Reactive transport mechanisms and transit times in unsaturated fractured crystalline rocks have been determined from field-scale observations using high-resolution monitoring over a three-year period. In the Roselend natural laboratory (French Alps), a tunnel provided access to the heart of the fractured-rock unsaturated zone, at 55 m depth, where two sampling sites provided access to dripwater samples from zones with contrasting contributions from matrix and fracture flow. Solute concentrations and fluid fluxes were determined, taking advantage of two events during which water with elevated concentrations of chloride (Cl) and other solutes infiltrated into the fractured medium. In one event, approximately 500,0001 of water with high NaCl concentrations flooded an area of 10-100 m above the sampling sites in the tunnel. The other event involved sampling after a Cl-rich rainfall event, which is unusual at this location. Elevated.
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