment

Aquatic plants

Aquatic inverterates

Fish, whole body

Bird, whole body

Bird liver

Bird breast

Bird egg

Uncompahgre 

Project

4.7 [5]

27,400 [2]

7,300 [5]

32,000 [1]

49,900 [5]

13,700 [33]

98,000 [1]

--[0]

14,700 [34]

Adjacent to the _ . 

.. . Grand Uncompahgre

Project '

10 [1] 7.4 [4]

-- [0] 17,700 [2]

23,700 [2] -- [0]

- [0] -- [0]

33,700 [4] 13,000 [2]

18,300 [13] 10,000 [10]

30,300 [14] 23,900 [11]

20,400 [5] - [0]

13,400 [25] 6,700 [20]

At wetlands adjacent to the Uncompahgre 

Project, the selenium concentration of the single 

water sample was 10 (ig/L, and, in whole-body fish 

samples, the geometric mean concentration was 

33,700 (ig/kg (table 37). These concentrations 

indicated a bioaccumulation factor of 3,370 (33,700 

divided by 10) between fish and water. At wetlands 

in the Uncompahgre Project, the geometric mean 

selenium concentrations were 4.7 (ig/L for water 

and 49,900 (ig/kg for whole-body fish, indicating a 

bioaccumulation factor of 10,600. For wetlands 

in the Grand Valley, geometric mean selenium 

concentrations were 7.4 (ig/L for water and 

13,000 jig/kg for whole-body fish (table 37), 

indicating a bioaccumulation factor of 1,800. Lemly 

and Smith (1987) reported that, where selenium in 

water has been concentrated from 100 to more than 

30,000 times, fish were affected by chronic toxicity.

Bioaccumulation factors between bottom 

sediment and all biota matrices and among the successive biota trophic levels were much smaller than the 

bioaccumulation factors between water and bottom 

sediment and between water and biota. These data 

indicate that bottom sediment had accumulated 

selenium at ratios similar to biological tissues. The 

bioaccumulation factors among biota trophic levels 

are similar to the factors of 2 to 6 between successive 

trophic levels of a food chain reported by Lemly and 

Smith (1987, p. 9).

The three areas were quite similar regarding 

bioaccumulation factors, indicating that the relative 

rates of selenium accumulation between sample 

matrices are similar in the Uncompahgre Project area 

and in the Grand Valley, although selenium concentrations were somewhat variable among the areas. The 

highest bioaccumulation factor for selenium was about 

20,900 between water and bird-liver concentrations 

for the irrigated area of the Uncompahgre Project.

IMPLICATIONS FOR HUMAN HEALTH

Selenium is a necessary nutritional component 

for humans in quantities from 50 to 200 (ig/d, 

depending on sex, age, and weight (National Academy 

of Sciences, 1983). However, too much intake of 

selenium can result in selenosis and can be detrimental 

to human health (Beath, 1962; Lo and Sandi, 1980; 

Shamberger, 1981; Levander, 1984; Combs and 

Combs, 1986).

Sweitzer Lake (site SWL in fig. 52), located near 

Delta in the Uncompahgre Project, was posted in 1977 

by the Colorado Department of Health advising the 

public not to eat fish caught from the lake. Selenium 

concentrations in catfish-fillet samples collected in 

1988-89 averaged 28 (ig/g dry weight selenium (about 

5.9 (ig/g wet weight) (Butler and others, 1991), or 

about 167 (ig of selenium per ounce of fillet. The 

National Academy of Sciences (1980) considered an 

upper limit of 200 (ig/d selenium to be safe for human 

intake. Levander (1984) suggested 500 (ig/d selenium 

to be the safe upper limit for humans.

