Abstract
There is no doubt that some elements in all waste materials disposed of on land sooner or later react with soil constituents, and reach the food chain via surface and underground water, resulting in a potential threat to the health of human beings. Recently, because of the increasing quantity and variety of wastes,this threat has been enhanced. This paper presents information on the movement of trace elements involving radionuclides, based on data obtained from the ongoing research program, the author* s experience, and reviews of the literature. A number of abiotic (physical, chemical) and biotic reactions take place, which affects the rate of movement of contaminants from sources to the surrounding soil. Although sharp distinctions of these processes cannot be made, relatively simple examples are described. Physical processes in the subsurface environment, composed of advection, dispersion, preferential flow, and so on, affect the movement of contaminants, though for low ground water flow less than ca.10 3cm/s molecular diffusion may dominate the contaminant transport. It is noted that the dispersion coefficient itself is not a parameter representing the dispersion phenomena,but the ratio of the coefficient divided by the interstitial velocity is related to the extent of the diffusivity. Among biological processes involving microorganisms, the three important mechanisms influencing migration of trace metals are oxidation—reduction, mineralization—immobilization, and production of organic constituents. Chemical processes affect contaminant transport by causing interactions between ionic compounds and geologic materials, such as chemical adsorption and ion exchange processes, or by affecting the form of the contaminant,i.e., hydrolysis and precipitation. For nonionic compounds, the adsorption is commonly hydrophobic with organic carbon on the soil surface. Based on the information on the environmental processes mentioned above, it is pointed out that an improved understanding about transport/interaction mechanisms of individual/mixed contaminants is needed to limit any risks from subsurface contamination in the future.
