Assessing patterns of dissolved methane in shallow aquifers related to Carboniferous and Triassic sedimentary basins, Nova Scotia, Canada

Abstract

The study examines patterns of groundwater methane in shallow aquifers located in Carboniferous and Triassic sedimentary basins in Nova Scotia to improve our understanding of the factors influencing the observed distribution. A combined total of over 800 dissolved methane samples were collected from water wells during surveys conducted in 1975 and 2013. Statistical analyses of the methane data did not detect a significant difference between groupings of methane concentrations for aquifer type, bedrock group, and distance to wetlands. A significant difference, however, was observed between sedimentary basins and bedrock formations, which was largely attributed to localized higher methane concentrations found in the Stellarton Formation/basin compared to other on-shore sedimentary basins of the province. A significant difference was also found between groupings of methane data based on the distance to major stream systems, which was used to indicate topographic position (i.e., valley vs. upslope). The low sample density and percentage of detectable methane concentrations, and the multiple sources of dissolved methane in shallow groundwater in sedimentary basins made it difficult to detect and interpret statistical and spatial trends. Geochemical classification indicates that elevated dissolved methane in well water is associated with sodium dominated groundwater. RÉSUMÉL' étude porte sur les schémas de méthane dans les eaux souterraines des aquifères peu profonds des bassins sédimentaires du Carbonifère et du Trias en Nouvelle-Écosse, afin d'améliorer notre compréhension des facteurs qui influent sur la distribution observée. En tout, plus de 800 échantillons de méthane dissous ont été recueillis dans des puits d' eau lors de levés réalisés en 1975 et en 2013. Les analyses statistiques des données sur le méthane ne révèlent pas une différence importante entre les groupes de concentrations de méthane par type d'aquifère, par groupe de substratum rocheux ou par distance des terres humides. On a toutefois observé une différence importante entre les bassins sédimentaires et les formations de substratum rocheux, qui a été en grande partie attribuée aux concentrations de méthane localisées plus élevées dans la formation et le bassin de Stellarton par comparaison avec d'autres bassins sédimentaires terrestres de la province. On a également découvert une différence importante entre les groupes de données sur le méthane fondés sur la distance des principaux chevelus hydrographiques, que l' on a utilisée pour indiquer la position topographique (vallée ou hauteurs). La faible densité des échantillons et le faible pourcentage de concentrations de méthane détectables, ainsi que les multiples sources de méthane dissous dans les eaux souterraines peu profondes des bassins sédimentaires, ont rendu difficiles la détection et l'interprétation des tendances statistiques et spatiales. La classification géochimique indique que les taux élevés de méthane dissous dans les eaux des puits sont associés aux eaux souterraines à conc...

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

“…These studies, and many others, have investigated the relationship between dissolved methane in groundwater and other factors such as topography, geological units, groundwater geochemical type, and distance from a conventional or unconventional gas well in regions where exploitation is already occurring. Conclusions vary widely between studies, but factors that have commonly been associated with higher methane concentrations include 1) proximity to gas wells (Jackson et al, 2013;Osborn et al, 2011b;contradicted by Siegel et al, 2015a), 2) topographic lows (Harkness et al, 2017;Kennedy and Drage, 2015;Molofsky et al, 2016;Molofsky et al, 2013), 3) water wells drilled within or just above shallow organic-rich shale or coal-bed units, which provide substrates for microbial methane production (Humez et al, 2016a;McIntosh et al, 2014;Molofsky et al, 2013), and 4) geochemically evolved groundwater types such as Na-HCO3 and Na-Cl (Harkness et al, 2017;Kennedy and Drage, 2015;McIntosh et al, 2014;McMahon et al, 2017;McPhillips et al, 2014;Molofsky et al, 2016;Molofsky et al, 2013;Warner et al, 2012b).…”

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

“…This context (i.e. a period of a few years without oil and gas activities), has provided an opportunity to carry out environmental scientific studies that shed light on the environmental issues related to unconventional hydrocarbon development (Considine et al, 2012;Kennedy and Drage, 2015;Loomer et al, 2016;Moritz et al, 2015;Nowamooz et al, 2015;Pinti et al, 2013;Siegel et al, 2016).…”

Shallow groundwater temperature patterns revealed through a regional monitoring well network

“…Issues related to groundwater quality have been previously documented in Nova Scotia due to the weathering of soil and rock containing naturally occurring groundwater contaminants such as arsenic (Kennedy & Drage, 2016), with other more localized concerns regarding saltwater intrusion (Beebe, 2011). In recent years, portions of Nova Scotia have also experienced severe drought conditions due to rainfall deficits resulting in groundwater shortages for homes relying on dug wells (Kennedy et al, 2017). Nova Scotia has many rivers with water temperatures that occasionally exceed critical thresholds for brook trout, Atlantic salmon and other important cold‐water aquatic species (e.g.…”