In January 1971, the last of nine plutonium production reactors using direct discharge of once-through cooling waters into the Columbia River was closed. Sampling was conducted at three stations on the Columbia River to document the decline of radionuclides in the biota of the Columbia River ecosystem.Concentrations of 6oCo in seston, periphyton and invertebrates did not decrease to the degree that the other radionuclides did; this is partially related to the seepage of 6oCo into the river from a disposal trench near the operating N Reactor. Levels of 6oCo in fish showed some decreases, but obvious trends were not present. Zinc-65 was present in the biota in highest concentrations. The amounts in seston and periphyton decreased rapidly and were measurable only until the spring of 1973. Zinc-65 in caddisfly larvae was not measurable by February 1973, but concentrations in McNary chironomids fluctuated between unmeasurable levels to 24pCilg dry weight (DW) and this was related to ingestion of contaminated sediments rather than larval concentrations. In suckers and squawfish, 65Zn decreased to fairly low, constant levels of 1 and 3 pCi/g DW, respectively.The data show that in a river-reservoir complex, the measurable concentrations of fission-produced radionuclides decreased to extremely low or unmeasurable levels within 18-24 months after cessation of discharge of reactor once-through cooling water into the river. On the basis of data from the free-flowing sampling station, we believe that the decrease would be even more rapid in an unimpounded river.

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