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Prepared as part of the
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Water-Quality Characteristics of Quaternary Unconsolidated-Deposit Aquifers and Lower Tertiary Aquifers of the Bighorn Basin, Wyoming and Montana, 1999-2001

Scientific Investigations Report 2004-5252

By Timothy T. Bartos, Cheryl A. Eddy-Miller, Jodi R. Norris, Merry E. Gamper, and Laura L. Hallberg

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Abstract

As part of the Yellowstone River Basin National Water Quality Assessment study, ground-water samples were collected from Quaternary unconsolidated-deposit and lower Tertiary aquifers in the Bighorn Basin of Wyoming and Montana from 1999 to 2001. Samples from 54 wells were analyzed for physical characteristics, major ions, trace elements, nutrients, dissolved organic carbon, radionuclides, pesticide compounds, and volatile organic compounds (VOCs) to evaluate current water-quality conditions in both aquifers.

Water-quality samples indicated that waters generally were suitable for most uses, and that natural conditions, rather than the effects of human activities, were more likely to limit uses of the waters. Waters in both types of aquifers generally were highly mineralized, and total dissolved-solids concentrations frequently exceeded the U.S. Environmental Protection Agency (USEPA) Secondary Maximum Contaminant Level (SMCL) of 500 milligrams per liter (mg/L). Because of generally high mineralization, waters from nearly one-half of the samples from Quaternary aquifers and more than one-half of the samples from lower Tertiary aquifers were not classified as fresh (dissolved-solids concentration were not less than 1,000 mg/L). The anions sulfate, fluoride, and chloride were measured in some ground-water samples at concentrations greater than SMCLs. Most waters from the Quaternary aquifers were classified as very hard (hardness greater than 180 mg/L), but hardness varied much more in waters from the lower Tertiary aquifers and ranged from soft (less than 60 mg/L) to very hard (greater than 180 mg/L).

Major-ion chemistry varied with dissolved-solids concentrations. In both types of aquifers, the predominant anion changes from bicarbonate to sulfate with increasing dissolved-solids concentrations. Samples from Quaternary aquifers with fresh waters generally were calcium-bicarbonate, calcium-sodium-bicarbonate, and calcium-sodium-sulfate-bicarbonate type waters, whereas samples with larger concentrations generally were calcium-sodium-sulfate, calcium-sulfate, or sodium-sulfate-type waters. In the lower Tertiary aquifers, samples with fresh waters generally were sodium-bicarbonate or sodium-bicarbonate-sulfate type waters, whereas samples with larger concentrations were sodium-sulfate or calcium-sodium-sulfate types.

Concentrations of most trace elements in both types of aquifers generally were small and most were less than applicable USEPA standards. The trace elements that most often did not meet USEPA secondary drinking-water standards were iron and manganese. In fact, the SMCL for manganese was the most frequently exceeded standard; 68 percent of the samples from the Quaternary aquifers and 31 percent of the samples from the lower Tertiary aquifers exceeded the manganese standard. Geochemical conditions may control manganese in both aquifers as concentrations in Quaternary aquifers were negatively correlated with dissolved oxygen concentrations and concentrations in lower Tertiary aquifers decreased with increasing pH.

Elevated nitrate concentrations, in addition to detection of pesticides and VOCs in both aquifers, indicated some effects of human activities on ground-water quality. Nitrate concentrations in 36 percent of the wells in Quaternary aquifers and 28 percent of the wells in lower Tertiary aquifers were greater than 1 mg/L, which may indicate ground-water contamination from human sources. The USEPA drinking-water Maximum Contaminant Level (MCL) for nitrate, 10 mg/L, was exceeded in 8 percent of samples collected from Quaternary aquifers and 3 percent from lower Tertiary aquifers. Nitrate concentrations in Quaternary aquifers were positively correlated with the percentage of cropland and other agricultural land (non-cropland), and negatively correlated with rangeland and riparian land. In the lower Tertiary aquifers, nitrate concentrations only were correlated with the percentage of cropland.

Concentrations of some naturally occurring radionuclides exceeded USEPA primary drinking-water standards. Gross-alpha activities in 9 of 24 samples from Quaternary aquifers and 6 of 26 samples from lower Tertiary aquifers were larger than the USEPA MCL of 15 picocuries per liter (pCi/L). Uranium concentrations were greater than the MCL of 30 micrograms per liter (mg/L) in 2 samples from Quaternary aquifers. Radon concentrations generally were large in samples from both types of aquifers. All samples from Quaternary aquifers and all but 2 samples from the lower Tertiary aquifers had concentrations larger than the proposed MCL of 300 pCi/L. One sample from the lower Tertiary aquifers had a radon concentration larger than the proposed alternative MCL of 4,000 pCi/L.

