An electric plasma arc apparatus and method produces nitrogen compounds, solely using ambient air extracted in proximity to the apparatus. The nitrogen compounds are brought into contact with a water processing system, forming nitrate on-site. Hydrogen sulfide present in the water processing system is removed, and the production of hydrogen sulfide by sulfate-reducing bacteria (SRB) is eliminated by introducing nitrate into the system, whereby denitrifying microorganisms, using the nitrate, outcompete the sulfate-reducing bacteria for the available carbon nutrients, thus preventing the SRB from producing hydrogen sulfide. Nitrate ions generated in the water processing system which contains the denitrifying microorganisms can enhance oil recovery by means of microbial enhanced oil recovery mechanisms. Further, the electric plasma arc apparatus and methodology eliminate major costs of conventional treatment techniques, including the transportation and storage of nitrate and the need for a continuous supply of natural gas and water.