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The goal of this study is to explain the origin of 234 U– 238 U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3 He/ 4 He ratios, to gain insight regarding the evolution of groundwater in the region. ( 234 U/ 238 U) activity ratios, or ( 234 U/ 238 U) act , were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest ( 234 U/ 238 U) act , 1.14 ± 0.01, was measured in Ca–HCO 3 -type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234 U– 238 U isotopic fractionation. The ( 234 U/ 238 U) act increases to 6.07 ± 0.14 in Na–HCO 3 –Cl-type groundwater. Preferential migration of 234 U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between ( 234 U/ 238 U) act and 3 He/ 4 He ratios has been observed. This relationship reflects the mixing of newly recharged water, with ( 234 U/ 238 U) act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water ( 14 C ages of 6.6 kyrs), with ( 234 U/ 238 U) act of ≥6.07 and large amounts of radiogenic 4 He, in excess of the steady-state amount produced in situ . The simultaneous fractionation of ( 234 U/ 238 U) act and the addition of excess 4 He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4 He and 234 U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4 He excesses measured in groundwater. [ABSTRACT FROM AUTHOR]

One hundred ninety-eight groundwater wells were sampled to measure the 222Rn activity in the region between Montreal and Quebec City, eastern Canada. The aim of this study was to relate the spatial distribution of 222Rn activity to the geology and the hydrogeology of the study area and to estimate the potential health risks associated with 222Rn in the most populated area of the Province of Quebec. Most of the groundwater samples show low 222Rn activities with a median value of 8.6 Bq/L. Ninety percent of samples show 222Rn activity lower than 100 Bq/L, the exposure limit in groundwater recommended by the World Health Organization. A few higher 222Rn 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 222Rn 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. 226Ra, the parent element of 222Rn could be easily adsorbed on clays, creating a favorable environment for the production and release of 222Rn 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. [ABSTRACT FROM AUTHOR]

Abstract: Brines in Cambrian sandstones and Ordovician dolostones of the St-Lawrence Lowlands at Bécancour, Québec, Canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path, in addition to quantifying their residence time. Major ion chemistry indicates that the brines are of Na–Ca–Cl type, possibly derived from halite dissolution. 87Sr/86Sr ratios and Ca excess indicate prolonged interactions with silicate rocks of the Proterozoic Grenville basement or the Cambrian Potsdam sandstone. The brines constrain a 2–3% contribution of mantle 3He and large amounts of nucleogenic 21Ne∗ and 38Ar∗ and radiogenic 4He and 40Ar∗. 4He/40Ar∗ and 21Ne∗/40Ar∗ ratios, corrected for mass fractionation during incomplete brine degassing, are identical to their production ratios in rocks. The source of salinity (halite dissolution), plus the occurrence of large amounts of 40Ar∗ in brines constrain the residence time of Bécancour brines as being older than the Cretaceous. Evaporites in the St-Lawrence Lowlands likely existed only during Devonian–Silurian time. Brines might result from infiltration of Devonian water leaching halite, penetrating into or below the deeper Cambrian–Ordovician aquifers. During the Devonian, the basin reached temperatures higher than 250°C, allowing for thermal maturation of local gas-prone source rocks (Utica shales) and possibly facilitating the release of radiogenic 40Ar∗ into the brines. The last thermal event that could have facilitated the liberation of 40Ar∗ into fluids and contributed to mantle 3He is the Cretaceous Monteregian Hills magmatic episode. For residence times younger than the Cretaceous, it is difficult to find an appropriate source of salinity and of nucleogenic/radiogenic gases to the Bécancour brines. [Copyright &y& Elsevier]

Abstract: The concentration and the isotopic ratios of noble gases He, Ne, Ar, Kr and Xe were measured in porewater trapped in shallow sediments of the estuary of the St-Lawrence River, Québec, Canada. The gases are atmospheric in origin but most samples have gas concentrations 1.7–28 times higher than those expected in solution in water at equilibrium with the atmosphere. Elemental fractionation of heavier noble gases Kr and Xe compared to Ar strongly suggests that noble gases were adsorbed on sediments or organic matter and then desorbed into porewaters due to depressurization, as collected samples were brought to the surface. Atmospheric Ar in porewater is used as a reference to measure the N2-fluxes at the water–sediment interface. Ignoring the Ar enrichments observed in porewater could lead to a severe underestimation of the denitrification rate in oceans and estuaries. [Copyright &y& Elsevier]

The assessment of groundwater quality in shallow aquifers is of high societal relevance given that large populations depend directly on these water resources. The purpose of this study was to establish links between groundwater quality, groundwater residence times, and regional geology in the St. Lawrence Lowlands fractured bedrock aquifer. The study focuses on a 4500 km 2 watershed located in the St. Lawrence Lowlands of the province of Quebec in eastern Canada. A total of 150 wells were sampled for major, minor, and trace ions. Tritium ( 3 H) and its daughter element, 3 He, as well as radiocarbon activity (A 14 C) were measured in a subset of wells to estimate groundwater residence times. Results show that groundwater evolves from a Ca–HCO 3 water type in recharge zones (i.e., the Appalachian piedmont) to a Na–HCO 3 water type downgradient, toward the St. Lawrence River. Locally, barium (Ba), fluoride (F), iron (Fe), and manganese (Mn) concentrations reach 90, 2, 18, and 5.9 mg/L respectively, all exceeding their respective Canadian drinking water limits of 1, 1.5, 0.3, and 0.05 mg/L. Release of these elements into groundwater is mainly controlled by the groundwater redox state and pH conditions, as well as by the geology and the duration of rock–water interactions. This evolution is accompanied by increasing 3 H/ 3 He ages, from 4.78 ± 0.44 years upgradient to more than 60 years downgradient. Discrepancies between calculated 3 H/ 3 He and 14 C water ages (the latter ranging from 280 ± 56 to 17,050 ± 3410 years) suggest mixing between modern water and paleo-groundwater infiltrated through subglacial recharge when the Laurentide Ice Sheet covered the study area, and during the following deglaciation period. A linear relationship between 3 H activity and corrected 14 C versus Mg/Ca and Ba support a direct link between water residence time and the chemical evolution of these waters. The Ba, F, Fe, and Mn concentrations in groundwater originate from Paleozoic rocks from both the St. Lawrence Platform and the Appalachian Mountains. These elements have been brought to the surface by rising hydrothermal fluids along regional faults, and trapped in sediment during their deposition and diagenesis due to reactions with highly sulfurous and organic matter-rich water. Large-scale flow of meltwater during subglacial recharge and during the subsequent retreat of the Laurentide Ice Sheet might have contributed to the leaching of these deposits and their enrichment in the present aquifers. This study brings a new and original understanding of the St. Lawrence Lowlands groundwater system within the context of its geological evolution. [ABSTRACT FROM AUTHOR]

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 δ13C 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 δ13C 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. [ABSTRACT FROM AUTHOR]