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Tens of thousands of municipal solid waste (MSW) landfill sites worldwide hold a high risk of contaminating groundwater. This study aimed to establish a practical hydrogeological classification system for MSW landfill sites and explores the correlation between the classification and the risk of groundwater contamination. Hydrogeological information and groundwater contamination data from 80 MSW landfill sites in China were collected and analyzed, and a general hydrogeological model was proposed. The key hydrogeological parameters in the model were identified and analyzed, including the relative depth to the water table, the ratio of the length of the MSW site's recharge boundary to the combined length of the discharge boundary and hydrobalance boundary, the hydraulic conductivity of the bearing layer, and the background hydraulic gradient. On the basis of the general model, hydrogeological conditions at the landfill sites were categorized into seven subtypes. By using chloride, ammoniacal nitrogen and chemical oxygen demand as the characteristic contaminants, the migration features of groundwater contaminants within the seven subtypes of landfill sites were revealed. It was found that the maximum contaminant migration distance could be 2,000 and 300 m at the landfill sites with 'plain and intensive runoff' type and 'valley and weak runoff' type, respectively, and the corresponding concentration gradients of the characteristic contaminants were less than 10 mg/(L × m) and greater than 10 mg/(L × m). This work provides a guide for implementing cost-effective site investigation and environmental risk management at landfill sites with different types of hydrogeological conditions. [ABSTRACT FROM AUTHOR]

Disposal of solid waste is a challenging task in developing countries as this type of waste can seriously degrade the environment and pose threat to human health. To mitigate the environmental impact, appropriate techniques must be used in accordance with standards for the disposal of solid wastes. Leachate that originates from the decomposition of solid waste within waste disposal dumping sites and landfills may infiltrate into soil, groundwater and surface water resulting in contamination. A combination of clay materials and geomembrane has been widely utilized as a suitable permeability barrier to prevent contamination. In the case of uncontrolled landfills, leachate may be responsible for serious and irreversible pollution. According to the standards utilized in Turkey, the thickness of the clay layer used as an impermeable material is 60 cm, which is constructed as two 30 cm thick compacted clay layers. This paper evaluates the optimum clay thickness to prevent leachate contamination of soil or water. Experimental models were prepared involving 20, 40, 60 and 80 cm thick clay layers with 20 cm height leachate to determine the optimum clay thickness permeability barrier for a waste disposal dumping site. The initial results indicate that a 40 cm thick clay layer is sufficient as an impermeable barrier in these sites. [ABSTRACT FROM AUTHOR]