Community analysis of groundwater and surrogate sediment samples during electron donor and electron acceptor injections into a chromium-contaminated site in Hanford, Washington, USA
Bowen De Leon, Kara Beth
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Heavy-metal contamination is a common problem in the industrialized world today. Bioremediation, the use of microorganisms or microbial processes to degrade or reduce contaminants, has been found to be successful for many different contaminants and environments. As part of the World War II Manhattan Project, eight single-pass reactors in the Hanford Nuclear Reservation in Washington were used for plutonium production for ~43 years. Columbia River water, used to cool these reactors, was contaminated with heavy metals and radionuclides and stored in retention basins before discharge or leakage into the ground or river. Hexavalent chromium (Cr(VI)), a carcinogen and mutagen, was used as a corrosion inhibitor in the cooling systems and is a common contaminant at the Hanford Site. In 2008, a polylactate compound was injected into Cr(VI)-contaminated groundwater in the Hanford 100-H area to stimulate the resident microbial community and resulted in Cr(VI) levels below background levels for ~3 months. Temporal and spatial community analyses of the groundwater and surrogate sediment samples via small-subunit rRNA gene pyrosequencing indicated an enrichment of Pseudomonas and fermentative organisms upon injection. The sediment and groundwater communities, while different pre-injection, converged on a similar, fermentative community after injection. Correlation analyses suggested that nitrogen-metabolism and fermentation might be driving community changes. Nitrate is another common contaminant at Hanford and an influx of nitrate, a competing terminal electron acceptor, into the Hanford 100-H site may be detrimental to the bioremediation process. To test this, nitrate was injected into the system following a lactate injection to stimulate the resident community. The nitrate injection resulted in a drastic community shift from metal-reducing organisms to denitrifiers and an increase in Cr(VI) concentrations to above background levels. Long after the nitrate injection, denitrifiers continued to be predominant in the groundwater community; however, the sediment community was comprised of denitrifying and metal-reducing populations. This suggests that the effects of an episodic nitrate event are long lasting, but the sediment community is more resilient than the groundwater community. These community analyses have revealed organisms and metabolic properties of interest during polylactate or nitrate perturbation and can be used to improve future bioremediation strategies.