Watershed based non-point source (NPS) studies have usually ignored the impact of on-site septic systems (OWS) on the quality of surface water. However, OWS can be a substantial source of nutrients to surface waters. Point discharges from Wastewater Treatment Plants (WWTPs) can also contribute to stream pollution from nutrients if treatment efficiency at the WWTP is low. Several models can handle point source discharges from WWTPs. On other hand, evaluating nutrient loading from septic systems to streams is a challenging task because most NPS models are not specifically developed to track the fate and transport of nutrients from on-site systems. The Watershed Analysis Risk Management Framework (WARMF) model is adapted to simulate on-site septic systems loading. The 47.2-square-mile Turkey Creek water­shed is located in Jefferson County, southwest of Denver, Colorado. The topography is mostly steep with elevations ranging from about 1800 m to 3200 m. WARMF model hydrologic calibration was performed using data from the gage station located at Turkey Creek at mouth of canyon near Morrison. Water-quality data collected during flow calibration period was used to guide water quality calibration.   After establishing a base case, scenarios related to OWS were tested using WARMF. These scenarios included the conversion of existing OWS to centralized sewers. The scenario runs provided information to evaluate the trade-offs between OWS and centralized sewer systems and the impact of OWS and centralized sewers on surface water quality. The study showed that for this watershed and for the existing treatment efficiencies at the WWTPs, OWS is more efficient in removing wastewater pollutants than WWTPs. Thus, the results indicate that in order for the conversion of OWS to be beneficial a very high level of treatment would be required at the WWTP.