An increasing number of scientists and disciplines around the world are benefiting from the application of stable water isotope techniques (2H and 18O) –, especially in ecohydrology. Stable isotope analysis of soil pore water is generally performed to investigate water flow paths through the unsaturated zone. For this reason, several water extraction methods for stable isotope analysis exist. A critical issue, however, is that we know little about which soil water extraction method captures which soil water pool (mobile to tightly bounded water) and how well mixed these soil water pools are. Early experiments with soil columns revealed the existence of a weakly bound, easily exchangeable soil water pool, which is isotopically different from mobile soil water.

In this study, we would like to test the following hypotheses:

• H1: The isotopic composition of field wet soils differs when analyzing the soil water with different extraction methods. (Simple field sampling; Figure 1, orange dots)
• H2: The outflow of an intensely flushed soil column contains an isotopic mixture of the initial isotopic composition (see H1) and the continuously added water, but the outflow eventually approaches the isotopic composition of the added water. (Outflow sampling of column experiment; Figure 1, blue triangles)
• H3: The isotopic composition of an intensely flushed soil does not differ when analyzing the soil water with different extraction methods. (Soil sampling after column experiment; Figure 1, green dots).

Figure 1. Hypothesis visualized in dual isotope space (adapted from Araguás-Araguás et al., 1995).

Against this background, the potential candidate would:

• Conduct a soil sampling campaign
• Setup a column experiment
• Sample pore water isotopes with different extraction methods
• Determine isotopic differences between different methods/columns
• Analyze and interpret the isotope data

The student/s should be interested in conducting field and lab work and be familiar with statistical software, such as R or Python.

Natalie Orlowski, University of Freiburg, Tel. +49761 203 9283, natalie.orlowski@hydrology.uni-freiburg.de

Matthias Sprenger, Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Spain, mspreng@ncsu.edu

English/German

• Araguás-Araguás, L., Rozanski, K., Gonfiantini, R. and Louvat, D.: Isotope effects accompanying vacuum extraction of soil water for stable isotope analyses, Journal of Hydrology, 168(1–4), 159–171, doi:10.1016/0022-1694(94)02636-P, 1995.
• Orlowski, N., Pratt, D. L. and McDonnell, J. J.: Intercomparison of soil pore water extraction methods for stable isotope analysis, Hydrol. Process., 30(19), 3434–3449, doi:10.1002/hyp.10870, 2016.
• Sprenger, M., Herbstritt, B. and Weiler, M.: Established methods and new opportunities for pore water stable isotope analysis, Hydrol. Process., 29(25), 5174–5192, doi:10.1002/hyp.10643, 2015.
• Sprenger, M., Seeger, S., Blume, T. and Weiler, M.: Travel times in the vadose zone: Variability in space and time, Water Resour. Res., 52(8), 5727–5754, doi:10.1002/2015WR018077, 2016.