Interactions between landfill leachate and the Ruabon Marl, North Wale

The interactions between Ruabon Marl and Coal Measures clays with natural and synthetic leachates have been investigated in the laboratory. The bulk clays were characterised before and after exposure to leachate and the crystalline components were found to consist of kaolinite, illite, chlorite, and quartz. Two natural leachates from sites situated in the Ruabon Marl in North Wales were used; acetogenic and methanogenic leachates from Pen-Y-Bont and Gardden Lodge sites near Wrexham. Synthetic acetogenic and methanogenic leachates were also used. Three groups of leachate-clay reaction experiments took place for periods of one set of 22 weeks and subsequently two sets of 17 weeks. Leachate composition was monitored regularly and in the acetogenic leachates showed a transition from acetogenesis to methanogenesis. This was evident from the degradation of organic acids, and the decrease in concentration of iron, calcium, sulphate and ammonium. The composition of the methanogenic leachates altered little over time. Exposure to all the leachates caused the clay minerals in the Ruabon Marl samples to break down. XRD analysis showed the clay mineral peaks to have virtually disappeared in most of the samples after exposure to leachates of varying composition. Where peaks were still present, they had become asymmetric and disordered. Hydrolysis reactions are likely to have caused a grain size reduction in the bulk clays, which means the clay minerals were not detectable using X-Ray diffraction. It is likely, however, that they may be re-precipitating out in a different form such as amorphous gels. Calcite was found to have precipitated from solution in the Ruabon Marl that was exposed to natural and synthetic acetogenic leachates. It did not precipitate from the methanogenic leachates. Calcite precipitation may have implications for landfill sites in the future as it has the potential to clog leachate drainage pathways. The coal washings material was found to be more stable than the two Ruabon Marl samples in the presence of leachate; the clay mineral peaks did not disappear from the X-ray diffraction traces in this sample, although they did become more disordered and asymmetric. This material may have proven to be more stable in the presence of leachate than the Ruabon Marl because it has not been previously weathered. This may have implications for landfill practice in circumstances where coal washings are available for site remediation using landfills.