STABLE COUNTERPARTS OF COMMON RADIOACTIVE CONTAMINANTS IN GROUNDWATER AT THE SAVANNAH RIVER SITE, SC

Several radioactive contaminants have naturally occurring non-radioactive counterparts with chemical behavior identical to the radioactive contaminants. An isotopic study to investigate naturally occurring non-radioactive species was conducted in the Southeastern Atlantic Coastal Plain shallow aquifers of Eocene age. The study was conducted using background wells at the Savannah River Site (SRS) Department of Energy Facility in South Carolina in areas not contaminated by past operations. Radioactive contaminants in the groundwater at SRS include ⁹⁰Sr, ¹²⁹I, ¹³⁷Cs, and ⁶⁰Co. The most abundant natural counterparts for these contaminants are ⁸⁸Sr, ¹²⁷I, ¹³³Cs, and ⁵⁹Co, and they are typically present at mass concentrations much greater than their associated radionuclides. These natural constituents compete with their radioactive counterparts in natural attenuation reactions as well as in remediation systems designed to remove the radionuclides from groundwater. Hence, the concentrations of the natural species are important to understanding the migration and remediation of these radionuclides.

Groundwater concentrations of ⁸⁸Sr ranged from 3.7 to 369.4 ug/L; for ¹²⁷I from 1.5 to 14.3 ug/L; for ¹³³Cs from <0.001 to 0.4 ug/L; and for ⁵⁹Co from 0.004 to 1.5 ug/L. Concentrations of ⁸⁸Sr, ¹³³Cs, and ⁵⁹Co were significantly different (P£0.006) in the two shallow aquifers. Results from this study also show that ⁸⁸Sr, ¹²⁷I, ¹³³Cs, and ⁵⁹Co concentrations vary depending on aquifer mineralogy and pore water chemistry. For example, elevated ⁸⁸Sr concentrations corresponded with wells with carbonates in or near the screen zones. Leaching analyses performed on core samples corresponding to the depths and location of groundwater samples indicated that higher concentrations of ⁸⁸Sr, ¹³³Cs, and ⁵⁹Co can be expected in acidic plumes.

Results of modeling various reactions illustrate the effects of the natural constituents on radionuclide contaminant behavior. Findings indicate that precipitation and co-precipitation reactions are influenced by the natural constituents more than surface complexation reactions. Understanding the geochemical behavior of these isotopes is integral to designing appropriate remedial systems and modeling the migration of radionuclides in coastal plain sediments.