Prospects and Limitations of Chemical and Isotopic Groundwater Monitoring to Assess the Potential Environmental Impacts of Unconventional Oil and Gas Development

With the advent of shale oil and shale gas development facilitated by hydraulic fracturing it has become increasingly important to develop tracer tools to scientifically determine potential impacts of stray gases, formation fluids, or fracturing chemicals on shallow aquifers. Based on a multi-year monitoring program conducted in Alberta (Canada), we demonstrate that a multi-isotope approach (δ²H_H2O; δ¹⁸O_H2O; δ¹³C_CH4; δ²H_CH4; δ¹³C_C2H6) in concert with chemical analyses is highly capable of identifying potential contamination of shallow aquifers with stray gases or saline fluids from intermediate or production zones, provided that sufficient baseline data have been collected. At baseline conditions, we found that methane in shallow groundwater in southern Alberta is ubiquitous and predominantly of biogenic origin. Novel approaches of in-situ concentration and isotope measurements for methane during drilling of a 530 m deep well yielded a mud-gas profile characterizing natural gas occurrences in the intermediate zone. The assembled data set provides evidence that potential stray gas contamination by isotopically distinct deeper thermogenic gases from the intermediate or from production zones can be effectively detected by suitable monitoring programs.