The long-term weathering of pulverised fuel ash and its implications for groundwater pollution.

The current, non-marketed, production of PFA in the UK amounts to approximately 6.5 x 106 tonnes per year. This waste is disposed of in lagoons, land-fill sites or mounds. Many trace elements are concentrated in PFA and their association with either unstable high temperature glass phases or surfaces of particles creates potential environmental problems of leachates contributing to groundwater. Natural weathering of PFA are not well documented and form the basis of this study. PFA samples were taken from boreholes in ash mound associated with two UK power stations. The PF A on these mounds date back 17 and approximately 40 years. In addition, effluents samples were taken from one of the ash mounds.
Reaction of PFA with the infiltrating porewaters in the boreholes are indicated from the depth-related trend of elements both in the porewater and in the PFA. This allows an assessment of the mobility of element. Losses of elements from the PFA to the porewater are also detected in the whole PFA and this provides confirmation of some of the pore water depth trend. Although the chemical analyses demonstrated that reactions are taking place, secondary reaction products have not been unambiguously detected using SEM and XRD analyses. The feedback based investigations have been supplemented in the laboratory using batch and column leaching tests. The performance of weathered and fresh PF A has been compared with the result from the field investigation and also the feasibility of weathered PFA providing some amelioration for land-fill leachate has been investigated.
The results from the field and laboratory investigations imply that not only fresh ash, but also weathered ash, yields elemental concentrations that exceed the drinking water quality standards. More importantly, the release of elements from the weathered ash decreases slowly with time and therefore weathered PFA, could be a potential source of groundwater pollution, without proper protection, even after a long period of disposal.