Distribution coefficients, K(d)s, for iodide in Canadian Shield lake sediments under oxic and anoxic conditions

In a series of laboratory studies, we determined the solid to liquid distribution coefficient, K-d, for iodine (I-125) in Canadian Shield lake sediments and the loss rate of I-125 from water to sediment for more accurate computer model predictions of the fate of I-129 released from an underground disposal vault. The K-d values were determined under oxic and anoxic (reducing) conditions at sediment-to-water ratios of 1, 10 and 50% for organic sediment (OS), peaty sediment (PS), clayey silt/sand sediment (CS/SS) and sand sediment (SS). Under oxic conditions and shaking for 48 h, the geometric mean (GM) (divided by) (x) geometric standard deviation (GSD) K-d values for the ratios were 247 divided by x 1.1, 109 (divided by) (x) 1.3 and 21 (divided by) (x) 1.3 lkg(-1) for OS, 104 (divided by) (x) 1.3, 68 (divided by) (x) 1.1 and 14 (divided by) x 1.1 lkg(-1) for PS, 32 (divided by) (x) 1.1, 15 (divided by) (x) 1.2 and 2.9 (divided by) (x) 1.1 lkg(-1) for CS/SS and 0.1 (divided by) (x) 2.7, 0.5 (divided by) (x) 2.2 and 0.2 (divided by) (x) 1.3 lkg(-1) for SS. Alkalinity of water ranging from < 1.0 to 250 mg CaCO3 had only a minor effect on K-d, with values changing less than a factor of two. Treating OS with a fungicide (nystatin), bactericide (streptomycin and penicillin), irradiation (1.72 Gy s(-1) for 4 h) or heat (80 degrees C for 4 h) decreased K-d values by a factor of 1.1, 2.3, 7.5 and 22, respectively. Thus, bacteria appear to be important in the sorption of I-125 to sediment, although chemical and/or physical changes as a result of radiation or heat treatment may also be important. Under anoxic conditions, K-d values were small (generally <1 lkg(-1)) and little I-125 was lost from the water. Over a 48-day period K-d values increased from about 690 lkg(-1) on day 6 to 3 840 lkg(-1) on day 48 under oxic conditions and from about 0.5 to 16 lkg(-1) under anoxic conditions. Most of the I-125 adsorbed to sediment under oxic conditions is released back into the water under reducing conditions. On the basis of our K-d values for OS, the water to sediment loss rate for iodine in Canadian Shield lakes would be about 0.6 (divided by) (x) 7.4 year(-1). Oligotrophic lakes would be better represented by a loss rate of 3.0 (divided by) (x) 2.6 year(-1). We conclude that anoxic organic lake sediment is not a particularly effective sink for long-lived I-129 from an underground disposal vault and that iodine in the water column is slowly lost from water to sediment. Crown Copyright (C) 1997 Elsevier Science Limited.