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USGS Nebraska Water Science Center Publication

Age and Quality of Ground Water and Sources of Nitrogen in the Surficial Aquifers in Pumpkin Creek Valley, Western Nebraska, 2000

By G.V. Steele, J.C. Cannia, S.S. Sibray, and V.L. McGuire

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2005-5157

Prepared in cooperation with the North Platte Natural Resources District


Abstract

Ground water is the source of drinking water for the residents of Pumpkin Creek Valley, western Nebraska. In this largely agricultural area, shallow aquifers potentially are susceptible to nitrate contamination. During the last 10 years, ground-water levels in the North Platte Natural Resources District have declined and contamination has become a major problem for the district. In 2000, the U.S. Geological Survey and the North Platte Natural Resources District began a cooperative study to determine the age and quality of the ground water and the sources of nitrogen in the aquifers in Pumpkin Creek Valley.

Water samples were collected from 8 surface-water sites, 2 springs, and 88 ground-water sites during May, July, and August 2000. These samples were analyzed for physical properties, nutrients or nitrate, and hydrogen and oxygen isotopes. In addition, a subset of samples was analyzed for any combination of chlorofluorocarbons, tritium, tritium/helium, sulfur-hexafluoride, carbon-14, and nitrogen-15.

The apparent age of ground water in the alluvial aquifer typically varied from about 1980 to modern, whereas ground water in the fractured Brule Formation had a median value in the 1970s. The Brule Formation typically contained ground water that ranged from the 1940s to the 1990s, but low-yield wells had apparent ages of 5,000 to 10,000 years before present. Data for oxygen-18 and deuterium indicated that lake-water samples showed the greatest effects from evaporation. Ground-water data showed no substantial evaporative effects and some ground water became isotopically heavier as the water moved downgradient. In addition, the physical and chemical ground-water data indicate that Pumpkin Creek is a gaining stream because little, if any, of its water is lost to the ground-water system.

The water-quality type changed from a sodium calcium bicarbonate type near Pumpkin Creek's headwaters to a calcium sodium bicarbonate type near its mouth. Nitrate concentrations were largest in the alluvial system (median = 5 mg/L) and smallest in the surface-water system (median = 1 mg/L). Most nitrate concentrations exceeding the U.S. Environmental Protection Agency maximum contaminant level for drinking water of 10 mg/L as nitrogen were adjacent to irrigated fields and in areas where alluvial sediments are less than 50 ft thick.

Sources of nitrogen in the ground water of the study area included naturally occurring nitrogen, commercial fertilizer, and animal waste. Based on nitrate concentration and delta nitrogen-15, the nitrogen in 65 percent of the water samples appears to have originated from a mixture of commercial fertilizers and animal waste. Some of the smallest nitrate concentrations in the ground-water samples contained some of the largest delta nitrogen-15 values (greater than 10 per mil), which suggests animal waste as the likely source. Commercial fertilizers were the likely source of most of the nitrogen in water samples with nitrate concentrations that exceeded 10 mg/L. The source of the nitrogen in water samples with nitrate concentrations exceeding 10 mg/L, but with delta nitrogen-15 values close to 10 per mil, could not be determined.

Contents

Abstract

Introduction

Purpose and Scope

Previous Investigations

Description of Study Area

Location and Physiography

Land Use

Hydrogeology

Acknowledgments

Methods

Well Installation

Site Identification

Sample Collection

Water-Sample Analysis

Data-Analysis Procedures

Age Dating

Chlorofluorocarbons

Tritium, Tritium/Helium, Carbon-14

Hydrogen and Oxygen Isotopes

Nitrogen and Nitrogen Isotopes

Aquifer Properties

Creation of Water-Table Map and Geologic Sections

Quality Assurance and Quality Control

Age and Quality of Ground Water

Age of Ground Water

Water Quality

Spatial Distribution of Nitrate

Temporal Distribution of Nitrate

Sources of Nitrogen

Summary

References

Appendix

Suggested Citation:

Steele, G.V., Cannia, J.C., Sibray, S.S., and McGuire, V.L., 2005, Age and quality of ground water and sources of nitrogen in the surficial aquifers in Pumpkin Creek Valley, western Nebraska, 2000: U.S. Geological Survey Scientific Investigations Report 2005-5157, 68 p.


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Send questions or comments about this report to the author, G.V. Steele (402) 328-4129.

For more information about USGS activities in Nebraska, visit the USGS Nebraska Water Science Center home page.

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