The mobility of anthropogenic ¹²⁹I in a shallow sand aquifer at Sturgeon Falls, Ontario, Canada

Summary

The transport and partitioning of ¹²⁹I 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 interval ¹²⁹I concentrations decreased from 1.9 × 10⁶ atoms/L at 9 m, to approximately 1.9 × 10⁵ atoms/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 of ¹²⁹I were evident in waters sampled near the location of the above-ground nuclear weapons peak.

The ¹²⁹I values in the lower aquifer are 1 to 2 orders of magnitude greater than published estimates of pre-bomb ¹²⁹I (10³ to 10⁴ atoms/L), and indicate contribution of ¹²⁹I from the fissiogenic decay of ²³⁸U in the basal granite bedrock or from leaching of ¹²⁹I from the aquifer grains.

A precipitation sample collected at Sturgeon Falls had concentrations of stable iodine and ¹²⁹I of 0.2 ppb and 8.5 × 10⁷ atoms/L, respectively. Concentrations of ¹²⁹I in Sturgeon Falls groundwaters are lower than those reported in other studies in central Canada, suggesting that the levels of ¹²⁹I 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 of ¹²⁹I 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 of ¹²⁹I ranged from approximately 4.3 × 10⁸ atoms/g in the uppermost soil litter layer to 5.6 × 10⁷ atoms/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.