Investigations into some hydrogeologic parameters of fractured sandstone bedrock at Fredericton, New Brunswick

Abstract

Evidence suggests that groundwater flows into the alluvial Fredericton aquifer from the underlying bedrock. Little is known about the hydro geology of the bedrock surrounding Fredericton and this project investigated its porosity, and the spacing and orientation of fractures. Two independent methods were used to estimate the porosities of bedrock samples. The hydrostatic method gave a porosity for the grey sandstones of2.2% ± 0.5. The SEM and image analysis method gave a porosity for the sandstones of 4.0% ± 3. 1. The t-test, at the 95% confidence level, showed that the mean porosities of the grey sandstones for the two methods were not significantly different. The measured porosities were consistent in the samples collected over a kilometer scale. Fracture trace analysis was carried out on rock cuts along the upgraded Trans-Canada Highway. There were two trends in the orientation of the sub-vertical fractures, northeast-southwest and northwest-southeast. Two fracture sets had spacings of 3 m and 10 m. Solute transport modeling was used to investigate the effects of three parameters, the hydraulic gradient, the porosity and the fracture spacing, on conservative solute transport in Fredericton bedrock. The larger hydraulic gradients produced a significant decrease of the arrival times of a solute. The low porosity of the Fredericton bedrock indicated that a solute traveling through the fracture network would not likely be attenuated significantly by diffusion from the fractures into the bedrock. The spacings were large enough to allow for continual solute mass loss by diffusion into the matrix.