ABSTRACT
Groundwater samples from 29 municipal drinking water sources and 137 private supplies were sampled across Southern Ontario and analysed for the radionuclides; radon (Rn), radium - 226 (Ra-226) and uranium (U). Considering yearly average concentrations, results from all locations sampled fall within Ontario drinking water objectives for Ra-226 and U. Currently there is no Provincial objective for Rn levels in drinking water.
In Southeastern Ontario, private well sampling was focussed in two areas in the Grenville Province of the Precambrian Shield:
Kennebec - Sharbot Lake, an area containing anomalous airborne gamma-ray spectral signatures in Lennox, Addington and Frontenac Counties, and
Bancroft area in northern Hastings County which contains economic deposits of uranium. Radionuclide concentrations in Southeastern Ontario ranged accordingly: Rn, 0.037-1650 Bq/L (l-44,600pCi/L), Ra-226, <37-925 mBq/L (<l-25 pCi/L) and U, <3-110 µg/L.
450In Southwestern Ontario, sampling was focussed on municipal and private wells which were finished in the Middle Devonian limestone and dolostone of the Detroit River Group which consists of a lower carbonaceous reef facies carbonate overlain by an evaporitic carbonate sequence containing anhydrite. Groundwaters, which recharge through up to 30m of glacial till with variable amounts of kame moraine and spillway sands, are predominantly calcium-bicarbonate to calcium-sulphate in character. Rn levels ranged from 5.18-407 Bq/L (140-11,000 pCi/L) Ra-226 ranged from <37-680 mBq/L (<1-18.4 pCi/L) and U concentrations ranged from <3-39μg/L.
Additional samples from 24 domestic wells from elsewhere across the Province were sampled. Wells finished in overburden and Middle Ordovician limestone generally contained low radionuclide levels.
Samples taken from discrete water bearing horizons within a municipal well show variations in radionuclide concentrations vertically within the stratigraphy. A conceptual model is presented for radionuclide distribution in the Detroit River Group in Southwestern Ontario whereby uranium is mobilized in groundwater under oxidizing conditions. At discrete sites containing carbonaceous material, uranium is reduced and precipitated with the eventual release of Ra and Rn to the groundwater.
