ATTENUATION OF NITRATE AND PHOSPHATE IN UTAH LAKE WETLANDS THAT RECEIVE INPUTS FROM URBAN RUNOFF AND TREATED WASTEWATER

Wetlands are critical for wildlife habitat, recreation, and improving water quality. The purpose of this project is to evaluate nutrient attenuation in wetlands that receive nutrient inputs from urban runoff and wastewater effluent. The wetlands are important for reducing nitrate and phosphate concentrations in the water prior to discharge to Utah Lake. Eutrophication due to excess nitrogen and phosphorous inputs has long been a concern on Utah Lake, threatening the health of the ecosystem. In this project, we are investigating nitrate and phosphate inputs to Utah Lake from storm drains and treated wastewater in Provo City. Mill Race Creek (MRC), which collects most of the storm runoff and treated wastewater in the city, has nitrate concentrations exceeding 25 mg/L and phosphate concentrations exceeding 2 mg/L. These concentrations are the highest of any tributary entering Utah Lake. Historical data from the National Water Quality Monitoring Council database shows that nitrate concentrations in MRC increased from roughly 2 mg/L in 1993 to 17 mg/L in 2009. Our results indicate that nitrate and phosphate concentrations are still increasing. A small fraction of the nutrient load in MRC comes from a spring and storm drains (<5 mg/L nitrate, <0.2 mg/L phosphate), while the majority of the nutrient loading is due to effluent from the Provo Wastewater Treatment Plant (>25 mg/L nitrate, >2.5 mg/L phosphate). However, nutrient concentrations decrease substantially downstream of the treatment plant where the water is channeled to an extensive wetland system prior to entering Utah Lake. Our results show that nitrate and phosphate concentrations decrease to <10 mg/L and <1 mg/L, respectively, during transit through the wetlands, indicating that the wetlands are improving water quality by naturally removing a substantial fraction of the nutrients. These findings have important implications for finding solutions to excessive nutrient loads to Utah Lake and elsewhere.