EVALUATION AND MODELING OF SUBSURFACE BIOBARRIER FORMATION AND PERSISTENCE
|A. B. Cunningham, G. James, and B.K. Warwood, Center for Biofilm Engineering, College of Engineering, Montana State University, Bozeman MT 59717||
One of the first steps for remediation of ground water pollution is often containment of the contaminant plume. Current containment technologies include sheet-pilings and grout curtains. A novel strategy for the containment of ground water contamination is the use of microbial biobarriers to manipulate the permeability and mass transport properties of the aquifer matrix. This approach has the potential to reduce the migration of contaminants from hazardous waste sites and can be coupled with in situ biodegradation or biosequestration of contaminants.
Subsurface biobarrier formation has been studied in relation to secondary oil recovery, where microbial plugging near the well bore has a deleterious effect by reducing the injection rate during water flood operations. Conversely, the addition of bacteria and/or nutrients to oil-bearing formations can result in selective plugging of high permeability zones which enhanced secondary oil recovery from zones of lower permeability. Bio-barrier formation is governed by biofilm accumulation processes including microbial cell growth, adsorption, desorption, attachment, detachment, and filtration.
Although biobarrier technology is a promising bioremediation strategy, several important questions must be answered for successful scale-up to field applications. Previous biobarrier research has utilized bench-scale reactors and often sterilized porous media. Although this approach was important in demonstrating the role of microbial growth in porous media plugging, indigenous microbial communities may have significant affects on biobarrier formation. Furthermore, little is understood of the factors influencing long-term biobarrier stability and the resistance of these barriers to typical ground water contaminants.
The project reported herein utilized relatively large scale reactors (three-foot packed-sand columns and 3x4 foot Iysimeters) and non-sterile conditions as a step towards scale-up. Biobarrier formation was compared in columns of different initial hydraulic conductivities (0.26-4.8 cm/min). The columns were equipped with a series of piezometers to determine changes in hydraulic conductivity along the length of the flow path. The persistence of biobarriers was examined by challenging established barriers with starvation conditions, dissolved strontium and cesium, and dissolved organic solvents.
The results of this study indicate biobarriers are a) resistant to the presence of strontium and cesium, b) resistant to dissolved carbon tetrachloride, and c) can significantly reduce hydraulic flow to near zero for extended periods of time without continuous nutrient addition. The creation of an effective hydraulic barrier throughout the length of the reactors suggests biobarriers are a viable technology for the containment of ground water contaminants and that further scale-up research is warranted.
Key words: ground water, biodegradation, subsurface biobarrier, reactors, hydraulic conductivity
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