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Isotopic Fingerprinting of Shallow and Deep Groundwaters in Southwestern Ontario and its Applications to Abandoned Well Remediation


Abandoned hydrocarbon wells in southwestern Ontario can act as conduits for sulphur water, brines, and hydrocarbons from intermediate to deep Paleozoic bedrock aquifers. Such leakage may pose a threat to shallow groundwater aquifers and the surface environment. Cost-effective plugging of these wells requires knowledge of the sources of the leaking fluids. This study characterizes the isotopic compositions (δ18OH2O, δ2HH2O, δ34SSO4, δ18OSO4, δ13CDIC, 87Sr/86Sr, δ37Cl and δ81Br) of groundwaters in the region, which show distinctive differences between bedrock formations, allowing determination of unique ‘fingerprints’ of each formation. The geochemical data also improve our understanding of groundwater origin and evolution. A brackish to saline aquifer system containing dissolved H2S is present at intermediate depths of up to 450m, recharged by down-dip infiltration of meteoric water from shallow fresh water aquifers. At greater depths, a series of confined brine aquifers contain residual evaporated Paleozoic seawater, modified by rock-water interaction and other processes. A Bayesian mixing model, SIAR, was applied to these data to develop a tool for identifying the source(s) of leaking fluids. This model determines the possible range of proportions for each source and the probability distribution therein; hypothetical test mixtures and a few real-world examples indicate that it is able to predict proportions with acceptable accuracy.