Combining reductive and sequestering technologies for the remediation of the perched water zone at Hanford, WA

A perched water zone at the Hanford Site located in southeastern Washington State contains high concentrations of contaminants associated with the operations of Plutonium production from 1944 to 1986. Contamination from these sources has spread into the vadose zone, with some of the more mobile contaminants reaching the groundwater. Contaminants such as technetium (Tc) (55,000 pCi/L), uranium (U) (150 mg/L), and nitrate (NO₃) (200 mg/L), serves as a long-term source of contamination to the underlying aquifer. The current response action is water extraction at low flow rates, but limitations on pumping dictate that other methods be developed to supplement this method once water extraction is no longer viable. Due to the mixed properties of Tc, U, and NO₃, a combined technology approach is needed because the presence of other contaminants may either 1) interfere with remedy effectiveness; or 2) mobilize another contaminant while sequestering the targeted contaminant. Consequently, the remediation strategy is to evaluate a two-step approach involving successive applications of reductive and sequestering chemical treatments to immobilize co-mingled contaminants. The goal of combining these approaches is to reduce and encapsulate the contaminants of concern so that they do not re-oxidize post-treatment. To this end, batch studies have been conducted using zero valent iron (ZVI), sulfur modified iron (SMI) and calcium polysulfide as the reductant technologies, coupled with apatite or calcite formation to either coat the reduced species or incorporate them into the mineral phase. Additionally, the use of a dilute base (NaOH) is used to identify if the re-dissolution of aluminosilicates from the sediment can form coatings on the reduced contaminant species and prevent re-oxidation. This information will be used to develop site-specific treatment strategies that address the specific contaminant mixtures in the perched water zone at Hanford.