Organic compounds (pesticide compounds and VOCs), which generally are indicators of effects from human activities on ground-water quality, were detected in both types of aquifers. At least one pesticide compound was detected in 15 of 25 well samples from Quaternary aquifers and in 4 of 29 well samples from lower Tertiary aquifers. Most detections in both types of aquifers were herbicides or herbicide breakdown products—only one insecticide was detected. All pesticide concentrations were much smaller than applicable USEPA standards and most measured concentrations were at trace levels (less than 0.1 mg/L). Most of the pesticides detected in ground water in both types of aquifers are commonly associated with agriculture. However, the detection frequency of some pesticide compounds in ground water did not necessarily match frequency of use in the Bighorn Basin. Prometon and tebuthiuron, which were among the most frequently detected pesticides, were not used exclusively for agricultural purposes.

VOCs were detected in both types of aquifers and more frequently in Quaternary aquifers. At least one VOC was detected in samples from 21 of 25 wells in Quaternary aquifers and from 14 of 29 wells in lower Tertiary aquifers. Nine different VOCs were detected in samples from the Quaternary aquifers, six of which commonly are associated with gasoline. The VOCs detected in lower Tertiary aquifers were different from those detected in Quaternary aquifers as only two detections in lower Tertiary aquifers were of gasoline compounds. All concentrations were much smaller than USEPA MCLs. In fact, most detected VOC concentrations were smaller than the laboratory reporting level and the lowest laboratory calibration standard. The relatively high frequency of VOC detection in both aquifers was unexpected and may be attributable to use of very sensitive analytical methods with very low laboratory reporting levels. Despite very stringent quality-assurance/quality-control procedures, some very low VOC detections may be attributable to random sample contamination.


Contents

Abstract

Introduction

Purpose and scope

Acknowledgments

Description of the study area

Geographic, physiographic, and geologic setting

Hydrogeologic setting and ground-water use

Climate

Methods of investigation

Ancillary data

Land use and land cover

Site selection and well construction

Sample collection

Chemical analyses

Quality assurance and quality-control samples

Blank samples

Replicate samples

Field-matrix spike samples

Major-ion balances

Statistical methods

Ground-water quality

Ground-water age

Physical characteristics

Major ions and related water-quality characteristics

Trace elements

Nutrients and dissolved organic carbon

Radionuclides

Pesticide compounds

Volatile organic compounds

Summary

References

Appendix 1-1.—Land use and land cover mapped within 500-meter radius of sampled wells, Quaternary aquifers, 1999-2000

Appendix 1-2.—Land use and land cover mapped within 500-meter radius of sampled wells, lower Tertiary aquifers, 2000-2001
Appendix 2-1.—Replicate data for major ions, trace elements, nutrients, and dissolved organic carbon, samples from Quaternary aquifers.

Appendix 2-2.—Replicate data for major ions, trace elements, nutrients, and dissolved organic carbon, samples from lower Tertiary aquifers

Appendix 2-3.—Concentrations and calculated relative-percent differences of pesticides detected in replicate samples from the Quaternary aquifers

Appendix 2-4.—Concentrations and calculated relative-percent differences of volatile organic compounds (VOCs) detected in replicate samples using unrounded data, Quaternary aquifers

Appendix 2-5.—Concentrations and calculated relative-percent differences of volatile organic compounds (VOCs) detected in replicate samples from the lower Tertiary aquifers

Appendix 3-1.—Graph showing pesticide field-matrix spike recoveries, samples from Quaternary aquifers

Appendix 3-2.—Graph showing pesticide field-matrix spike recoveries, samples from Quaternary aquifers

Appendix 3-3.—Graph showing pesticide field-matrix spike recoveries, samples from lower Tertiary aquifers

Appendix 3-4.—Graph showing pesticide field-matrix spike recoveries, samples from lower Tertiary aquifers

Appendix 3-5.—Graph showing volatile organic compound (VOC) field-matrix spike recoveries, samples from Quaternary aquifers

Appendix 3-6.—Graph showing volatile organic compound (VOC) field-matrix spike recoveries, samples from Tertiary aquifers

Appendix 4-1.—Analyses for physical characteristics, samples from Quaternary aquifers

Appendix 4-2.—Analyses for physical characteristics, samples from lower Tertiary aquifers

Appendix 5-1.—Analyses for major ions and related water-quality characteristics, samples from Quaternary aquifers

Appendix 5-2.—Analyses for major ions and related water-quality characteristics, samples from lower Tertiary aquifers

Appendix 6-1.—Analyses for trace elements, samples from Quaternary aquifers

Appendix 6-2.—Analyses for trace elements, samples from lower Tertiary aquifers

Appendix 7-1.—Analyses for nutrients and dissolved organic carbon, samples from Quaternary aquifers

Appendix 7-2.—Analyses for nutrients and dissolved organic carbon, samples from lower Tertiary aquifers

Appendix 8-1.—Analyses for radionuclides, samples from Quaternary aquifers

Appendix 8-2.—Analyses for radionuclides, samples from lower Tertiary aquifers

Appendix 9-1.—Analyses for pesticides, samples from Quaternary aquifers

Appendix 9-2.—Analyses for pesticides, samples from lower Tertiary aquifers

Appendix 10-1.—Analyses for volatile organic compounds (VOCs), samples from Quaternary aquifers

