Vertical profiles of the concentration and isotopic composition (delta C-13) of methane (CH4) and dissolved inorganic carbon (DIC), as well as of ancillary parameters, were obtained in the top 25 cm of a sediment column in a seasonally anoxic basin from an oligotrophic boreal lake. Modeling the profiles of CH4 and DIC concentrations and those of their delta C-13 signatures with reaction-transport equations allowed us to determine the organic matter (OM) degradation rates according to various reactions and to constrain the in situ isotopic fractionation factors and diffusivity coefficients of CH4 and DIC. This exercise reveals inter alia that (i) CH4 production occurs below a depth of 5 cm, with the highest production rate between 5 and 7.5 cm depth, (ii) all CH4 is produced through hydrogenotrophy, and (iii) methanogenesis yields a production rate of CH4 about three times greater than that of DIC. This latter observation indicates either that fermentation of OM is not the exclusive source of H-2 sustaining hydrogenotrophy, or that the commonly assumed model molecule CH2O does not adequately represent the fermenting OM, since its fermentation yields identical rates of CH4 and DIC production. The porewater profiles of Fe and SO42- suggest that some H-2 may be produced during the reoxidation of reduced sulfur by Fe(III), but the rate of H-2 production via this process, if active, would be insignificant in comparison to that required to sustain the estimated rate of hydrogenotrophy. We deduce that the imbalance between CH4 and DIC production rates is rather due to the fermentation of organic substrates that are more reduced than CH2O, i.e., having a negative average carbon oxidation state (COS). From the constraints on reaction rates and on fermentation pathways imposed by the delta C-13 data, we infer that the organic substrate fermenting between depths of 5 and 7.5 cm should have a COS of -1.87. We thus submit that CH4 is produced in the sediments of the seasonally anoxic basin of our boreal lake through hydrogenotrophy coupled to the fermentation of reduced organic substrates that can be represented by a mixture of fatty acids (e.g. C16H32O2; COS of -1.75) and fatty alcohols (e.g., C(16)H(3)4O; COS of -2.00). This study emphasizes the importance of characterizing the sedimentary OM undergoing mineralization in order to improve diagenetic model predictions of CH4 cycling in boreal lakes and of its significance in climate change. (C) 2018 Elsevier Ltd. All rights reserved.

The increasing number of studies on the determination of natural methane in groundwater of shale gas prospection areas offers a unique opportunity for refining the quantification of natural methane emissions. Here methane emissions, computed from four potential sources, are reported for an area of ca. 16,500km(2) of the St. Lawrence Lowlands, Quebec (Canada), where Utica shales are targeted by the petroleum industry. Methane emissions can be caused by 1) groundwater degassing as a result of groundwater abstraction for domestic and municipal uses; 2) groundwater discharge along rivers; 3) migration to the surface by (macro- and micro-) diffuse seepage; 4) degassing of hydraulic fracturing fluids during first phases of drilling. Methane emissions related to groundwater discharge to rivers (2.47×10(-4) to 9.35×10(-3)Tgyr(-1)) surpass those of diffuse seepage (4.13×10(-6) to 7.14×10(-5)Tgyr(-1)) and groundwater abstraction (6.35×10(-6) to 2.49×10(-4)Tgyr(-1)). The methane emission from the degassing of flowback waters during drilling of the Utica shale over a 10- to 20-year horizon is estimated from 2.55×10(-3) to 1.62×10(-2)Tgyr(-1). These emissions are from one third to sixty-six times the methane emissions from groundwater discharge to rivers. This study shows that different methane emission sources need to be considered in environmental assessments of methane exploitation projects to better understand their impacts.

