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Scientific Investigations Report 2007-5228


Publications Warehouse SIR 2007-5228

Prepared as part of the National Water-Quality Assessment Program

Occurrence of Agricultural Chemicals in Shallow Ground Water and the Unsaturated Zone, Northeast Nebraska Glacial Till, 2002–04

By Jennifer S. Stanton, Gregory V. Steele, and Jason R. Vogel

SIR 2007-228 Contents | Citation

Abstract

Agricultural chemicals applied at the land surface in northeast Nebraska can move downward, past the crop root zone, to ground water. Because agricultural chemicals applied at the land surface are more likely to be observed in the shallowest part of an aquifer, an assessment of shallow ground-water and unsaturated zone quality in the northeast Nebraska glacial till was completed between 2002 and 2004. Ground-water samples were collected at the first occurrence of ground water or just below the water table at 32 sites located in areas likely affected by agriculture. Four of the 32 sites were situated along a ground-water flow path with its downgradient end next to Maple Creek. Twenty-eight sites were installed immediately adjacent to agricultural fields throughout the glacial-till area. In addition to those 32 sites, two sites were installed in pastures to represent ground-water conditions in a non-cropland setting. Ground-water samples were analyzed for physical properties and concentrations of nitrogen and phosphorus compounds, selected pesticides and pesticide degradates, dissolved solids, major ions, trace elements, and dissolved organic carbon. Chlorofluorocarbons (CFCs) or sulfur hexafluoride (SF6) concentrations were analyzed at about 70 percent of the monitoring wells to estimate the residence time of ground water. Borehole-core samples were collected from 28 of the well boreholes. Sediment in the unsaturated zone was analyzed for nitrate, chloride, and ammonia concentrations.

Analytical results indicated that the agricultural chemicals most often detected during this study were nitrates and herbicides. Nitrate as nitrogen (nitrate–N) concentrations (2003 median 9.53 milligrams per liter) indicated that human activity has affected the water quality of recently recharged ground water in approximately two-thirds of the wells near corn and soybean fields. The principal pesticide compounds that were detected reflect the most-used pesticides in the area and included parent or degradate compounds of acetochlor, alachlor, atrazine, and metolachlor. Overall, pesticide concentrations in ground-water samples collected in 2003 and 2004 were small and did not exceed public drinking-water standards where established. On average, more pesticides were detected in the flow-path wells than in the glacial-till network wells. The presence of a perennial stream within 1,640 feet of a well was correlated to smaller nitrate–N concentrations in the well water, and the presence of a road ditch within 164 feet of the well was correlated to the presence of detectable pesticides in the well water. All other variables tested showed no significant correlations to nitrate–N concentrations or pesticide detections.

Unsaturated zone soil cores collected in 2002 from well boreholes indicated that nitrogen in the forms of nitrate–N and ammonia as nitrogen (ammonia–N) was available in the unsaturated zone for transport to ground water. Concentrations of nitrate–N and ammonia–N in these soil cores were inversely correlated to depth, and nitrate–N concentrations were correlated to chloride concentrations.

Posted January 2008

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Contents

Foreword

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Study Area

Hydrogeologic Setting

Land-Use Setting

Methods of Investigation

Site Selection and Well Installation

Sample Collection and Analysis

Data Treatment

Quality Control

Field-Blank Samples

Replicate Samples

Environmental-Matrix Spike Samples

Occurrence of Agricultural Chemicals in Shallow Ground Water

Nutrients

Pesticides

Occurrence of Chemicals in the Unsaturated Zone

Factors Related to Occurrence of Agricultural Chemicals in Shallow Ground Water

Water Depth

Aquifer-Sediment and Soil Characteristics

Land Use

Geologic Unit

Well Type

Summary

References Cited

Appendix 1. Pesticides analyzed in ground-water samples collected from northeast Nebraska glacial till, 2003–04

Appendix 2. Unsaturated-zone borehole-sediment sample results, northeast Nebraska glacial till, May–October 2002

Appendix 3. Nitrate as nitrogen and pesticide concentrations in ground-water samples collected from domestic wells, northeast Nebraska glacial till, 1994–95



Report Citation

Stanton, J.S., Steele, G.V., and Vogel, J.R., 2007, Occurrence of agricultural chemicals in shallow ground water and the unsaturated zone, northeast Nebraska glacial till, 2002–04: U.S. Geological Survey Scientific Investigations Report 2007–5228, 51 p.

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