ESTIMATING URBAN CONTRIBUTIONS TO SURFACE WATER THROUGH CL AND 87SR/86SR MIXING MODELS WITHIN THE FOURCHE CREEK WATERSHED, LITTLE ROCK, ARKANSAS

The Fourche Creek Watershed is an urban watershed in Central Arkansas, serving as the primary drainage basin for most of metropolitan Little Rock. The land cover within this basin is about 52.8% green space and 45.4% urban land. This watershed has been urbanizing at a rate of about 3.1 square kilometers (0.7%) per year. As the land surrounding these streams is developed, the risk of negatively impacting water quality through the addition of contaminants like metals, sewage, and pesticides increases. In this project, we utilized Cl concentrations and the 87Sr/86Sr in order to determine the percent event flow contribution (minimum urban contribution) to surface water during general conditions through hydrograph separations. Dissolved load was calculated for each sampling site for every event; then each location was averaged to get an estimated yearly load. Load measurements reported to the EPA’s Enforcement and Compliance History Online (ECHO) within this watershed were less than the estimated dissolved load measurements, indicating that permitted discharges of specific urban elements (Mn, Zn, Cu, Cr) alone do not explain their increase in load. This difference in load indicates that another source must exist for these urban elements. Chloride and 87Sr/86Sr mixing models were used to quantify estimated urban contributions into the Fourche Creek Watershed. Hydrograph separations were used to divide total discharges of individual sampling events into two components (baseflow and event flow) to estimate component percentages of mixing. The hydrograph separations revealed that event flow contributions ranged from 17.7–79.2% during conditions not singularly baseflow, meaning that estimated event flow Cl concentrations ranged from 2.8–14.1 mg/L. The mixing analyses of Cl and 87Sr/86Sr indicated that the mixing of these waters cannot be from only baseflow and rainfall (0.2 ppm Cl; 0.70725 87Sr/86Sr); instead, event flow must be chemically a combination of rainfall and other inputs from the urban environment. Urban contributions into any watershed impact water quality and influence total load calculations; this simplified mixing technique provides a basic approach to estimating urban influence and provides insight into future investigations to quantify a more exact urban contribution.