Decrease of Radionuclides in Columbia River Biota Following Closure of Hanford Reactors

Abstract

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.

Literature and data review for the surface-water pathway: Columbia River and adjacent coastal areas

“…Deep areas (e.g.,Gray's Point),where water depths approach 100 feet (Neal 1972), can be found in the estuaryapart from the main channel. (Cushing et al 1981). …”

“…The number and diversity of fish samples routinely collected and the associated radionuclide concentrations declined steadily over the years, following , the decline of radioactivity in the river as a result of the shutdown of the reactors (Cushing et al 1981). The primary constituents of concern in the river environment included strontium-90, tritium, and the gamma-emitters.…”

Survey of Potential Hanford Site Contaminants in the Upper Sediment for the Reservoirs at McNary, John Day, The Dalles, and Bonneville Dams, 2003

“…The largest releases of radioactivity and chemicals to the Columbia River occurred from 1956 to 1965 when the single-pass (i.e., Columbia River water was used for direct cooling) nuclear reactors were operating at Hanford (HEDR 1994;DOE 2002). Since the shutdown of the single-pass reactors in the early 1970s, the contaminant burden in the surface sediments has been decreasing as a result of radioactive decay and the subsequent deposition of uncontaminated sediment (Cushing et al 1981). Radionuclides with long half lives deposited during the operational period of the single pass reactors are still present in the deep sediment of the lower reservoirs and are now covered with layers of more recently deposited sediment.…”

Summary of Radiological Monitoring of Columbia and Snake River Sediment, 1988 Through 2004

“…The ninth reactor, 100-N Area reactor, used a closed-loop cooling system that virtually eliminated direct releases of radionuclides and chemicals to the river but still discharged a significant amount of heated water into the river. With the shutdown of the single-pass reactors, which took place from 1965 to 1971, direct discharges of contaminants to the Columbia River were nearly eliminated (Cushing et al 1981;Becker 1990). …”