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

Post, V. E. A.; Zhou, T.; Neukum, C.; Koeniger, P.; Houben, G. J.; Lamparter, A.; Šimůnek, J.

Estimation of groundwater recharge rates using soil-water isotope profiles: a case study of two contrasting dune types on Langeoog Island, Germany | Abschätzung der Grundwasserneubildungsrate mit Hilfe von Isotopenprofilen des Bodenwassers: eine Fallstudie zweier unterschiedlicher Dünentypen auf der Insel Langeoog, Deutschland Estimation des taux de recharge des eaux souterraines à l’aide de profils d’isotopes dans les eaux des sols: un cas d’étude pour deux types de dunes contrastées sur l’île de Langeoog, Allemagne Estimación de las tasas de recarga de las aguas subterráneas utilizando los perfiles isotópicos del agua del suelo: un estudio de caso de dos tipos de dunas diferentes en la isla de Langeoog, Alemania 使用土壤水同位素剖面估计地下水补给率:以德国Langeoog岛两种不同沙丘类型为例 Estimação de taxas de recarga de águas subterrâneas utilizando perfis isotópicos de água no solo: estudo de caso de dois tipos contrastantes de dunas na Ilha Langeoog, Alemanha

2022

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.

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