Sorption, insolubility, and related processes are largely responsible for controlling the bioavailability of many pollutants to microorganisms in soils and sediments. The turnover rates of forms of a chemical that are not bioavailable are usually slower than those for the same chemical form in solution. In addition, for many chemicals with limited bioavailabilities, the rate-limiting step in their biodegradation is mass transfer from an unavailable to available form.