The study site is underlain by 3 meters of clayey-sand overburden. The upper 20 meters of bedrock at the site consist of flat-lying, thinly bedded shale that contains numerous limestone interbeds. Two extensive horizontal fractures are associated with the sharp contacts found between the limestone and shale.
Targeted Environmental Media:
 Fractured Bedrock
- Flow
- Tracer (dye) Test
- Other (Point dilution tests; tracer experiments)
Comments:
Point dilution tests provide a direct method for measuring ground water velocity and were used to monitor changes in the magnitude and distribution of velocity in the fracture. The approach involves injecting a pulse of conservative tracer into an isolated section of a borehole and monitoring the decrease in concentration as the ground water flow gradually dilutes the tracer. The rate at which the concentration of the tracer decreases can be related to the ground water velocity within the fracture. These tests were performed simultaneously in the 12 borehole locations with mixing systems and were repeated at routine intervals during the experiment.
Tracer experiments were conducted on a routine basis during the experiment and just before the nutrient was injected. The tests were conducted in an injection-withdrawal format that involved injecting two tracers bromide and Lissamine FF (fluorescent dye) into the injection well and monitoring their arrival in the withdrawal wells and intermediate boreholes. The results were used to monitor changes in the average velocity between the two wells and to identify the presence of obstructions in the natural flow paths.
- Other (Biological Barrier)
Comments:
The use of biological barriers (biobarrier) has been suggested to control and limit ground water movement in fractured rock aquifers, where the remediation options are limited. Biobarriers consist of biofilms that develop after indigenous bacteria or injected bacteria are stimulated to grow, have attached on rock surfaces, and have secreted exopolymeric substances.
A demonstration of the biobarrier concept was undertaken in Mississauga. The site configuration consists of 29 vertical boreholes that provide access to a fracture in a 25- by 25-meter area. Each borehole was equipped with pneumatic packers to hydraulically isolate the fracture from other influences and to minimize well-bore storage. The packer systems were equipped with a variety of instrument combinations to monitor development and effectiveness of a biobarrier in the fracture. A pressure transducer and thermistor were installed to measure changes in hydraulic head and temperature within the fracture.
Twelve boreholes were fitted with a mixing system and sampling lines. The mixing systems were required to conduct the various tests (point dilution tests and tracer experiments) that were used to measure changes in the flow system as the biofilm developed. The sampling lines permitted ground water samples to be collected from the fracture so that changes in microbial and physiochemical conditions could be monitored.
Biostimulation was conducted by injecting molasses and a nitrate source at a rate of 5 milliliters per minute (mL/min) for 3 weeks. The biostimulation was then stopped for 1 week for tracer experiments. Injection of nutrients then resumed for 3 more weeks.
This study aimed at (1) measuring the efficiency of biostimulation to bioclog a planar fracture, and (2) testing the reliability of monitoring tools, such as point dilution tests, as indicators of biobarrier development.
In situ monitoring and monthly ground water sampling are under way to document the efficiency of biobarrier development.
References:
Ross, Nathalie; Greg Bickerton; John Voralek; Suzanne Lesage; Kent Novakowski; Louise Deschenes; Rejean Samson. 2003. Initiating a Field Demonstration of the Development of a Biological Barrier in Fractured Shale. The Seventh International In Situ and On-Site Bioremediation Symposium, Orlando, Florida, June 2-5.
|
