The use of nitrogen and sulfur isotopes in a shallow outwash aquifer

摘要

A study site in the East Fork of the White River in southern Indiana, USA, has many shallow wells in an aquifer that exceed maximum contaminant levels (MCLs). Nitrogen isotopes were analyzed to differentiate nitrogen sources, including inorganic fertilizers used on cropland, effluent from septic systems, and animal wastes generated at two confined feedlots. NO_3-N concentrations are highest (up to 52.1 mg/L) and isotopically heavy (δ~(15)N > +13.7 per thousand) in shallow monitoring wells located immediately down-gradient from the larger of the two confined feedlots. Several up-gradient wells also have NO_3-N concentrations above drinking water standards and are isotopically light (δ~(15)N < 0 per thousand, indicating leachate from inorganic fertilizers. Sulfate concentrations ranged from 14 to 225 mg/L and sulfur isotopes (δ~(34)S per thousand) were analyzed to determine sulfur sources. Unique sulfur isotopic signatures were determined for potential sources, such as ammonium sulfate fertilizer, sulfate generated at the feedlots, and the oxidation of pyrite within the aquifer.