Establishing baseline concentration and δ¹³C signature of methane in shallow ground waters of the St. Lawrence Lowlands, QC, Canada: A tool for determining shale gas contaminationud

摘要

With the increased interest in non-conventional energy sources, major environmental questions are being raised concerning the possible impacts of shale gas exploitation on the quality of ground waters. The extraction of shale gas is done by hydraulic fracturing, which utilizes large volumes of water and fracturing fluids to break the source rocks that entrap gases such as methane, ethane and propane. Because it is uncertain whether these fracturing fluids and gases can contaminate shallow ground waters, it is important to assess the baseline concentration of these gases before hydraulic fracturing activities are initiated to be able to determine the source of future ground water contaminations. In this work, we measured the natural concentrations and sources of hydrocarbons dissolved in ground waters of the St. Lawrence Lowlands, QC, Canada, in 130 ground water samples collected from private (n=81), municipal (n=34) and observation (n=15) wells. Localized pools of high concentrations of methane of mostly bacterial origin (biogenic) were found throughout the study area. Three samples showed high concentrations of methane bearing a δ¹³C signature specific to deep sources (thermogenic) but a gas wetness ratio (C1/(C2+C3)) falling outside the thermogenic gas window. This result suggests mixing with gas from other sources, migration through the bedrock, or partial oxidation of the gas. The results obtained in this study will be used as a reference for future research projects and monitoring activities on the impact of shale gas exploration and exploitation on ground water quality.