PNNL

Abstract

Laboratory tests were conducted to determine sediment geochemical properties needed to develop a design for implementation of the in-situ oxidation–reduction (redox) manipulation (ISRM) technology for chromate (CrO42–) remediation at a Superfund site and three other sites. A generalized hydrogeologic description of the Superfund site consist of a silty clay upper confining layer to a depth of ~6.71 m, the A1 unit from ~6.71 m to ~8.23 m, the A2 unit from ~8.23 m to ~10.67 m, and the A3 unit from ~10.67 m to ~12.19 m below ground surface. The A/B aquitard was encountered at a depth of ~12.19 m. The A1, A2, and A3 hydrostratigraphic units are all sandy gravels, but with considerable difference in fines content and subsequently, hydraulic conductivity. Hydraulic tests conducted in pilot test site monitoring wells indicate that the A1 unit has a 10 times lower hydraulic conductivity than the A2 unit, while the A3 unit hydraulic conductivity is significantly higher than that observed in the A2 unit (i.e., a trend of increasing permeability with depth). Calculated hydraulic conductivities, based on sieve analysis, show this same spatial trend. Results from a tracer injection test and electromagnetic borehole flow meter tests conducted at the site indicate a relatively high degree of formation heterogeneity. Laboratory experiments showed that chemical reduction yielded a redox capacity (0.26% iron(II)) that falls within the range of values observed in sediments analyzed from sites where field-scale deployment of the ISRM technology is currently in progress or being considered (0.1% Hanford 100D area, 0.24% Ft Lewis, 0.4% Moffett Federal Airfield). There was relatively little spatial variability in reducible iron (Fe) content between the three aquifer units. This mass of reducible Fe represents a sufficient quantity for a treatment zone emplaced to remain anoxic for 430 pore volumes, which would be expected to last tens of years, depending on aquifer flow rates and the concentration of oxidizing species in the groundwater. The geochemical analysis also indicated relatively low spatial variability in reducible Fe content although some depth dependent variability was indicated. Variation in the CrO42– concentration and flow rates between the A1 and A2 aquifer units indicated the necessity for greater reduction in the A2 aquifer unit, in order that both aquifer units prevent offsite CrO42– migration for the same amount of time. Results from these laboratory analyses of sediment core samples are used in conjunction with: (1) site specific geologic information obtained during installation of monitoring wells, (2) results from hydraulic tests conducted at the site, (3) electromagnetic borehole flow meter testing results, (4) results from a conservative tracer injection test, and (5) results of a series of S2O42– injection simulations of the field site, to develop a S2O42– injection strategy for deployment of the ISRM technology at sites to prevent offsite CrO42– migration.