Résumé
Responsible mining includes groundwater management. This implies a good knowledge of groundwater flow to manage the risk of groundwater inflow, design the pumping capacity for open pits, or safeguard tailings storage facilities. Here we propose a permeability model for ultramafic rocks weathered in a tropical climate based on field observations and field experiments. Field data, from which the model relies on, have mainly been acquired on the Koniambo massif (New Caledonia) which is a worldwide known lateritic Ni deposit. A positive correlation is observed between the cumulated width of weathered fractures observed in a given depth interval and the hydraulic conductivity measured by packer test in the same interval. Therefore, we propose a two-scale model of weathered fractures being deduced from outcrop data and from a remote sensing analysis. Hydraulic conductivity is 10 (super -5) m/s in the fractures and 10 (super -8) m/s in the matrix. Permeability fields are computed for synthetic fractures network models and results are compared to packer tests. When both sets of large and small fractures are included, the proposed model reproduces reasonably in situ measured data. The decrease of hydraulic conductivity with depth is discussed, using additional data from the Tiebaghi Massif. The proposed model for peridotites aquifers follows the one currently admitted for granitic rock aquifers in the way that weathering creates permeability. It relies on weathering of a pre-existing serpentine-bearing fractures network which controls permeability in the weathering profile itself, but also deeper in the fresh and non-weathered rock. The proposed conceptual model allows deriving quantitative estimate of hydraulic conductivity from field data which are of primary interest for managing the groundwater resource of peridotite aquifers. Since mining operations commonly involve removing most of the weathering profile, occurrence of high conductivity weathered fractures in the basement should be considered. In New Caledonia ultramafic massifs constitute a major interest for water supplies and a Ni resource which is actively mined. The present study could allow for a better management of water resource for the common benefit of New Caledonian people