Articles | Volume 7, issue 3
Method article
01 Jun 2016
Method article |  | 01 Jun 2016

Structural geology and geophysics as a support to build a hydrogeologic model of granite rock

Lurdes Martinez-Landa, Jesús Carrera, Andrés Pérez-Estaún, Paloma Gómez, and Carmen Bajos

Abstract. A method developed for low-permeability fractured media was applied to understand the hydrogeology of a mine excavated in a granitic pluton. This method includes (1) identifying the main groundwater-conducting features of the medium, such as the mine, dykes, and large fractures, (2) implementing this factors as discrete elements into a three-dimensional numerical model, and (3) calibrating these factors against hydraulic data . A 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 sampling, geophysics, hydrogeochemistry, and local hydraulic tests aided in locating all structures. Integration of these data yielded a conceptual model of the site. A preliminary calibration of the model was performed against short-term (< 1 day) pumping tests, which facilitated the characterization of some of the fractures. The 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 excellent agreement with the observations. Model results confirmed 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.

Short summary
In the present study we present a methodology of how structural geology and geophysics techniques, together with hydrochemical and hydraulic data, can help in identifying the main fractures that conduct most of the groundwater flow in a granitic pluton (low-permeability fractured media). Using the values of transmissivities obtained from 3-D numerical models of the local hydraulic test interpretation, we have been able to reproduce the effect of a large-scale and long-term pumping test.