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Advantages & Challenges of Soil Water Isotopes for Groundwater Recharge Assessment - Field Study in Canada

This research study explores the potential of using soil water isotopes to assess groundwater recharge, focusing on a field study conducted in Canada. The advantages, challenges, and methodology are discussed, along with the results obtained from the study.

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Advantages & Challenges of Soil Water Isotopes for Groundwater Recharge Assessment - Field Study in Canada

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  1. Advantages and challenges of using soil water isotopes for assessing groundwater rechargeIllustration with a field study located in Canada Romain Chesnaux1and Christine Stumpp2 1Université du Québec à Chicoutimi, Chicoutimi (Québec) Canada 2University of Natural Resources and Life Sciences, Vienna, Austria

  2. Research issue and objectives • Issue :Assessing potential temporal recharge from volumetric water content (VWC) profiles and water stable isotopes in the vadose zone of an aquifer • Objective 1 : Using the water stableisotopes 2H/18O for determining the origin of infiltrated water at different depths BGS • Objective 2 :Assessing potential recharge associated to a specific period of time from the difference between the observed VWC and the residual VWC • Objective 3 :Comparing the values of recharge obtained at different sites and discuss the site effects

  3. Methodology • 1-Core logging and collecting core samples • 2-Analysing isotopes 2H/18O after water extraction from the samples – Profiles of the isotopic composition of pore water • 3- Measuring the VWC of the samples – VWC profiles • 4- Applying the peak shift method to determine temporal recharge • 5-Grain-size sieve analysis to evaluate the heterogeneity of the vadose zone and understand site effects

  4. Study site

  5. Study site

  6. Fieldworks: core sampling with a hand auger with a split-spoon sampler at two sites: Site C1 (2 m-thick vadose zone) and C2 (4 m-thick vadose zone) on October 16th, 2016 On the pictures: UQAC technician Mr. David Noël

  7. Two sites C1 and C2 Site C1 Site C2

  8. Samples collected every 10 cm

  9. Samples shipped at the laboratory of Helmoltz Zentrum München in Germany

  10. Isotopic analyses On the picture: HZM technician Ms. Petra Seibel Ratios (δ-value (‰)) 18O/16O et 2H/1H calculated relatively to the V-SMOW (Vienna-Standard Mean Ocean Water)

  11. VWC and grain-size analyses

  12. Isotopic composition of precipitation Mean isotopic signature during ''post-snowmelt period'' Mean isotopic signature during ''snowmelt period''

  13. Results: Isotopic profiles (example of Deuterium)

  14. Results: Isotopic profiles (example of Deuterium)The two y-axis designed to match the isotope ratios of the two soil profiles to represent a comparable timing of infiltration -Velocity of infiltration 2.78 times higher in C1 than in C2 -3 periods identified: Winter, Snowmelt, Summer/Early Fall

  15. Results: VWC profiles (same scale)

  16. Results: VWC profiles (adjusted scales)

  17. Results: Historical infiltration at C2

  18. Results: Historical infiltration at C1

  19. Calculating potential recharge from the peak-shift method during period of time T (plug-flow type) • with RT the recharge during time period T [LT-1] • z the depth below ground surface [L] • Θ(z) the VWC et z • Θr the residual VWC

  20. Results: Potential recharge values calculated for different periods • At C2: -Recharge post-snowmelt (June to mid-October, 4.5 months): 239 mm/571 mm, i.e. 42%; -Recharge during snowmelt (April and May, 2 months): 125mm/300mm, i.e. 42% • At C1: -Recharge from July 23rd to October 16th (3 months): 287 mm/239 mm, i.e. 120% !!!

  21. Results: Grain-size analyses

  22. Limitations and recommendations The method is accurate but only under specific conditions • Limitations: -Thickness of vadose zone -Site effects : Heterogeneity and topography • Recommendations: -Choice of the site : vadose zone thickness, topography (runoff), vegetation cover Chesnaux, R. and Stumpp, C. 2018. Advantages and challenges of using soil water isotopes to assess groundwater recharge dominated by snowmelt, Hydrological Sciences Journal, 63(5): 679-695

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