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.