Abstract: A method and apparatus for treating ground and ground water contaminated with hydrocarbon and halogenated hydrocarbon compounds. Treatment of contaminated water removed from the ground is accomplished by microorganisms which consume the contaminants. Growth of microorganisms is enhanced under controlled conditions by addition of nutrients and gases such as oxygen, nitrogen, carbon dioxide or a combination of these and other gases to the contaminated water. Treated water carrying microorganisms, nutrients and gases is returned for recirculation through the contaminated area of ground to leach out and biodegrade the contaminants deposited in the ground. Microorganism activity is also enhanced in the ground by injection of nutrients and oxygen or other gases into the ground and ground water in the contaminated area.