High resolution spatial and temporal evolution of dissolved gases in groundwater during a controlled natural gas release experiment

Fugitive gas comprised primarily ofmethane (CH4) with traces of ethane and propane ( collectively termed C1-3) may negatively impact shallow groundwater when unintentionally released from oil and natural gas wells. Currently, knowledge of fugitive gas migration, subsurface source identification and oxidation potential in groundwater is limited. To advance understanding, a controlled release experiment was performed at the Borden Research Aquifer, Canada, whereby 51 m(3) of natural gas was injected into an unconfined sand aquifer over 72 dayswith dissolved gasesmonitored over 323 days. During active gas injection, a dispersed plume of dissolved C1-3 evolved in a depth discrete and spatially complex manner. Evolution of the dissolved gas plume was driven by free-phase gas migration controlled by small-scale sediment layering and anisotropy. Upon cessation of gas injection, C1-3 concentrations increased to the greatest levels observed, particularly at 2 and 6 m depths, reaching up to 31.5, 1.5 and 0.1 mg/L respectively before stabilizing and persisting. At no time did groundwater become fully saturated with natural gas at the scale of sampling undertaken. Throughout the experiment the isotopic composition of injected methane ( delta C-13 of -42.2%) and the wetness parameter ( i.e. the ratio of C-1 to C2+) constituted excellent tracers for the presence of fugitive gas at concentrations > 2 mg/L. Atdiscrete times C1-3 concentrations varied by up to 4 orders of magnitude over 8 m of aquifer thickness (e.g. from < 0.01 to 30 mg/L for CH4), while some groundwater samples lacked evidence of fugitive gas, despite being within 10mof the injection zone. Meanwhile, carbon isotope ratios of dissolved CH4 showed no evidence of oxidation. Our results show that while impacts to aquifers from a fugitive gas event are readily detectable at discrete depths, they are spatially and temporally variable and dissolved methane has propensity to persist. (c) 2017 Elsevier B.V. All rights reserved.