Transient Versus Seasonal Geochemical Signals in Water Dripping From Fractured Rocks and Their Related Forcing

Solute transport in fractured porous media presents a large variability in space and time due to variations in water content, in flow pathway and regime, and chemical reactions. The dynamic of these systems in response to external stimuli such as meteorological, mechanical, hydrogeological and chemical disturbances remains poorly known. To build reliable predictive models, there is a strong need to obtain data from complex but well characterized natural observatories. We set up an experiment in a 150 m long tunnel where drippings give access to ground water flowing vertically from 50 m of variably saturated fractured gneiss. This tunnel is located close to and above the artificial Lake Roselend (France) where large variations in level induce reproducible deformation and hydrogeological disturbances. This area is also characterized by contrasted infiltration regimes with alternating snow, rain, and dry periods. In addition, we performed a tracer test with the surface injection of NaCl, 50 m vertically above the dripping water collecting points. In order to get water representative of different contributions of matrix porosity and fractures, we monitored several zones with different structures and flow rates, and looked for changes in flow rates and ion concentrations in water over several years. In addition, we monitored in the tunnel atmosphere the radon activity, independently known to respond to deformation events. Water composition reflects a yearly equilibrium. In spring, concentrations decrease by inputs of rain and melted snow along fractures at high flow rates with limited reactivity. In summer, concentrations increase by diffusion of saline water from matrix porosity and increased dissolution at low flow rates. We observed transient increases of SO4 and Mg concentrations associated with radon bursts, and that are not related to meteorology. Transient enhancement of conductance, with discharges of saline water and radon from the matrix porosity to the fractures, is thus a mechanism able to produce both ground-water and gas anomalies in response to hydrogeological or mechanical processes such as increases in pore pressure or changes in crack geometry. Before the tracer injection, Na, Mg, Ca and SO4 are contributed by the host rock, whereas Cl and K are contributed by meteoric sources, soils and surface biota. After the surface injection of tracers, NaCl breakthrough (Na being late relative to Cl) is associated with an important release of K, Ca, Mg, Ba, and F that were not injected and are contributed by the host rock by ion exchange reactions. By contrast, SO4 is generated by oxidative dissolution of pyrite enhanced by air entry on drought regime and subsequent flush on recharge. Natural and artificial stimuli, even minor, are thus able to trigger major changes in solute transport in fractured rocks, with transient, seasonal or long-term evolutions.