Appendix 10-2.—Analyses for volatile organic compounds (VOCs), samples from lower Tertiary aquifers

Figures

1. Map showing location of the Bighorn Basin study area in relation to the Yellowstone River Basin NAWQA study unit, Wyoming, Montana, and North Dakota

2. Map showing generalized geology and physiographic provinces, Bighorn Basin study area and Yellowstone River Basin NAWQA study unit, Wyoming, Montana, and North Dakota

3. Schematic geologic section across the central part of the Bighorn Basin in Wyoming

4. Map showing location of Tertiary-age Willwood and Fort Union Formations and sampled wells in the Bighorn Basin study area, Wyoming and Montana

5. Map showing location of Quaternary unconsolidated deposits and sampled wells in the Bighorn Basin study area, Wyoming and Montana

6. Simplified schematic diagram showing relation of domestic, stock, and monitoring wells to hydrogeology

7. Map showing distribution of land use and land cover on Quaternary unconsolidated deposits in the Bighorn Basin, Wyoming and Montana

8. Map showing distribution of land use and land cover on or overlying lower Tertiary rocks in the Bighorn Basin, Wyoming and Montana

9. Graph showing physical characteristics of ground-water samples collected from Quaternary and lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2001

10. Graph showing major-ion concentrations and related water-quality characteristics of ground-water samples collected from Quaternary and lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2001

11. Trilinear diagram showing major-ion composition and dissolved-solids concentrations for ground-water samples collected from Quaternary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2000

12. Stiff diagrams showing areal distribution of major-ion composition and dissolved-solids concentrations for ground-water samples collected from Quaternary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2000

13. Trilinear diagram showing major-ion composition and dissolved-solids concentrations for ground-water samples collected from lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 2000-2001

14. Stiff diagrams showing areal distribution of major-ion composition and dissolved-solids concentrations for ground-water samples collected from lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 2000-2001

15-17. Graphs showing:

15. Trace-element concentrations in ground-water samples collected from Quaternary and lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2001

16. Nutrient and dissolved organic carbon (DOC) concentrations in ground-water samples collected from Quaternary and lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2001

17. Radionuclide concentrations in ground-water samples collected from Quaternary and lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2001

18. Map showing pesticide detections in ground-water samples collected from Quaternary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2000

19. Map showing pesticide detections in ground-water samples collected from lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 2000-2001

20. Graph showing twenty most commonly used pesticides in the Bighorn Basin, Wyoming and Montana, 1994, and comparison with frequency of detection in ground-water samples collected from Quaternary and lower Tertiary aquifers, 1999-2001

21. Graph showing crop types and percentage of total crop production for the years 1994 and 2000 in the Bighorn Basin, Wyoming and Montana

22. Map showing volatile organic compounds (VOCs) detected in ground-water samples collected from Quaternary aquifers, Bighorn Basin, Wyoming and Montana, 1999-2000

23. Map showing volatile organic compounds (VOCs) detected in ground-water samples collected from lower Tertiary aquifers, Bighorn Basin, Wyoming and Montana, 2000-2001

Tables

1. Percent land use and land cover mapped within 500-meter radius of sampled wells.

2. Well construction and related ancillary information for sampled wells completed in Quaternary aquifers.

3. Well construction and related ancillary information for sampled wells completed in lower Tertiary aquifers.

4. Laboratory methods used to analyze ground-water samples

5. Pesticides analyzed, trade name, pesticide action, class, laboratory reporting levels, and U.S. Environmental Protection Agency health benchmarks.

6. Volatile organic compounds (VOCs) analyzed, laboratory reporting levels, and related U.S. Environmental Protection Agency standards.

7. Summary of major ions, trace elements, nutrients, and dissolved organic carbon detected in field-equipment blank samples and in environmental samples collected during sampling of Quaternary aquifers.

8. Summary of major ions, trace elements, nutrients, and dissolved organic carbon detectedin field-equipment blank samples and in environmental samples collected during sampling of lower Tertiary aquifers.

9. Summary of volatile organic compounds (VOCs) detected in field-equipment blank samples and respective environmental samples collected during sampling of the Quaternary aquifers.

10. Summary of volatile organic compounds (VOCs) detected in field-equipment blank samples and respective environmental samples collected during sampling of the lower Tertiary aquifers.

11. Summary of inorganic ground-water quality, Quaternary aquifers.

12. Summary of inorganic ground-water quality, lower Tertiary aquifers.

13. Dissolved-solids concentrations in waters from Quaternary and lower Tertiary aquifers compared to U.S. Geological Survey salinity classification.

14. Summary of pesticides detected in ground-water samples collected from Quaternary aquifers

15. Summary of pesticides detected in ground-water samples collected from lower Tertiary aquifers

16. Summary of volatile organic compounds (VOCs) detected in ground-water samples collected from Quaternary aquifers.

17. Summary of volatile organic compounds (VOCs) detected in ground-water samples collected from lower Tertiary aquifers.


Wyoming Water Science Center


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