Randy Hiebert¹, Robert Sharp², Al Cunningham³, and Garth James ¹

1 – MSE Technology Applications, Inc., Butte, MT, USA
2 – Environmental Engineering Department, Manhattan College, Riverdale, NY, USA
3 – Center for Biofilm Engineering, Montana State University, Bozeman, MT, USA

A novel technology for the containment of contaminated groundwater has been developed using bacterial biofilms to “plug” aquifer material, producing a subsurface biofilm barrier.  Installation of a biofilm barrier consists of injection of starved bacteria downstream from the contaminant plume via a series of injection wells.  The starved bacteria are environmental isolates selected from the contaminated site for their ability to produce copious amounts of biofilm.  These cells are mass-produced and then starved to reduce their size and enhance their subsurface transport properties.  The starved bacteria are introduced to the aquifer with injection water where they migrate through the porous media and adsorb along the flow paths.  A selective nutrient feed is injected into the aquifer following cell inoculation.  When the nutrient feed reaches the starved bacteria, they are “resuscitated” and begin to grow, attach, and produce a biofilm within the aquifer material.  As the biofilm develops, it fills the pores of the aquifer material, reducing the hydraulic conductivity of the aquifer by as much as 99.99%, attenuating the transport of contaminants further down stream.

A great deal of laboratory and pilot-scale research has been performed to thoroughly understand and develop the starved cell subsurface biological barrier to a point where it can be applied effectively in the field.  This research included: characterizing the transport and resuscitation properties of starved cells in porous media; understanding the benefits and limitations of using starved cells to produce sub-surface biofilm barriers; and developing a protocol for designing, installing and maintaining site-specific biofilm barriers.   The culmination of this research has been a full-scale demonstration of the starved cell biofilm barrier by MSE Technology Applications, Inc. (MSE) in Butte, Montana.  The test cell is a fully contained and hydraulically controlled man-made aquifer that is 180 feet long, 130 feet wide and 20 feet deep.  Prior to barrier formation, the effective ground water velocity through the site was established at approximately 1.0 foot/second.  The demonstration site was homogeneous and had an initial hydraulic conductivity of approximately 0.02 cm/sec., with a constant applied head of 0.002 ft/ft.  The highly permeable soil, low groundwater temperature, and relatively large applied hydraulic head represented a difficult plugging scenario.  In other words, if an effective barrier could be formed under these “worst-case” conditions, the average subsurface situation should be relatively easy to plug.  

In preparation for barrier formation, the bacterial cells were grown in large quantities and starved for 4-6 weeks using a highly effective, proprietary starvation procedure developed by MSE.  The biofilm barrier was installed by injecting concentrated starved cells and a low cost, high carbon, nutrient rich growth substrate into the aquifer using a series of injection wells.  Flow of fresh groundwater was maintained while the bacteria and nutrient solutions were being applied.  Hydraulic conductivity was monitored via a series of monitoring wells up-stream and down-stream of the barrier wall.  The installation and development of the biofilm barrier wall was achieved using a series of injection and recovery wells to concentrate the cell mass, and the subsequent biofilm growth, through the middle of the site.  The well-spacing and pumping rates used to achieve a relatively thin and continuous wall across the site were determined using hydrogeologic modeling software.  This also helped reduce the loss of the starved cell inoculum and nutrient feed due to the high flow rates through the system, thus reducing costs and installation time.   

The initial drop in hydraulic conductivity was rapid, but the full permeability reduction took approximately 8 weeks due to the high initial permeability and relatively large hydraulic head.  In addition, the groundwater temperature of about 7 degrees Celsius slowed the in-situ biofilm growth and development at the site.  However once the biofilm barrier was established a 99.8% reduction in over-all initial hydraulic conductivity across the site was achieved.

Due to the high initial permeability of the site, the relatively high flow rate, and the desire to establish a barrier quickly, the initial feeding of the barrier required more nutrient than initially predicted.  However, once the biofilm barrier was established it remained highly effective for many months without the need for additional nutrient. 

For the first six months after the biofilm barrier was established no nutrient was added and the biofilm barrier maintained an average of 99.7% reduction in hydraulic conductivity.  A small nutrient feed was added to the south end of the wall where the hydraulic conductivity was highest.  This nutrient addition was used as an opportunity to monitor the performance of the biofilm barrier wall in response to maintenance feeding.  After the addition of 0.2 pore volumes of nutrient to the system, the average hydraulic conductivity of the biofilm barrier wall dropped an additional 45% to achieve greater than 99.8% reduction in the initial hydraulic conductivity (from 0.02 cm/sec to 0.00004 cm/sec). 

The biofilm barrier is concentrated along a relatively thin wall across the site.  The hydraulic conductivity of the biofilm barrier increases at the ends of the wall due to side channeling and wall effects.  This diagram demonstrates the ability to effectively and accurately place the biofilm barrier wall in-situ.  The accuracy of the biofilm barrier placement is a function of starved cell subsurface transport characteristics and the combination of nutrient feed loadings, and the spacing and operation of the injection and recovery wells during the installation and development of the biofilm barrier wall.

The starved cell biofilm barrier demonstration is ongoing, and is planned to continue for at least the next 12 months.  During this time, tests will be performed on the barrier wall to determine its long-term performance, and the need and effectiveness of maintenance feeding.  In addition, tests are currently being conducted on the demonstration barrier wall to determine the ability of this particular barrier to carry out in-situ denitrification.  Future plans are being made to demonstrate the starved cell biofilm barrier technology in a number of applications including: containment of leaky underground storage tank plumes, redirection of groundwater plumes to enhance the effectiveness of existing treatment processes, construction of in-situ cut-off walls for reducing salt water intrusion into fresh water aquifers, and the plugging of high permeability zones in oil fields to enhance secondary oil recovery operations.   

The starved cell biofilm barrier wall is an attractive alternative to other barrier technologies (grout curtains, slurry walls, sheet piling, etc.), because it does not require excavation, it utilizes indigenous bacterial populations, it requires minimal maintenance, it has no depth limitations, and installation and maintenance costs are lower than other barrier technologies.  Under reasonable site conditions (hydraulic conductivity and hydraulic head) the barrier wall can likely be installed quickly and requires maintenance feeding only once every 12-24 months.  In addition, the biofilm barrier wall is self-healing with the addition of a small amount of nutrient, and can be constructed around all type of heterogeneities (pipes, clay lenses, rock formations etc.) and in all types of soil (high and low permeability, fractured rock, etc.).  Finally, with the use of horizontal wells, the starved cell biofilm barrier can also be placed under buildings and in areas where the use of large excavation equipment may not be feasible.

Ongoing research includes investigation of new applications of biofilm barriers.  One new type is single and multiple-species reactive barrier walls, which simultaneously reduce plume migration while degrading the contaminant.  Reactive biofilm barrier walls capable of denitrification, and degradation of BTEX, chlorinated aliphatics, and PAHs are currently being developed.

A large portion of this work was funded by the U.S. Department of Energy under contract number DE-AC22-96EW96405.

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