ASSESSING THE POTENTIAL CONSEQUENCES OF SUBSURFACE BIOREMEDIATION: FE-OXIDE BIOREDUCTIVE PROCESSES AND THE PROPENSITY FOR SECONDARY MINERAL PRECIPITATION AND MEDIA STRUCTURAL BREAKDOWN

Bioremediation technologies are often deemed as the most attractive technique for the restoration of groundwater contaminated with unacceptable levels of toxic metals and organics. This presentation discusses investigations on the post-remedial side effects associated with groundwater bioremediation technologies that induce anaerobic conditions to stimulate contaminant degrading microorganisms. Our goal is to provide new fundamental data and numerical simulations to assess the potential consequences of subsurface bioremediation on Fe-oxide bioreductive processes and the propensity for secondary mineral precipitation and media structural breakdown following bioremediation. Our approach involves the integration of laboratory-scale macroscopic fate and transport experiments, advanced micro-spectroscopic techniques, molecular microbiological analyses, and numerical modeling to assess the dynamic changes in subsurface hydrological, geochemical, and microbial processes resulting from subsurface contaminant bioremediation. Experimental and numerical results to date indicate that the rates and mechanisms of contaminant bioremediation may be strongly impacted by the type of Fe-oxide minerals that are present in the subsurface. The bioreduction of less crystalline Fe-oxides is controlled primarily by several specifically defined Fe-reducing bacteria and that more crystalline Fe-oxide bioreduction is controlled by other types of specifically defined SO₄^2- reducing bacteria. This suggests a possible Fe mineralogy dependence on the rates and mechanisms of subsurface contaminant degradation. X-ray Absorption Spectroscopy was used to quantify Fe mineralogy transformations as a function of dynamic flow in the different Fe-oxide media types. It was found that the formation of different types of secondary Fe minerals occurs along the flow path which possibly can impact the rates and mechanisms of contaminant in situ bioremediation. We also discuss the potential impacts of bioremediation on media structural breakdown in undisturbed media. Our goal is to provide site managers and risk assessors with improved knowledge and tools to make better decisions on the methods used to implement in situ bioremediation at contaminated sites.