High-Resolution Forensic Evaluation of Nitrate-Contaminated Groundwater

Determining the source(s) of nitrate contamination in agricultural areas is complicated; doing this as a forensic evaluation for litigation in one field week is especially challenging. The objective of this 2021 investigation, conducted for an agricultural producer who began applying animal waste in 2007, was to determine whether animal waste was the source of nitrate reported at downgradient residential wells. The test Site was a 180 acre, irrigated field that overlies a thick sand aquifer with the water table at about 20 ft below ground level (fbgl). We used multiple lines of evidence to resolve the source(s) of nitrate at three test locations at the Site using continuous vertical testing for hydraulic conductivity and high-resolution vertical testing of selected chemical and isotopic parameters in water at 5-ft depth intervals between 5 and 80 fbgl. We found that beneath the Site at the depth interval of most of the impacted residential wells: (1) groundwater travel time indicates nitrate at the impacted residential wells could not have originated from the Site, (2) the groundwater age based on tritium (³H) pre-dates the establishment of manure spreading, and (3) the nitrogen-15 concentrations (δ¹⁵N) in nitrate indicate the nitrate is not all, if in any part, due to animal waste. This high-resolution investigation clearly indicated that the agricultural producer was not responsible for the impacted residential wells and was completed in only 1 week.