SOUTHWEST PACIFIC, HYDRAULIC CONDUCTIVITY, HARD-ROCK AQUIFERS, PERIDOTITE NAPPE, GRANDE TERRE, CONCEPTUAL-MODEL, Ultramafic aquifer, New Caledonia, Boreholes data, Weathered fracture, Multi-scale fracture network, CONVERGENCE, OPHIOLITE, TECTONICS, NETWORKS
We study the relationship between fracture distribution and permeability in weathered peridotites exposed to tropical climate using the data from the Koniambo massif (New Caledonia, South Pacific). The vertical distribution of hydraulic conductivity is obtained from packer tests performed on four 200-m-deep boreholes. A positive correlation is observed between the cumulated width of weathered fractures and the hydraulic conductivity in a given depth-interval. Therefore, a two-scale model of weathered fractures is developed with small fractures (5 m radius) deduced from outcrop data and large fractures (50 m radius) from a remote sensing analysis. Hydraulic conductivity is 10(-5) m/s in the fractures and 10(-8) m/s in the matrix. Permeability fields are computed for the synthetic fractures network models, either from flow along the three axes of a periodic cube or from injection through a slit. Then, results are compared to packer tests and show that when both sets of large and small fractures are included, the proposed model reproduces reasonably in situ measured data. Analytical formulae are proposed for computing an approximate permeability from field data. The decrease of hydraulic conductivity with depth is also discussed. We show that the proposed model for peridotites aquifers is similar to the one currently admitted for granitic rock aquifers. 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, particularly where mining operations impact groundwater flow.