Estimation of groundwater recharge rates using soil-water isotope profiles: a case study of two contrasting dune types on Langeoog Island, Germany

Abstract

The creation of artificial dunes for coastal protection may have important consequences for freshwater lenses in coastal aquifers. The objective of this study was to compare the recharge processes below such a young dune with scant vegetation to an older dune covered by grass and herbaceous vegetation. To this aim, soil and water samples were collected from the unsaturated zone at two sites on Langeoog Island in northern Germany, and the soil water was analysed for stable water isotopes and chloride. Recharge rates were calculated by using a new version of HYDRUS-1D, which was modified to simulate isotope fractionation during evaporation. Both the model outcomes and the data highlight the importance of fractionation, which is slightly more pronounced at the older, more vegetated dune. At the newly constructed dune, vegetation dieback seemingly reduces the importance of transpiration during summer. Recharge occurs year-round, albeit predominantly during the winter months. Calculated recharge rates are consistent with lysimeter measurements, but are significantly higher than previously reported rates based on groundwater age data, which is primarily attributed to the absence of dune shrub at the sites investigated here. More data are needed to establish the importance of soil-water repellency and overland flow. Based on the results, it is proposed that repeated isotope sampling can yield important insights into the dynamics of recharge processes, including their response to climate change.

Estimating Characteristic Grain Sizes and Effective Porosity from Hydraulic Conductivity Data

“…The second data set consists of samples from different projects of the Federal Institute for Geosciences and Natural Resources (BGR), Germany (Table 1; Tewolde et al 2019; Houben et al 2020; Post et al 2022; BGR 2022). It includes dune and beach sediments from the island of Langeoog, Germany, as well as fluvial‐aeloian sediments from Namibia and fluviolacustrine‐lacustrine sediments from Chad and comprises in total 273 samples containing the representative grain diameters $$$ {d}_{\mathrm{10,20,50,60}} $$$ and 150 Beyer and Schweiger (1969)‐based and 96 Hazen (1892)‐based hydraulic conductivities that satisfy the preconditions for use of the proposed procedure.…”

The Impact of Soil Tension on Isotope Fractionation, Transport, and Interpretations of the Root Water Uptake Origin

“…These conditions make it a good site for studying the impacts of soil tension control on evaporation fractionation. The vegetated dune (Site LB01 in Post et al (2022), covered by grasses) was selected in this study to analyze the origin of its vegetation RWU.…”

“…The temporal distributions of relevant weather and vegetation growth variables are shown in Figures S1 and S2 in Supporting Information S1. More details about the study area and data collection methods can be found in (Post et al, 2022).…”

“…The initial soil water content (0.046 cm 3 /cm 3 ), temperature (20°C) and isotopic composition (−110 ‰ for 2 H and −10% for 18 O) are used throughout the soil profile. Post et al (2022) provide more details about the model setups.…”

Technical note: Removing dynamic sea-level influences from groundwater-level measurements

“…Estimating groundwater recharge on oceanic islands is challenging because groundwater levels in such systems are highly dynamic and can be influenced by multiple factors, such as periodic and aperiodic sea-level changes, coastal morphology, aquifer properties, precipitation, and withdrawals (Jiao and Post, 2019), that interact to influence near-shore groundwater levels (e.g., Patton et al, 2021). Several methods have been used for estimating groundwater recharge, such as lysimeters (e.g., Stuyfzand, 2017), tritium-helium age dating (e.g., Houben et al, 2014;Röper et al, 2012), and stable-isotope methods (e.g., 18 O, 2 H, see Koeniger et al, 2016;Post et al, 2022). However, temporal differentiation of the recharge, that is critical for understanding the dynamics of coastal groundwater systems, is costly and time intensive using these methods.…”