A system is provided for producing an oxidizing zone within a saturated zone of an aquifer for in-situ decontamination of groundwater carrying contaminants in a plume wholly located below a water table in said saturated zone of said aquifer. The system comprises a source of gas for providing a gas capable of producing an oxidizing environment in the groundwater which can oxidize the contaminant to purify the groundwater and the aquifer. Gas injectors are connected to the source of gas, for injecting the gas into the groundwater below the plume at a gas injection rate, thereby producing the oxidizing environment in the groundwater. Auxiliary apparatus is provided for testing decomposition of the contaminants in the oxidizing environment and for determining the gas injection rate.