Isotopic fingerprinting of groundwaters in southwestern Ontario: Applications to abandoned well remediation

• We created a tool to predict sources of groundwaters leaking from abandoned wells.
• Each of the main aquifers were characterized, for a total suite of over 130 samples.
• 8 isotopes (δ18O, δ2H, δ34SSO4, δ18OSO4, δ13C, 87Sr/86Sr, δ37Cl and δ81Br) were analyzed.
• Our tool well-predicted fluid origins in different real and hypothetical scenarios.
• Leaking fluids from wells sampled so far were from a shallow Devonian aquifer.

Southwestern Ontario has a legacy of unplugged oil and gas wells drilled in the late 1800s and early 1900s before the advent of government regulatory controls. A number of these wells exhibit artesian flow of salty and/or sulphurous water at the surface, creating a liability to landowners and a possible threat to potable groundwater aquifers and the surface environment. Cost-effective plugging of these wells is hindered by incomplete drilling records regarding well depths, well construction, and geologic information on the sources of the leaking fluids. This study seeks to fill this knowledge gap by characterizing distinct geochemical ‘fingerprints’ for each of the major bedrock water-producing zones that may be the possible sources of the leaking fluids. To this end, over 130 groundwater samples were collected and analyzed for a broad suite of isotopic parameters (δ18O, δ2H, δ34SSO4, δ18OSO4, δ13CDIC, 87Sr/86Sr, δ37Cl and δ81Br), and this dataset was combined with available data from previous studies in the area. A Bayesian mixing model, SIAR, was applied to these data to develop a statistical tool for identifying the probable source(s) of leaking fluids. Several hypothetical samples and one real-world example are presented here to demonstrate the model’s performance.