Publication Summary

The Total Maximum Daily Load (TMDL) study recommends reducing nutrients, especially nitrogen, in the Walla Walla River and its tributaries so that dissolved oxygen (DO) and pH water quality standards are met. Of 54 sites monitored, 34 did not meet pH criteria and 17 did not meet DO criteria. Near background nutrient levels need to be maintained along watershed streams (a 20 to 99 percent decrease in concentrations). With seasonal (May - October) point source diversion, citizen education, and best management practices, the DO and pH water standards may be met in 10 years from the completion of the Water Quality Implementation Plan.

Several stream reaches in the Walla Walla River basin were listed on the 303(d) list for non-attainment of pH criteria by Washington State. Salmon and bull trout are potentially affected by the poor water quality. The federal Clean Water Act requires that a Total Maximum Daily Load (TMDL) be developed for each waterbody on the 303(d) list to identify how much pollution needs to be eliminated to achieve clean water.

During 2002 and 2003, the Washington State Department of Ecology found pH and dissolved oxygen (DO) problems in the basin from May through October in the Walla Walla River and several tributaries. Of 54 sites monitored, 34 did not meet pH criteria and 17 did not meet DO criteria. The headwaters of the Touchet River and Mill Creek had relatively high levels of soluble reactive phosphorus (40 - 60 µg/L) and elevated pH (> 7.5).

Nitrogen is currently the limiting nutrient throughout most of the area. Excessive nutrients from point (discrete) and nonpoint (diffuse) sources, low streamflow in the summer and fall, and exposure to the sun cause excessive primary productivity. This causes the diel (24-hour) minimum DO and maximum pH values to be out of compliance with criteria.

pH and DO conditions can be improved by shading the streams more, decreasing point and nonpoint nutrient loads, and increasing streamflows. However, water quality modeling results suggest that even with these improvements, some stream reaches would still not meet Washington State water quality criteria, especially for pH.

This TMDL recommends seasonal nutrient wasteload allocations for the Dayton Wastewater Treatment Plant (WWTP). The College Place and Walla Walla WWTPs should continue their seasonal removal of effluent from Garrison and Mill Creeks during the May-October growing season. Subsurface transport from the Waitsburg WWTP wetland and nearby nonpoint sources need to be investigated as potential nutrient sources. The strategy to reduce nutrients from nonpoint sources consists of educating watershed residents and implementing best management practices.

Link to EIM data for User Study ID JJOY0003