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New York Citys drinking water supply is threatened by the potential development of the Catskill/Delaware watershed. A potential major non-point source of pollution within this watershed is on-site system effluent. The effectiveness of alternative on-site wastewater treatment systems in this drinking water supply watershed is being evaluated during the course of a one-year field study. Twenty-four on-site systems were designed and installed for individual sites throughout the Catskill/Delaware watershed. Fifteen systems utilize one of three alternative pretreatment technologies: peat filter, sand filter or aerobic treatment unit and were constructed with mounds. Five systems were constructed with septic tank effluent pumped to mounds while the remaining systems are conventional systems incorporating septic tanks and leach fields installed in native soils. Samples are collected at five or six points within the treatment train monthly and analyzed for carbon, nutrients, and fecal coliforms. Performance metrics within a system in terms of carbon, nutrient and fecal coliform removal are calculated to determine the effectiveness of each system. Observed breakouts has revealed hydraulic or treatment failure in over 40% of the systems with mounds. On average, peat and sand filters removed over 80% of the ammonia, total organic carbon, and fecal coliform, while the aerobic treatment unit removed less than 35% of each of these water quality constituents. In general, systems with pretreatment generate effluent of higher quality within the system before discharge to the environment.