Membrane effects in clay-lined inward gradient landfills

Inward gradient landfills are attractive because the inward flow of groundwater is believed to inhibit outward movement of contaminants. However, clay liners may act as semipermeable membranes and exhibit coupled solute and water flow behavior. Our initial evaluation of the membrane behavior of clays used as liners for inward gradient landfill cells suggests that the situation is not as simple as previously thought. The total volume flux (water plus dissolved solutes) through a clay liner may not always be directed inward during the lifetime of an inward gradient landfill even though an inward-directed hydrostatic head difference is maintained. Thus we recommend that synthetic liners be used at least on the basal portion of inward gradient landfill cells to limit outward coupled flux of the dissolved components of leachate and water and that groundwater monitoring may be more important than previously thought for inward gradient landfills. We also suggest that the membrane behavior of clay liners used at inward gradient landfills be incorporated in the design of the cell.