PNNL

Abstract

During operations at the U.S. Department of Energy Hanford Site, releases of chemical and radioactive waste resulted in contamination of the soil and groundwater beneath portions of the site, including inadvertent releases of carbon-14 in the 100-K Area. Ongoing environmental cleanup efforts in the 100-K Area are focused on hexavalent chromium [Cr(VI)] and thus have not yet addressed the two carbon-14 groundwater plumes. This is reflected in the positioning of the groundwater wells, where extraction wells are outside the carbon-14 plume areas and only monitoring wells are placed within the plume areas. Based on a literature review of studies on carbon-14-contaminated sites, this report provides context and offers conceptualized options for implementing targeted ex situ remediation strategies for carbon-14 using the ion exchange (IX) water treatment technology within the 100-K Area. Carbon-14 has a relatively long half-life, is persistent in the environment, and is readily available for biological uptake, so it is important to implement a remedy for carbon-14 groundwater plumes that will meet regulatory requirements, such as the drinking water standard. Carbon-14 detected in the groundwater will likely be in the form of (bi)carbonate anions, which can be removed using commercial IX resins. Commercially available anion exchange resins (sometimes advertised as dealkalizers) are designed for capture and removal of the anions associated with alkalinity in water (i.e., bicarbonate and carbonate). While aqueous carbon-14 is not a common contaminant of concern, there are a few examples of IX treatment to remove carbon-14 (Carlton and Baumann 1991; Nuclear Decommissioning Authority 2014). At the end of the resins’ treatment life, there are multiple options for long-term storage, immobilization, or disposal of spent IX resins loaded with carbon-14. This report outlines three options for implementing IX resin technology for treatment of carbon-14 in groundwater in the 100-K Area. The first option consists of modifying the pre-existing aboveground treatment facilities for the 100-KX and -KW pump and treat (P&T) systems, in which an additional IX treatment train for carbon-14 removal is operated before the existing IX treatment trains that use the SIR 700-HP resin for Cr(VI) removal. The second option consists of replacing SIR-700-HP resin from some of the existing IX vessels with a carbon-14 targeted IX resin, which has the benefit of minimal infrastructure modifications. The feasibility of option 2 depends on whether (i) the current adjustment from pH 8 to 7 [implemented to optimize SIR-700-HP resin effectiveness for Cr(VI) removal] will impact carbon-14 removal by the new IX resin, and (ii) fewer SIR-700-HP IX vessels in the treatment train is still adequate to meet the requirements for Cr(VI) treatment. The advantage of options 1 and 2 would be leveraging the established network of P&T injection and extraction wells and the existing aboveground treatment facility. The third option is to use a mobile skid for IX treatment of carbon-14 at impacted wells. The advantage of the mobile skid is deploying the treatment at targeted wellheads rather than undergoing modifications to the 100-KX and -KW P&T facilities. To support a decision on these implementation options, some uncertainties should be addressed regarding candidate commercial resin performance under conditions relevant to 100-K Area and P&T system operational parameters. Since the findings in this report are based on a literature review, laboratory experiments are recommended to resolve these site-specific uncertainties so calculations can be performed for developing an intermediate-scale IX design with operational specifications for IX treatment of carbon-14. This initial screening evaluation determined IX treatment of carbon-14 is possible with commercial IX resins designed for (bi)carbonate removal....