Origin, fate, and transport of dissolved organic gases in bedrock aquitards; Saskatchewan, Canada

Abstract

Dissolved gases including methane (CH4), ethane (C2H6), and propane (C3H8) are natural components of groundwater. Knowledge of the origin, distribution, and fate of these gases is crucial for assessing the impacts of oil and gas development on shallow groundwater systems. This study provides one of the first suites of baseline profiles of dissolved CH4, C2H6, and C3H8 through a bedrock aquitard and provides insight into their origin (using carbon (C) and hydrogen (H) stable isotopes), fate, and transport. Core samples were collected over 240 m of continuously-cored Quaternary-aged Battleford till (80 m) and underlying Cretaceous units (160 m) from an aquitard research site 140 km southwest of Saskatoon, SK, Canada. Physical properties (moisture content, density, grain size and carbon forms), stable H and oxygen isotopes of pore water (d2H and d18O), pore water chemistry, and gas concentrations (CH4, C2H6, and C3H8) and their stable C and H isotope values (d13CCH4, d2HCH4, d13CC2H6, and d13CC3H8) were measured on these cores. Results of physical parameters and d2H and d18O measurements agreed with previously reported data. The high-resolution profiles extend existing data sets into the Ardkenneth aquifer, the upper layer of which contains high percentages of sand and abrupt increases in values of d2H, and chloride (Cl-) and CH4 concentrations; these peaks were likely introduced laterally from an alternate source. One-dimensional numerical modeling of the d2H confirms that transport across the till-shale interface occurred over ~20-30 ka Before Present (BP), and from the base of the oxidized till (~5 m below ground surface, BGS) into the till over ~10 ka BP. CH4 concentrations through the till were below detection limit (BDL; < 5 ppmv); concentrations increase from 0.04 mg L-1 below the till-clay interface (at ~85 m BGS), to 49 mg L-1 at 240 m BGS. One-dimensional numerical modeling of the CH4 concentration profile suggests that the timing of deposition of the till cannot be constrained using CH4, and that a geochemical reaction impacts its transport to ~30 m below the till-clay interface. Concentrations of C2H6 and C3H8 are minor throughout the profile. Depleted 13C and 2H values of CH4, ranging between –52 and –92‰ VPDB and –173 to –269‰ VSMOW, respectively, and the 13C-depleted nature of d13CC2H6 (–45 to –85‰ VPDB) support a bacterial origin for the gases.