Abstract

Subaqueous sediments and landfill waste disposal sites are both natural locations for anaerobic microbiological metabolism, where sulphates are reduced to sulphide, and organic matter (particularly acetate) is oxidized and degraded to produce ultimately CO2 and CH4. The microbiologically mediated geochemical processes in each case are similar. However, natural sedimentary porewaters can be difficult to sample and monitor, whereas landfill sites are relatively easily accessible, yielding leachate samples that are routinely monitored over periods of time. In both environments, carbonate precipitation is predicted to be a normal consequence of anaerobic microbiological activity, consistent with changes in leachate composition and the predictions of geochemical modelling. Initial investigation of scale from a landfill drainage system confirms the presence of calcite, together with gypsum and iron minerals.

Calcium oxalate minerals (whewellite and weddelite) are widespread in their occurrence within septarian concretions from a number of geological formations; whewellite is reportedly stable up to 160°C. The solubility of calcium oxalate is very low, and precipitation readily takes place when oxalate (oxalic acid) is present (derived from fungal processes in sediments and soils). Calcium oxalate minerals should be expected wherever vegetable matter is decomposing; this link can be demonstrated for natural sediments, but has yet to be proven for landfill.

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