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A Four-Year Mobility Study of Selected Trace Elements and Heavy Metals

Environ. Sci. Branch, AECL Research, Whiteshell Laboratories, Pinawa, MB R0E 1L0 Canada.

^* Corresponding author.

ABSTRACT

The environmental impact of accidental or chronic releases of contaminants to the unsaturated soil zone, either through surface spills or contaminated groundwater, has become a major concern. This study was designed to provide data after 1 and 4 yr to compare the leaching and upward transport of Tc, I, Np, Cs, U, Th, Cr, and Mo in cores of the same soil in outdoor lysimeters. Over the 4-yr study a broad range of climatic, soil moisture, and temperature conditions existed, which were representative of long-term means as well as some extremes. The amount of rainfall leached through the cores decreased with time. The leaching resulted in a downward movement of fine soil particles, especially in the lower horizons. Element mobility in the leaching cores for the 4 yr decreased in the order I > Tc >> Cr > Np > Mo > Cs U > Th. This differs slightly from the ranking predicted after 1 yr. After 1 yr, Mo appeared to be more mobile than Cr or Np. After 4 yr, I, Tc, Np, and Cs migrated to the soil surface from depth. Analysis of the soil solid/liquid partition coefficient, K_d, values indicates the retention of these elements is primarily related to soil organic matter content and dependent on porewater concentration. The K_d model is appropriate for I in all soil horizons and for Tc, Np, and Cs in the Ae horizon of the groundwater cores. The Freundlich isotherm or a kinetic expression best describes the sorption for all other elements and horizons.