Transport and Accumulation of Cesium-137 and Mercury in the Clinch River and Watts Bar Reservoir System

C. R. Olsen, I. L. Larsen, P. D. Lowry, C. R. Moriones, C. J. Ford, K. C. Dearstone, R. R. Turner, B. L. Kimmel, C. C. Brandt

Date Issued--June 1992

Prepared by Environmental Sciences Division Oak Ridge National Laboratory ESD Publication 3471

Prepared for U.S. Department of Energy Office of Environmental Restoration and Waste Management under budget and reporting codes CD 10 72 and EW 20

OAK RIDGE NATIONAL LABORATORY, Oak Ridge, Tennessee 37831-6285 managed by MARTIN MARIETTA ENERGY SYSTEMS, INC. for the U.S. DEPARTMENT OF ENERGY under contract DE-AC05-84OR21400

Executive Summary

Operations and waste disposal activities on the U.S. Department of Energy's Oak Ridge Reservation have introduced cesium-137 (¹³⁷Cs) and mercury (Hg) into local streams that ultimately drain into the Clinch River and Watts Bar Reservoir system. The highest discharges for both ¹³⁷Cs and Hg occurred during the mid-1950s. Measurements of the partitioning of ¹³⁷Cs and Hg between dissolved and particulate phases in the reservoir water column indicate that both contaminants have particles in this aquatic system. About 190 surface-sediment grab samples and more than 60 sediment cores were collected in Watts Bar Reservoir to (1) determine the extent of downstream contamination and (2) document particle and particle-associated contaminant accumulation patterns. The vertical distributions of ¹³⁷Cs and Hg in these sediment cores are strongly correlated (r² = 0.87), and both contaminants exhibit an large subsurface peak coincident with their peak discharge histories. Concentrations of ¹³⁷Cs and Hg as high as 80 pCi/g (3.0 Bq/g) and 47 µg/g, respectively, occur in this subsurface peak. A preliminary screening of the contaminants that may contribute to human health and environmental risk (Hoffman et al. 1991; Suter 1991) showed that these ¹³⁷Cs and Hg concentrations do not pose an imminent risk, especially if deep sediments are not disturbed. Further study, however, is warranted to determine the need for remediation. The sediment depth of this subsurface peak and the thickness of contaminated sediment varies with location in the reservoir and depends on the rate of sediment accumulation. The total accumulation of ¹³⁷Cs in Watts Bar Reservoir sediments has been estimated by measuring the ¹³⁷Cs inventory in each sediment core and extrapolating these data spatially with the ARC:INFO Geographic Information System software package. Results indicate that about 304 Ci (1.12 x 10¹³ Bq) of ¹³⁷Cs now reside in the reservoir sediments. Discharge records indicate that a decay-corrected total of about 335 Ci (1.24 x 10¹³ Bq) of¹³⁷Cs have been released into the river system between 1949 and 1986. Some ¹³⁷Cs was released before 1949, but discharges prior to 1949 were not monitored for ¹³⁷Cs. Sediment core profiles of ¹³⁷Cs in the Clinch River and Watts Bar Reservoir suggest that the annual amounts discharged from White Oak Dam before 1949 were considerably less than the amount discharged in 1949. The comparison between measured and discharged ¹³⁷Cs indicates that almost 91% of the total ¹³⁷Cs released to the Clinch River and Tennessee River system has been retained by accumulation in Watts Bar Reservoir sediments. Using the strong correlation between the vertical distribution of ¹³⁷Cs and Hg in sediment cores, it has been estimated that about 76 metric tons of Hg have also accumulated in the sediments of Watts Bar Reservoir. The vertical distribution of ¹³⁷Cs and Hg in dated sediment cores was also used to document levels of contamination in the reservoir water column during the past 40 years.

ORNL Clinch River Environmental Restoration Program / The Visualization Group