Susceptibility of residential wells to VOC and nitrate contamination.

Water quality of residential wells is vital to public health and a complex issue for regulatory agencies. South Carolina, a typical southeastern rural state, has no required testing of residential well water quality after initial well construction. This study used site-selection criteria to identify susceptible residential wells based on a combination of geologic vulnerability and potential contaminant loading. Geologic vulnerability was defined as increased probability of contaminants being transported from the land surface into the groundwater based on geological properties. As a surrogate for potential general contaminant loading, wells were located within 800 m of an EPA Toxics Release Inventory facility reporting VOC emissions, thus sampling was nonrandom. Seventy private residential wells were sampled for volatile organic compounds (VOCs) and nitrate-nitrogen (NO3-N) and analyzed using gas chromatography/mass spectrometry and the cadmium reduction method, respectively. Geographic Information Systems (GIS) was used to quantify four explanatory variables that affect well susceptibility to nitrate: population density, land cover, local relief (percent slope), and soil texture. VOCs were detected in 11 wells, and two sites exceeded the MCL for 1,1-dichloroethylene (36.1 microg/L) and trichloroethylene (9.0 microg/L). Elevated NO3-N (defined as > or = 1.0 mg/L) was measured in 20 wells. Logistic regression identified grassland/cultivated land cover as a variable that significantly increased the probability of NO3-N contamination (p = 0.003). Using easily accessible databases to identify factors that increase the probability of groundwater pollution could lead to more effective programs for locating residential wells that are susceptible to contamination. Increased monitoring of well water quality, as is being considered in some states, is warranted to reduce potential human exposure to contaminated drinking water.