an arrangement for retaining harmful substances contained in water seeping from a waste dumping ground body includes an impervious layer of mineral sealing materials , such as clay , with an adjacent adsorption layer containing materials capable of binding harmful substances by physical and / or chemical adsorption . the materials may be in the form of admixtures . when seepage water flows very slowly through a layer of mineral sealing materials , it is possible to remove and retain the harmful substances . the permeability of the arrangement reduces with time due to the adsorption process with its sealing effect improving over geological time periods .