Ground Water Quality beneath Twin Cities Metropolitan Communities Served by Individual Sewage Treatment Systems

Individual Sewage Treatment Systems (ISTS or septic systems) can contaminate ground water with nitrogen, carbon, chloride, synthetic organic chemicals, and pathogens. Providing municipal sewer services to the large number of people in the Twin Cities Metro Area currently served by septic systems, however, would be very expensive. We conducted a study in 2000 and 2001 to determine ground water quality in 14 Metro Area locations served by septic systems. In each community, we collected samples from private wells for nitrate, chloride, Kjeldahl nitrogen, organic carbon, volatile organic chemicals (VOCs), and coliform bacteria. In five communities, we also collected samples from temporary wells completed in the upper few feet of each aquifer. Nitrate was the chemical of greatest concern. Nitrate concentrations in private wells were greatest in Ramsey East, Credit River, and three Lakeland communities. The occurrence of nitrate was strongly correlated with the presence of aerobic conditions within each sampled aquifer. Within oxygenated aquifers, the distribution of nitrate followed distributions found in other studies from nonsewered communities in Minnesota. Age of septic system appeared to relate to nitrate concentrations in ground water, with concentrations in some communities increasing with age. There was no affect of septic system characteristics and compliance on the occurrence of nitrate in ground water. Different management strategies for nitrate exist in areas with different aquifer sensitivity. In areas highly sensitive to nitrate contamination, management strategies include providing municipal services, drilling into deeper protected aquifers for drinking water, blending water supplies from different sources, utilizing lot sizes greater than 2 acres, and employing technologies that enhance denitrification. In areas where shallow ground water is sensitive to nitrate contamination, management for nitrate in these aquifers includes drilling deeper private wells, minimizing mixing of aquifers by preventing overpumping, utilizing lot sizes greater than one acre, and utilizing technologies that enhance denitrification. In areas where ground water is not sensitive to nitrate contamination, no particular management strategies are required.