NATURAL WEATHERING OF PULVERIZED FUEL ASH AND POREWATER EVOLUTION

Borehole samples of pulverized fuel ash (PFA) were taken from the unsa turated zone in a disposal mound at a decommissioned power station in the UK. The aim was to investigate the long-term natural weathering re actions of PFA and the chemical evolution of the contained porewaters. Concentrations of most species, including Al, Na, K, Ca, SO4 B, Co, C r, Li, Mo, Ni, Pb and Sr in the porewaters, increase with borehole dep th, consistent with these elements being released from the PFA through continued weathering reactions with infiltrating porewaters. The conc entration 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 deplet ion near-surface, consistent with the higher porewater concentrations with depth. Using mass balance calculations for these two elements, ap proximate infiltration rates are obtained. Other elements which are de pleted in near-surface samples are Cu, Mn, Ni, Pb and Zn. Higher conce ntrations, particularly of Na2O and K2O, in near-surface borehole samp les demonstrate, however, that the ash was probably not homogeneous at the time of emplacement. Other elements in solution, such as Cl and N O3, show peak concentrations in the depth profiles, which are thought to represent a time-dependent migration of an anthropogenic input, pro bably fertiliser. No significant changes were detected in the mineralo gy using XRD and SEM. Porewater analyses were processed using a geoche mical modelling program, WATEQ4F, to investigate equilibrium relations hips and to identify potential solubility controlling solid phases. Se veral solid phases were identified, including Al(OH)(3) for Al, Fe(OH) (3)(am) for Fe and CaSO4 . 2H(2)O (gypsum) for Ca and SO4 (C) 1997 Els evier Science Ltd.