Physico-chemical assessment of a fixated flue-gas desulfurization sludge cap emplaced along with other coal-combustion residues to abate acid mine drainage

Abstract

Long term monitoring of the physical and chemical effects of using coal-combustion residues (CCRs), in particular fixated flue gas desulfurization (FGD) sludge, as a major component in the reclamation of a pyritic refuse deposit was undertaken to determine the beneficial and detrimental consequences of placing these controversial materials in an unrestricted environment. Monitoring wells, neutron probe access tubes, and weirs were installed before and after reclamation to observe hydrologic conditions and determine how the use of FGD sludge as a recharge barrier was affecting hydrochemical response to ambient weather conditions. Data were collected for six months prior to reclamation and then for an additional 13years (more intensively during the first 5years). Statistical analyses of water levels in the pyritic refuse deposit indicate a shift from precipitation- to barometric-controlled fluctuations. These findings, along with minimal variability in soil moisture within the CCR cap and transient perching of groundwater above the cap, are evidence that recharge of the refuse aquifer has been minimized. Statistically significant improvements in the quality of groundwater on-site and surface water leaving the site include long-term declines in acidity, As, and Fe concentrations within the refuse aquifer, attributed to a decrease in recharge of oxygenated water as supported by an analysis of calculated mineral saturation indices. Long-term declines in acidity and associated trace metals discharging from the site are attributed to the post-reclamation loss of sulfate salts brought to the surface by capillary forces. The results of this study indicate that strategic usage of CCRs in reclamation programs can produce beneficial effects, including acid drainage reductions, that are beyond those achieved using traditional reclamation approaches such as the utilization of mine spoil as capping and fill material.