LEGACY EFFECTS OF NITRATE CONTAMINATION IN A PRAIRIE RIPARIAN ECOSYSTEM

There is a need to better understand the long-term fate of groundwater nitrate dynamics in contaminated ecosystems, especially within the context of the subsurface (e.g., anaerobic denitrification, oxidation of pyrite, etc.), and their eventual impact on surface water quality. This study sought to explore this topic through the regular monitoring of groundwater chemistry along King’s Creek, a small stream in which one bank was once cultivated intensively and subject to regular applications of nitrate fertilizers while the other bank was left unplowed. This contrasting land use history provided an ideal context in which to study the long-term transport and processing of the excess nitrate through the subsurface. To elucidate groundwater flow paths and groundwater-surface water interactions groundwater levels, temperature, and conductivity were measured continuously on five-minute intervals from 2016 to 2018 from a total of 8 nested wells on the formally cultivated and prairie stream banks. Between November 2017 and October 2018, monthly water samples were collected from these same wells and analyzed for major anions and cations, pH, temperature, DO, and conductivity to quantify how the seasonal changes in water table position and flow paths governed the transport and transformation of nitrate. We then compared this data to previous studies of the same system to evaluate how the geochemical trends observed in the past have evolved over time.

In 2017-2018, nitrate concentrations on the cultivated side of King’s Creek still remain higher than those on the uncultivated bank. Considering that agriculture ceased on the cultivated bank in the 1990s, this suggests a relatively long residence time for the remaining excess nitrate. Significant correlations observed between nitrate concentrations and dissolved oxygen, chloride and sulfate may indicate anoxic denitrification and pyrite oxidation play a role in nitrate removal in the subsurface. Ultimately, this study hopes to provide novel insights into the long-term ramifications of nitrate contamination of groundwater chemistry and its ecological impacts on riparian systems.