A conceptual hydrogeological model of ophiolite hard-rock aquifers in Oman based on a multiscale and a multidisciplinary approach

Ophiolites are found all over the world: from the Alps to the Himalayas, in Cuba, Papua-New Guinea, New Caledonia, Newfoundland, etc. They are composed of hard rocks-basalt, dolerite, gabbro and peridotite, which are formed at the mid-oceanic ridges, with specific ridge-related tectonic fracturing and intense hydrothermal alteration. Their geological and thus their hydrogeological properties differ from those of both granite or "classical" gabbro and "classical" basaltic lava. A conceptual hydrogeological model of these hard-rock aquifers was developed based on the convergent results of a multidisciplinary approach at several spatial scales, from rock-sample (centimetre) to catchment (kilometre), on well-preserved ophiolite rocks in Oman. In ophiolite rocks, groundwater circulation takes place mostly in the fissured near-surface horizon (approximate to 50 m thick), and, to a lesser degree, in the tectonic fractures. Hydrograph analysis (Water Resour Res 34:233-240, 1977), interpretation of numerous pumping tests using both classical Theis and dual porosity models [Water Resour Res 32:2733-2745, 1996; Comput Geosci J (in press)], and mercury porosity and hydraulic conductivity lab-measurements support the aquifer parameter estimates. The hydraulic conductivity K of the fissured horizon is estimated at 10(-5) to 10(-6) m/s for gabbro and dolerite, and 10(-7) m/s for peridotite. The storage coefficient S of the peridotite aquifer is estimated at 10(-3) and appears to be controlled mainly by microcracks (20 to 100 mu m wide). Tectonic fractures in the ophiolite have similar hydrodynamic properties regardless of lithology (10(-1) T < 10<(-4) m(2)/s and 10(-1)< S < 10(-3)) though the probability of obtaining productive wells is two to three times greater in gabbro and dolerite than in peridotite. Some of the tectonic fractures produce small hydrothermal, hyperalkaline springs in the peridotite. The water budget and hydrochemistry of the Oman ophiolite are characterized and support the conceptual hydrogeological model. Despite low annual rainfall, a relatively low hydraulic conductivity and a significant storage coefficient explain why most of the streams in peridotite are perennial.