A process for in situ groundwater decontamination within a subsurface region of earth includes: directing a liquid feed stream through a first conduit; injecting a treatment gas into the first conduit and/or a second conduit; discharging the liquid feed stream downwardly from the first conduit through an orifice restriction into the second conduit to form a confined plunging liquid jet gas and liquid contact zone in the second conduit; dissolving at least a portion of the treatment gas into the liquid feed stream in the contact zone to form an enhanced gas concentration liquid stream; passing the enhanced gas concentration liquid stream downwardly from the contact zone; and passing the enhanced gas concentration liquid stream from the second conduit into the groundwater. A device for performing this process is also disclosed.