The highest selenium concentrations in fishfillet and bird-breast samples collected for the detailed 

study from the Uncompahgre Project area and from 

the Grand Valley are listed in table 38. The quantity of 

each sample that an adult human could safely consume 

without exceeding 200 (ig/d selenium was computed 

and is listed in table 38. The consumption limits do 

not account for selenium consumed in other food 

sources. Amounts of fillet or breast meat that could 

safely be consumed on a daily basis were quite small 

and ranged from 0.68 oz of mallard breast taken from 

Ferrier Pond to 3.7 oz of coot breast taken from 

Fruitgrowers Reservoir and to 3.7 oz of catfish fillet 

taken from Highline Reservoir in the Grand Valley. 

Ferrier Pond and Fruitgrowers Reservoir (sites FRP 

and FGR in fig. 52) are located near irrigated areas 

adjacent to the Uncompahgre Project, but are not in

124 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
the irrigated area served by the project. Highline 

Reservoir (fig. 53) is in the upper Mack Wash 

Basin, and most of the inflow into the reservoir 

is irrigation water from the Government Highline 

Canal. Apparently, selenium accumulates in fish and 

birds in the study area to levels that indicate human 

consumption of such items need to be limited if eaten 

on a daily basis. It is unlikely that residents eat fish 

and game birds on a daily basis; therefore, selenium 

toxicity probably is not a major issue regarding human 

health in the Uncompahgre Project area and in the 

Grand Valley.

Table 38. Selenium concentrations in fish-fillet and birdbreast samples from the Uncompahgre Project area and 

the Grand Valley, and the quantity of each sample that 

could be consumed without exceeding the maximum 

recommended daily quantity for an adult human

[Selenium concentrations in micrograms per gram wet weight; 

recommended daily consumption limit for selenium is 200 micrograms 

(National Academy of Sciences, 1980); daily consumption limit is the 

quantity of the food item, in ounces per day, that would need to be 

consumed to equal the daily consumption limit of 200 micrograms]

Type 

of 

sample

Yellow perch 

fillet

Brown trout 

fillet

Channel catfish 

fillet

Largemouth 

bass fillet

Mallard breast

Mallard breast

Mallard breast

Coot breast

Coot breast

Sampling Selenium 

location concentration

Crawford 

Reservoir

Cummings 

Gulch

Highline 

Reservoir

Highline 

Reservoir

Ferrier Pond

Ferrier Pond

Ferrier Pond

Fruitgrowers 

Reservoir

Fruitgrowers 

Reservoir

3.1

2.6

1.9

2.2

7.4

5.6

10.3

1.9

2.2

Daily 

consumption 

limit

2.3

2.7

3.7

3.2

.95

1.3

.68

3.7

3.2

SUMMARY

The U.S. Department of the Interior began a 

program in 1985 to determine the effects of irrigation 

drainage on water quality and on fish and wildlife 

resources in the Western United States. This report 

describes results of a study done in 1991-93 of irrigation drainage associated with the Uncompahgre 

Project area and the Grand Valley in west-central 

Colorado. The focus of the report is on the sources,

distribution, movement, and fate of selenium in the 

hydrologic and biologic system. The information 

collected for the study is for use in identifying 

potentially serious water-quality problems related 

to irrigation drainage and to provide scientific 

understanding needed to develop alternatives to 

mitigate or resolve the problems.

In irrigated areas of the Uncompahgre Project, 

residual soils and alluvium primarily derived from 

Mancos Shale contained the highest concentrations of 

total and water-extractable selenium. Geometric mean 

selenium concentrations for soils derived from the 

Mancos Shale were 10 to 13 times greater than the 

geometric mean for soils in the Western United States. 

Selenium concentrations in 128 alfalfa samples ranged 

from less than 0.3 to 9.5 p,g/g dry weight, and only 

5 samples had concentrations that exceeded the 

recommended dietary limit of 4 p,g/g for consumption 

by livestock. These results indicate that irrigation 

practices might be mobilizing and redistributing 

selenium in the Uncompahgre Project.

