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Scientific Investigations Report 2012–5235

Prepared in cooperation with the Delaware Department of Natural Resources and Environmental Control and the
Delaware Geological Survey

Residence Time, Chemical and Isotopic Analysis of Nitrate in the Groundwater and Surface Water of a Small Agricultural Watershed in the Coastal Plain, Bucks Branch, Sussex County, Delaware

By John W. Clune and Judith M. Denver

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Nitrate is a common contaminant in groundwater and surface water throughout the Nation, and water-resource managers need more detailed small-scale watershed research to guide conservation efforts aimed at improving water quality. Concentrations of nitrate in Bucks Branch are among the highest in the state of Delaware and a scientific investigation was performed to provide water-quality information to assist with the management of agriculture and water resources. A combination of major-ion chemistry, nitrogen isotopic composition and age-dating techniques was used to estimate the residence time and provide a chemical and isotopic analysis of nitrate in the groundwater in the surficial aquifer of the Bucks Branch watershed in Sussex County, Delaware. The land use was more than 90 percent agricultural and most nitrogen inputs were from manure and fertilizer. The apparent median age of sampled groundwater is 18 years and the estimated residence time of groundwater contributing to the streamflow for the entire Bucks Branch watershed at the outlet is approximately 19 years. Concentrations of nitrate exceeded the U.S. Environmental Protection Agency drinking-water standard of 10 milligrams per liter (as nitrogen) in 60 percent of groundwater samples and 42 percent of surface-water samples. The overall geochemistry in the Bucks Branch watershed indicates that agriculture is the predominant source of nitrate contamination and the observed patterns in major-ion chemistry are similar to those observed in other studies on the Mid-Atlantic Coastal Plain. The pattern of enrichment in nitrogen and oxygen isotopes (δ15N and δ18O) of nitrate in groundwater and surface water indicates there is some loss of nitrate through denitrification, but this process is not sufficient to remove all of the nitrate from groundwater discharging to streams, and concentrations of nitrate in streams remain elevated.

First posted December 6, 2012

For additional information contact:
Maryland-Delaware-D.C. Water Science Center
5522 Research Park Drive
Baltimore, MD 21228

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Suggested citation:

Clune, J.W., and Denver, J.M., 2012, Residence time, chemical and isotopic analysis of nitrate in the groundwater and surface water of a small agricultural watershed in the Coastal Plain, Bucks Branch, Sussex County, Delaware: U.S. Geological Survey Scientific Investigations Report 2012–5235, 15 p.,




Sampling and Analytical Methods

Estimated Residence Time of Groundwater

Chemical and Isotopic Analysis of Nitrate

Summary and Conclusions


References Cited

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