New York City’s drinking water supply is threatened by the potential development of the Catskill/Delaware watershed. The effectiveness of alternative on-site wastewater treatment systems in this drinking water supply watershed is being evaluated during the course of a oneyear 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 to receive the treated wastewater. Five systems were constructed with septic tank effluent pumped to mounds while the remaining four systems are conventional systems incorporating septic tanks and leach fields installed in native soils. Samples were collected at five or six points within each treatment system monthly and analyzed for carbon, nutrients, and fecal coliform. A least squared difference (LSD) analysis was used to determine the significances of treatment at the distribution box, the point of collect after pretreatment, but before the leach field, and lysimeter 4, the deepest point of collection in the leach field. The data used in the LSD analysis of lysimeter 4 was first adjusted to remove the affects of dilution as this factor was not consistent across system types. Adjustments for dilution were made using conductivity, a conservative tracer that was collected throughout the study. The LSD analysis of the distribution box data presented evidence that pre-treatment units provide a significant amount of treatment to wastewater. Generally, sand and peat filters treat waste to the same levels, but sand filters removed phosphorus to a greater degree. Aerobic treatment units removed the greatest amounts of total nitrogen, but failed to convert a significant amount of ammonia to nitrate + nitrite. In comparison to both sand and peat filters, aerobic treatment units failed to remove significant amounts of fecal coliform. Dilution affects were found to remain fairly constant throughout raised bed leach fields. In comparison they increased with depth in traditional leach fields. As a result, conventional systems, the only system type installed with a traditional leach field, appeared to have to greatest constituent removal by lysimeter 4. After the lysimeter 4 data had been adjusted for dilution it became apparent that peat and sand systems had the greatest removal rates by lysimeter 4.