Fate and Transport of Contaminants

Abstract

Release of toxic chemicals into the environment cannot always be avoided completely. As a result organisms, man included, will be exposed to chemicals via the environment. Given the release of certain chemicals into the environment, their exposure concentrations in air, water and soil would depend on the rates at which they are removed from the environment. This chapter deals with the transport and transformation processes that affect concentrations in the environment, with emphasis on the modeller's perspective. Being interested primarily in the effects that processes have on concentrations of chemicals in environmental media, we focus on a quantitative description of the rates at which losses from the environment take place, and on how these rates differ for different chemicals. We systematically formulate process rate constants for each transport or transformation process. Eventually, the rate constants combine into a mass balance model which allows us to describe and predict how releases into the environment result in exposure concentrations of organisms. PROCESSES AND MECHANISMS After entering the environment, chemicals are transported, distributed over the various environmental compartments and may be transformed into other chemicals. Transport can occur within a compartment, such as in air or in soil, or between compartments (e.g. between air and water, air and soil or water and soil). Transformation processes in the environment involve chemical degradation or biodegradation. Process rates (i.e. the mass flows of substance that result from them) generally depend on two independent factors: (i) the concentration of the substance in the environmental medium (driving force) and (ii) the likelihood of occurrence of the process (rate constant). When process rates are directly proportional to concentrations, process kinetics are called first order (first power concentration). Non-linear relationships apply in cases of higher or lower order kinetics. In the case of first-order kinetics, the mass M of chemical in the environmental compartment of origin falls exponentially with time t: