MULTIPLE ISOTOPE (δ15N, δ18O, δ11B) TRACING OF NITRATE CONTAMINANT SOURCES IN SURFACE AND GROUNDWATER

The global population has been increasing exponentially and this has caused several challenges surrounding sustainability, including the need for greater food production. To meet these demands and boost agricultural productivity, mechanized, more efficient agricultural practices and synthetic fertilizers are used. Synthetic fertilizers and other sources of nutrients have resulted in water pollution and water quality degradation. Much of the Great Miami River Watershed’s streams and aquifers in southwestern, Ohio are impacted by nitrate contaminants originating from anthropogenic sources including use of synthetic fertilizers for agriculture, organic fertilizers, human wastes (domestic, industrial, and municipal wastes), and urbanization. High nitrate concentrations cause ecological disturbances affecting organisms across all trophic levels. Nitrate levels greater than 10 mg/l also pose a danger to human health, if the contaminant reaches drinking water sources. Although networks of water quality monitoring stations in the watershed report nitrate concentrations in surface and groundwater, a nitrate contaminant source has not been identified. Here we used isotope ratios of nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) in nitrates to identify nitrate sources for surface and groundwater. Initially we fingerprinted the isotopic composition of the main nitrate contaminant sources in the watershed. Our result shows a distinct low δ¹⁵N for commercial synthetic fertilizers (0.4±4‰) and high δ¹⁵N for animal and human waste (13.0±1.3‰). However, as δ¹⁵N of human and animal waste overlaps, further analysis of boron isotopes (δ¹¹B) is used to distinguish anthropogenic sources from natural sources. Further sampling along the Great Miami River, Mad River, and Stillwater River within the watershed provides insights into contaminant sources contributing to high levels of nitrate in each river. In general, the δ¹⁵N from river samples collected during the low river flow lies within a range of human and animal waste, whereas δ¹⁵N values of groundwater suggest that the nitrates might have been derived from synthetic fertilizers. This research provides a regional baseline for nitrate contaminant source tracing and helps to better inform state and local water quality and nutrient management planning.