TEMPORAL INVESTIGATION OF THE SOURCES AND FATE OF NITRATE CONTAMINATION AT A RESIDENTIAL SITE
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 δ13CDIC 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.