Although these changes will profoundly influence groundwater recharge and discharge in mountainous environments (Hayashi, 2019), they have largely been ignored so far (Somers et al., 2019). Since surface water and groundwater resources are closely coupled, an improved understanding of surface water-groundwater interactions is highly relevant for a sustainable water governance as well as for water-dependent ecosystems in mountainous regions (e.g., Holman, 2006;Krause et al., 2014;Schilling et al., 2020).Within the last two decades, studies on river-aquifer exchange dynamics have substantially improved the understanding of the drivers (e.g., river discharge) and controls (e.g., riverbed hydraulic conductivity) of water exchange patterns and their impact on biogeochemical cycling of solutes (e.g., reviews by Boano et al., 2014;Brunner et al., 2017; Lewandowski et al., 2019 and references therein). Particularly, the continued recognition and investigation of riverbed dynamics as key controls on river-aquifer exchange have brought substantial scientific progress in the field of surface water-groundwater interactions (e.g., Mutiti & Levy, 2010;Tang et al., 2018). However, the spatiotemporal dynamics of surface water-groundwater interactions still remain elusive, mainly due to a lack of high-resolution field data (Barthel & Banzhaf, 2016;

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In the Haldimand sector of Gaspé, Québec, Canada, a study was carried out to assess the potential risk on a shallow fractured rock aquifer system due to development of a tight sandstone petroleum reservoir. Petroleum exploration wells are being drilled in the forested core of a hilly 50 km2 peninsula by the sea (up to 200 m amsl) and where local residents rely on groundwater wells for their water supply. The study used existing hydrogeological, geological and petroleum exploration data and more recently acquired field characterization data. Groundwater and surface water sampling within a 2 km radius of a proposed new drill pad. All samples were subject to chemical analyses. Fracturing controls groundwater flow especially in the upper 15 m of the rock aquifer. Recharge occurs on topographic highs where the glacial till cover is thin. Quite wide variations in groundwater geochemistry were encountered. Groundwater residence times can thus be quite long. Methane is of mixed origin but is preferentially associated with the relatively more evolved water types. The SALTFLOW model was used to simulate density-dependent groundwater flow and salt transport within the peninsula as well as the adjacent highlands along a 2D vertical section.

Labeling groundwater by injecting an artificial tracer is a standard and widely used method to study groundwater flow systems. Noble gases dissolved in groundwater are potentially ideal artificial tracers, as they are not subject to biogeochemical transformations, do not adsorb onto the aquifer matrix, are colorless, and have no negative impact on the quality of groundwater resources. In addition, combining different noble-gas species in multi-tracer tests would allow direct analysis of the spatio-temporal heterogeneity of groundwater flow systems. However, while the handling of noble gases is safe and straightforward for injection into groundwater, conventional methods to analyse dissolved noble gases tend to be impractical for groundwater tracer tests. The sampling and subsequent lab-based analysis of dissolved noble gases are laborious, expensive and time intensive. Therefore, only researchers with access to specialized noble-gas labs have attempted such tracer tests. The recently developed gas-equilibrium membrane-inlet mass spectrometers (GE-MIMS) allow efficient on-site analysis of dissolved gases at high temporal resolution. The GE-MIMS instruments thereby eliminate most of the analytical and logistical constraints of conventional lab-based techniques and therefore provide new opportunities for groundwater tests using artificially injected gases. We used a GE-MIMS to systematically test the applicability of He, Kr, and Xe as artificial groundwater tracers. These gas species were injected into groundwater as Dirac-like pulses at three piezometers located at various locations upstream of a pumping well, where dissolved gas concentrations were continuously monitored with the GE-MIMS instrument. The groundwater travel times observed in these tracer tests ranged from a few hours to several weeks, and were consistent with the groundwater flow field at the experimental test site. Travel times determined from the noble gas tracer tests were also consistent with those obtained traditional dye tracers.

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