In the Blue River Watershed of Summit County Colorado, we combined supplemental fieldwork with publicly available data to evaluate the potential anthropogenic effects on water quality. This evaluation was focused on two locations: (1) Frisco Terrace, located near Dillon Reservoir; and (2) Blue River Estates, located along the upper reaches of Blue River. Both are served by on-site wastewater treatment systems (OWS). Hydraulic conductivity (K) estimated from driller's pumptest data and K values estimated from particle analysis of soils suggest relatively fast chemical transport times. However, during 2002 (a dry year), the hydraulic head of the ground water in portions of the Blue River Estates was lower than the nearby streams, implying the streams were locally recharging the ground water, making OWS pollution of the stream unlikely at that location. In contrast, at the site near the mouth of the watershed (Frisco Terrace development), water levels in wells and the stream were similar, and it was not clear whether ground water was recharging the stream. Surface water samples from the head of the Blue River, Pennsylvania Creek and Swan River showed chemical signatures of un-impacted natural background with elevated sulfate from mineralized bedrock. Samples from portions of the Blue River associated with suburban areas show anthropogenic impact with elevated chloride and nitrate. The similarity of the hydrochemical data in surface and ground water samples at the study sites established there was local hydraulic connection. The Frisco Terrace study site exhibits both elevated SO 4 associated with mine tailings and elevated chloride and nitrate from OWS.