GEOCHEMICAL FACTORS AFFECTING TRACE-ELEMENT MOBILITY

The mobility of trace elements in soil reflects the chemical reactivit y of the elements in the soil matrix. The result is that trace-element mobility is both specie and site dependent. An overview of selected g eochemical factors that influence the concentration of trace elements in the soil solution is presented. Factors affecting both cationic and anionic trace-element mobility are given in the context of multiphase equilibria. Chemical processes such as ion exchange, adsorption, soli d-phase formation, and redox are considered and related to control of trace-element concentration in solution and hence their mobility in so il. The limitations inherent in the use of a thermodynamic database fo r determining partition functions used in solute transport modeling ar e emphasized. The importance of microbially mediated biomethylation of trace elements such as Pb, Hg, Cd, and Se is discussed.