Pulsing of electron donor and electron acceptor for enhanced biotransformation of chemicals

A method for purifying an underground water supply containing halogenated aliphatic compounds by in-situ biodegradation of said compounds, comprising:

• injecting at least two water aliquots into an aquifer zone, at least one of said aliquots containing an electron donor capable of being utilized by an indigenous population of microorganisms contained in said groundwater supply as a primary growth substrate, and at least one of said aliquots containing an electron acceptor for oxidation of said primary growth substrate, wherein each of said aliquots are injected in said aquifer zone at separate points of addition into said aquifer zone, to stimulate growth of said microorganisms and to distribute them throughout said aquifer zone to cause in-situ degradation of said halogenated aliphatic compounds by metabolic activity of said microorganisms.