Dechlorinating and Iron Reducing Bacteria Distribution in a Trichloroethene Contaminated Aquifer
The Operable Unit 5 (OU 5) area of Hill Air Force Base currently has two trichloroethene-contaminated groundwater plumes underneath residential areas in Sunset and Clinton, Utah. Bioremediation by biological reductive dechlorination can be an important mechanism for the removal of chlorinated compounds from the plumes. The presence of suitable bacteria to carry on reductive dechlorination is the key in the bioremediation process. The goal of this study was to determine the distribution and population density of the 16S rRNA genes of Bacteria, Dehalococcoides ethenogenes, Desulfuromonas michiganensis, Geobacter spp and Rhodoferax ferrireducens-like bacteria, as well as the functional genes trichloroethene reductive dehalogenase (tceA) and vinyl chloride reductase gene (vcrA). This study also evaluated the influence of the physical-chemical properties of the OU 5 aquifer material on the observed bacterial distribution. Twenty OU 5 soil cores were obtained from a 14-ha area that included a trichloroethene (TCE) plume. DNA was extracted from each core. Molecular analysis with qRT-PCR was used to quantify the densities of the mentioned 16S rRNA and functional genes. Separately, total arsenic and iron in the II and II+III oxidation states were extracted by two methods (HCl and hydroxylamine HCl in HCl) in each core. Concentrations of TCE, cis-dichloroethene (cis-DCE), vinyl chloride (VC) and ethane in well water were included. Dehalococcoides population density was low and its distribution was uneven with densities lower than 3.2x104 copies/g (detection limit of 2.5x103). D. michiganensis distribution was not uniform but was clustered near the TCE-source area with densities of 7.9x103-1x105 copies/g (detection limit of 6.3x103). Geobacter spp. distribution was uneven but broader, with densities of 4x103-1.6x106 copies/g (detection limit of 3.2x103). The vcrA gene distribution was relatively uniform and broad. Densities were the lowest measured (detection limit of 63 copies/g). TceA was measured in two cores with densities close to detection limit of 1.6x103 copies/g. Rhodoferax ferrireducens-like bacteria had a broader distribution with the highest densities of 1.6x106-1.3x108 copies/g (detection limit of 7.9x103). Total Bacteria were measured in all cores with densities 3.2x104-3.2x106 copies/g (detection limit of 1x103). Three multivariate statistical methods were used to determine the effect of physical-chemical properties on the target gene distributions. Cluster and discriminant analysis selected five properties (pH, sand and silt content, NO3--N and NH4+-N) as the most discriminating factors among thirty-one physical-chemical properties. However, no effect in the bacterial distribution was observed. The tree classification method identified nine variables that described higher or lower densities of the target genes. Variables such as Bacteria, Fe(II+III) (by hydroxylamine HCl in HCl), organic matter and cis-DCE were selected by the method.. All methods agreed on the selection of pH and sand content as the physical/chemical factors most influencing in the bacterial distribution. Based on the findings of low densities of dechlorinating bacteria and dechlorinating-associated functional genes, low available carbon donor and sandy mineral composition, the partial TCE-dechlorination at this site can be ascribed to the nature of the site and incomplete set of required factors for complete reductive dechlorination.
School:Utah State University
School Location:USA - Utah
Source Type:Master's Thesis
Keywords:tce functional genes rhodoferrax ferrireducens bacteria distribution clustering tree clasification method
Date of Publication:12/01/2008