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

Abstract

Reliable groundwater recharge quantification at the regional scale (e.g., watershed or subwatershed) is fundamental to sustainable water resource management. Although modeling at the watershed scale is gaining wide support, the long-term monitoring needed for model calibration is often not readily available, as many watersheds worldwide remain ungauged. In response to this situation, we propose a new approach to estimate groundwater recharge at the watershed scale. This approach is fast and accurate and takes into account the existing variability without requiring long-term monitoring. Only a single field campaign to acquire soil water content and pore water isotopic composition depth profiles is needed. The principle is to extend a physically based, one-dimensional unsaturated zone flow model from the local (i.e., profile) to the watershed scale, using an index method for distributed recharge based on a GIS. The methodology was validated in a gauged watershed, where previous studies have estimated recharge using a spatialized water balance model calibrated using long-term discharge monitoring data. Scaling was investigated by comparing recharge values obtained using the local-scale approach at 10 study sites within the watershed with coinciding values obtained at the watershed scale. Recharge values were similar in terms of both dynamics and quantity. Using the pore water isotopic fingerprint of ungauged watersheds is therefore confirmed to be a suitable approach for understanding spatiotemporal recharge variability.

Technical note: Unresolved aspects of the direct vapor equilibration method for stable isotope analysis (<i>δ</i><sup>18</sup>O, <i>δ</i><sup>2</sup>H) of matrix-bound water: unifying protocols through empirical and mathematical scrutiny

“…In all these examples DVE-LS analyses were performed on soil or rock samples. Although generally conceivable (Millar et al, 2018(Millar et al, , 2019, we are not aware of any published field study employing DVE-LS on plant samples.…”

“…They also tested different sample storage containers and favored Ziploc® freezer bags, which when doubled they found to reliably hold sample water and prevent significant evaporitic enrichment of heavy isotopes for up to 10 d. Comparing the DVE-LS method with analyses of liquid water squeezed from low-permeability samples, Nakata et al (2018) found the former to represent water from open pores only. Millar et al (2018) analyzed plant samples from a controlled environment in a direct comparison of the DVE-LS method against five quantitative water extraction methods. They found the former to be superior in terms of limited co-extraction of volatile organic compounds (VOCs), rapid sample throughput, and near-instantaneously returned stable isotope results.…”

Vadose zone modeling to identify controls on groundwater recharge in an unconfined granular aquifer in a cold and humid environment with different meteorological data sources

“…The results from the hydro-isotopic water budget and from the model used in the present study were within the same order of magnitude and in good agreement. Mattei et al (2020a) estimated GR in the same region using water isotopic profiles. The estimated GR at SLZ for 2017 varied between 380 and 395 mm/y and from 158 to 214 in 2018.…”

“…Under a wide variety of conditions, GR is still considered to be one of the most challenging water-balance components to quantify (Dripps and Bradbury 2007) because it is difficult to measure directly and it is strongly influenced by climate, soil heterogeneity and land use. A wide range of methods (Scanlon et al 2002) has been shown to be useful in assessing the quantity of water actually replenishing an aquifer, including isotopic profiles (Barbecot et al 2018;Mattei et al 2020a, b), remote sensing techniques (Jackson 2002), chloride mass balance (Szilagyi et al 2011), base flow separation (Hung Vu and Merkel 2019), unsaturated zone modeling (Hu et al 2019;Mattei et al 2020a), water balance models (Dubois et al 2021) and water table fluctuations (Crosbie et al 2005). The reliability of these methods depends on a variety of factors, including the availability of field data to describe site characteristics, such as soil type, vegetation and depth to the water table (Scanlon et al 2002).…”

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

“…The chemical and isotopic study of groundwater, usually termed "hydrogeochemistry" is an essential complement to physical studies to identify recharge and discharge zones, flow paths and water chemical evolution [1]. As it reflects inputs from recharge areas [2], as well as confinement conditions [3], hydrogeochemistry is also used in certain cases as a proxy for groundwater vulnerability [4]. Yet, hydrogeochemistry studies suffer the same limitations as those in physical hydrology: flow path reconstructions or chemical evolution of water are often based on limited spatial access to the aquifer, through the measurements, when available, in piezometers, water wells and natural sources.…”