Ground penetrating radar surveys for near surface charactarization: example from the Canada Creosote Site, Calgary

Several ground penetrating radar (GPR) traverses were collected across a contaminated site near downtown Calgary, Alberta. At this location, known as the Canada Creosote Site, dense nonaqueous phase liquid (DNAPL) accumulated in the ground during creosote treatment of railroad ties and power poles between 1924 and 1962. In the late 1980's, these liquids were observed to be leaking into the Bow River just west of the downtown core, and a large-scale investigative program was begun by Alberta Environment. It was considered that DNAPL was accumulating in bedrock lows beneath surficial glacial tills, gravels and recent river sediments which are up to 10 m thick. About 1.5 km of GPR data were collected, including profiles along both north and south banks of the Bow River. It was found that good energy penetration was obtained through gravels beside the river, with radar reflections from bedrock recorded from depths of up to 5 m. However, on the flood plains above the river banks, energy penetration was significantly less probably due to the presence of near-surface clays and fill materials which are highly attenuative. For GPR profiles at river level, good agreement was obtained between drillhole data, refraction seismic profiles and interpreted GPR sections. Of particular interest is an amplitude anomaly in the GPR data that was mapped over a limited area. The drillhole data at this location showed DNAPL contamination in bedrock fractures.