Isotope fractionation of sandy-soil water during evaporation – an experimental study

ABSTRACT

Soil samples containing water with known stable isotopic compositions were prepared. The soil water was recovered by using vacuum/heat distillation. The experiments were held under different conditions to control rates of water evaporation and water recovery. Recoveries, δ¹⁸O and δ²H values of the soil water were determined. Analyses of the data using a Rayleigh distillation model indicate that under the experimental conditions only loosely bound water is extractable in cases where the recovery is smaller than 100 %. Due to isotopic exchange between vapour and remaining water in the micro channels or capillaries of the soil matrix, isotopic fractionation may take place under near-equilibrium conditions. This causes the observed relationship between δ²H and δ¹⁸O of the extracted water samples to have a slope close to 8. The results of this study may indicate that, in arid zones when soil that initially contains water dries out, the slope of the relationship between δ²H and δ¹⁸O values should be close to 8. Thus, a smaller slope, as observed by some groundwater and soil water samples in arid zones, may be caused by evaporation of water before the water has entered the unsaturated zone.

KEYWORDS:

• Evaporation
• hydrogen-2
• isotope fractionation
• isotope hydrology
• incomplete extraction
• oxygen-18
• sandy soil
• soil water

Acknowledgements

We thank Zhiguo Su, Jiang Sun, Xue Sun and Huifang Zhou, Yaodong Pan and Xiao Wang for the field assistance and experimental help. Thanks to Luis J. Araguás-Araguás (International Atomic Energy Agency) and two anonymous reviewers for providing suggestions that led to the improvement of the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study is financially supported by the National Natural Science Foundation of China [Grant Nos. 41273015, 40973001, 41271041].