BIOGEOCHEMICAL CHARACTERIZATION OF A GASOLINE-CONTAMINATED AQUIFER

Hydrocarbon-contaminated aquifers can be remediated by indigenous microbial communities, which use the carbon along with electron acceptors such as oxygen for energy and respiration. Oxygen quickly becomes depleted, forcing microorganisms to use less energetically favorable electron acceptors and creates reducing conditions. Metals such as ferric iron and arsenate are solids, or sorb to sediments under oxidizing conditions, but form soluble species under reducing conditions. Soluble ferrous iron and arsenite species then contaminate groundwater and move with hydrocarbon plumes. Arsenite solubility may be caused by direct reduction of As(V) by arsenate-respiring microorganisms, or through iron reduction, which releases arsenic species adsorbed onto iron oxides. This study investigates the cause of a localized increase in arsenic concentrations in a gasoline-contaminated groundwater in Irving, Texas.

Water and sediment samples were collected along the flow path of the contaminated aquifer. Unstable parameters and redox-sensitive species were tested in the field and groundwater and sediment samples taken at various, corresponding depths. PCR-DGGE was utilized to look for changes in community composition, and backscatter scanning electron (BSE) microscopy was used to look for patterns in colonization of sediments.

Groundwater chemistry shows reducing conditions are occurring at contaminated sites, causing iron reduction. PCR-DGGE data is pending. BSE microscopy showed microbial communities from gasoline-contaminated wells preferentially adsorbed to sediments with higher concentrations of iron and trace amounts of arsenic, suggesting that arsenic release could be related to active reduction of either iron or arsenic, or both. Future work will determine differences in community composition, including the presence or absence of arsenate-respiring microorganisms, in contaminated and uncontaminated sites.