Can groundwater sampling techniques used in monitoring wells influence methane concentrations and isotopes?
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Methane concentrations and isotopic composition in groundwater are the focus of a growing number of studies. However, concerns are often expressed regarding the integrity of samples, as methane is very volatile and may partially exsolve during sample lifting in the well and transfer to sampling containers. While issues concerning bottle-filling techniques have already been documented, this paper documents a comparison of methane concentration and isotopic composition obtained with three devices commonly used to retrieve water samples from dedicated observation wells. This work lies within the framework of a larger project carried out in the Saint-Édouard area (southern Québec, Canada), whose objective was to assess the risk to shallow groundwater quality related to potential shale gas exploitation. The selected sampling devices, which were tested on ten wells during three sampling campaigns, consist of an impeller pump, a bladder pump, and disposable sampling bags (HydraSleeve). The sampling bags were used both before and after pumping, to verify the appropriateness of a no-purge approach, compared to the low-flow approach involving pumping until stabilization of field physicochemical parameters. Results show that methane concentrations obtained with the selected sampling techniques are usually similar and that there is no systematic bias related to a specific technique. Nonetheless, concentrations can sometimes vary quite significantly (up to 3.5 times) for a given well and sampling event. Methane isotopic composition obtained with all sampling techniques is very similar, except in some cases where sampling bags were used before pumping (no-purge approach), in wells where multiple groundwater sources enter the borehole.
KeywordsGroundwater Sampling techniques Dissolved methane Shale gas Monitoring
The authors would like to thank Dr. Mathieu Duchesne of the GSC and Pr. Erwan Gloaguen of INRS for their advices and contribution related to the representation of data with Matlab. Our gratitude goes out to Mrs. Marianne Molgat, formerly of Talisman Energy, without whom this project would likely not have taken place. We would also like to deeply thank the Ministère du Développement durable, de l’Environnement et de la Lutte contre les Changements climatiques (MDDELCC), land and well owners that allowed work to be performed on their property, the Municipality of Saint-Édouard, the MRC de Lotbinière and the Ministère des Forêts, de la Faune et des Parcs du Québec. The authors also want to sincerely thank Nicolas Benoit and the anonymous reviewer for their review. This paper is GSC contribution # 20170288.
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