HOW TOXIC ARE TOXIC-CHEMICALS IN SOIL

Assessments of the hazard of toxic chemicals in soil are made without concern about the possibility that their bioavailability may change with time. The patterns of disappearance of persistent compounds in the field and laboratory studies show a declining availability to microorganisms with residence time in soil. Changes in extractability with residence time and the kinetics of sorption and desorption suggest that the compounds are becoming sequestered in inaccessible microsites within the soil matrix. Diminishing toxicity as chemicals age in soil is evident in a limited number of assessments. Such findings suggest that the hazard and risk from toxic chemicals diminish as the compounds persist in soil.