Publication Summary

Prompted by elevated concentrations in fish tissues, a TMDL assessment was done for DDT and PCBs in the lower Okanogan River basin. Results indicate there is little external loading to the mainstem Okanogan River through tributaries or sewage treatment plants (STPs). Most tributaries and STPs are below load/waste load allocations needed to meet criteria. Contaminated sediments appear to be the primary source for accumulation in fish tissues Significant decreases in sediment DDT and PCB concentrations will be necessary to reduce fish tissue concentrations below criteria. Recommendations are made to augment data from Osoyoos Lake and in certain fish species.

The Washington State Department of Ecology (Ecology) Environmental Assessment Program prepared an assessment of total maximum daily loads (TMDLs) of DDT and PCBs in the lower Okanogan River basin, including Osoyoos Lake. Sampling conducted during 2001-2002 examined DDT and PCB concentrations in the water column of the mainstem Okanogan River, water in tributary streams, sewage treatment plant (STP) effluent and sludge, and cores of bottom sediments. Composite samples of three species of fish - carp (Cyprinus carpio), mountain whitefish (Prosopium williamsoni), and smallmouth bass (Micropterus dolomieui) - also were analyzed for DDT and PCBs. Data from these samples were used in conjunction with historical data to develop the TMDLs.

Results suggest that only small loads of DDT and PCBs are delivered to Osoyoos Lake and the lower Okanogan River through tributary streams and STPs. Combined, measurable DDT and PCB loads from tributaries and STPs averaged approximately 200 mg t-DDT/day and 3 mg t-PCB/day, respectively. This contrasts sharply with the measured loads in several reaches of the lower Okanogan River (1,500 - 4,300 mg t-DDT/day; no measurable PCBs), the assimilative capacities of the river (1,300 - 6,700 mg t-DDT/day; 230 - 1,100 mg t-PCB/day), and theoretical loads based on fish tissue concentrations (13,000 - 32,000 mg t-DDT/day; 0 - 6,500 mg t-PCB/day). The loading analysis showed that the bulk of loading was internal, presumably through bottom sediments. Load allocations and waste load allocations were developed for tributaries, STPs, and sediments.

Recommendations for further study are to expand the sampling effort around the Osoyoos Lake basin, including a re-assessment of DDT and PCB concentrations in Osoyoos Lake fish. Carp from the Monse reach of the Okanogan River also should be analyzed for DDT and PCBs. Investigation of DDT loading through erosional processes also is recommended, although it appears improbable that DDT loading can be substantially reduced through best management practices.

Link to EIM data for User Study ID DSER0009