NOVELTY - System (100) for removing contaminant from aqueous medium comprises gas injector (106) to electrically excite gas and to inject the gas into the medium, where injected gas comprises gaseous oxidant generated by electrical excitation and having persistence time in gas of less than 5 seconds, oxidant is operative to oxidize contaminant from first chemical state to second chemical state; and precipitating agent in contact with medium, having capacity to remove contaminant from medium, where capacity is greater for contaminant in second state than in the first state.    USE - As non-thermal plasma device for treating or cleaning water such as drinking water for human use, by removing toxic mineral contaminants e.g. arsenic from groundwater, well water, surface waters such as water from lakes, ponds and wetlands, agricultural waters, wastewater from industrial processes, and geothermal fluids.    ADVANTAGE - The non-thermal plasma (NTP) system is advantageous for removing dissolved arsenic and other inorganic contaminants from water intended directly or indirectly for human consumption or other use. The system can also be used to treat or enhance the treatment of certain organically contaminated liquid feeds, and particularly organic contaminants susceptible to treatment that includes oxidative processes. The NTP system provides reduced raw material use as well as disposal or recycling cost associated with the removal or sequestration of toxic contaminants. The NTP injection device can be applied in humid as well as dry conditions and ambient environments, a characteristic that is particularly important for operation in or adjacent an aqueous medium. The NTP can increase operational life or enable continuous or improved duty cycle treatment of process stream. The oxidizing capabilities of oxygen-bearing NTP injection device reduce organic contamination and thus extend the operational lifetime of some fluid treatment systems by oxidizing organic or biological contaminants, that otherwise can clog filtration components, or coat or spoil catalytic or reactive surfaces of known treatment materials or components.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for removing arsenic in +3 oxidation state from aqueous medium involving oxidizing arsenic from +3 oxidation state to +5 oxidation state by injecting gaseous oxidant into medium; and precipitating arsenic from medium by contacting medium with precipitating agent having greater capacity to precipitate arsenic present in +5 oxidation state, than its capacity to precipitate arsenic present in +3 oxidation state.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of fluid treatment system.    System (100)    Gas injector (106)    Gas handling system (112)    Power supply (116)    Generation unit (118)