TEMPORAL INVESTIGATION OF THE SOURCES AND FATE OF NITRATE CONTAMINATION AT A RESIDENTIAL SITE

Shallow groundwater aquifers are highly susceptible to contamination from natural and anthropogenic factors. This is especially true in residential areas where applications of nitrate fertilizers can potentially affect the groundwater quality. We have monitored shallow groundwater at a residential site in Stillwater, Oklahoma; taking samples every two months to quarterly from 2008 to 2012. The first aim of the study was to assess the source(s) of nitrate contamination of the groundwater. The second aim was to assess the long-term behavior and fate of the nitrates in the contaminated groundwater. We conducted in-situ measurements of pH, temperature, oxidation reduction potential (ORP), total dissolved solids (TDS) and dissolved oxygen (DO) in monitoring wells. Water samples were collected for analysis of major cations and anions, as well as dissolved inorganic carbon (DIC) and the stable isotope ratio of DIC (δ¹³C_DIC).

Nitrate contamination of the groundwater occurred from the periodic applications of nitrate fertilizers and a leaky sewer pipe located adjacent to the residence. The contamination by nitrate fertilizers showed peak concentration during the growing seasons of fall and spring. Contamination by nitrate in sewage showed peaks during periods of a low water table when the drainage from the sewer line flowed into the groundwater. Spatially across the site, nitrate concentrations in groundwater decrease in the direction of groundwater flow. The decrease in nitrate corresponds to the increase in the TDS and in the concentrations of alkalinity, Ca, Mg, and DIC. This suggests that nitrate attenuation by microbial denitrification was responsible for the elevated ion concentration from the weathering of aquifer minerals and for DIC from organic carbon mineralized during denitrification. Microbial minerlization of organic carbon is consistent with the negative excursion in the δ¹³C_DIC at locations with active denitrification. We conclude that a combined monitoring of nitrate, major ions, and DIC and stable carbon isotopes over space and time is a useful way to assess the sources and fate of nitrate contamination in shallow groundwater.