Publication Summary

The National Water Quality Assessment (USGS, 1989-90) found high levels of the organochlorine pesticide DDT in resident fish from the lower reaches of the Yakima River. Further evaluations by Ecology in 1994-95 demonstrated a high correlation between sediment loads eroded from irrigated agricultural lands and DDT in the water column. The Lower Yakima River Suspended Sediment and DDT Total Maximum Daily Load study, finalized in 1998, required effectiveness monitoring to determine whether targets are being achieved. This effectiveness monitoring study found improved turbidity and reduced sediment loads in the study area, and most, but not all, TMDL targets were achieved.

The Washington State Department of Ecology is required, under Section 303(d) of the federal Clean Water Act and U.S. Environmental Protection Agency regulations, to develop and implement Total Maximum Daily Loads (TMDLs) for impaired waters, and to evaluate the effectiveness of these water clean-up plans to achieve the needed improvement in water quality.

The Lower Yakima River Suspended Sediment and DDT TMDL was developed to reduce suspended sediment, turbidity, and the pesticide, DDT, in the lower reaches of the Yakima River. TMDL implementation is scheduled over 20 years with interim targets set at five-year intervals.

The fifth-year (2003) targets included meeting Washington State water quality criterion for turbidity in the lower Yakima River during the irrigation season. Also included was the requirement that non-Yakama Reservation tributaries to the lower mainstem -- especially the major tributaries of Moxee Drain, Granger Drain, Sulphur Creek, and Spring Creek -- achieve a maximum 90th percentile turbidity of 25 NTU at their mouths during the irrigation season. This report presents an assessment of the turbidity within the project area and the effectiveness of the TMDL in reducing agriculturally related sediment.

Sampling during the 2003 irrigation year demonstrated that sediment loads have been reduced in the agricultural drains and river, but improvement is needed to meet all of the targets. Of the four major agricultural drains, three met the criteria for turbidity, while the fourth failed to do so even though it had a sediment load reduction of approximately 80%. Mainstem turbidity requirements at the TMDL compliance point of Kiona Gauge did not meet the state water quality criterion of "5 NTU over background," and neither did the intermediate mainstem sampling sites at Sunnyside-Mabton Road and Euclid Bridge. However, comparing suspended sediment data at the Kiona site collected during 1995 and 2003, both loads and concentrations were greatly reduced in 2003.

Link to EIM data for User Study ID CCOF0001