Abstract: A bioremediation process employing an improved bioreactor comprises pumping contaminated ground water from a recovery well in a contamination site, passing the water through a fully pressurized bioreactor wherein the water is enriched with nutrients and oxygen and treated with microorganisms native to the contamination site, and returning the enriched and treated water and microorganisms in a finely dispersed state to the contamination site, thereby enhancing bioremediation in a subsurface bioactive zone. The recirculation is accomplished with only the recovery well pump. The bioreactor is a fully pressurized fluidized bed bioreactor. The bioreactor includes a recirculation loop driven by a fluidization pump. The loop has a first branch that returns a portion of the recirculating water directly to the bioreactor inlet.