Phosphorus Source Assessment and Reduction for Moses Lake

Moses Lake is a shallow warm water lake located in central Washington. The 6800 acre lake is a valuable resource for recreation, boating and fishing. In 2004 it was placed on the 303(d) list for total phosphorus, pH, and temperature. Washington Department of Ecology (WA DOE) has assessed the lake and recommended phosphorus load reductions. This study presents preliminary assessme n ts of the impacts of municipal and residential wastewater disposal on groundwater loading to Moses Lake. The Larson Waste Water Treatment Plant disposes an average flow of 324,000 gallons per day into rapid infiltrations basins that are located 1 to 3 miles from the lake. The basins are on top of very gravelly soils deposited during glacial floods. Groundwater concentrations of total phosphorus below the infiltration basins averaged 2079 ug/L TP. Total phosphorus concentrations in wastewater effluent averaged 2614 ug/L. Chloride, sodium, and boron in the groundwater was also similar to concentrations measured in the wastewater effluent. Septic systems in this rapidly growing area are installed in very gravelly (Type 1A) soils. These soils are generally calcareous and coarse grained and are not expected to remove significant amount of phosphorus. The estimated groundwater load to Moses Lake from the WWTP is 601 kg; the estimated load from existing septic systems is 466 kg TP. The recommended groundwater load reduction from WA DOE is 752 kg. Groundwater loading and removal should be better quantified but this evaluation shows that removal of TP at the WWTP and a sewer collection system would result in significant load reductions to Moses Lake.