In the Uncompahgre Project area and in the 

Grand Valley, distribution and concentrations of 

dissolved selenium in ground water differ according 

to the hydrogeologic setting. The highest concentrations of dissolved selenium (maximum 1,300 p,g/L) 

were in water from wells completed in alluvium 

overlying Mancos Shale. The lowest concentrations 

of dissolved selenium were in water from wells 

completed in terrace deposits on the western side 

of the Uncompahgre Valley and in water from wells 

completed in Mancos Shale residuum. Dissolvedselenium concentrations in ground water of the 

study area generally were higher during summer 

when irrigation water was applied than during 

winter. Selenium concentrations decreased in winter 

when irrigation ceased and reducing geochemical 

conditions tended to remove selenium from solution. 

Dissolved-selenium concentrations were low where 

the platinum-electrode potential was low and the 

hydrogeologic system was more reducing. Selenium 

in the Uncompahgre Project and in the Grand Valley 

could be mobilized by oxidation of reduced selenium, 

desorption of selenium from aquifer sediments, ion 

exchange, and by dissolution of selenium. Dissolved 

solids and dissolved selenium are positively correlated 

in water from wells completed in alluvium, possibly 

because of desorption and ion-exchange processes, but 

dissolved solids and selenium are negatively 

correlated in water from wells completed in shale

SUMMARY 125
residuum. The application of irrigation water and 

perhaps nitrate fertilizers on alluvium and shale 

residuum in the study area mobilizes selenium by 

desorption, ion exchange, or oxidation into more 

mobile chemical forms where the selenium is 

transported by ground water to streams and irrigation 

drains tributary to the Gunnison, Uncompahgre, and 

Colorado Rivers.

Median selenium concentrations in the 

Gunnison and Colorado Rivers upstream from the 

Uncompahgre Project area and the Grand Valley were 

less than or equal to 1 |lg/L. Downstream from the 

irrigated areas, the median selenium concentrations 

(1975-92) were 7 |lg/L for the Gunnison River at 

Whitewater (about 10 mi from the confluence with the 

Colorado River) and 5 |ig/L for the Colorado River 

near the Colorado-Utah State line. About 64 percent 

of the samples from the Gunnison River and almost 

50 percent of the samples from the Colorado River 

exceeded the U.S. Environmental Protection Agency's 

aquatic-life criterion of 5 |lg/L. There apparently was 

a significant decreasing trend in selenium concentrations in the Colorado River near the Colorado-Utah 

State line during 1975-92. The Uncompahgre Project 

and the Grand Valley may account for as much as 

75 percent of the selenium load in the Colorado River 

near the State line.

The highest selenium concentrations in surfacewater samples were collected in areas primarily 

irrigated on Mancos Shale. The selenium concentrations exceeded 5 |lg/L in almost all surface-water 

samples collected during the nonirrigation season 

(November through March) from the eastern side of 

the Uncompahgre Project and from the Grand Valley. 

For nonirrigation-season samples, the median 

selenium concentration was 105 |lg/L for the eastern 

side of the Uncompahgre Project and 50 |Ag/L for the 

Grand Valley. The largest selenium concentrations in 

surface water in the Uncompahgre Project were in the 

Loutsenhizer Arroyo Basin and in the area between 

the Garnet Canal and Peach Valley Arroyo. In the 

Grand Valley, the highest selenium concentrations 

were in parts of the Salt Creek, Reed Wash, and Leach 

Creek Basins. The maximum selenium concentrations 

in surface-water samples collected in 1991-93 were 

600 (ig/L in the Uncompahgre Project area and 

380 (ig/L in the Grand Valley. Selenium concentrations generally were lower on the western side of 

the Uncompahgre Project and in the Redlands and 

Orchard Mesa areas in the Grand Valley because these

areas primarily are irrigated on soils derived from 

terrace deposits that contain less shale detritus than 

soils in Mancos Shale areas.

