CHARACTERIZATION OF NUTRIENT SOURCES, TRANSPORT PATHWAYS, AND TRANSFORMATIONS USING STABLE ISOTOPE AND GEOCHEMICAL TOOLS IN THE BIG CREEK WATERSHED OF NORTHWEST ARKANSAS

Agricultural, residential, and urban land use has the potential to provide excess phosphorus and nitrogen nutrients to watersheds. Stable isotopes are a powerful tool in tracing pathways and identifying sources of these nutrients: for example, the isotopic signatures of nitrate, δ¹⁵N-NO₃ and δ¹⁸O-NO₃, allow for delineation of synthetic and organic fertilizers as well as atmospheric sources. The addition of traditional geochemical methods such as basic common ion and trace element analyses may further distinguish between nutrient sources such as septic effluent and manure from various livestock. The recent establishment of a concentrated animal-feeding operation near Big Creek, a tributary of the Buffalo National River, has raised concern for degradation of water quality in the watershed. We have established an isotopic reference library of nutrient sources, specifically, δ¹⁵N-NO₃, δ¹⁸O-NO₃, and δ¹⁸O-PO₄ in the Big Creek watershed by direct sampling of possible sources, including septic system effluent, poultry, swine, and cattle manure, storm runoff, and agricultural runoff. Isotopic signatures in Big Creek water samples are related to source signatures through consideration of fractionating processes and mixing relations coupled with geochemical characteristics of samples. Understanding of the sources responsible for nutrient input to the stream will enable best management practices to be improved in order to protect the watershed.