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, 129 I, tritium, 14 C and 13 C/ 12 C ratios in groundwater and geologic materials (sands and peats), was initiated at this site to examine the partitioning of 127 I and 129 I amongst the various reservoirs in this system and the controlling factors.The maximum iodine concentration and 129 I inside the groundwater contaminant plume at the recharge and discharge sites were 67.0 ng/ml and ∼ 8.3 × 10 11 atoms/liter, and 32.4 ng/ml and ∼ 2.9 × 10 11 atoms/liter, respectively, with positive correlations between iodine, 14 C (0.82), and tritium (0.87). Maximum total iodine concentrations for in-plume, rechargesite sands and discharge-site peats were 190.1 µg/kg and 14 100 µg/kg, respectively. 129 I analyses on these same samples showed concentrations of 2.3 × 10 7 and 6.4 × 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 127 I and 129 I 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 127 I being more strongly sorbed than 129 I, probably as the result of kinetically controlled sorption mechanisms and the differing residence times of stable and radio-iodine in this hydrologic regime.