Biobarriers & Fractured Rock

Fractured rock aquifers are among the most difficult sites to remediate, and to date, no known technologies have been completely successful.

The complexity of such sites is due to the fracture framework, which governs groundwater flow, and to the diffusion of contamination in the rock matrix. “Back diffusion,” where the contamination is released into the groundwater, may last for decades. Traditional remediation may include the cost of excavating. However, recent work on a biobarrier concept, showed it has the potential for controlling the transport of contaminants in fractured environments.

Biostimulation of groundwater microbes

The biobarrier concept is based on stimulating groundwater bacteria by injecting nutrients into the fracture framework. The biostimulation promotes the growth of bacteria and production of extracellular polymeric substances (EPS). These secreted substances form the skeleton of a biofilm that fills microfractures. The gelatinous matrix may act as a cut-off wall by decreasing the hydraulic conductivity, or as a treatment wall by breaking down the contaminants via embedded bacteria.

Biofilms are extensively studied in disciplines such as medicine (e.g. for cystic fibrosis) and biotechnology (e.g. for producing ethanol). Studies have focused on avoiding detrimental biofilms (e.g. deterioration of metal and dental surfaces) or generating beneficial biofilms (e.g. biofilm reactors for wastewater treatment).

In the environmental industry, biofilm applications range from water containment to pollutant degradation, including contaminant sorption and binding. Although the biobarrier concept has been investigated for the control of groundwater in porous aquifers, limited information is available on the biobarrier’s potential application in fractured media.

Promising recent laboratory scale studies, however, have shown that groundwater bacteria, when biostimulated with molasses, may develop biofilms as thick as 1.1 mm on a rock surface. Biostimulated in a limestone fracture (having an aperture of 0.5 mm, a width of 5 mm, and a length of 50 cm), groundwater bacteria developed a biofilm that led to a decrease of 4 orders of magnitude in groundwater velocity (initial velocity 2 metres per minute).

A closer look at the bacterial population revealed the presence of as much as 60% of ultramicrobacteria (UMB), which are bacteria having a diameter of 0.3 micrometres (figure 2). Ultramicrobacteria are of interest in biobarrier formation because their low surface hydrophobicity facilitates their transport, and their size allows them to reach smaller fractures compared to “normal” size bacteria (typically 1 to 10 micrometres). This first experimental phase suggested that the bacteria producing extracellular polymeric substances (EPS) are numerous enough in groundwater to sustain the development of a biobarrier providing the nutrient delivery is optimum.

Field test in Southern Ontario

A well characterized and uncontaminated site in Southern Ontario was selected for measuring the extent of bioclogging in a fracture plane, the stability of a biobarrier over time, and the resistance of bacteria to starvation.

Previous work at the site has identified two extensive horizontal fractures; the lowest (approximately 10.5 metres below ground surface) was selected to evaluate the application of the biobarrier.

The configuration included 29 vertical boreholes 76 mm in diameter and cased through the entire thickness of the overburden. The fracture was isolated using a double-packer system, which was equipped with a variety of instrument combinations for monitoring.

The effects of bioclogging on the environment were measured in terms of changes in the flow regime and changes in the physicochemical, microbiological and ecotoxicological conditions. To measure changes in the flow pattern within the fracture, forced-gradient tracer experiments and the point dilution technique were used. Environmental regulations require that the biosafety of in situ bioremediation approaches are addressed. Therefore, at different intervals during the development of the biobarrier, analysis for a battery of potential pathogens, as well as biotests for ecotoxicological changes, will be done.

Reduced flow

After two weeks of applying biostimulation with molasses to the bedrock, measurements indicated a reduction of 70% to 90% in groundwater velocity and typical permeability reductions between 33% and 93% throughout the borehole grid. Ongoing testing aims at measuring the stability of this bioclogging several months after stopping the injection of nutrients. Ecotoxicological assessment revealed no adverse effects on a battery of organisms including local amphibians.

For the next development phase, the research team has built a partnership with firms from the consulting engineering industry likely to apply the biobarrier at contaminated sites, namely Golder Associates and Kinectrics. Both laboratory and field scale work will be undertaken with the goal of investigating the fate of groundwater bacteria in a rock matrix, corroborating the matrix bio-sealing approach, and monitoring the bioclogging of a complex fracture framework.

The proposed concept of biobarriers is attractive not only because of its expected low cost and maintenance, but also for its ability to contain the contamination while being environmentally sustainable and easily dismantled.

Nathalie Ross, Ph.D. is a research scientist with the National Water Research Institute of Environment Canada. She is located in Burlington, Ontario. E-mail nathalie.ross@ec.gc.ca

The research was carried out by the National Water Research Institute, Burlington, Ontario (Dr. Nathalie Ross, M. Greg Bickerton, and Dr. Suzanne Lesage), Queen’s University (Prof. Kent Novakowski), and cole Polytechnique de Montral (Prof. Louise Deschnes and Prof. Rjean Samson). The work was funded by the Program on Energy Research and Development (Natural Resources Canada) and the partners of the NSERC Industrial Chair in Site Remediation.