Steep Spatial Chemical Gradients in Shallow Groundwater Contaminated with Nitrate at a Residential Site

Abstract

Shallow groundwater at a residential property in north-central Oklahoma was assessed to determine the extent of NO3- contamination and the effect of nitrate attenuation on groundwater chemistry. We hypothesized that denitrification at the site is the primary process controlling NO3- attenuation. The objectives of this investigation were to identify the potential sources and processes which control NO3- distributions, and observe the impact denitrification had on groundwater chemistry. Groundwater was sampled quarterly over a three-year period and analyzed for physical, chemical, and stable carbon isotopic composition. Persistent high concentrations of NO3- were observed with non-uniform distributions both temporally and spatially. The high persistence in NO3- contamination was attributed to upgradient application by residents, on-site application of fertilizers on flower beds and for lawn care, and contamination from a leaky sewer pipe. Variable NO3- concentrations were observed across the 40 m x 60 m site over a the study period. Temporal and spatial variations in NO3- concentrations were attributed to non-uniform fertilizer application, focused rain recharge with lower NO3- concentrations and denitrification. Nitrate concentrations were relatively low at a location suspected of effluent contamination, indicating NO3- attenuation at that location was accelerated. The stable carbon isotope ratios of dissolved inorganic carbon (δ13CDIC) showed consistent depletion in groundwater at locations where it was suspected denitrification was fueled by organic matter. Marked increase in HCO3- and DIC and decreases in δ13CDIC suggests that weathering of carbonates coupled to organic matter mineralization by microbial activity was occurring in groundwater at the site.