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Scientific Investigations Report 2014–5171

Prepared in cooperation with the City of Durham, Public Works Department, Stormwater Services Division

Using Stable Isotopes of Nitrogen and Oxygen to Identify Sources of Nitrate in Three Creeks, Durham County, North Carolina, 2011–12

By Kristen Bukowski McSwain, Megan B. Young, and Mary L. Giorgino

Thumbnail of and link to report PDF (7.16 MB)Abstract

A preliminary assessment of nitrate sources was conducted in three creeks that feed nutrient impaired Falls and Jordan Lakes in the vicinity of Durham County, North Carolina, from July 2011 to June 2012. Cabin Branch, Ellerbe Creek, and Third Fork Creek were sampled monthly to determine if sources of nitrate in surface water could be identified on the basis of their stable isotopic compositions. Land use differs in the drainage basins of the investigated creeks—the predominant land use in Cabin Branch Basin is forest, and the Ellerbe and Third Fork Creek Basins are predominantly developed urban areas. Total nutrient concentrations were below 1 milligram per liter (mg/L). All measured nitrate plus nitrite concentrations were below the North Carolina standard of 10 mg/L as nitrogen with the highest concentration of 0.363 mg/L measured in Third Fork Creek. Concentrations of ammonia were generally less than 0.1 mg/L as nitrogen in all creek samples. More than 50 percent of the total nitrogen measured in the creeks was in the form of organic nitrogen. Total phosphorus and orthophosphate concentrations in all samples were generally less than 0.2 mg/L as phosphorus. The isotopic composition of surface water (δ 2HH20 and δ18OH2O) is similar to that of modern-day precipitation. During July and August 2011 and May and June 2012, surface-water samples displayed a seasonal difference in isotopic composition, indicating fractionation of isotopes as a result of evaporation and, potentially, mixing with local and regional groundwater. The dominant source of nitrate to Cabin Branch, Ellerbe Creek, and Third Fork Creek was the nitrification of soil nitrogen. Two stormflow samples in Ellerbe Creek and Third Fork Creek had nitrate sources that were a mixture of the nitrification of soil nitrogen and an atmospheric source that had bypassed some soil contact through impermeable surfaces within the drainage basin. No influence of a septic or wastewater source was found in Cabin Branch. Results from this study suggest that it is possible to distinguish sources of nitrogen and biogeochemical processes on nitrate using stable isotopes of nitrogen and oxygen in small creeks of Durham County, North Carolina.

First posted October 1, 2014

For additional information, contact:
Director, North Carolina Water Science Center
3916 Sunset Ridge Road
Raleigh, NC 27607
http://nc.water.usgs.gov/

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Suggested citation:

McSwain, K.B., Young, M.B., and Giorgino, M.L., 2014, Using stable isotopes of nitrogen and oxygen to identify sources of nitrate in three creeks, Durham County, North Carolina, 2011–12: U.S. Geological Survey Scientific Investigations Report 2014–5171, 22 p., http://dx.doi.org/10.3133/sir20145171.

ISSN 2328–0328 (online)



Contents

Abstract

Introduction

Methods of Data Collection and Analysis

Water-Quality Sampling Results

Summary and Conclusions

References


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