Structural geology and geophysics as a key to build a hydrogeologic model of granite rock to support a mine
Abstract. A methodology developed for low permeability fractured media has been applied to understand the hydrogeology of a mine excavated in a granitic pluton. This methodology consists of (1) identifying the main ground water conducting features of the medium, such as the mine, dykes and large fractures, (2) implementing them as discrete elements into a three-dimensional numerical model, and (3) calibrating them against hydraulic data (Martínez-Landa and Carrera, 2005b). The key question is how to identify preferential flow paths in the first step. Here, we propose a combination of several techniques. Structural geology, together with borehole samples, geophysics, hydrogeochemistry and local hydraulic tests aided in locating all structures. Integrating these data yields a conceptual model of the site. A preliminary calibration of the model was performed against short-term (less than a day) pumping tests, which helped in the characterization of some fractures. Their hydraulic properties were then used for other fractures that, according to geophysics and structural geology, belonged to the same families. Model validity was tested by blind prediction of a long-term (4 months) large-scale (1 km) pumping test from the mine, which yielded an excellent agreement with observations. Model results confirm the sparsely fractured nature of the pluton, which has not been subjected to glacial loading-unloading cycles and whose waters are of Na-HCO3 type.