EVALUATING SANITARY SEWERS AS A POTENTIAL SOURCE OF SODIUM AND CHLORIDE IN AN URBAN AQUIFER SYSTEM

Although road salt is an obvious source of sodium (Na⁺) and chloride (Cl^-) that may impact groundwater, other sources may provide a substantial contribution. In urban areas with hard water, residents may use water softening devices that discharge brine to the sanitary sewer during back flushing. These brine discharges may result in very high concentrations of Na⁺ and Cl^- in the sanitary sewer effluent. Previous research has demonstrated that 10% of sanitary sewer effluent may be lost due to leakage and ultimately impact the underlying aquifer. If road salt were the source of Na⁺ and Cl^-in an aquifer, a seasonal variation in these ions would be expected. If these ions originated from leaking sewers the concentrations would exhibit a temporal consistency.

This project evaluated the relative contributions of road salt and sanitary sewer effluent to groundwater Na⁺ and Cl^- concentrations in Madison, Wisconsin. Sewer effluent samples contained an average of 250 mg/l for sodium and 400 mg/l for chloride. Analysis of groundwater samples from nested wells at a field site adjacent to a public supply well revealed elevated concentrations of several major ions, including Na⁺ and Cl^-. These groundwater concentrations remained elevated throughout the year and in one well were equivalent to those detected in the sewer effluent. Based on leakage estimates, it is likely that the sewers are contributing more than a million kg/yr of Na⁺ and Cl^- to the aquifer. Road salt usage in Madison from 2000-2009 averaged approximately 10 million kg/yr and has a substantial impact on surface water quality. However, only a fraction of the road salt used directly impacts groundwater. The relative contributions of road salt and leaking sewers may be equivalent. In areas with rising concentrations of chloride in groundwater it is important to evaluate all potential sources of contamination to these systems.