Instead of injection the oxidant into the subsurface (In-Situ Chemical Oxidation (ISCO)), it sometimes makes more sense to pump the water out of the ground and treat it. The treated water can then be reinjected back into the subsurface or simply discharged at the surface depending on the local conditions.
Below is an illustration of the options.
The pump and treat option can be a good one for ozone based oxidation processes, especially advanced oxidation processes. First, the ozone can be efficiently mixed with the water under controlled conditions at the surface. Excess gas can be released without becoming a possible source of blockage subsurface. In the case of advanced oxidation, contaminated water can be in intimate contact with hydroxyl radicals allowing for efficient reaction. In the subsurface the oxidants have to travel to the contaminant. In the case of short lived oxidants such as hydroxyl radicals, it is easier for the water with contaminant to travel versus the hydroxyl radical. Finally, UV based processes can be employed at the surface where they are not available subsurface.