Microbiological processes in the Severnyi deep disposal site for liquid radioactive wastes

Local monitoring of physicochemical, radiochemical, and microbiological parameters was performed in the deep horizons of the Severnyi site used for disposal of liquid radioactive waste (LRW). Analysis of the chemical and radiochemical composition of the wastes and formation fluid revealed that the boundary for migration of radionuclides lagged behind that for nonradioactive waste components (sodium nitrate) and tritium. The physicochemical and radiochemical conditions in deep horizons did not prevent microbial growth. The numbers of microorganisms (aerobic organotrophs, denitrifying, fermentative, sulfate-reducing, and methanogenic) were low, as were the rates of sulfate reduction and methanogenesis; they increased in the waste dispersion zone. The microorganisms from deep horizons were able to produce gases (CH₄, CO₂, N₂, and H₂S) from possible waste components. Denitrifying bacteria belonged to different Pseudomonas species and reduced nitrate to dinitrogen under the conditions of pH, salinity, temperature, and radioactivity found in the disposal site. These results suggest the need for control of microbiological processes in deep disposal site for liquid RW.