Natural weathering of pulverized fuel ash and porewater evolution

Borehole samples of pulverized fuel ash (PFA) were taken from the unsaturated zone in a disposal mound at a decommissioned power station in the UK. The aim was to investigate the long-term natural weathering reactions of PFA and the chemical evolution of the contained porewaters. Concentrations of most species, including Al, Na, K, Ca, SO4, B, Co, Cr, Li, Mo, Ni, Pb and Sr in the porewaters, increase with borehole depth, consistent with these elements being released from the PFA through continued weathering reactions with infiltrating porewaters. The concentration of Ba shows a near-constant value throughout the depth range investigated and this element is thought to have achieved equilibrium with respect to a sulphate phase. The Ca and S in the PFA show depletion near-surface, consistent with the higher porewater concentrations with depth. Using mass balance calculations for these two elements, approximate infiltration rates are obtained. Other elements which are depleted in near-surface samples are Cu, Mn, Ni, Pb and Zn. Higher concentrations, particularly of Na2O and K2O, in near-surface borehole samples demonstrate, however, that the ash was probably not homogeneous at the time of emplacement. Other elements in solution, such as Cl and NO3, show peak concentrations in the depth profiles, which are thought to represent a time-dependent migration of an anthropogenic input, probably fertiliser. No significant changes were detected in the mineralogy using XRD and SEM. Porewater analyses were processed using a geochemical modelling program, WATEQ4F, to investigate equilibrium relationships and to identify potential solubility controlling solid phases. Several solid phases were identified, including Al(OH)3 for Al, Fe(OH)3(am) for Fe and CaSO4.2H2O (gypsum) for Ca and SO4.