Silica in landfill leachates: implications for clay mineral stabilities

The SiO2 contents of landfill leachates have been monitored (together with other constituents) for up to 21 sampling locations at 3 landfill sites in Cheshire, England, at 6 month intervals over a 2 a period. The observed SiO2 values average 26.9 mg/l SiO2 (s.d. 12.1 mg/1), and show no variation that can be attributed to the transition from acetogenesis to methanogenesis. The youngest (<2 a) leachates have highest SiO2 levels (27–47 mg/I). Geochemical modelling shows that most samples are supersaturated with respect to both quartz and chalcedony, and with respect to calcite. Ion activity diagrams suggest that alkali and SiO2 activities are controlled by reactions which involve clay minerals, especially smectites, probably as constituents of the waste rather than the site containment. Ammonium also appears to be controlled by exchange with K within clays. The information gained in this study suggests that SiO2 should be routinely determined for leachates, to permit a more detailed interpretation of the processes and reactions that influence their compositions.