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Leachate water containing dissolved chemicals; applied particularly to fluids escaping from waste-disposal sites. They are by definition designed to minimise adverse effects of waste disposal. However, many were poorly designed and are leaking liquids, generically termed leachate, which are contaminating groundwater. Actually, landfill leachate is the liquid that is the product of the liquid content of the waste, infiltrating precipitation, and groundwater if the waste is below the groundwater table. These liquids mix with the waste and dissolve both inorganic and organic constituents.

Leachate is a complex mixture of dissolved and colloidal organic matter and inorganic compounds and ions. Materials placed in landfills include such things as municipal garbage and trash, demolition debris, sludge from wastewater treatment plants, incinerator ash, foundry sand and other foundry waste, and toxic and hazardous materials. To minimise the amount of leachate generated, modern landfills are built in sections, with a low permeability cover placed over the waste as soon as possible to limit the infiltration of rainwater. Present-day landfills also have low permeability liner systems and collection pipes to remove the leachate that forms so that it can be taken to a waste water treatment plant. A modern landfill that is properly sited with respect to the local geology and that has a properly designed and constructed liner, leachate collection system, and low permeability cover has limited potential to contaminate groundwater.

In Bangladesh sources of leachate are generated in the municipal waste dumping sites in large cities. Also considerable amounts of industrial leachates are generated from industrial sources. In Dhaka city, and all other big cities of the country, municipal wastes are dumped in low-lying areas without following recognised landfilling procedures. Municipal leachates are rich in chlorinated hydrocarbon solvents (CHS) which are very toxic and carcinogenic. No study has so far been conducted to identify the CHS in Dhaka groundwater. However, recent investigations have found remarkable quality deterioration under landfill leachates and identified a number of CHS in groundwater from the industrial areas of Hazaribag and tejgaon. It is very likely that the same contaminants may also be present in groundwater in other large cities and industrial zones of Bangladesh. However, there is no recorded information on the situation of groundwater pollution for industrial and landfill leachate in other cities of the country. [Sifatul Quader Chowdhury]