Two different types of soil were selected to observe the migration and/or degradation of petroleum contaminants. One soil sample was composed of fine clay and was contaminated by an electrical insulating oil. The other soil sample was a coarse soil contaminated by fuel oil number 2 and number 6. A portion of each was cultured in a minimal medium to generate large concentrations of indigenous microbes capable of degrading each petroleum contaminant. Four columns fabricated of plexiglass with five ports at 6 inch intervals were filled with the contaminated soil which had been air dried and autoclaved for sterilization. These columns simulated unsaturated soil in the vadose zone.
Two types of experiments were carried out during this research. In one type of experiment, the petroleum was added just once in order to simulate a petroleum spill. In the other type of experiment, petroleum was added to the column repeatedly at measured intervals in order to simulate a petroleum leak. To one of the two columns operated for each soil, the contaminating petroleum and water were added. To the second column operated for each soil, the contaminating petroleum, water, and cultured microbes were added. From these experiments, the biodegradability was observed to be affected by soil composition and conditions, concentrations of petroleum contaminants, and microbial concentrations. Better microbial degradation occurred in the fine soil for the simulated spill case than in the coarse soil. In this soil with low porosity, the microbes were confined to the top of the column and the petroleum concentration was low. In the simulated leak case, the biodegradation took place at a higher rate in the coarse soil than in the fine soil. Here, the high rate of advection distributed the microbes throughout the depth of the column and had a diluting effect on the high petroleum cumulative concentrations.
From these experiments, a mass balance of petroleum contamination could be determined at all time intervals at all column depths. The sterile column behavior exhibited only physical changes such as adsorption and evaporation and could be compared to results in columns of biotic conditions. These differences permitted the development of a computer model to simulate and predict physical and biological processes in fine and coarse soils.
Comparison of an unsaturated soil zone model (SESOIL) predictions with a laboratory leaching experiment