Groundwater recharge, flow and stable isotope attenuation in sedimentary and crystalline fractured rocks: Spatiotemporal monitoring from multi-level wells

Abstract

The complexities of fractured rock systems necessitate the use of multi-level monitoring wells for the measurement of isotopic tracers with depth. These methods, however, are rarely employed in groundwater flow studies, particularly involving temporal isotope measurements in multiple rock types. To understand the use of isotopic snowmelt tracers for exploring surface connections and recharge, an investigation was undertaken at three geologically-distinct field sites in south-eastern Ontario, Canada. Results of constant head injection tests at 2-m intervals were used to identify and isolate high transmissivity zones (2-3 in each well) for the construction of eight multi-level wells (22 intervals) for groundwater sampling. Deuterium and oxygen-18 were measured in rain (n=64), snow (n=67), soil water (n=118) and groundwater (n=594) on a bi-weekly to monthly basis over two seasonal snowmelt cycles (> 1.5 years). In combination with stable isotopes, measurements of hydraulic head, groundwater temperature, specific conductance and two rounds of tritium sampling contributed to the development of a conceptual model for snowmelt recharge and flow in sandstone, limestone and crystalline rock. Seasonal variation of ẟ 2 H and ẟ 18 O was observed in all rock types indicating recent recharge and strong surface connections. The dampening and residence time of the snowmelt tracer increased with depth in crystalline rock. Attenuation was highest in higherporosity sedimentary rock and strong confining conditions were observed to inhibit recent recharge.Substantial isotopic attenuation measured in the overburden indicated that the strong snowmelt signal measured in groundwater stems from water infiltrating through areas with thin or no overburden.Identification of the periodic trends and distinct isotopic horizons in this study would not have been possible without the use of multi-level wells.

Use of Multidisciplinary Approaches for Groundwater Recharge Mechanism Characterization in Basement Aquifers: Case of Sanon Experimental Catchment in Burkina Faso

“…The selection of specific methods depends on several factors such as the objective of the study, the consideration given to spatial and temporal scales and variability of the considered processes, the characteristics of the aquifer, and the availability of hydro-climatic data [15,30]. However, a reliable characterization of the aquifer recharge mechanism might help in developing a robust conceptualization of the recharge process [17,18,31,32] and support the development of aquifer protection policies [33,34].…”

Identification of Recharge Controls For Groundwater Aquifer System By Stable Water Isotopes

“…Other than that, it can be influenced by geological control, and the result cannot be validated accurately. Studies using a water stable isotope method found that the results could be verified more accurately than the other techniques, as Darling stated [7]- [9].…”

Impacts of warming winters on recharge in a seasonally frozen bedrock aquifer

“…Sharp rises in hydraulic head corresponding to decreases in groundwater temperature indicate that rapid midwinter recharge can occur despite the presence of thin basal ice (1-2.5 cm) and shallow (<10 cm) frozen ground. The considerable decreases in groundwater temperature (up to 4℃, Figure 3c) during recharge events indicates that cold water from snowmelt or winter rain is being transported advectively from surface along relatively short preferential flow paths (Gleeson et al, 2009;Wright and Novakowski, 2019). Less temperature sensitivity in the well would be evidence of longer flow paths, as more heat is transferred from the soil or rock matrix to the cold infiltrating water (Bou Jaoude et al, 2018).…”

“…This is despite bedrock aquifers in seasonally frozen environments providing, in some areas, nearly all municipal drinking water (Golder Associates Ltd., 2003;Nunes et al, 2017;Singer et al, 2003) and sustaining low-flows for ecologically sensitive habitats (Levison et al, 2014). Unlike unconsolidated aquifers, the low storativity of bedrock aquifers and high sensitivity to surface conditions can limit the capacity to buffer climatic changes (Cai and Ofterdinger, 2016;Hugman et al, 2012;Rathay et al, 2018;Wright and Novakowski, 2019). This leaves shallow bedrock aquifers particularly susceptible to impacts from climate change, especially in areas where winter precipitation is a dominant source of recharge.…”