Environ. Sci. Technol. 2004, 38, 643-649
Mobilizing Particles in a Saturated Zone during Air Sparging YIH-JIN TSAI* AND DA-FENG LIN Department of Environmental Resources Engineering, Diwan College of Management, No. 87-1, Nansh Li, Madou, Tainan, Taiwan
The mobilization of soil particles changes the porosity of saturated zone during air sparging. Soil porosity is shown to be correlated with soil electrical resistivity. This study performs porosity-resistivity tests to establish the relationship between porosity and resistivity of quartz sand. Experiments, involving a large sandbox to simulate the saturated zone, are then performed to compare the resistivity of compacted sand before air injection with that after air injection. The relevant data enable the mobilization of quartz sand particles to be quantified. Results of the experiments indicate the mobilization of sand particles and an increase in porosity directly proportional to the rate at which air is injected. Besides, a layer of fine-grained particles covered the compacted sand at the upper boundary of sandbox after each air injection experiment. This is direct evidence that finer particles were transported upward during air sparging. Two methods were applied to verify the results of this study. The first verification method indicated that changes in porosity increased directly proportional to the air injection rate, which is consistent with shear theory. The other validation method indicated that the mass of sand in the tank did not change after air sparging, which indicates that the resistivity-porosity method is unbiased.
Introduction Soil scientists have focused on particles mobilized by the infiltration of rainfall (1-6). Soil particles are mobilized by both chemical and physical perturbations. In model systems that consist of fluids flowing over spherical particles attached to flat surfaces, the hydrodynamic shear force (Fshear) depends on the fluid’s viscosity (µ), the particle’s radius (r), and the flow velocity (Ur) (1):
Fshear ) 1.7009(6π)µrUr
(1)
According to eq 1, an increase in velocity (Ur) increases the shear force. The particle will be mobilized if the hydrodynamic shear force exceeds the gravity and resistance. Similarly, particle concentrations eluted from repacked lysimeters were directly related to the kinetic energy of moving water (2). Changes in the chemical composition of interstitial water, caused by the wetting and drying of soil particles can strongly affect the mobilization of particles (3). Strong sorption of hydrophobic and hydrophilic particles at the air-water interface has been observed (4). This sorption is fast and * Corresponding author phone: 886-6-5718888, ext 872; fax: 8866-5722858; e-mail:
[email protected]. 10.1021/es030325q CCC: $27.50 Published on Web 12/10/2003
2004 American Chemical Society
irreversible, suggesting that significant immobilization of particles can occur in soils with a stationary air phase. Accordingly, air bubbles may move and carry sorbed fine particles. In air sparging, the velocity of flowing air or flowing water is 10-100 times greater than that due to rainfall infiltration. Soil particles can be reasonably expected to be transported by flowing air and water during air sparging. If soil particles are transported during air sparging, then the distributions of the porosity and the reservoir permeability will be changed, altering the flow path of the air. Some theoretical studies and numerical simulations of air sparging, with reference to airflow paths, have been conducted (7-14). However, the effect of particle mobilization on airflow path has never been considered. Furthermore, flow visualization experiments performed in the laboratory have not addressed the transport of soil particles (15). Observing this phenomenon is difficult because the grain sizes of soil particles used in experiments are greater and the flow rate is lower than in the field scale (15). In this study, a set of laboratory sandbox experiments was performed to simulate in-situ air sparging and thus elucidate the transport of soil particles in air sparging. These tests extend the understanding of particle mobilization in air sparging.
Experimental Methods Relationship between Resistivity and Compacted Soil. Relationships between electrical conductivity (σo) and other properties of compacted soils have been extensively examined. Electrical conductivity has been shown to be correlated with water content and saturation (16). One model of the electrical conductivity of soil is
σo ) σw(aθ2 + bθ) + σs
(2)
where σw represents the electrical conductivity of the pore water (S/cm); σs is the apparent conductivity of the soil particle (S/cm); θ is the volumetric water content, and a and b are dimensionless material constants (17). For a given packing material, a and b are constants. High resistivity may correspond to low water content, high air-filled porosity, or high sand content. Theoretical relationships imply that the porosity of compacted soils can be evaluated by measuring electrical resistivity (18, 19). Porosity-Resistivity Tests. Experiments were performed in a conductivity cell to establish the relationships between resistivity and porosity of compacted soil. The porous media used in all the experiments were clean quartz sand with a density of 2.55 g/cm3. Table 1 lists the grain sizes of the sand used, which includes sands (99.3%) and silts (0.7%). The quartz sand was compacted into a conductivity cell. The conductivity cell was 5 cm thick by 10 cm wide by 10 cm high (internal dimensions) and was considered to represent a single unit of the sandbox (Figure 1). The porosity could be determined from the density and weight of compacted sand and the volume of the cell. The quartz sand in the cell was saturated with 0.1 M NaCl solution. A twoelectrode method was used to measure electrical resistivity by connecting the current and potential leads. Several tests at different porosities were performed. The relationships between resistivity (or electrical conductivity) and porosity are plotted as in Figure 2. Sandbox and Experimental Setup. Airflow through a saturated porous media is a three-dimensional phenomenon. Experiments were performed in a thin Plexiglas sandbox to VOL. 38, NO. 2, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Distribution of Grain Sizes for Quartz Sand grain sizes (mm) 0.84-0.59 0.59-0.42 0.42-0.297 0.297-0.21 0.21-0.149 0.149-0.105 0.105-0.074 0.074-0.053
