Coal combustion byproducts (CCBs) have potential as structural fill and capping materials in abandoned mine land (AML) reclamation because of their acid-neutralization capacity. However, the potential for these materials to leach constituents of potential concern (COPCs) into groundwater hinders their use by AML program managers. In 1996, the Midwestern AML site in southwestern Indiana was reclaimed using fixated scrubber sludge (FSS) cap composed of flue gas desulphurization sludge, fly ash, and lime. The cap was placed over a pyritic refuse deposit, an unreclaimed spoil ridge, and highwall lakes that were filled with ponded ash. Post reclamation analyses of groundwater chemistry showed that alkalinity and pH levels increased below the cap, indicating that the CCBs helped to neutralize the preexisting acid mine water. Concentrations of COPCs initially increased below the cap, but dropped back to near or below pre-reclamation values over the past 10 years. For example, boron that initially increased to 13,000 µg/L decreased to pre-reclamation values (300 μg/L) with time. X-ray diffractometry was conducted on core samples containing FSS and ponded ash collected from the site in 2011 to identify mineralogical changes since emplacement. Oxidation of hannebachite (calcium sulfite hemihydrate) produced gypsum and calcite in the upper 1 to 4 cm of the 1 to 3 m thick FSS cap. No oxidation zone was observed at the bottom of the cap. Sediment cores of ponded ash reveal little change with depth, possibly because leaching had already occurred as a result of stockpiling at the power plant. The results of this study indicate that FSS did contribute COPCs, including boron, molybdenum, chromium, and arsenic, to groundwater for short periods and in the immediate vicinity of emplacement. Nevertheless, a lack of significant oxidation and declining trends in groundwater contamination represent long-term favorable outcomes for use of CCBs in AML reclamation.