Temporal variations of methane concentration and isotopic composition in groundwater of the St. Lawrence Lowlands, eastern Canada

Abstract

Dissolved methane concentrations in shallow groundwater are known to vary both spatially and temporally. However, the extent of these variations is poorly documented and this knowledge is critical for distinguishing natural fluctuations from anthropogenic impacts stemming from oil and gas activities. This issue was addressed as part of a groundwater research project aiming to assess the risk of shale gas development for groundwater quality over a 500 km 2 area in the St. Lawrence Lowlands (Quebec, Canada). A specific study was carried out to define the natural variability of methane concentrations and carbon and hydrogen isotope ratios in groundwater, as dissolved methane is naturally ubiquitous in 2 aquifers of this area. Monitoring was carried out over a period of up to 2.5 years in seven monitoring wells. Results showed that for a given well, using the same sampling depth and technique, methane concentrations can vary over time from 2.5 to 6 times relative to the lowest recorded value. Methane isotopic composition, which is a useful tool to distinguish gas origin, was found to be stable for most wells, but varied significantly over time in the two wells where methane concentrations are the lowest. The use of concentration ratios, as well as isotopic composition of methane and dissolved inorganic carbon (DIC) helped unravel the processes responsible for these variations. This study indicates that both methane concentrations and isotopic composition, as well as DIC isotopes, should be regularly monitored over at least one year to establish their potential natural variations prior to hydrocarbon development.

Geochemistry of groundwater in the Saint-Édouard area, Quebec, Canada, and its influence on the distribution of methane in shallow aquifers

“…Both methanogenic pathways result in an increase in the DIC concentration and δ 13 C-DIC of the carbon pool (Clark and Fritz, 1997). If the groundwater system is closed or significantly restricted in terms of input of recent carbon, the δ 13 C-DIC values can increase past 0‰ and values as high as +25-30‰ have been observed (Sharma and Baggett, 2011;Martini et al, 1998;Rivard et al, 2017a). Generally, values above +10‰ are considered to unequivocally show the occurrence of methanogenesis (Sharma and Baggett, 2011).…”

“…To better understand the origin of the methane found in groundwater, the scientific community is widely calling for baseline groundwater characterization of targeted regions before unconventional hydrocarbon exploitation begins (Jackson et al, 2013b;Vengosh et al, 2014), although there has not yet been a consensus on the extent, methods, or duration/frequency of sampling involved in such baseline studies. Without such proper baseline studies, it could be very difficult to conclude whether groundwater quality has been impacted by unconventional hydrocarbon development activities, especially since methane can be naturally present in shallow groundwater of many regions, and because natural methane concentrations can vary greatly in space and time (Rivard et al, 2017a). The Upper Ordovician Utica Shale found in the St. Lawrence Lowlands (southern Quebec, eastern Canada), is known to contain important quantities of thermogenic gas (Chen et al, 2014).…”

A multi-isotope approach to determine the origin of methane and higher alkanes in groundwater of the St. Lawrence Platform, Saint-Édouard area, eastern Canada

“…Results reported in this paper correspond to the geometrical mean of all samples collected from a given sampling point. Obtaining multiple samples over time should be part of any baseline study, as both the methane concentrations and isotopic composition can vary over time, with some wells varying very little, and others exhibiting major variations (Rivard et al, 2018b).…”

Advancing knowledge of gas migration and fugitive gas from energy wells in northeast British Columbia, Canada

“…Unfortunately, few mandatory predevelopment groundwater characterization programs are conducted in regions of intense energy resource development prior to drilling. A key exception to this is the work performed in Quebec, Canada where a moratorium on development is currently in place . Nonetheless, natural subsurface and groundwater methane conditions in regions of historic, ongoing or planned development remain poorly understood.…”

“…A key exception to this is the work performed in Quebec, Canada where a moratorium on development is currently in place. [49][50][51] Nonetheless, natural subsurface and groundwater methane conditions in regions of historic, ongoing or planned development remain poorly understood.…”