Electromagnetic Alteration of Hydraulic Conductivity of Soils
Hydraulic conductivity is a measure of the rate at which water flows through porous media. Because of the dipole properties of water molecules, any electric field can alter the alignment of these dipole molecules and affect the hydraulic conductivity. In this study, the effect of the Radio-Frequency (RF) waves on the hydraulic conductivity was investigated.
Bentonite clay and sand samples were tested in rigid-wall, cylindrical permeameters and stimulated using a CPVC-cased monopole antenna, vertically centered in the permeameters. The permeameters were encased within RF cavities constructed of aluminum mesh in order to prevent interference from outside and to confine the RF wave to the medium. Falling-head and constant-head tests were performed to measure the hydraulic conductivity of the clay and sand samples, respectively.
The experimental results showed a correlation between the change in the hydraulic conductivity and the characteristics of the RF stimulation. Using RF frequencies of 80, 94, and 153 MHz, water flow through the clay sample was sharply decreased for a relatively short period of time and then stabilized at a higher hydraulic conductivity; nevertheless, the stable value was lower than that of the unstimulated case. Using a frequency of 153 MHz, water flow through the sand sample was sharply increased for a relatively short period and then stabilized at a hydraulic conductivity higher than the unstimulated case. At 153 MHz, the test was replicated with a decrease of the input RF power from 30 to 20 W, and then to 10 W, on the clay sample. The results revealed that the change in the hydraulic conductivity was larger at the higher input RF power. Measurements of the electric field component of RF waves were performed to map the electric field. In addition, the electric field was simulated using COMSOL Multiphysics for better three-dimensional (3D) visualization and analysis. The possibility of a dielectrophoretic phenomenon governing this effect was then studied for the simulated data. The result of this study was compared with the experimental result of the hydraulic conductivity tests. The result was in agreement with the results from the experimental field investigation for the bentonite clay sample but in disagreement for the sand soil sample.
A finite difference numerical model was developed to obtain a nodal hydraulic conductivity pattern of the medium and to find a potential correlation between the hydraulic conductivity and electric field magnitude. The results suggested a spatially varying hydraulic conductivity due to the spatially varying electric field pattern.