The eastern side of the Uncompahgre Project 

is the primary source of selenium loading from the 

Uncompahgre Project. Loutsenhizer Arroyo and 

Cedar Creek, on the eastern side of the project, 

and Dry Creek, on the western side of the project, 

discharged the largest selenium loads to the 

Uncompahgre River. The highest mean selenium load 

from a tributary stream was 14.0 Ib/d in Loutsenhizer 

Arroyo. Much of the selenium load measured during 

1991-92 in west-side tributaries, such as Dry Creek 

and Roubideau Creek, probably was transported into 

these basins in the irrigation water and winter water 

by the Ironstone Canal. The Uncompahgre Project 

also contributes substantial selenium loads to the 

Gunnison River between Peach Valley Arroyo and 

the Uncompahgre River.

The largest selenium loads from the Grand 

Valley were in Salt Creek, Reed Wash, Big Salt Wash, 

and Leach Creek, which drain large areas of irrigated 

land in the western one-half of the valley. The highest 

selenium load from the Grand Valley during 1991-92 

was 12.8 Ib/d in Reed Wash. Generally, selenium 

loads were less than 0.2 Ib/d in washes and ditches 

in the Redlands area, the area east of Grand Junction, 

and on Orchard Mesa. In some Grand Valley 

streams, such as Reed Wash, selenium loads were 

considerably larger during the irrigation season than 

the nonirrigation season.

Irrigation drainage from the Uncompahgre 

Project and from the Grand Valley is a major source 

of dissolved solids in the Upper Colorado River 

Basin. Median dissolved-solids concentrations for 

nonirrigation-season samples were 1,160 mg/L for the 

western side of the Uncompahgre Project, 4,110 mg/L 

for the eastern side of the Uncompahgre Project, and 

3,600 mg/L for the Grand Valley. There are positive 

correlations between selenium and dissolved-solids 

loads for tributary streams in the study area. For 

15 basins in the Grand Valley, the correlation coefficient was 0.91 between selenium yield and dissolvedsolids yield. Salinity-control projects that were 

designed to decrease dissolved-solids loads from 

irrigated areas also could have decreased selenium 

loading.

Much of the gain in dissolved solids in 

irrigation-drain water in the study area was from 

gains of calcium, sodium, magnesium, and sulfate

126 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
ions. An analysis of chloride and selenium data 

indicated that selenium concentrations in irrigation 

drainage in the Grand Valley were 5 to 100 times 

greater than the selenium concentrations predicted 

from evaporative concentration of the irrigation 

source water. Based on ratios of sulfate to 

selenium, selenium was most readily mobilized in 

the Loutsenhizer Arroyo Basin; in the area between 

the Garnet Canal and Peach Valley Arroyo; and in 

the Salt Creek, Reed Wash, Leach Creek, and Indian 

Wash Basins. Low selenium concentrations and 

higher sulfate to selenium ratios for pond-water 

samples might be indicative of removal of selenium by 

chemical and biological processes in ponds. Some 

of the highest selenium concentrations in surface 

water were associated with high nitrite-plus-nitrate 

concentrations, and there are significant correlations 

of selenium to nitrite-plus-nitrate concentrations in 

surface-water samples from the study area.

Many concentrations of other potentially toxic 

trace constituents in surface water were equal to or 

less than analytical reporting limits and did not exceed 

aquatic-life criteria. The highest uranium concentrations in surface water in each area were in samples 

from reference sites; the maximum uranium concentration was 83 flg/L for Big Salt Wash upstream from 

the Government Highline Canal in the Grand Valley. 

A one-time sampling for 26 organophosphate and 

carbamate insecticides in the Grand Valley indicated 

that concentrations of most compounds were at or less 

than reporting limits.

Selenium concentrations in bottom-sediment 

samples from eight streams and four ponds exceeded 

the base-line concentration of 1.4 fig/g for soils in the 

Western United States, and most of the concentrations 

exceeded the level of concern of 4 |ig/g for fish and 

wildlife. The maximum selenium concentration in 

bottom-sediment samples was 47 flg/g. Ponds and 

wetlands probably are sinks for selenium in the study 

area. Soluble and adsorbed selenium accounted for 

less than 10 percent of the total selenium in seven of 

eight bottom-sediment samples from streams.

