What is the primary function of a high-density polyethylene geomembrane within a modern composite liner system for a municipal solid waste landfill?

The primary function of a high-density polyethylene (HDPE) geomembrane within a modern composite liner system for a municipal solid waste landfill is to act as the principal hydraulic barrier, effectively containing leachate and preventing its migration into the surrounding environment. An HDPE geomembrane is a synthetic, highly impermeable sheet made from high-density polyethylene polymer, specifically engineered for its low permeability, excellent chemical resistance, and high strength. Leachate is the contaminated liquid that forms when precipitation infiltrates the waste, dissolving soluble materials and collecting hazardous substances. By forming a continuous, virtually impenetrable barrier, the HDPE geomembrane prevents the bulk flow, or advective transport, of this leachate. While it significantly reduces advective flow, it also offers substantial resistance to diffusive transport, which is the molecular movement of contaminants through the barrier due to concentration differences. In a composite liner system, which combines two or more barrier components, the HDPE geomembrane works in conjunction with a compacted clay liner (CCL) or a geosynthetic clay liner (GCL). The geomembrane serves as the primary, most effective barrier against advective leachate flow, whereas the underlying compacted clay or geosynthetic clay liner provides a secondary, less permeable barrier, offering redundancy and further attenuating contaminant migration, particularly slowing down diffusive transport and acting as a critical backup should the geomembrane be damaged.