Modeling the Effect of Population Growth on Stream Nutrient Concentration in Turkey Creek Watershed using the WARMF Model

Abstract

Population growth and water quality in the foothills of Jefferson County are of concern to Countys Board of County Commissioners and the Planning and Zoning Department. The Department desires to meet the needs of residents for adequate supplies of good quality water. The Turkey Creek watershed is representative of the foothills portions of Jefferson County. In 2001 population in the Turkey Creek watershed is estimated at 11,064 residents with projected population growth, using a 2-percent per year rate, at 13,186 residents in 2010, and 15,313 residents in 2020. Wtershed 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. 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 122.2 square-kilometer 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. The base case scenario is established at year 2001. The scenarios in this study are population at 1990 and projected population at year 2010. The scenario runs will also provide the contributions of different water treatment options (OWS and WWTPs) in addition to the impact of population growth on stream nutrient concentration and loadings.