Publication Summary

The Washington State Department of Ecology studied DDT in the lower Mission Creek basin during 2003 to address federal Clean Water Act section 303(d) listings and develop a TMDL assessment.

DDT was analyzed in orchard soils, bed sediments, suspended particulate matter, surface water, and groundwater. Sampling found that orchard soils contain substantial amounts of DDT. In streams, DDT loads will require reductions of 69-98% to meet the target criterion of 1 ng/l total DDT.

The report includes DDT load estimates, target loads, load allocations, and recommended load reductions.

The Washington State Department of Ecology studied DDT in the lower Mission Creek basin during the spring and early summer of 2003. The study was conducted to address federal Clean Water Act section 303(d) listings and develop a total maximum daily load (TMDL) assessment.

DDT and ancillary parameters were analyzed in orchard soils, bed sediments, suspended particulate matter (SPM), and surface waters of Mission, Brender, and Yaksum creeks. Groundwater sampling also was done at two locations.

Results showed that orchard soils contain 5-10 kg DDT/hectare, and concentrations are much higher than in streambed sediments. DDT concentrations and composition in bed sediments are more comparable to SPM, suggesting sediment re-suspension as the primary form of instream transport under a spring flow regime. Approximately 75% of the DDT in the water column is particle-bound. No DDT was detected in groundwater at either of the two sampling locations.

Loads measured during 2003 were generally lower than those found during 2000, probably due to the lower streamflows in 2003. Yaksum Creek continues to deliver at least 80% of the DDT load to Mission Creek.

DDT-TSS regression equations predict that total suspended solids (TSS) concentrations in Brender and Yaksum creeks will need to be reduced to < 1 mg/l in order to meet a target DDT criterion of 1 ng/l t-DDT. Mission Creek should achieve a target DDT load if TSS in Yaksum Creek can be reduced by approximately 40%.

Recommendations include (1) reducing TSS in Yaksum and Brender creeks by preventing bank erosion or by other means of limiting transport of upland soils to streams, (2) conducting detailed assessments of soil input to streams, (3) assessing the influence of the Icicle and Peshastin Canals, and (4) evaluating the possibility of DDT transport through groundwater in lower Brender Creek.

Link to EIM data for User Study ID DSER0011