# Characterization of Ground-Water Quality, Upper Republican Natural Resources District, Nebraska, 1998-2001

**By Jill D. Frankforter and Daniele T. Chafin**

U.S. GEOLOGICAL SURVEYScientific Investigations Report 2004-5013

Prepared in cooperation with the Upper Republican Natural Resources District

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## Abstract

Nearly all rural inhabitants and livestock in the Upper Republican Natural   Resources District (URNRD) in southwestern Nebraska use ground water that can   be affected by elevated nitrate concentrations. The development of ground-water   irrigation in this area has increased the vulnerability of ground water to the   introduction of fertilizers and other agricultural chemicals. In 1998, the U.S.   Geological Survey, in cooperation with the Upper Republican Natural Resources   District, began a study to characterize the quality of ground water in the Upper   Republican Natural Resources District area with respect to physical properties   and concentrations of major ions, coliform bacteria, nitrate, and pesticides,   and to assess the presence of nitrogen concentrations in the unsaturated zone.   At selected well sites, the ground-water characterization also included tritium   and nitrogen-isotope analyses to provide information about the approximate age   of the ground water and potential sources of nitrogen detected in ground-water   samples, respectively.

In 1998, ground-water samples were collected from 101 randomly selected domestic-well   sites. Of the 101 samples collected, 26 tested positive for total coliform bacteria,   exceeding the U.S. Environmental Protection Agency's Maximum Contaminant Level   (MCL) of zero colonies. In 1999, ground-water samples were collected from 31   of the 101 well sites, and 16 tested positive for coliform bacteria.

Nitrates were detected in ground water from all domestic-well samples and from   all but four of the irrigation-well samples collected from 1998 to 2001. Eight   percent of the domestic-well samples and 3 percent of the irrigation-well samples   had nitrate concentrations exceeding the U.S. Environmental Protection Agency's   MCL for drinking water of 10 milligrams per liter. Areas with nitrate concentrations   exceeding 6 milligrams per liter, the URNRD's ground-water management-plan action   level, were found predominantly in north-central Chase, western and south-central   Dundy, and south-central Perkins Counties. Generally, these concentrations were   detected in samples from wells located in upland areas with permeable soils   and a high percentage of cropland.

In 1999, 31 of the ground-water samples collected from irrigation wells were analyzed for pesticides, and 14 samples (45 percent) had detectable concentrations of at least one pesticide compound. In 2000, all of the 23 irrigation-well samples analyzed had one or more pesticides present at detectable concentrations. In 2001, 12 of 26 domestic-well samples (46 percent) had detectable concentrations. Although the analytical method used during the study was changed to increase the number of pesticides included in the analyses, the pesticides detected in the ground-water samples from domestic and irrigation wells were limited to the commonly used herbicide compounds acetochlor, alachlor, atrazine, metolachlor, prometon, propachlor, propazine, trifluralin, and the atrazine degradation product deethylatrazine. Of the compounds detected, only atrazine (3.0 micrograms per liter) and alachlor (2.0 micrograms per liter) have MCLs established by the U.S. Environmental Protection Agency. None of the ground-water samples from the URNRD study area had concentrations that exceeded either MCL.

Tritium age-dating analyses indicate water from about one-third of the sites entered the ground-water system prior to 1952. Because the increase in agricultural practices occurred during the 1950s and 1960s, it can be assumed that this water was not influenced by agricultural practices. Nitrogen-isotope speciation analyses for samples from three irrigation wells indicated that the source of nitrates in the ground water probably is synthetic fertilizer; however, the source at most irrigation wells probably is either naturally occurring or a mixture of water from various anthropogenic sources (such as synthetic fertilizer and animal waste).

## Contents

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Study Area

Climate

Physiography

Soils

Land Use

Surface Water

Ground Water

Methods

Site Identification

Domestic-Well Selection and Sampling Strategy

Irrigation-Well Selection and Sampling Strategy

Soil-Coring Site Selection and Sampling Strategy

Water-Quality Sampling and Laboratory Analytical Methods

Physical-Property and Major-Ion Measurements

Coliform Bacteria and Nitrate

Herbicides and Other Common-Use Pesticides

Tritium

Nitrogen Isotopes

Soil-Core Sampling and Laboratory Analyses

Data Analysis

Quality Assurance and Quality Control

Characterization of Ground-Water Quality

Domestic-Well Network

Physical-Property and Major-Ion Measurements

Coliform Bacteria

Nitrates

Herbicides and Other Common-Use Pesticides

Irrigation-Well Network

Physical-Property and Major-Ion Measurements

Nitrates

Herbicides and Other Common-Use Pesticides

Tritium

Nitrogen Isotopes

Soil Nitrogen

Summary and Conclusions

References

**Suggested Citation:**

Frankforter, J.D., and Chafin, D.T., 2004, Characterization   of ground-water quality, Upper Republican Natural Resources District, Nebraska,   1998-2001: U.S. Geological Survey Scientific Investigations Report 2004-5013,   24 p.

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Send questions or comments about this report to the author,   J.D. Frankforter (402) 437-5152.

For more information about USGS activities in Nebraska, visit the USGS Nebraska District home page.

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