Fractured-rock hydrogeophysics with electrically conductive and neutrally buoyant tracers

Alexis Shakas (1) , Niklas Linde (1) , Ludovic Baron (1) , Tanguy Le Borgne (2) , Nicolas Lavenant (2) , Marie-Françoise Gérard (2)

1 ISTE - Institut des sciences de la terre [Lausanne]
2 GR - Géosciences Rennes

Artificial tracer tests help to characterize and understand the dynamics of groundwater systems. This remains a challenging task, especially when dealing with highly heterogeneous formations in which flow can be very localized and the interpretation of tracer breakthrough curves may be ambiguous. As a complement to tracer tests, ground-penetrating radar (GPR) and electrical resistivity tomography can map the space-time migration of electrically conductive tracers. In hydrogeophysics, the most common tracer is dissolved table salt in water. However, conventional salt tracers lead to density effects that are often ignored. Even less than 1% density variations can have a dramatic effect on transport behavior and affect tracer tests in complex ways. Such effects have been demonstrated in our previous experiments that used single-hole GPR to monitor saline push-pull tests in fractured granite. It is possible to model density effects, but this leads to computational complexity and field dynamics that are not necessarily representative of the natural responses of the system. To minimize density effects, we performed a new set of push-pull tests using a neutrally buoyant and electrically conductive tracer at the same test site located close to Ploemeur, France. This novel tracer consists of a mixture of salt (NaCl), water and pure ethanol. Ethanol has a density of 789 g/L at 20° C and is used to counter-act the salt-induced density increase. Our GPR time-lapse images and tracer breakthrough data indicate a largely reversible transport process that confirms the neutral buoyancy of the tracer. Ethanol is biodegradable and does not pose significant environmental issues. Furthermore, calibration of the neutral-buoyant mixture is straightforward to perform in the field using Archimedes principle. Based on these results, we argue that neutrally buoyant ethanol-salt-water mixtures are ideal for a wide variety of hydrogeophysical tracer tests in porous or fractured media.

Domaines: Hydrologie

Alexis Shakas, Niklas Linde, Ludovic Baron, Tanguy Le Borgne, Nicolas Lavenant, et al.. Fractured-rock hydrogeophysics with electrically conductive and neutrally buoyant tracers . American Geophysical Union Fall Meeting 2016, Dec 2016, San Francisco, United States. pp.H31D-1400, 2016. ⟨insu-01416936⟩