GEOCHEMIAL, MICROBIOLOGICAL, AND GEOPHYSICAL ASSESSMENTS OF ANAEROBIC BIOREMEDIATION OF HEAVY METALS IN GROUNDWATER

Bioremediation methods that precipitate the contaminants in situ as solid (mineral) phases are considered as the most promising technologies because they provide cost-effective options for the removal of toxic metals. Two field experiments were conducted to demonstrate that indigenous bacteria can be stimulated to remove metal contaminants by injection of water-soluble nutrients and sucrose into a shallow water-table aquifer. Groundwater at the investigation site is under aerobic conditions and contained high levels of lead, cadmium, zinc, copper, and sulfuric acid (pH=3.1) derived from a car-battery recycling plant. Lead, cadmium, zinc, and copper were almost completely removed by precipitation of solid sulfide phases as pH increased and Eh dropped. Energy-dispersive X-ray analysis (EDAX) of filtered material from treated groundwater resulted in peaks for Cu, Zn, Fe, and S, indicating the presence of newly formed solid sulfide materials. Pre-injection samples contained genetic sequences resembling Bacillus niacini, which is a spore former and a facultative anaerobe. B. niacini apparently lowered the aquifer redox conditions by consuming O2 after sucrose was added. After injection, DNA analysis of groundwater identified two main organisms: Clostridium acetobutylicium and Desulfosporosinus orientis. C. acetobutylicium is a strict anaerobe that continued to lower groundwater redox state after aerobes turn the system anaerobic. D. orientis is a rapid sulfate-reducing anaerobe and may have been dormant under the initial aerobic condition, but became active and caused iron sulfide minerals to precipitate in the lower Eh conditions as a consequence of nutrient injection. Surface electromagnetics and resistivity surveys show conductivity low or resistivity high anomalies extending downgradient from the injection well to the southwest. Magnetic data also show slightly higher anomalies along the same hydrologic trend. The geophysical results suggest that bioremediation effectively lowers electrical conductance of groundwater by removing sulfate and dissolved metals. Our field study demonstrate that integrated geochemical, microbiological, geophysical analyses, combined with theoretical modeling, provide useful assessment tools for the progress of subsurface bioremediation.