An Evaluation of Conventional Septic systems and Their Impact on Groundwater Quality in the Central Wisconsin Sand Plain

Abstract

A four year study evaluated the impact of unsewered subdivisions, conventional septic systems and a residential lawn on groundwater quality in the Central Wisconsin Sand Plain. Literature concerning unsewered subdivisions and the associated impact on ground water quality is reviewed. Multiple well designs were utilized to determine the nature and extent of groundwater contamination from these sources. Groundwater monitoring revealed nitrate-N concentrations in excess of 50 mg/I, 125 feet downgradient of a conventional septic system. These data suggested minimal dilution or mixing in the saturated zone and resulted in relatively thin, narrow plumes of impacted groundwater. Nitrogen:chloride ratios in the septic tank and in downgradient groundwater were compared for evidence of denitrification or nitrogen loss. Little or no evidence of denitrification was found to be occurring in the study area. Contamination of groundwater with reactive phosphorus in excess of 5 mg/I was also documented at several 10-20 year old septic systems. These systems were located on sandy soils with approximately 20 feet of unsaturated material above the water table. Concentrations of reactive phosphorus in excess of 7 mg/I were measured at a horizontal distance of 95 feet from one suspected source drainfield. These data suggests that the current Wisconsin septic system setback distance from a surface water body of SO feet offers little protection from the potential addition nutrients such as nitrogen and phosphorus. Two subdivisions located near Stevens Point, Wisconsin were evaluated for the cumulative impact of unsewered development on groundwater quality in areas underlain by sand. Results indicate that septic systems and lawns can and do impact shallow groundwater quality and can result in nitrate-N concentrations in excess of 10 mg/I downgradient of subdivisions. Recommendations are made for evaluation of alternative septic system designs which enhance or encourage natural processes resulting in the degradation of nitrate-N. Examples of environmental variables worthy of consideration when designing and developing unsewered subdivisions are also presented.