Physical and hydrogeochemical investigation of fractured bedrock aquifers by thermal logging and sequential sampling, Quebec, Canada

Guillaume Meyzonnat – 2020

This research concerns the vertical physical and the hydrogeochemical investigation of carbonate fractured aquifers over a depth of 100 m. In Quebec, the exploitation of fractured aquifers ensures the water supply of more than a third of the municipalities and nearly 80% of the population in rural areas from individual wells. Since 2008, major investigation campaigns have been conducted in Quebec and have already led to the acquisition of remarkable knowledge on regional hydrogeological and hydrogeochemical systems. In relation with this momentum, the objective of this research is to detail the heterogeneity of aquifers at the scale of boreholes, and to identify, if occurring, the stratification of water bodies according to the depth. The research conducted includes a first physical component focusing on temperature surveys. The second component involves the development of passive samplers dedicated to the sequential collecting of hydrogeochemical data. The coupling of these two methods enables to draw a portrait of the vertical stratification of fractured aquifers located in the St. Lawrence Lowlands region (Potsdam, Beekmantown and Trenton Groups) as well as some wellbores in the Appalachian’s range. A total of 21boreholes ranging from 22 to 128 m depth were investigated by high-resolution temperature logging. Fifteen (15) of these wells also included spinner flowmeter and optical surveys and solicited by pumping. Nine (9) among twenty-one (21) wellbores investigated were influenced by natural passive flow or by nearby municipal pumping. Ten wells (18) were investigated for hydrogeochemistry. Twenty-nine (29) samples were sequentially collected for major ions and stable isotopes of water, seventeen (17) for tritium, thirteen (13) for 14C and 13C, fifteen (15) for noble gases, and four (4) for SF6 and CFC-12. Borehole logging results show a large heterogeneity in the distribution of fractures and their transmissivity. Eighty percent (80%) of the productive zones identified (n=54 for 15 boreholes) are discrete fractures or conduits, with apparent apertures ranging from 1 to 40 cm. The transmissivity of the fractures is very uneven (1.9 10-2 to 2.2 10-6 m2/s, harmonic mean 2.6 10-5 m2/s) and show no discernible trend according to lithology or depth. Thermal investigations under dynamic conditions have identified that half of the fracture drain aquifer horizons deeper than the fractures themselves, and the other half likely drain water from the surface or shallower horizons. Investigations regarding the calco-carbonic system reports the dominance of carbonates weathering. However, these “modern” waters showed rather limited evolution in a closed system conditions towards CO2. Isotopic exchange between DIC and carbonate matrix and cation exchange are evident and still testifies about a certain evolution of groundwater within the aquifers. Highest water mineralization relates to the presence of highly soluble materials such as sulfate from gypsum dissolution and saline inputs. Groundwater dating reports the prevalence of “modern” water bodies (i.e. 3H/3He and SF6 ages ranges from 16 to 44 years). Half of these water bodies would have recharged post-1960s, and the other half would be a binary mixture with proportions of up to 75% of pre-1960s waters. Interpretation regarding 14C ages does not report significant contribution of “old” water bodies, especially when the wellbores are influenced by nearby municipal pumping which are modifying groundwater flows apart from their natural dynamic. “Modern” residence times do not appear to influence the composition (water type and total mineralization) of the groundwater. Little or no variation of mineral composition is observed towards the depth, and when it does occur, is essentially site dependent and related to the presence of soluble minerals (sulfate and saline inputs). The data collected report heterogeneous distribution of the productive zones according to depth. Such “modern” water bodies are related to relatively “quick” flows, enhanced by nearby pumping whatever the depth, suggesting that the fracture networks do not induce apparent confining conditions, or at least, it is not apparent regarding water composition and residence time. These “modern” and mixed flows (pre/post 1960s) do not follow evident vertical stratification of water masses over a range of between 0 to 100 m depth for the wells investigated.