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

Abstract

Summary 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, 129I, tritium, 14C and 13C/12C ratios in groundwater and geologic materials (sands and peats), was initiated at this site to examine the partitioning of 127I and 129I amongst the various reservoirs in this system and the controlling factors. The maximum iodine concentration and 129I inside the groundwater contaminant plume at the recharge and discharge sites were 67.0 ng/ml and ∼8.3 × 1011 atoms/liter, and 32.4 ng/ml and ∼2.9 × 1011 atoms/liter, respectively, with positive correlations between iodine, 14C (0.82), and tritium (0.87). Maximum total iodine concentrations for in-plume, recharge-site sands and discharge-site peats were 190.1 µg/kg and 14100 µg/kg, respectively. 129I analyses on these same samples showed concentrations of 2.3 × 107 and 6.4 × 109 atoms/g of soil, respectively. K D values (concentration on porous medium/concentration in co-existing water) calculated from the contaminant plume data for 127I and 129I were 1.3 and 1.6 l/kg, respectively, at the recharge site and 486 and 93 l/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 127I being more strongly sorbed than 129I, probably as the result of kinetically controlled sorption mechanisms and the differing residence times of stable and radio-iodine in this hydrologic regime.