Chemical characterisation of landfill leachate and its potential mobility through the Cape Flats sand

Master Thesis


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University of Cape Town

Researchers have expressed concern about pollution of groundwater at Coastal Park, a large, general waste landfill situated on the False Bay coastline above the Cape Flats Aquifer. The landfill was constructed without a liner, but with an average 2m separation of calcareous sand providing a "buffer" zone between the waste pile and the water table. Water balance studies and application of a model, FLOW, have predicted that leachate will be generated seasonally. This study was initiated as a result of uncertainties about hydrological and geochemical aspects, such as the hydraulic conductivity of the soil in the buffer zone and the degree of leachate attenuation occurring in this zone. The Coastal Park soil was classified as an aeolian, calcareous, medium quartzitic sand with negligible organic carbon content. Extreme clay-depletion would render the soil almost incapable of leachate attenuation, although calcite and aragonite, found by X-ray diffractometry, would impart a significant pH buffering capacity to the soil. The solid phase of a locally-derived landfill leachate (sampled from Vissershok landfill, about 35 km NW of Cape Town) was found to contain amorphous sulfides of iron and heavy metals, and green rusts which are mixtures of Fe²⁺ - Fe³⁺ hydroxides, in addition to organic matter. The solid phase was isolated by centrifugation, freeze-dried, and analyzed by XRF and XRD. Distribution coefficients of heavy metals in the leachate (at pH 7.7) demonstrated the high affinity of heavy metals, such as Cu, Zn, Cr, Ni and Pb, for the solid phase. The leachate solid phase consists of amorphous solids, with high Ca and Cl concentrations in the liquid phase leading to halite and calcite formation upon evaporation of the liquid phase. According to locally specified requirements by Department of Water Affairs and Forestry, a landfill liner material must have a hydraulic conductivity (K) not higher than 1 x 10⁻⁷cm.s⁻¹. Air dried samples of Coastal Park soil were treated with various amendments to test their efficacy as landfill liners. An 8 % kaolinite plus 4 % gypsum treatment was the most effective, maintaining a minimum K of 10⁻⁴⁵ cm.s⁻¹, which, however, is still higher than the local requirement. Amendment with 8 % Na-bentonite initially achieved a minimum K of 10⁻⁷·⁸ cm.s⁻¹, but the high electrical conductivity (EC) of the leachate (⁻¹) caused shrinking and severe side-wall seepage, which rapidly enhanced hydraulic conductivity, reaching a maximum K of about 10⁻⁴·⁷ cm.s⁻¹. Both treatments of the sand do show promise as possible liners, although the use of higher percentage concentrations of clay should be investigated further. LEACHW (the water regime submodel of LEACHM) was used to predict leachate discharge from the Coastal Park landfill, assuming a hypothetical capping system of 1 or 2 m soil depth with 0, 50, 70, or 90 % vegetation cover (Acacia cyclops), and based on the assumption that drainage from this layer into the waste pile contributes directly to leachate generation. The model predicted that under average rainfall conditions the landfill, with a 2 m soil depth and 0 % vegetation cover, would not generate leachate. However, under the wettest conditions not even a 90% vegetation cover and 2 m soil cover would be sufficient to prevent the landfill from generating leachate, suggesting that, under such conditions, a more effective leachate management strategy, such as leachate collection sumps, should be implemented. This exercise demonstrated the use of LEACHM as an alternative means of predicting leachate discharge from landfill sites.