Using GIS to investigate septic system sites and nitrate pollution potential

A geographic information system (GIS) was used to identify the impact that septic-system nitrates may have on water quality in the upper Blue River watershed in Summit County, Colorado. GIS proves to be a useful tool for exploring relationships between permitting data, geology, soil types, and the results of water quality sampling for residential wells. Its uses include identification of areas of groundwater that are susceptible to nitrate pollution, priorization of water quality improvement projects, and assessment of the suitability of sites for septic-system placement. For each of 39 unique combinations of soil and geology, the authors defined nitrate source areas as adjacent land parcels with on-site wastewater treatment. The nitrate pollution potential of each source area was rated according to the DRASTIC model, a standard method of the U.S. Environmental Protection Agency. Ratings ranged from 35 to 88 on a scale of 100. The study found no statistically significant relationships between nitrate concentrations in well water samples and site factors or DRASTIC values. The authors suggest that these relationships be further investigated with a finer scale of geologic data, water table information, and additional unbiased samples.