Biota collected from the eastern side of 

the Uncompahgre Project generally had higher 

selenium concentrations than biota from the 

western side. Selenium concentrations in some 

aquatic-plant and aquatic-invertebrate samples 

exceeded 3 |ig/g dry weight, a dietary guideline 

for fish and wildlife. Most whole-body fish samples 

from the Uncompahgre Project and adjacent areas

had selenium concentrations that exceeded the 

1984 NCBP 85th percentile (0.73 ng/g wet weight). 

The NCBP 85th percentile is used for comparative 

purposes and does not necessarily indicate a hazard 

to fish. A trout sample from Crawford Reservoir had 

a selenium concentration of 18 fig/g dry weight.

Selenium concentrations in some aquatic plants 

and aquatic invertebrates from the Grand Valley 

exceeded the dietary guideline (3 fig/g dry weight) 

for protection of fish and wildlife. All selenium 

concentrations in whole-body fish samples collected 

from tributary streams and washes in the Grand Valley 

exceeded the 1984 NCBP 85th percentile. Egg and 

liver samples from catfish from Highline Reservoir 

and Salt Creek had selenium concentrations that 

exceeded levels of concern for reproductive success 

of freshwater anadromous fish. There was a general 

trend of increasing selenium concentrations in 

roundtail chubs and speckled dace in a downstream 

(east to west) direction for tributary streams in the 

Grand Valley.

The mean selenium concentration of 7.1 jig/g 

dry weight for fish samples collected in 1992 from the 

Gunnison River Basin (one site was on the North 

Fork) was significantly higher (probability 0.05) 

than the mean concentrations for fish from the 

Uncompahgre River (5.1 fig/g dry weight) and from 

the Colorado River (4.6 fig/g dry weight). Selenium 

concentrations in about 71 percent of the fish samples 

from the Gunnison and North Fork of the Gunnison 

Rivers, about 64 percent of the fish samples from 

the Uncompahgre River, and about 55 percent of the 

fish samples from the Colorado River exceeded a 

guideline of 4 fig/g dry weight for selenium in wholebody fish. Speckled dace contained significantly 

(probability 0.05) higher selenium concentrations than 

roundtail chubs, mottled sculpin, or suckers. Mean 

selenium concentrations in suckers from the Gunnison 

River at Delta and Whitewater were significantly 

higher than the mean concentrations in suckers from 

the Gunnison River at Austin and the North Fork near 

Lazear. Suckers from the Colorado River collected 

upstream from the Grand Valley had significantly 

(probability 0.05) less selenium than suckers collected 

in and downstream from the Grand Valley.

Using selenate and selenite data for five streams 

in the Grand Valley to calculate risk factors for 

endangered fish, selenate concentrations would pose a 

theoretical moderate risk to the swim-up life stage 

(0 to 98 days old) of endangered Colorado squawfish

SUMMARY 127
and to all three of the tested life stages of endangered 

razorback suckers. Because selenite concentrations 

were much smaller than selenate concentrations, 

selenite would be a lower risk to the endangered fish 

than selenate.

Except for selenium, copper concentrations in 

whole-body fish from the study area most often 

exceeded the 1984 NCBP 85th percentile. Some 

copper concentrations in aquatic plants and invertebrates from the Uncompahgre Project area and in 

fish from the Gunnison River were indicative of 

copper contamination. Relatively few concentrations 

of arsenic, cadmium, lead, mercury, and zinc in 

whole-body fish exceeded the 1984 NCBP 

85th-percentile concentrations. About 16 percent 

of the fish from the Colorado River exceeded the 

1984 NCBP 85th percentile for mercury. Risk assessments for Colorado squawfish and razorback suckers 

to boron, vanadium, and zinc for five tributary streams 

in the Grand Valley most often were in the moderate 

risk category. Zinc concentrations in Adobe Creek, 

Leach Creek, and Indian Wash were in the high-risk 

category for the swim-up stage of Colorado squawfish.

