Mobility and Plant Uptake of Inorganic 14C and 14C-labelled Pcb in Soils of High and Low Retention

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Quantifying and understanding the mobility of 14C and organic pollutants in soils is important, especially in the context of underground waste disposaI. We studied migration of 14C applied as NaHCO3 (14C-CO3) and as 2,2′,5,5′ tetrachlorobiphenyl (14C-PCB) in carbonated, high-organic-matter-content and acidic, low-organic-matter-content undisturbed soil cores. The mobility of 14C-PCB depends on the profile distribution and amount of soil organic matter, whereas the mobility of 14C-CO3 depends primarily on the soil carbonate content. The solid/liquid partition coefficients (Kd) for 14C-CO3 were 6.7 and 1.2 mL g−1 for the two soils, respectively. For the 14C-PCB, the corresponding Kd values were 49 and 22 mL g−1. Plant/soil concentration ratios (CR) for inorganic 14C have previously been derived using overly conservative assumptions. Using plants grown in outdoor lysimeters, CR values for 14C-CO3 of 0.7 and 1.3, on a dry-weight basis, were measured for the two soils. These values are about 25-fold lower than the currently used values. The corresponding CR values for 14C-PCB were 0.014 and 0.088. For both 14C sources, there was evidence of atmospheric transfer from the soil to the plants. This was especially important for 14C-CO3, where it may have been dominant. Detailed modelling of 14C transport from underground waste disposal should include volatilization as a loss process from soil as well as a source for plants.

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