SMALL-SCALE DELINEATION AND MODELING OF GROUNDWATER FLOW IN A FRACTURED DOLOSTONE

Groundwater flow in fractured rock is greatly dependent on the nature of the fractures. Different types of fractures, and even different fractures of a same type, can act either as preferential flow paths or barriers to groundwater flow.

Our study involved structural characterization, geophysical surveying and hydrogeological investigation of a flat-lying dolostone formation in order to determine the type of fractures most likely to conduct groundwater flow at the site scale. The field study was undertaken at the St-Eustache quarry near Montréal, Canada. In the quarry, two different types of fractures intersect the dolostone: stylolitic bedding planes and vertical joints. Detailed structural analysis of the quarry walls and floors revealed four different sets of joints based on their orientation. All joints terminate on the bedding planes.

Three vertical boreholes were diamond drilled on the quarry floor, where a series of geophysical measurements were conducted. Pumping tests and pulse tests were also conducted with inflatable packers to evaluate the connectivity between the wells. Integration of geophysical and hydraulic measurements suggests that the aquifer can be divided in two high transmissive zones. The first zone is a surficial aquifer, where vertical joints connect bedding planes. This surficial aquifer is drained by a high-transmissivity bedding plane of great lateral extent, located at a depth of 13 m. The second high-transmissivity zone is a bedding plane located at a depth of 23 m that can be regarded at the site scale as a perfectly confined aquifer. Although transmissive zones have been identified in the dolostone, it is still not clear which type of fracture is more active for groundwater flow.

To investigate whether the bedding plane, the joints, both or part of each type of fracture act as preferential groundwater flow paths, a groundwater flow model is used to simulate hydraulic tests in the dolostone. The discrete fracture model FRAC3DVS is selected and allows the representation of discrete features in the system, as well as the treatment of the dolostone as an equivalent porous medium. Simulation results from different scenarios will be presented to investigate the effect of the bedding planes and the vertical joints on the hydraulic response that could be observed in a well located in the dolostone.