Using Water Stable Isotopes in the Unsaturated Zone to Quantify Recharge in Two Contrasted Infiltration Regimes

Abstract

A reliable estimate of recharge is needed for the sustainable management of groundwater resources. Water stable isotope (d 18 O and d 2 H) profiles in the unsaturated zone are frequently used to quantify groundwater recharge based on the seasonality of water isotopic compositions in precipitation. A very simple approach consists of integrating the soil water content between peak values of soil water isotopic composition, typically corresponding to precipitation signatures from warm and cold seasons. When precipitation isotopic compositions are available, a conceptual surface water isotopes budget and lumped parameter dispersion model can be computed. These models were applied on two field sites with similar permeable soils with grass cover but contrasting recharge regimes and seasonality, one in the Paris Basin (France) with continuous recharge from autumn to spring and the other in the St. Lawrence Lowlands (Quebec, Canada) with episodic recharge in fall and after snowmelt. For the two sites, the peak-topeak method and isotope surface budget led to comparable recharge intensities. At least at the Paris Basin site, evaporation was shown to slightly modify the average unsaturated zone and hence groundwater isotope composition. The proposed parameterization of isotope fractionation due to evaporation allows qualitative estimation of the fraction of evaporation, at least during the recharge seasons. In spite of its simplifications and limitations, the proposed parsimonious model can give estimates of recharge in a variety of sites even if they are not well characterized, as it benefits from the large availability of monthly isotopic compositions in precipitation.

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

“…Plant water source studies in which the xylem water isotopic composition does not spread within the range of the sources' isotopic compositions often acknowledge that a relevant source of water may not have been sampled (Bowling et al, 2017;Geris et al, 2017). Not many studies sample rock moisture, but it has been shown that water stored in rocky layers can contribute to plant transpiration, sometimes more than saturated soil layers (Barbeta and Peñuelas, 2017, and references therein). Indeed, rock moisture can represent up to 27 % of annual rainfall and can be taken up by trees during the dry season (Rempe and Dietrich, 2018).…”

“…3). Non-monotonic soil isotopic profiles would imply that both soil water δ 18 O and δ 2 H are more depleted in a certain layer, as they should be proportional (Barbecot et al, 2018). Thus, if root water uptake was sourced in a soil layer with relatively depleted soil water, xylem water should be depleted in both isotopes, not just in δ 2 H. In addition, non-monotonic soil isotopic profiles may occur under some circumstances, which is probably the reason why the fit of the soil water lines changes over time and space (Table S3).…”

Depth–Sequential Investigation of Major Ions, δ18O, δ2H and δ13C in Fractured Aquifers of the St. Lawrence Lowlands (Quebec, Canada) Using Passive Samplers

“…All the groundwater samples have isotopic composition (δ 18 O and δ 2 H) that are depleted compared to the yearly average of precipitations (Figure 15a). This would be the result of more intense recharge periods during spring snow melt and fall rainfall as suggested by recharge studies in Quebec [56], whose isotopic composition being depleted compared to other precipitations periods in the study area [47]. Different isotopic compositions are observed from one well to another, but do not show depth-dependent variability within each well (Figure 15b).…”

Pore water isotope fingerprints to understand the spatiotemporal groundwater recharge variability in ungauged watersheds

“…At the profile scale, and based on a single field campaign, pore water stable isotopes have been shown to be valuable tools to obtain insights into vertical water flow under various climatic, geological, and land cover conditions (Barbecot et al., 2018; Sprenger, Seeger, Blume, & Weiler, 2016; Stumpp, Maloszewski, Stichler, & Maciejewski, 2007; Stumpp, Stichler, Kandolf, & Šimůnek, 2012). Easy to sample in the field, and fast to measure in the laboratory, thanks to the development of H 2 O liquid –H 2 O vapor equilibration methods (Mattei et al., 2019; Wassenaar, Hendry, Chostner, & Lis, 2008), pore water isotopes are also relevant to calibrating unsaturated zone models (Sprenger, Volkmann, Blume, & Weiler, 2015), even without temporal monitoring (Mattei et al., 2020).…”