Experimental investigation of the petroleum hydrocarbons migration in unsaturated porous media using image analysis

The groundwater contamination by hydrocarbon products has been a common problem in industrial areas in recent decades. Leakage from storage tanks or during the transportation seeps through the soil and pollutes the groundwater resources. Materials such as benzene, toluene, ethyl-benzene and xylene (BTEX) are common compounds of gasoline and can be found frequently in subsurface layers. In order to understand the behavior and to analyze the distribution of this kind of contamination in subsurface layers, a laboratory experimental were carried out as part of this research. An experimental sand tank was constructed in University of Tehran Laboratory to simulate the movement of BTEX in unsaturated and saturated porous media. The dimensions of the tank were 70, 50, and 8 cm for length, height, and thickness, respectively. The walls of the tank were made of Plexiglas to make the movement of the plume visible and possible for photography during the experiment. Since BTEX is colorless, SUDAN III, a red soluble substance, was added to BTEX. Image processing technique was used to estimate the concentration of BTEX in porous media in different time steps. In this experiment, 200 milliliters of BTEX with a flow rate of 15 milliliters per minute injected into the soil layer 2 cm below the surface. A groundwater flow in the saturated zone was applied with the rate of 10.4 cm per day. The upgradient depth of water was 12 cm and that for the downgradient was kept at 11.8 cm.
The profiles showed that the maximum degree of saturation happened 50 seconds after the start of the experiment with 0.92. The thickness of BTEX plume in the unsaturated zone reached 4 cm after 650 seconds. The degree of saturation and BTEX velocity were decreased with depth and increase in water saturation. After stopping the BTEX injection, the degree of saturation decreased in upper layer until it reached the residual saturation. The degree of saturation was increased above the water table and since it could not penetrate the saturated zone, it spread laterally.
The image processing technique using MATLAB software was used to estimate the BTEX degree of saturation at different location of the plume and for different time steps. The concept behind image processing algorithm is that the intensity of the pixel in a photograph is directly related to the degree of saturation of the plume. High degree of saturation means that the presence of the BTEX is higher in that location and the red color has higher intensity. The correlation between the pixel color intensity and BTEX saturation degree was obtained by calibrating the pixel intensity verses the measured saturation. The calibration process showed that there is a linear relationship between saturation degree and pixel intensity. The saturations extracted by image processing technique were used to produce the iso-lines of BTEX saturation for different time steps using Surfer 10.0 software. These plumes were compared to photographs for each time step to additionally confirm the accuracy of the technique.
The results of the image processing technique and comparison with the laboratory results showed that the proposed technique can accurately estimate the plume distribution. In addition, the results showed that in the initial stages of the BTEX injection, the gravitational force is the dominate force and lateral movement is negligible compare to vertical movement. Once the plume reaches the capillary fringe, BTEX may migrate laterally as a continuous, free-phase along the upper boundary of the water saturated zone due to gravity and capillary forces. BTEX saturation and accumulated volume increase in the capillary fringe and they depend on the depth of the capillary zone, BTEX viscosity, hydraulic head of the injection source, and volume of the injected BTEX. BTEX depressed the capillary fringe by approximately 4 cm. Ultimately, BTEX plume stopped above the water table.
The importance of groundwater, as the source of agricultural, industrial, and drinking water, makes it imperative to protect this precious resource. Understanding the behavior of the petroleum hydrocarbons in saturated and unsaturated zone can help to better prevent the pollution or remediate the contaminated groundwater. The successful application of the image processing technique in BTEX saturation estimate proved that this method can reduce the cost of laboratory experiment and can be used in evaluation of the numerical models, as well