This project will develop and test a methodology for assessing the water quality impacts of decentralized wastewater systems including individual and cumulative effects on local water supply wells as well as downstream receiving waters. The project will include field and laboratory work, modeling, and stakeholder involvement. Field and laboratory work will enhance the understanding of the transport/fate of microbes and chemicals in soil-based wastewater treatment systems and enable site-scale model development. The site scale processes will then be incorporated into an existing watershed model (WARMF) used to simulate cumulative effects and water quality responses in a watershed or sub-watershed. The modified WARMF model will be used as a decision support system to engage stakeholders in the development of TMDLs for onsite wastewater system discharges in the Blue River tributary of the Lake Dillon watershed in Colorado. The project will be carried out in two phases by a collaborative team involving the Colorado School of Mines (CSM), Electric Power Research Institute (EPRI), US. Geological Survey (USGS), and the Summit County Environmental Health Department (SCEH). Phase I will include model preparation and calibration, base simulations with WARMF, development of onsite system input expressions and adaptation of WARMF, laboratory experimentation and field sampling and analyses to define key transport/fate parameters, and model simulations and comparisons to existing water quality data. Phase II will encompass an evaluation and assessment of the onsite system input expressions and modified WARMF model through comparative analysis, modeling, and large-scale laboratory and/or field testing.