Partitioning of 127I and 129I in an unconfined glaciofluvial aquifer on the Canadian shield

At AECL - Chalk River Laboratories, Ontario, Canada solid, low-level radioactive wastes from industrial, academic and medical applications have been stored in trenches above unconsolidated sandy glacial tills and permeable very-fine to fine-grained sands overlying crystalline bedrock. The sandy aquifer system drains into a swamp comprised of approximately 3 m of sphagnum peat. A comprehensive field and analytical program, involving measurements of total iodine, I-129, tritium, C-14 and C-13/C-12 ratios in groundwater and geologic materials (sands and peats), was initiated at this site to examine the partitioning of I-127 I and I-129 amongst the various reservoirs in this system and the controlling factors. The maximum iodine concentration and I-129 inside the groundwater contaminant plume at the recharge and discharge sites were 67.0 ng/ml and similar to 8.3 x 10(11) atoms/liter, and 32.4 ng/ml and similar to 2.9 x 10(11) atoms/liter, respectively, with positive correlations between iodine, C-14 (0.82), and tritium (0.87). Maximum total iodine concentrations for in-plume, recharge-site sands and discharge-site peats were 190.1 mug/kg and 14100 mug/kg, respectively. I-129 analyses on these same samples showed concentrations of 2.3 x 10(7) and 6.4 x 10(9) atoms/g of soil, respectively. K-D values (concentration on porous medium/concentration in co-existing water) calculated from the contaminant plume data for I-127 and I-129 were 1.3 and 1.61/kg, respectively, at the recharge site and 486 and 931/kg, respectively, at the discharge area, indicating that both stable and radio-iodine are preferentially sorbed to the organic rich, aquifer materials at the discharge sites. Incremental leach experiments on these same geologic materials have borne out these differences, with I-127 being more strongly sorbed than I-129, probably as the result of kinetically controlled sorption mechanisms and the differing residence times of stable and radio-iodine in this hydrologic regime.