USGS Scientific Investigations Report 2008-5021

Prepared in cooperation with the North Carolina Department of Environment and Natural Resources, Division of Water Quality, Aquifer Protection Section

Factors Affecting Nitrate Delivery to Streams from Shallow Ground Water in the North Carolina Coastal Plain

By Stephen L. Harden and Timothy B. Spruill

U.S. Geological Survey Scientific Investigations Report 2008-5021, 39 pages (Published online only, March 2008)

This report is available in PDF format: SIR 2008-5021 (Opens the PDF file in a new window. ) (1.1 MB)

Cover thumbnailAn analysis of data collected at five flow-path study sites between 1997 and 2006 was performed to identify the factors needed to formulate a comprehensive program, with a focus on nitrogen, for protecting ground water and surface water in the North Carolina Coastal Plain. Water-quality protection in the Coastal Plain requires the identification of factors that affect the transport of nutrients from recharge areas to streams through the shallow ground-water system. Some basins process or retain nitrogen more readily than others, and the factors that affect nitrogen processing and retention were the focus of this investigation to improve nutrient management in Coastal Plain streams and to reduce nutrient loads to coastal waters.

Nitrate reduction in ground water was observed at all five flow-path study sites in the North Carolina Coastal Plain, although the extent of reduction at each site was influenced by various environmental, hydrogeologic, and geochemical factors. Denitrification was the most common factor responsible for decreases in nitrate along the ground-water flow paths. Specific factors, some of which affect denitrification rates, that appeared to influence ground-water nitrate concentrations along the flow paths or in the streams include soil drainage, presence or absence of riparian buffers, evapotranspiration, fertilizer use, ground-water recharge rates and residence times, aquifer properties, subsurface tile drainage, sources and amounts of organic matter, and hyporheic processes. The study data indicate that the nitrate-reducing capacity of the buffer zone combined with that of the hyporheic zone can substantially lower the amount of ground-water nitrate discharged to streams in agricultural settings of the North Carolina Coastal Plain.

At the watershed scale, the effects of ground-water discharge on surface-water quality appear to be greatly influenced by streamflow conditions and the presence of extensive riparian vegetation. Streamflow statistics that reflect base flow and the general hydrologic dynamics of a stream are important in understanding nutrient transport from a watershed and may be useful indicators of watersheds that are likely to have higher yields of nutrients and water. Combining streamflow statistics with information on such factors as land use, soil drainage, extent of riparian vegetation, geochemical conditions, and subsurface tile drainage in the Coastal Plain can be useful in identifying watersheds that are most likely to export excessive nitrogen due to nonpoint-source loadings and watersheds that are effective in processing nitrogen.

CONTENTS

Abstract

Introduction

Purpose and Scope

Description of Study Sites

Methods

Site-Scale Factors Affecting Nitrate Concentrations along Flow Paths

Site 1--First-Order Stream with Stream Buffer and Well-Drained Soil

Site 4--First-Order Stream with Stream Buffer and Poorly Drained Soil

Site 2--Second-Order Stream with and without Stream Buffer and Well-Drained Soil

Site 3--Third-Order Stream without Stream Buffer and Poorly Drained Soil

Site 5--First- and Third-Order Streams with Stream Buffers and Well to Poorly Drained Soils

Summary of Nitrate-Reducing Capacity in Different Hydrogeologic Settings

Watershed-Scale Factors Affecting Nitrogen and Water Yields--A comparison of Two Watersheds

Summary and Conclusions

Acknowledgments

References Cited

Appendix

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Suggested citation: Harden, S.L., and Spruill, T.B., 2008, Factors affecting nitrate delivery to streams from shallow ground water in the North Carolina Coastal Plain: U.S. Geological Survey Scientific Investigations Report 20085021, 39 p. (online only at http://pubs.water.usgs.gov/sir2008-5021)

For more information, please contact Stephen L. Harden.

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