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Element Leaching and Capillary Rise in Sandy Soil Cores: Experimental Results¹

ABSTRACT

Disposal of hazardous wastes by burial is a common practice. This practice usually involves several barriers to the migration of the wastes, one of which is the unsaturated soil zone. The long-term consequences of capillary rise of water and its dissolved contaminants through the unsaturated soil is not easy to assess. Similarly, downward migration after a surface contamination event, such as irrigation with contaminated groundwater, is a complex process. This study examines both the leaching and capillary rise of Tc, I, Np, U, Th, Cs, Cr, and Mo through a sandy soil. Eighty undisturbed soil cores from a forest floor were contaminated, half were contaminated 10 cm from their base to simulate contact with contaminated groundwater (groundwater cores), and the other half were contaminated below the litter layer to simulate soil surface deposition (leaching cores). Cups at the base of the cores served as receptacles for groundwater or for leachate. The cores were placed in the ground with their surfaces exposed to natural rainfall. After 1 yr, half of the cores were exhumed. Soil moisture and temperature conditions in the cores reflected the trends of moisture and temperature at the coring site. Leachate analyses and exhumation data of the leaching cores indicated element mobility as Tc I >> Mo Cr >>> U Np, Cs, Th, with Tc the most mobile and Th the least mobile. Only Tc, I, Mo, and Cr passed through the cores in the leachate in the first year. The groundwater cores showed that Tc, I, Mo, and Cr are most likely to reach surface soils from contaminated groundwater based on one year of data. Partition coefficient (K_d) values for Tc, I, and U from batch sorption experiments were compared with K_d values obtained from the exhumation porewater and soil, by soil horizon.

Key Words: Solute transport • hazardous waste • environmental assessment • element redistribution • nuclide • groundwater • radionuclide • sorption coefficients • K_d • technetium • iodine • neptunium • uranium • thorium • cesium • chromium • molybdenum

¹ Contribution no. AECL-9284 from the Environmental Research Branch, Atomic Energy Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, MB, Canada R0E 1L0.

² Research officer and technologists, respectively.

Received for publication November 20, 1986.