Bioremediation is a classical method for the reduction of TPH levels is soil. However, these methods will not achieve high level reduction. It can give lower than 10,000 PPM or 1%.Biomederation requires the addition of different types of nutrient to keep the bacteria life.  The principle is enhancement of the bacteria which already exist in soil. Our challenge is to balance the nutrient & oxygen requirement to activate the bacteria.  Heavy Hydrocarbon ends require more research to specify the right nutrients & proper atmosphere.  In all cases, soil temping is required periodically to provide oxygen to the bacteria.