Bioavailability of Organic Compounds Sequestered in Soils

The pollutants present in soil and sediments usually are not recent introductions, rather they were typically added one, two and frequently more than three decades ago. This fact was one of the concerns as we devised our initial studies. The studies were also prompted by a number of long- and medium-term field studies that showed that persistent pesticides (such as DDT, dieldrin, heptachlor and kepone) and a number of other compounds initially disappear from soil at reasonable rates, but the rate subsequently slows appreciably. Indeed, the subsequent disappearance is often so slow that the rate of decline in concentration sometimes cannot be estimated. Because the compounds were initially subject to biodegradation by microorganisms (as well as loss by volatilisation or other abiotic processes in some instances), clearly something was occurring in soil that was slowly but progressively making the compounds less and less bioavailable, at least to microorganisms (Alexander 1995).