Publication Summary

To better manage water quality, a total maximum daily load assessment for Moses Lake and its major sources was made based on a review of data from previous studies of the lake. An in-lake total phosphorus (TP) criterion was proposed based on the water quality target established in 1978 to reduce the seasonal, in-lake TP concentration to 50 ug/L. Although this TP value exceeds the lake nutrient criterion established in Washington State water quality standards for surface water, it is recommended that 50 ug/L TP be the new criterion for Moses Lake.

Over the last 50 years, Moses Lake and its watershed, including groundwater, have been altered permanently by human activities, especially the use of Columbia River water for irrigation farming. During the last 30 years, at least 13 studies of Moses Lake indicate that anthropogenic activities, particularly agricultural practices, have contributed to a hypereutrophic state for the lake. It has been estimated that over 75% of the total phosphorus (TP) load to Moses Lake originates from agricultural fertilizers and farm animal wastes. As of 1989, it was reported that only 20% of the irrigated land had implemented recommended water and nutrient controls, resulting in a nutrient concentration reduction of less than 2% in the Crab Creek inflows to the lake. In addition to these agricultural impacts, climatic variation, internal loading, and adding dilution water to the lake can also have a significant influence on the water quality of Moses Lake.

To better manage water quality, a total maximum daily load (TMDL) assessment for Moses Lake and its major sources was made based on a review of data from previous studies of the lake. An in-lake TP criterion was proposed based on the water quality target established in 1978 to reduce the seasonal, in-lake TP concentration to 50 ug/L. Although this TP value exceeds the lake nutrient criterion established in Washington State water quality standards for surface water, it is recommended that 50 ug/L TP be the new criterion for Moses Lake.

Preliminary load allocations to achieve this TP concentration in the lake were recommended for the following major sources to the lake: Rocky Coulee Wasteway, Rocky Ford Creek, Crab Creek, and groundwater. Phosphorus loading from these sources was considered manageable; however, load allocations for nonpoint and point sources within each of these sources were not established in this study. This study recommends allocations be established in the future, based on the discharge characteristics of point sources and land-use classifications of nonpoint sources.