Chapter 16 Reactive Transport and Residence Times in Unsaturated Fractured Rocks from Field-Scale Experiments

Abstract

Abstract 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 500,000 l of water with high NaCl concentrations flooded an area of 10–100 m 2 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 Na concentrations from the NaCl spill triggered cation exchange reactions, which apparently occurred mainly in the soil above the bedrock, releasing Ca, Mg, and K. Concentrations of Cl and the exchanged cations during transport through the fractured rock were controlled primarily by dilution, although Na–Mg exchange occurred to a limited extent. Chloride concentrations significantly above background were observed one day after the beginning of the NaCl spill. Average travel times estimated from moment calculations of breakthrough curves from the two events ranged from 25 to 100 days at the sampling site dominated by flow through major fractures and 200–300 days at the site dominated by matrix or sub-order fracture flow. Comparison with results of flow-model simulations conducted using reconstructed fracture networks indicated that rapid flow occurs at near-saturation conditions in the fractures and that slower flow rates occurred at lower degrees of saturation. Thus, it is important to consider both variable saturation and fracture versus matrix flow in order to understand fluid fluxes and reactive transport in unsaturated, fractured media. This study illustrates how long-term, high-resolution monitoring of solute concentrations and fluid fluxes can provide valuable insight into flow and transport in these complex systems.