Geochemical controls on the fate and mobility of coal combustion product-derived elements in a dolomitic aquifer

Coal combustion products (CCPs) can release elements of concern into groundwater, posing risks to drinking water and ecosystems, yet their behavior in carbonate aquifers remains poorly understood. This study investigated the fate and mobility of seven elements (As, B, Cd, Cu, Cr, Mo, Ni) at a CCP disposal site overlying a dolomitic aquifer in the southeastern United States. CCP-impacted wells showed Ca-Mg-SO₄ type waters with elevated sulfate concentrations (460-635 mg/L) compared to background Ca-Mg-HCO₃ waters. Boron (0.13-4.5 mg/L) and Mo (0.10-2.9 mg/L) were the most mobile elements and exceeded groundwater protection standards. Sequential extraction showed that ∼76% of leachable Mo was associated with water-soluble and amorphous oxide fractions, indicating weak sorption. In contrast As, Cd, Cr, Cu, and Ni were predominantly associated with aquifer solids and occurred as low dissolved concentrations. Geochemical speciation models predicted HAsO₄^2-, B(OH)₃, and MoO₄^2- as the dominant aqueous species under the site's alkaline and oxidizing conditions. However, SEM-EDS identified As-Ca-Mg-O associations and sequential extraction showed that ∼95% of the leachable As was associated with carbonate-dominated fractions, indicating that As attenuation occurs through adsorption and/or structural incorporation onto carbonate minerals, rather than Fe-oxide attenuation pathways. Chromium was predominantly associated with resistant residual fractions, consistent with precipitation/co-precipitation of Cr(III) oxide or spinel phases. Cadmium, Cu, and Ni showed no plume enrichment and were interpreted as predominantly geogenic. Overall, the empirical and modeling results suggest B and Mo are the major CCP-derived hazards in this carbonate aquifer, whereas As is likely immobilized by carbonate adsorption and/or structural incorporation.