Use of the DRASTIC Model to Evaluate Groundwater Pollution Sensitivity from On-Site Wastewater Systems in the Mesilla Basin
J. Steven Walker, Christopher Brown, Sam Fernald
Groundwater contaminated by septic systems can have many undesirable impacts on both human and environmental quality of life. High concentrations of septic-system effluent that have reached groundwater without being fully treated at the environmental level can lead to harmful algal, bacterial, and chemical conditions in ground and surface water. Effluent exiting densely packed septic-system groups is a nonpoint source pollutant that travels underground, and detecting a source from high extraction concentrations is generally problematic. We have addressed this difficulty by adopting the DRASTIC model (a preliminary mapping system for hydrogeology components that govern groundwater pollution transmittance), modified to account for the unique geology of the Mesilla Basin, in an attempt to locate and map areas of high sensitivity to pollutants and cross reference them with areas of high septic-system density. The spatial variability of these areas of sensitivity and risk were used to determine appropriate venues for community outreach and septic-system training within the study area. The highest pollution sensitivity values occurred within the Rio Grande floodplain, where low depth to water values, high amount of recharge from agriculture, high hydraulic conductivities, and relatively flat sand and gravel hydrogeology are located. Groups of parcels with the highest risk values were dispersed around and up to 7.5 miles (12.1 km) outside of the floodplain, because septic-system density combined with pollution sensitivity were above median values. Pollution risk from septic systems was found to increase as parcel size decreased. The DRASTIC model may underestimate sensitivity in arid areas and can be further improved for assessment of septic-system pollution by adjusting for a pollutant that begins underground and is delivered with its own source of water.
Keywords: aquifer, DRASTIC, geographic information systems (GIS), groundwater, Mesilla Basin, pollution sensitivity, on-site wastewater systems