An anomalously high toxaphene concentration 

of 0.82 jj-g/g wet weight was reported for a flannelmouth sucker from Dry Creek in the Uncompahgre 

Project. The highest concentrations of several 

polycyclic aromatic hydrocarbons (PAH) were in a 

white sucker collected from the North Fork of the 

Gunnison River near Paonia.

Selenium concentrations in food items of 

migratory birds in the Uncompahgre Project area 

greatly exceeded a recommended guideline of 

3 H-g/g dry weight for protection of consumer wildlife. 

Fathead minnows from ponds had very high concentrations, including 59 jig/g dry weight in a sample 

from Brozina Pond (located in the Uncompahgre 

Project) and 72 and 110 |lg/g dry weight in two 

samples from St. George Pond (located outside 

the project north of the Gunnison River). Selenium 

concentrations in bird eggs and livers in the 

Uncompahgre Project area exceeded normal concentrations, and some samples had concentrations in the 

high-risk category for adverse biological effects. The 

geometric mean selenium concentration of 41.9 ug/g 

dry weight for five avocet eggs from Brozina Pond 

was similar to selenium concentrations associated 

with a 5-percent incidence of teratogenic effects. 

The geometric mean selenium concentration of 

6.7 |J-g/g dry weight for bird eggs from the Grand 

Valley was about one-half the mean concentration

for eggs from the Uncompahgre Project area. The 

selenium concentrations in bird eggs and livers from 

the Grand Valley were in the range of uncertainty 

regarding biological risk. Concentrations of other 

trace constituents in biota from wetlands generally 

were less than adverse-effect levels or were in normal 

concentration ranges.

Bird eggs were collected in 1992 and incubated 

in a laboratory, and of the 65 eggs incubated, only one 

had any visual deformities. Of the 25 yellow-headed 

blackbird nests monitored at Ferrier Pond (east of 

the Uncompahgre Project) in 1992, one prefledgling 

bird was found dead, and that bird had 43 jig/g dry 

weight of selenium. Two dead prefledgling mallards 

(selenium concentrations of 26 and 27 jig/g dry 

weight) found at Markley Pond on the eastern side 

of the Uncompahgre Project did not have abnormalities or deformities. Also from Markley Pond, two 

dead coot hatchlings had a selenium concentration 

of 19 |lg/g dry weight (combined sample), and two 

coot eggs incubated at the laboratory that had dead 

embryos when opened (never hatched) had a selenium 

concentration of 39 jig/g dry weight (combined 

sample).

Chronic toxicity water tests on Ceriodaphnia 

and fathead minnow lavae using irrigation drainage 

from five Grand Valley streams indicated no significant difference between control solutions and test 

solutions, except for chronic toxicity to fathead 

minnows for the water sample from Leach Creek. 

A growth and survival test using Hyallela on bottomsediment samples indicated no significant difference 

between test sediment and control sediment.

The highest bioaccumulation factors of 

selenium in the study area were from water to biota. 

For samples collected at wetlands in the Uncompahgre 

Project irrigated area, the bioaccumulation factor 

between water and fish was about 10,600. The highest 

bioaccumulation factor was about 20,900 from water 

to bird livers collected in the Uncompahgre Project. 

Bioaccumulation of selenium between bottom 

sediment and biota and in biota trophic levels were 

much smaller than between water and biota.

Selenium accumulates in fish and birds in the 

study area to levels that would limit human consumption if eaten on a daily basis. Because it is unlikely 

that residents eat fish and birds taken in the study area 

on a daily basis, selenium probably is not a human 

health concern in the study area.

128 Detailed Study of Selenium and Other Constituents in Water, Bottom Sediment, Soil, Alfalfa, and Biota Associated with 

Irrigation Drainage in the Uncompahgre Project Area and in the Grand Valley, West-Central Colorado, 1991-93
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