Use of nitrogen isotopes and other geochemical tools to evaluate the source of ammonium in a confined glacial drift aquifer, Ottawa County, Michigan, USA

This study utilized several isotopic tracers and geochemical parameters to characterize the source and hydrogeochemical setting of elevated ammonium (NH4+) concentrations (>2 mg/L) in a confined glacial drift aquifer in Ottawa County, Michigan. Results from this investigation indicate that NH4+ is attributed to degrading in situ organic matter and not from surficial anthropogenic sources. Tritium, δ18O, and δ2H values suggest that primary recharge in the confined aquifer occurred during the middle to late Holocene and before the mid-1900's. Geochemical constituents, including redox parameters and a suite of common ions, demonstrate a compositional difference between the confined and unconfined aquifers in the study area and therefore further support the isolation of the confined aquifer from a significant amount of recent recharge (post 1950's). Insignificant δ15N- NH4+ fractionation (+0.81‰ to +1.38‰) implies that NH4+ in the confined aquifer originated from degrading in situ organic matter. A complete Rotosonic glacial sediment core, collected to characterize the physical setting of the NH4+-rich aquifer, confirmed the presence of in situ organic matter in the confined aquifer. A limited set of δ15N-NO3 and δ18O-NO3 values suggest that the application of manure to row crops may be contributing nitrate (up to 10 mg/L) to nearby shallow residential wells. As implemented in this study, the use of multiple geochemical and isotopic parameters, including δ15N-NH4, can be applied to evaluate the potential source of NH4+ in an aquifer.