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The mechanisms of water transport in the capillary fringe: sandbox experiments and numerical studies

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Abstract

Capillary fringe (CF) is the transition area between unsaturated zone and unconfined aquifer; the mechanisms of lateral seepage in CF are the basis to accurately assess water flow and solute transport in the subsurface environment. In this study, the characteristics of lateral seepage in CF were studied via sandbox experiments. Based on the established two-dimensional capillary tube model, force analysis of the capillary tube was implemented. In addition, numerical simulations were performed by HYDRUS software. The accuracy of the numerical simulation was determined by comparing the simulated results with sandbox experiment results. Then, the spatial distributions of water potential and flow velocity in CF were further analyzed. The results indicated that, in the recharge area of CF, due to forces like self-gravity, atmospheric pressure, wall tension, matric suction, friction, etc., the water column in the capillary tube had an upward velocity component which decreased during the process of capillary rise. Similarly, in the discharge area, a downward velocity component of water flow was also caused by the forces mentioned above and the velocity increased from up to down in the capillary tube. Moreover, the water finally moved across the water table into the saturated zone. In the middle part of CF, the water moved horizontally, and the higher the distance from the water table, the lower the flow rate. Besides, the hydraulic conductivity was proportional to the fourth power of the capillary tube radius. The comparison between the simulated data and the measured data revealed that the Brooks–Corey model could fit well with the experimental results. The spatial distributions of water potential and the flow velocity in CF showed that the flow rate in the recharge area was lower than that in the discharge area resulting from the smaller hydraulic gradient in the recharge area. Overall, this study contributes to an in-depth understanding of the mechanisms of lateral seepage in CF.

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Acknowledgements

This project was supported by Wuhan Zondy W&R Environmental Technology Co., Ltd. and the China Scholarship Council (201708420145). We also acknowledge the assistance of Prof. Junwei Wan from the China University of Geoscience (Wuhan).

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Authors and Affiliations

  1. School of Environmental Studies, China University of Geoscience, Wuhan, 430074, China

    H. Peng

  2. Department of Hydrology, University of Bayreuth, 95440, Bayreuth, Germany

    H. Peng & T. Lu

Authors
  1. H. Peng
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  2. T. Lu
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Contributions

HP: Conceptualization, methodology, investigation, writing- original draft preparation; TL: formal analysis, writing- original draft preparation, writing- reviewing and editing.

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Correspondence to T. Lu.

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Editorial responsibility: Maryam Shabani.

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Peng, H., Lu, T. The mechanisms of water transport in the capillary fringe: sandbox experiments and numerical studies. Int. J. Environ. Sci. Technol. 19, 5791–5802 (2022). https://doi.org/10.1007/s13762-021-03609-3

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  • DOI: https://doi.org/10.1007/s13762-021-03609-3

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