Uranium series isotopes in the Avon Valley, Nova Scotia

An U-series isotopic study was carried out in the waters of the Avon Valley, Nova Scotia. The fresh and acidic recharge waters flow rapidly through the watershed composed of a granitic highland and a sedimentary, largely carbonate, lowland plain, before draining to the sea. There is no significant anthropogenic pollution; but, naturally elevated U levels can be encountered within the bedrock. Nonetheless, the U concentrations of the surface and groundwater are low (generally within the range of several hundredths to several tenths of a μg l^-1), except in the proximity to weathering of U mineralization. The dissolved U in the surface waters appears to be stabilized by organic rather than inorganic complexes. Both the groundwaters and surface waters have similar ²³⁴U/²³⁸U activity ratios that rarely deviate from secular equilibrium by more than 20% throughout the watershed. The magnitude of the ²³⁴U/²³⁸U activity ratio is not determined by lithology but rather by the weathering mechanism, the high rate of flushing, and the leaching of local U mineralization. Dissolved Ra is consistently absent. The dissolved Rn concentrations, though variable, are measurable even in surface waters. This may be due to a continual degassing from the U-enriched bedrock or release from local sites of U mineralization underlying the surface water sources.