Publication Summary

In support of a TMDL analysis for Moses Lake, a study was conducted to examine the role of groundwater discharge in the lake′s nutrient budget. Samples where collected from a network of lake-bed piezometers, springs, and wells, and tested for phosphorus and nitrogen nutrients. Phosphorus concentrations were higher than the target water quality criteria for the lake (0.050 mg/L TP) at 75% of the stations tested. Data indicate that a combination of urban wastewater sources are the likely origin for the elevated phosphorus measured in the discharging groundwater. The results of this study are incorporated into a comprehensive analysis of lake loading published separately.

The related publication, Moses Lake TMDL Phosphorus Study, is not yet online or printed.

Moses Lake has historically exhibited eutrophic or hypereutrophic conditions, and is on the 1996 federal Clean Water Act 303(d) list as an impaired waterbody. Phosphorus has been identified as the limiting nutrient for the lake. Based on characteristic uses of the lake, an in-lake total phosphorus concentration target of 0.050 mg/L has been proposed to manage water quality concerns. In order to develop an allocation strategy for phosphorus loading to the lake, a Total Maximum Daily Load (TMDL) study was conducted by the Washington State Department of Ecology (Ecology) Environmental Assessment Program, at the request of Ecology′s Eastern Regional Office.

To better characterize the concentration and potential source of nutrients in groundwater directly discharging to the lake, 12 lake-bed piezometers were installed in the littoral zone of the lake along portions of the shoreline judged from regional data to be receiving groundwater discharge. Water quality samples were collected from the piezometers, as well as from one near-shore domestic well and two surface springs, during May, July, and October of 2001. Samples were analyzed for orthophosphate as P (OP), total dissolved phosphorus, nitrate+nitrite-N, ammonia-N, total persulfate nitrogen, chloride, total dissolved solids, and dissolved iron and manganese.

The majority of stations (75%) exhibited groundwater OP concentrations above the 0.050 mg/L surface water target criteria. Concentrations of OP in groundwater generally increased from north to south, paralleling increases in concentrations of parameters that indicate anthropogenic (human-caused) impact to water quality. A statistically significant relationship was established between OP concentration and the relative percentage of urban development upgradient of each station. These findings suggest that urban releases of wastewater to the aquifer are the primary source of phosphorus entering the lake via groundwater discharge. Loading calculations predict an annual OP mass flux to the lake from approximately 400 to 40,000 kg_op per year via groundwater discharge, with a value from 10,000 to 20,000 kg_op per year considered the best estimate of field conditions.

Link to EIM data for User Study ID CHPI001