Performance of in situ bioremediation was demonstrated at a shallow water site with subsurface contamination of used crankcase oil, diesel fuel, gasoline, and other oily phase products. Treatment involved ground water amendment by hydrogen peroxide and nutrients by a recharge gallery. The system was operated for 2 1/2 years. During treatment benzene and total BTEX concentrations in ground water were reduced in magnitude exceeding ten and twenty times, respectively. Vertical profile core samples showed a two feet thick interval near the water table containing significant amounts of hydrocarbons. The center of the interval had not been depleted of BTEX compounds, but the surrounding cortex had been physically and biologically weathered. The interval material was fine textured that restricted flow of remedial fluids which resulted in little opportunity for bioremediation.