3.3 Site Characteristics
The site characterization program was carried out in two phases and involved a number of different techniques to optimize data retrieval and management decision-making.
Three boreholes were drilled using rotary coring at locations upgradient and approximately 30m and 115m downgradient of the source area. Boreholes were drilled to depths of approximately 55m bgs in the upgradient hole and 42m bgs in the downgradient holes. Cores were retrieved by wireline at 1.5 m intervals and logged for lithology and fracture analysis. During drilling, hydraulic conductivity testing using pump tests and falling head tests were undertaken on packered sections of the borehole. On completion, each borehole was logged using optical and acoustic methods to identify the location, orientation and aperture of fractures for the purpose of locating monitoriing ports
Continuous Multichannel Tubing (CMT) multilevel systems manufactured by Solinst Canada Ltd were installed in each borehole. The CMT system comprises continuously extruded 43mm O.D. polyethylene tubing, which contains seven discrete channels. Monitoring ports are cut in place, sealed with expandable plugs and wrapped with stainless steel screens onsite. The entire system is then inserted into the borehole as a continuous string. In this project, the monitoring ports were backfilled with sand and sealed above and below by bentonite to a tolerance of + 2 cm.
Water levels were measured. Groundwater samples were collected using dedicated inertial lift pumps. Samples were analyzed for pH, Eh, electric conductivity, dissolved oxygen, total dissolved inorganic carbon, major ions, nitrogen species, Fe2+, Mn2+, S2- , petroleum hydrocarbon species, and oxygenates.
Water level measurements showed no significant vertical gradients affecting groundwater flow. Hydraulic conductivity profiling identified zones of higher hydraulic conductivity within the depth range of 27-35m bgs A contaminant plume comprising BTEX and MTBE/TAME extending to a depth of approximately 10 m below the water table was identified 30m downgradient from the tank source area. At 115m downgradient only the MTBE/TAME plume remains. Between 30m and 115m, the BTEX plume is attenuated.
Given the lack of downward vertical gradients to transport contamination to depth, the occurrence of contamination well below the water table could have resulted from displacement of LNAPL in vertical fractures and fluctuations in the water table. Theoretical calculations show that a continuous LNAPL column of 6.3 m in the spill area would be sufficient to drive contaminants to the observed depth in the source zone.
Calculated groundwater velocities suggest that groundwater has traveled 2000m since contaminant release. The limited extent of the BTEX plume combined with the distribution of redox sensitive species in groundwater suggests that BTEX is being aerobically degraded within a short distance of the source and that MTBE may be aerobically degraded to tertiary butyl alcohol (TBA) further downgradient.
While fractures control groundwater flow in the chalk aquifer, mass balance calculations suggest that as much as 90% of groundwater contamination could have diffused into the chalk matrix. As contaminant concentrations decrease in the fractures, diffusion from the chalk matrix back into the fractures will provide a long term source of groundwater contamination.
A revised conceptual diagram is shown in Figure 4.
Detailed information defining aquifer characteristics, geochemistry and contaminant distribution, conducted by an experienced team using strategically placed multilevel systems, has demonstrated that a contaminant plume resulting from a significant release of gasoline into a fractured rock aquifer can be managed by natural attenuation.
The process has resulted in a workable and cost effective solution that would have been difficult to reach with the same certainty and at the same cost as a program using a traditional approach of single well completions.