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. T he observed SiO2 values average 26.9 mg/l SiO2 (s.d. 12.1 mg/l), and s how no variation that can be attributed to the transition from acetoge nesis to methanogenesis. The youngest (<2 a) leachates have highest Si O2 levels (27-47 mg/l). Geochemical modelling shows that most samples are supersaturated with respect to both quartz and chalcedony, and wit h respect to calcite. Ion activity diagrams suggest that alkali and Si O2 activities are controlled by reactions which involve clay minerals, especially smectites, probably as constituents of the waste rather th an the site containment. Ammonium also appears to be controlled by exc hange with K within clays. The information gained in this study sugges ts that SiO2 should be routinely determined for leachates, to permit a more detailed interpretation of the processes and reactions that infl uence their compositions. (C) 1997 Elsevier Science Ltd.