Evaluation of the Soil–Water Retention Curve of Arsenic-Contaminated Soil by the Filter Paper Method

Abstract

Arsenic is a metalloid frequently found in contaminated sites due to its easy mobilization in the environment. This has attracted the researchers who have studied this phenomenon from an environmental engineering perspective. Although there is evidence indicating that soil contamination impacts its geotechnical behavior, there is no available information about the changes in the soil’s suction after its contamination. The objective of this paper was to evaluate the soil–water retention curve of arsenic-contaminated soil. An unsaturated soil sample was contaminated with arsenic at two different concentrations and a filter paper calibration curve was developed for each contaminant concentration. Soil contamination decreased the BET area and increased its pore sizes. At a saturation rate of 0.8, the matric suction of the contaminated soils decreased (from 1681 kPa to 260 kPa), while the osmotic suction increased (from 23 kPa to 530 kPa), compared with the natural soil’s condition. Furthermore, the soil’s suction values obtained with the ASTM D5298 calibration curve were higher than those obtained using the calibration curve developed for the contaminated state. Thus, to avoid misunderstanding the mechanical behavior of the unsaturated soils under contaminated conditions, the osmotic suction should be considered and evaluated with the corresponding calibration curve.

Estimating the Soil-Water Retention Curve of Arsenic-Contaminated Soil by Fitting Fuentes’ Model and Their Comparison with the Filter Paper Method

“…The methodology used to achieve the objective of the present study was performed in two stages: (i) estimation and comparison of the SWRC using Fuentes' model (including the fractal, Burdine, and Mualem restrictions) and (ii) evaluation of the hydraulic conductivity function (HCF) using the van Genuchten equation via the Mualem model. In the first stage, it was necessary to use Equation (1) to adjust the ψ m data previously reported in [29] to compare it with the SWRC obtained by Fuentes' model. This allowed the researchers to collate the ψ m with the SWRC using Fuentes' model.…”

“…This indicates that the retention potential does not reflect the behavior of the evaluated soils. Specifically, the HCF of the soil is modified due to the arsenic contamination, which, in turn, increases the permeability coefficient by increasing the specific surface and pore-size distribution [29], indicating a modification of the soil's structure. In contrast, the simulated HCF for the contaminated soils does not reflect that change either by the SWRC or the HCF.…”

“…A frequently used method to evaluate soil suction is the filter paper method due to its reliability [26][27][28][29] and the suction range it covers (from 0 to 1 × 10 6 kPa and from 1000 to 500,000 kPa for the total and matric suction, respectively) [27]. This test method is described by the ASTM D5298 standard [30].…”

“…Nonetheless, after the literature review, no available evidence was obtained on the applicability of such models to evaluate the SWRC of the contaminated soils. When assessed through the filter paper method, it has been reported that the SWRC tends to change its position and retention potential once the soil becomes contaminated [23,29]. Hence, there is uncertainty about whether such models can be directly used to evaluate the SWRC of the contaminated soils.…”

“…Hence, there is uncertainty about whether such models can be directly used to evaluate the SWRC of the contaminated soils. Thus, the objective of the present study was to elucidate if Fuentes' model [36] can be used to estimate the SWRC of arsenic-contaminated soil, by comparing it with the reported filter paper method data [29] at the wetting path.…”