Determination of nonpoint-source pollution using GIS and numerical models

A geographic information system (GIS) was utilized to apply a modified DRASTIC method to the assessment of groundwater contamination sensitivity in Goshen County, Wyoming. Several basic environmental characteristics, identified as influencing contaminant transport through the vadose zone to groundwater systems, were mapped, automated, and analyzed. These characteristics include : depth to groundwater, net recharge, hydrogeologic setting, vadose zone soil properties, land surface slope, and saturated hydraulic conductivity. Sensitivity ratings were developed for each parameter based on a combination of mathematical functions and the inherent capacity of each characteristic to influence transport of contaminants. A raster-based overlay analysis was performed to derive a map that portrays cumulative aquifer sensitivity ratings across the county, providing a relative indication of groundwater vulnerability to contamination. A process-based numerical model was used to simulate water flow and solute transport in the vadose zone and groundwater systems. The model incorporated soil and hydraulic properties produced with the GIS into the simulations. Numerical simulations described the time and spatial distributions of contaminants. Chemical mass stored in the soil and leaching out from the vadose zone were computed to characterize groundwater contamination. Groundwater sensitivity indexes, which were developed based on the numerical modeling results, were compared with the GIS sensitivity map and used to verify the reliability of the map.