NITROGEN ISOTOPE FRACTIONATION AS A TOOL FOR IDENTIFYING NITRATE SOURCES IN LOWER ESOPUS CREEK, ULSTER COUNTY, NEW YORK

Surface water chemistry is influenced by a range of natural and anthropogenic factors, with nitrogen isotope fractionation serving as a valuable tracer for identifying nitrate sources and transformation processes. This study investigated nitrate sources in the Lower Esopus Creek, located in Ulster County, New York, focusing on contributions from agricultural runoff, including inputs from the Hudson Valley Farm Hub, as well as other potential sources such as precipitation, soil nitrogen mineralization, septic systems, manure, fertilizers, and industrial effluents.

Water samples (n=24) were collected annually over three years from the Lower Esopus Basin, including tributaries influenced by agricultural land use. Isotopic analyses of nitrogen (¹⁵N and ¹⁴N) as well as oxygen (¹⁸O and ¹⁶O) were conducted to differentiate between nitrate sources and identify biogeochemical processes such as denitrification and assimilation. The findings revealed three distinct isotopic signatures, indicating varied nitrate origins.

Results suggest that nitrate inputs from the Hudson Valley Farm Hub are predominantly associated with manure-based fertilizers, while areas without known anthropogenic inputs exhibited isotopic evidence of natural processes such as denitrification and assimilation. Samples collected from areas unaffected by agricultural land use indicated that nitrate inputs were primarily derived from atmospheric deposition through precipitation.