Publication Summary

During October 2000 through September 2001, intensive sampling for phosphorus was conducted in Moses Lake and its tributaries. The lake experienced low loading and high dilution during the study period, resulting in water quality with a mean total phosphorus concentration of 38 ug/L from May through September.

This report recommends establishing a seasonal water quality total phosphorus criterion of 50 ug/L for Moses Lake. A hydrodynamic water quality model was developed and used to evaluate the capacity of the lake to assimilate total phosphorus loads and meet the recommended water quality criterion during a critical season.

Moses Lake and its watershed have been altered by human activities, especially irrigation, lake phosphorus dilution with irrigation feed water, and urban development. The last intensive water quality study of Moses Lake was completed in 1988, summarized by Welch et al. (1989; 1992). This current report updates that work and complements the historical review and preliminary phosphorus loading capacity evaluation by Carroll et al. (2000).

Intensive sampling of Moses Lake and its vicinity was conducted from October 2000 through September 2001 to assess the status of the lake and its tributaries. The mean total phosphorus (TP) concentration for the whole lake from May through September was 38 ug/L. This report recommends establishing a seasonal water quality TP criterion of 50 ug/L for Moses Lake.

A hydrodynamic, unsteady-state water quality model was developed for Moses Lake and calibrated to the 2001 water quality data. The model was used to estimate the capacity of the lake to assimilate TP loads from point and nonpoint sources and meet the recommended water quality criterion.

Using critical loading conditions, the lake model showed that a 35% load reduction in TP from Rocky Ford Creek, Crab Creek, Rocky Coulee Wasteway baseflow, and groundwater was necessary to meet the proposed TP criterion with only a 10% exceedance probability. Further reductions in external phosphorus loads only marginally reduced TP concentrations in the lake because under these conditions internal sources begin to dominate in-lake concentrations.

Link to EIM data for User Study ID CARR0002