Characterization and Well Remediation Strategies for Improved Long-term Pump and Treat System Injection Well Performance - 17272
In June of 2012 the CH2M Hill Plateau Remediation Company completed construction of a 2,500 gallon-per-minute (gpm) pump and treat (P and T) system designed to remove technetium-99, uranium, carbon tetrachloride, trichloroethene, nitrate, and total and hexavalent chromium from incoming groundwater. The entire pump and treat system consists of the following primary components: 1) extraction wells (20+); 2) a radionuclide removal facility; 3) a biological system; 4) sludge processing; 5) vapor processing; and 6) injection wells. Overall efficiency of the 200 West P and T has been adversely impacted due to fouling in vertical injection wells where treated water is being reinjected into the subsurface. Fouling of the wells is thought to be caused by bacteria producing extracellular polymeric substances (EPS), as well as high manganese and iron in the biofilm. Due to decreased specific injectivity, all injection wells are on a maintenance schedule requiring rehabilitation. During rehabilitation, the wells are out of service affecting the overall capacity of the P and T, plus the rehabilitation process adds hundreds of thousands of dollars of cost to operate the P and T each year. Chemical and biological foulants may be coming from the P and T or are already present in the subsurface and active pumping has stimulated growth of the microbes present. Chemical and microbiological characterization efforts are in progress for the biological plant and injection well system to determine causative agents in the biofouling. P and T effluent tank samples, injection well distribution tanks, bailed samples from the fouled well, and post-rehabilitation samples were analyzed. Chemical constituents analyzed include, sulfate, phosphorus, chemical oxygen demand, total organic carbon, and iron. Microbial characterization included total heterotrophs, anaerobic heterotrophs, nitrate reducing bacteria, iron reducing bacteria and sulfate reducing bacteria. Molecular biological tools employed included 16S rRNA meta-genomes, and functional gene analysis using qPCR. Chemical analysis of the P and T effluent and injection well tanks showed the presence of ∼6.5 mg/L nitrate, 96 mg/L sulfate and phosphorus levels below 1 mg/L. Bailed samples from the injection well showed similar concentrations for each chemical, but following cleaning, TOC, total iron, manganese and other constituents were higher than prior to cleaning. These data show that bacterial nutrients and metals that may precipitate are present in the well water and are likely the causative agent in the biofouling. (authors)