Partition coefficients (Kd values) for Tc in shield lake sediments under oxidizing and reducing conditions

In a laboratory study, the authors determined the solid-liquid partition coefficient (Kd) for technetium (95mTc) in Canadian Shield lake sediments and the loss rate of 95mTc from water to sediment for more accurate assessment model predictions. The Kd values were determined under oxic and anoxic (reducing) conditions at sediment-water ratios of 1, 10 and 50% for organic sediments and 10% for sand and clayey silt/sand sediments. Kd values for organic sediment at ratios of 1, 10 and 50% had geometric means (GMs) geometric standard deviations (GSDs) of 4.3 2.6, 6.9 1.3 and 13.0 0.1 litres kg−1 dry weight (dw), respectively, under oxic conditions, and 13.7 2.4, 9243 0.2 and 206 0.2 litres kg−1 dw, respectively, under anoxic conditions. Kd values for sand and clayey silt/sand sediments have GMs of 0.5 litres kg−1 dw under both oxic and anoxic conditions. On the basis of the Kd values for organic sediments, the water-to-sediment loss rate for technetium in well-oxygenated Canadian Shield lakes would have a GM of about 0.15 a−1 and GSD of 2.3, whereas those lakes whose bottom waters experience periods of anoxia may be better represented by a GM of 1.3 a−1 and a GSD of 5.6. Therefore, anoxic organic lake sediments should form an effective barrier for the transport of long-lived 99Tc from an underground disposal vault, although movement of 99Tc with organic colloids is a possibility.