Hydraulic fracturing is becoming an important technique worldwide to recover hydrocarbons from unconventional sources such as shale gas. In Quebec (Canada), the Utica Shale has been identified as having unconventional gas production potential. However, there has been a moratorium on shale gas exploration since 2010. The work reported here was aimed at defining baseline concentrations of methane in shallow aquifers of the St. Lawrence Lowlands and its sources using δ(13)C methane signatures. Since this study was performed prior to large-scale fracturing activities, it provides background data prior to the eventual exploitation of shale gas through hydraulic fracturing. Groundwater was sampled from private (n = 81), municipal (n = 34), and observation (n = 15) wells between August 2012 and May 2013. Methane was detected in 80% of the wells with an average concentration of 3.8 ± 8.8 mg/L, and a range of <0.0006 to 45.9 mg/L. Methane concentrations were linked to groundwater chemistry and distance to the major faults in the studied area. The methane δ(1)(3)C signature of 19 samples was > -50‰, indicating a potential thermogenic source. Localized areas of high methane concentrations from predominantly biogenic sources were found throughout the study area. In several samples, mixing, migration, and oxidation processes likely affected the chemical and isotopic composition of the gases, making it difficult to pinpoint their origin. Energy companies should respect a safe distance from major natural faults in the bedrock when planning the localization of hydraulic fracturation activities to minimize the risk of contaminating the surrounding groundwater since natural faults are likely to be a preferential migration pathway for methane.

One hundred ninety-eight groundwater wells were sampled to measure the (222)Rn activity in the region between Montreal and Quebec City, eastern Canada. The aim of this study was to relate the spatial distribution of (222)Rn activity to the geology and the hydrogeology of the study area and to estimate the potential health risks associated with (222)Rn in the most populated area of the Province of Quebec. Most of the groundwater samples show low (222)Rn activities with a median value of 8.6 Bq/L. Ninety percent of samples show (222)Rn activity lower than 100 Bq/L, the exposure limit in groundwater recommended by the World Health Organization. A few higher (222)Rn activities (up to 310 Bq/L) have been measured in wells from the Appalachian Mountains and from the magmatic intrusion of Mont-Saint-Hilaire, known for its high level of indoor radon. The spatial distribution of (222)Rn activity seems to be related mainly to lithology differences between U-richer metasediments of the Appalachian Mountains and magmatic intrusions and the carbonaceous silty shales of the St. Lawrence Platform. Radon is slightly enriched in sodium-chlorine waters that evolved at contact with clay-rich formations. (226)Ra, the parent element of (222)Rn could be easily adsorbed on clays, creating a favorable environment for the production and release of (222)Rn into groundwater. The contribution of groundwater radon to indoor radon or by ingestion is minimal except for specific areas near Mont-Saint-Hilaire or in the Appalachian Mountains where this contribution could reach 45% of the total radioactive annual dose.

An academic consortium from UQAM, Geotop, Concordia University, INRS-ETE and Laval University has initiated a study as part of the strategic environmental assessment of shale gas production from the Utica Shale in the St. Lawrence Lowlands. The objective of the study is to contribute objective data on 1) the presence of light hydrocarbons (methane and alkanes) in groundwater, 2) the origin of these hydrocarbons (bio- or thermogenic), and 3) migration mechanisms of these gases through gas well casings. These data will be provided through geochemical and numerical modeling methods. Here we will focus on the first two objectives of the project, whereas the study related to the third objective is the object of another communication at this conference (Nowanooz et al.). Over a hundred private and public wells were sampled in 2012 south of the St. Lawrence River between Montreal and Quebec City. The area corresponds to the most prospective corridor for Utica shale gas production, which is where several vertical and horizontal exploration wells have been drilled, some of which were fracked. The area includes the Richelieu, Yamaska, Nicolet, Saint-Francois and Becancour River basins, between the piedmont of the Appalachians and the St. Lawrence River. Water was analyzed for light n-alkane concentrations (CH4, C2H6 and C3H8) and carbon isotopic composition (d13C) as well a for noble gas isotope ratios (4He, 40Ar and 222Rn). Methane isotopic composition is useful to distinguish biogenic methane produced within the Quaternary deposits from thermogenic methane originating from the deeper Lorraine or the Utica shales. To distinguish between the two shale formations, the d13C values of heavier alkanes (C2H6 and C3H8) are also measured to quantify the isotopic rollover (heavier gases show lighter d13C values) observed in this area as in other shale gas production regions. Noble gases, particularly 4He, are useful tracers of gas migration and cross-formational fluid flow. 222Rn is a radioactive element of the U-decay chain useful to discriminate between local sources of radiogenic 4He from deeper sources (for which Rn and He should be decoupled). In this communication, the strategies used for this environmental study are presented along with results on the spatial distribution of hydrocarbons and associated noble gases in the region, which will be detailed in two other communications in this conference (Moritz et al.; Pinti et al.).