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 deep Paleozoic bedrock aquifers. Such leakage may pose a threat to shallow groundwater and the 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) of groundwaters in the region, which are distinct in different bedrock formations. A Bayesian mixing model was applied to these data to develop a tool for identifying the source(s) of leaking fluids. The geochemical data also improve our understanding of groundwater origin and evolution. Shallow (~\u3c350m) aquifers are recharged by recent meteoric water. At greater depths, brine aquifers contain residual evaporated Paleozoic seawater, modified by rock-water interaction and mixing with meteoric water. These brines are likely related to long-distance fluid migration from deeper portions of the adjacent Michigan and Appalachian basins