A porewater-based stable isotope approach for the investigation of subsurface hydrological processes

J Garvelmann,C Külls,M Weiler

doi:10.5194/hess-16-631-2012

Abstract

Abstract. Predicting and understanding subsurface flowpaths is still a crucial issue in hydrological research. We present an experimental approach to reveal present and past subsurface flowpaths of water in the unsaturated and saturated zone. Two hillslopes in a humid mountainous catchment have been investigated. The H2O(liquid) – H2O(vapor) equilibration laser spectroscopy method was used to obtain high resolution δ2H vertical depth profiles of pore water at various points along two fall lines of a pasture hillslope in the southern Black Forest, Germany. The Porewater-based Stable Isotope Profile (PSIP) approach was developed to use the integrated information of several vertical depth profiles of deuterium along transects at the hillslope. Different shapes of depth profiles were observed in relation to hillslope position. The statistical variability (inter-quartile range and standard deviation) of each profile was used to characterize different types of depth profiles. The profiles upslope or with a weak affinity for saturation as indicated by a low topographic wetness index preserve the isotopic input signal by precipitation with a distinct seasonal variability. These observations indicate mainly vertical movement of soil water in the upper part of the hillslope before sampling. The profiles downslope or at locations with a strong affinity for saturation do not show a similar seasonal isotopic signal. The input signal is erased in the foothills and a large proportion of pore water samples are close to the isotopic values of δ2H in streamwater during base flow conditions indicating the importance of the groundwater component in the catchment. Near the stream indications for efficient mixing of water from lateral subsurface flow paths with vertical percolation are found.

Highlights

The stable isotope ratios of deuterium (2H) to hydrogen (1H) and of oxygen-18 (18O) to oxygen-16 (16O) of water have been used as natural tracers in many studies in order to explore hydrological processes at the hillslope and catchment scale (e.g. McDonnell, 1990; Kendall and McDonnell, 1998; Uhlenbrook et al, 2002; Vitvar et al, 2005; Tetzlaff et al, 2009)
In this study we further develop the H2O(liquid) – H2O(vapor) equilibration and laser spectroscopy method presented by Wassennaar et al (2008) to obtain stable isotope data of pore water of vertical depth profiles along two transects at a study hillslope
The vertical depth profiles of the pore water deuterium signature presented in this study provide time series information because one can see precipitation water from different seasons

Summary

Introduction

The stable isotope ratios of deuterium (2H) to hydrogen (1H) and of oxygen-18 (18O) to oxygen-16 (16O) of water have been used as natural tracers in many studies in order to explore hydrological processes at the hillslope and catchment scale (e.g. McDonnell, 1990; Kendall and McDonnell, 1998; Uhlenbrook et al, 2002; Vitvar et al, 2005; Tetzlaff et al, 2009). The stable isotope ratios of deuterium (2H) to hydrogen (1H) and of oxygen-18 (18O) to oxygen-16 (16O) of water have been used as natural tracers in many studies in order to explore hydrological processes at the hillslope and catchment scale In the absence of kinetic fractionation this variability determines the atmospheric boundary condition for hydrological stable isotope studies at the plot scale. The propagation and attenuation of the seasonal isotope signal with increasing soil depth allows for an investigation of vertical water movement in the unsaturated zone at the plot scale (Zimmermann et al, 1967; Maloszewski et al, 1983; Stewart and McDonnell, 1991; Gehrels et al, 1998). An enrichment of heavy isotopes in the subsurface may occur in the uppermost part of the soil column due to evaporation. Evaporation from plant interception or from ponding water in filled surface depressions entails kinetic fractionation effects, transpiration is a non-fractionating process from a hydrological perspective

Methods

Results

Discussion

Conclusion