Long-Term Transformation and Fate of Nitrogen in Mound-Type Soil Absorption Systems for Septic Tank Effluent

Abstract: The potential for groundwater contamination by fixed nitrogen forms originating from septic tank soil absorption systems is a widespread and growing concern. This concern is exacerbated by significant difficulties and uncertainties in predicting the long-term transformations and fate of nitrogen in any given soil-based wastewater infiltration system.

Twelve septic tank soil absorption systems of three different designs (conventional, pressurized-dosing, and mound systems) in Wisconsin previously studied in detail were selected for monitoring to assess their performances after years of service in attenuating the contamination of nitrate nitrogen to groundwater. Groundwater in the vicinity of these systems was sampled monthly from June 1993 to August 1994 and analyzed for its contents of total Kjeldahl nitrogen, ammonium nitrogen, and nitrate nitrogen entering from the septic systems. Indicator organisms (total coliforms, fecal coliforms, and fecal streptococci) were also examined to determine any potential threat of biological contamination to groundwater during the summer of 1994. Comparisons were made between data newly obtained and data from corresponding previous studies.