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Section 1: Investigation, hazard assessment and remediation of existing landfills |
Department of Geology and Applied Geology, University of Natal, Durban, South Africa
Two waste disposal sites in the greater Durban area used for codisposal of solid and liquid wastes are considered. One site is still operational and the other has been closed for a number of years.
An investigation of Site 1 revealed that much of the area is covered by Berea Red Sand. However, the clay content of much of this formation means that it generally has a low permeability. The sandstones and tillite which occur beneath the sand also are of low permeability or are virtually impermeable. This is necessary when the character of the leachate produced is considered. The very high values, for example, of chemical oxygen demand are attributable to a large extent to the disposal ofhop waste. Accordingly the leachate is conveyed from a sump at the toe of the landfill to a sewerage works.
At site 2 very little has been done since its closure to ensure that leakage from the landfill does not pollute the surrounding ground or surface waters. It was suspected that this was occurring. Small non-perennial and perennial streams occur in the area of the site. The site itself is underlain by sandstones and tillite. These rocks have been weathered to form a mantle of residual soil of variable thickness which is overlain by colluvium on the valley slopes and alluvium in the valleyfloor. An investigation was undertaken to determine the extent of the pollution and revealed that a plume extended some 300 m downstream of the leachate pond. Two main remedial measures were proposed. First, a subsoil drain should be constructed immediately downstream of the landfill site. This should be located in bedrock, its purpose being to intercept seepage from the landfill. The drain should lead into a pump chamber that would remove polluted water which would then be conveyed to thesewer system. Secondly, to make doubly sure that the pollution was contained, a cement-bentonite cut-off trench, founded in bedrock, should be constructed 3 m downstream of the subsoil drain.
