Isotopic and Geochemical Analysis of Nitrate Contaminated Groundwater in Edgar, Nebraska

High nitrate concentrations in the groundwater beneath Edgar, Nebraska have been measured for well over a decade, exceeding the EPA’s MCL of 10 mg-N/L. Investigating the isotopic signature of nitrate and groundwater have determined the likely source(s) of contamination. Apparent seasonal changes exists in nitrate concentration, isotope ratios, and geochemistry, suggesting that sources and flow paths change as a result of irrigation. Samples collected in December and May had very similar nitrate concentrations (average value of 13.7 mg-N/L) and isotopic ratios (δ15N 4.46‰ and δ18O-NO3 20.21‰), suggesting synthetic nitrate fertilizers are the major source of nitrate in the groundwater. August samples, indicated other sources of nitrate present in the groundwater. Nitrate concentrations from the same sites increased to an average value of 14.65 mg-N/L and shifted isotope ratios of δ15N to 9.30‰ and δ18O-NO3 to 15.45‰. The changes are associated with proximity to irrigation and/or municipal wells. Shallow wells screened near the water table as well as deeper wells follow the same pattern of drastic changes in the August samples. Two wells located inside the city limits and more than a half mile away from any major well saw little to no change in nitrate concentration or isotopic signature, indicating that they are unaffected by seasonal changes. Further evidence of mixing of waters can be seen with a subtle shift where water isotopes become more enriched in δ18O-H2O and more depleted in δD. Geochemistry supports an apparent mixing of waters. Advisors: David Loope and Daniel Sno