The mobility of anthropogenic¹²⁹I in a shallow sand aquifer at Sturgeon Falls, Ontario, Canada

Summary The transport and partitioning of129I has been examined for a shallow groundwater flow system at Sturgeon Falls in northeastern Ontario. Recharge into a sandy surficial postglacial deltaic aquifer occurs in the vicinity of a seasonally-wet boreal forest. Concentrations of stable iodine,129I, tritium, radiocarbon, stable isotopes and major ions were measured on samples of groundwater and precipitation. The present-day tritium profile delineates the position of the early 1960's above-ground nuclear weapons pulse at a depth of approximately 12 m. The concentrations of stable iodine for groundwaters above, near and below the above-ground nuclear weapons pulse range from 0.07 to 1.7 ppb. Over that same interval129I concentrations decreased from 1.9 × 106atoms/L at 9 m, to approximately 1.9 × 105atoms/L in tritium-depleted waters at 35 m, below the present-day depth of the recharging above-ground nuclear weapons peak at approximately 12.7 m. No substantial increases in the levels of129I were evident in waters sampled near the location of the above-ground nuclear weapons peak. The129I values in the lower aquifer are 1 to 2 orders of magnitude greater than published estimates of pre-bomb129I (103to 104atoms/L), and indicate contribution of129I from the fissiogenic decay of238U in the basal granite bedrock or from leaching of129I from the aquifer grains. A precipitation sample collected at Sturgeon Falls had concentrations of stable iodine and129I of 0.2 ppb and 8.5 × 107atoms/L, respectively. Concentrations of129I in Sturgeon Falls groundwaters are lower than those reported in other studies in central Canada, suggesting that the levels of129I in these groundwaters may have been attenuated by complexation or ion exchange with organic materials in the near-surface soil horizons. Such processes have been documented during a previous study on the behavior of129I in a shallow aquifer near a low-level, radioactive waste management area. Along a 30 cm soil profile at piezometer nest 28 at the Sturgeon Falls study site, the concentrations of129I ranged from approximately 4.3 × 108atoms/g in the uppermost soil litter layer to 5.6 × 107atoms/g in the siltier bottom soil horizons. Over that same profile, stable iodine varied from 4.7 ppm in the upper layers to 3.9 ppm in the lower layers.

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