Accumulation processes has been observed that sediments in rivers and the oceans have the property of adsorbing some types of dissolved substances present in the overlying water so that the concentration in the sediment is appreciably greater than that in the water with which the solid is in contact. The concentration of toxicants present in sediments is a measure of its concentration in the water over a period of time and is therefore a measure of the risk to creatures. When a creature is exposed to toxicants in the water or sediments in which it lives then the concentration of those toxicants in its tissues gradually increases as a function of exposure time and the concentration of toxicant in the water until the concentration in the tissues of the creature is many times that present in the water.