Degradation of water quality has become a key environmental issue over the past 40 years in the United States. While there has been great progress in cases where the sources of the degradation could be clearly identified (point sources), the effect of non-point sources, particularly those from anthropogenic sources, has proven more difficult to identify and quantify. The major objective of this study is to evaluate the impact of population growth on the water quality in a specific mountain watershed. We have chosen one typical watershed (Turkey Creek Basin, Jefferson County, Colorado) where population has more than doubled in the past twenty-five years and is projected to double again in the next 20 years. Irregular water sampling over the past 25 years in this 47 square-mile basin documents the changing water quality and provides detailed information usually unavailable in an area this size. Comparing baseline studies from predevelopment years with recent data helps to define the relationship between growth and water quality. This information can help formulate practices for attaining a sustainable balance between population growth and water quality in mountain watersheds. Using the insights gained from this study, impacts of growth on water quality for other mountain watersheds can be predicted. PHREEQC inverse models can characterize the background water chemistry in the basin that is derived solely from rock/water reactions. This background water chemistry is compared to the current water chemistry to determine the chemical signatures of possible anthropogenic sources that include transportation infrastructure, runoff from construction activities, commercial activities, agriculture, and individual home sources (i.e. septic tanks and leach fields). dD and d¹⁸O data from groundwater, surface water, and precipitation samples are used to further improve our understanding of basin scale flow paths, the amount of mixing within the flow system, and the travel of potential contaminants to the surface water.