The Effect of Soil Water Characteristic Curve (SWCC) Parameters on Contaminant Transport in Homogeneous Earth Dams

Document Type : Research Paper

Author

Assistant Professor of Civil Engineering,, Faculty of Engineering, Behbahan, Khatam Al-Anbia University of Technology,

Abstract

Contaminant transport in homogeneous earth dams is strongly influenced by the soil's hydraulic parameters, the most important of which is the Soil-Water Characteristic Curve (SWCC). This research was conducted to investigate the impact of five key SWCC parameters—saturated volumetric water content (θₛ), residual volumetric water content (θᵣ), the inverse of the air entry suction parameter (α), the pore size distribution parameter (n), and the shape parameter (m)—on contaminant transport in a homogeneous earth dam. Numerical modeling was performed using Geo-Studio 2024 software and the CTRAN/W module, and the influence of variations in each parameter on breakthrough time, concentration profile, and contaminant penetration depth was analyzed. The results indicated that an increase in θᵣ and a decrease in n significantly reduce the time for the contaminant to reach the downstream side, whereas an increase in α and θₛ has the opposite effect and slows down the transport. Parameter m demonstrated a moderate effect on breakthrough time. Regression analysis also confirmed the high sensitivity of the model to parameters θᵣ, θₛ, and n. This study emphasizes the importance of accurate calibration of SWCC parameters in modeling contaminant transport and assessing pollution risk in earth dams.

Keywords

Main Subjects

Introduction

 Contaminant transport in homogeneous earth dams is a critical environmental and geotechnical concern, significantly influenced by the soil's hydraulic properties. The Soil-Water Characteristic Curve (SWCC) is the fundamental relationship governing moisture dynamics in unsaturated soils, thereby controlling key transport parameters such as unsaturated hydraulic conductivity and the hydrodynamic dispersion coefficient. Accurate prediction of contaminant breakthrough time and concentration profiles depends heavily on the precise calibration of SWCC parameters. This study investigates the sensitivity of contaminant transport in an earth dam to variations in the five key parameters of the van Genuchten SWCC model: saturated volumetric water content (θₛ), residual volumetric water content (θᵣ), the inverse of the air entry suction parameter (α), the pore size distribution parameter (n), and the shape parameter (m).

Method

 Numerical modeling was conducted using Geo-Studio 2024 software, coupling the SEEP/W module for steady-state seepage analysis and the CTRAN/W module for contaminant transport simulation. A homogeneous earth dam with a height of 12 meters and a reservoir water level at 10 meters was modeled. A constant contaminant source concentration (100 kg/m³) was applied upstream. A systematic sensitivity analysis was performed by varying each of the five SWCC parameters individually while keeping others constant at baseline values (θᵣ=0.1, θₛ=0.45, α=50, n=2, m=0.5). The primary outputs analyzed were the contaminant concentration profile along the downstream slope at day 29, the concentration breakthrough curve at a critical downstream monitoring point (Point A) over 365 days, and the breakthrough time (defined as the time for the relative concentration to reach 10% of the source concentration).

Results

The results demonstrated a significant and varied impact of Soil-Water Characteristic Curve (SWCC) parameters on contaminant transport dynamics. Among the parameters, residual water content (θᵣ) and pore size distribution (n) exhibited the most pronounced effects. Increasing θᵣ from 0.05 to 0.25 accelerated transport by enhancing moisture connectivity in fine pores, reducing breakthrough time from 19 to 15 days. The parameter n was identified as the most sensitive factor; increasing n from 1.5 to 4 drastically reduced breakthrough time from 24 to 0.3 days, leading to rapid and widespread contaminant propagation. In contrast, increasing the inverse air entry suction (α) delayed breakthrough from 2 to 25 days and resulted in a more concentrated contamination plume near the dam toe. Saturated water content (θₛ) unexpectedly increased breakthrough time from 11 to 20.5 days, while shape parameter (m) showed a moderate retarding effect. Pearson correlation analysis confirmed the sensitivity ranking as θᵣ > θₛ > n > α > m, with θᵣ exhibiting the strongest negative correlation (R² = 0.99).

Conclusions

 This study conclusively demonstrates the decisive role of SWCC parameters in modeling contaminant fate in earth dams. Parameters θᵣ, n, and θₛ were identified as highly sensitive, requiring careful calibration for realistic risk assessment.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

The data of the present study can be accessed through a request from the authors.

Artificial Intelligence Declaration

The author confirms that no artificial intelligence tools have been used in any stage of this research.

Acknowledgements

The authors would like to thank anonymous reviewers for their constructive comments and valuable suggestions in manuscript revision.

Ethical considerations

The author avoided data fabrication, falsification, and plagiarism, and any form of misconduct.

Conflict of interest

The author declares no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 57, Issue 3
May 2026
Pages 531-547

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