Formatted Title
Subtle Geochemical Adjustments in Native Aquifer Soils: In Situ Groundwater Treatment Strategies for Cobalt at Coal Combustion Residual Sites
Background/Objectives
Cobalt has been observed in groundwater downgradient of coal combustion residual (CCR) management areas (units) at concentrations that require corrective measures. Native aquifer soils have the capacity to influence the geochemical transformation of groundwater as it flows through the hydrogeological system, and these soils play an important role in attenuation of cobalt and other CCR constituents. A novel approach of treating groundwater through subtle geochemical adjustments with reliance on native aquifer soils is based on recent work conducted to support remedy selection as part of the Federal CCR Rule.
Approach/Activities
Combined information from a hydrogeologic evaluation, groundwater and CCR porewater chemistry, and results of geochemical testing of aquifer materials were used with PHREEQC computer code to develop a geochemical model for the CCR unit. The model considered four geochemical processes that influence the prediction of groundwater chemistry: (1) mixing of porewater and upgradient groundwater, (2) thermodynamic equilibrium with select geochemically reactive mineral phases, (3) surface complexation, or adsorption, of CCR constituents in groundwater with iron and aluminum hydroxide minerals, and (4) cation exchange between positively charged ions in groundwater and positively charged ions in clay minerals. Based on the model results, an in situ treatability study consisting of baseline, reagent demand, microcosm, and dynamic flow-through column testing was conducted to evaluate the ability of different reagents to attenuate dissolved cobalt in the native aquifer soils. An alternatives analysis was performed that considered different reagent deployment methods and a pilot test was conducted to demonstrate the results of the bench-scale treatability testing in a field-scale setting and collect data for a full-scale design.
Results/Lessons Learned
Mineralogical analyses identified the existence of iron (ferrihydrite), aluminum (gibbsite), barium-sulfur (barite), calcium (calcite), and calcium-sulfur (gypsum) rich deposit fractions within the soil. Sequential extraction procedure testing indicated that the majority of cobalt in the site soils was associated with reactive minerals phases, particularly the exchangeable and metal hydroxide steps, and that a large fraction of the cobalt was bound to the soil such that it is likely to be easily mobilized as a result of decreasing pH. The reagent demand tests provided an early indication that increasing the pH of the site groundwater was effective at treating cobalt and not as likely to lead to secondary influences (i.e., mobilization of other metals) in the area downgradient of the CCR unit. The microcosm study results for soil mixed with groundwater indicated that native soil has the capacity to sorb cobalt, although at the level required to treat cobalt to below the groundwater protection standard (GWPS), there is the potential of mobilizing arsenic. Results from the column tests demonstrated that the dose of sodium hydroxide (NaOH) applied to the influent groundwater raised the pH to the target level of near 8, but ultimately the tested dose and resulting pH did not achieve the goal of lowering the effluent cobalt concentrations to below the GWPS. Overall, the treatability testing identified NaOH as the most effective reagent in reducing cobalt concentrations to below the groundwater protection standards (GWPS) while maintaining arsenic, cadmium, lithium, and molybdenum concentrations. The dominant mechanism of attenuation was the sorption and/or co-precipitation of cobalt to iron and manganese oxyhydroxides through pH adjustment. The treatability testing results demonstrated that pH adjustment could be utilized as a corrective measure to treat cobalt in groundwater downgradient of the CCR unit. The subsequent alternatives analysis identified injection of aqueous reagent (NaOH) into horizontal wells to establish a permeable reactive barrier as the preferred reagent deployment method. The conceptual design consists of two 700-foot horizontal wells each consisting of 25 individual injection lines terminating in 25-foot screened intervals. Pilot testing is currently in progress and results/conclusions are anticipated to be ready by the conference dates.