HR: 15:10h
AN: H52B-07    [PDF]
TI: Characterization of the Different Solute Transport Behaviours in a Fissured and Fractured Chalk by Multi-Tracer Tests and Modelling
AU: * Dassargues, A
EM: Alain.Dassargues@ulg.ac.be
AF: Hydrogeology Group, Dpt Georesources, Geotechnologies & Building Materials (GeomaC), University of Liege, B 52/3 Sart-Tilman, Liege, B-4000 Belgium
AU: Brouyere, S
AF: Hydrogeology Group, Dpt Georesources, Geotechnologies & Building Materials (GeomaC), University of Liege, B 52/3 Sart-Tilman, Liege, B-4000 Belgium
AB: Thirty-five injections of tracers, distributed between 11 sites, were performed under groundwater convergent flow conditions to pumping wells or towards a collecting gallery in a regional Cretaceous chalk aquifer near Liege (Belgium). It was known that the double porosity effect could have a strong influence on the transport behaviour in such a micro-fissured, fissured, fractured and even locally slightly karstified aquifer. The high chalk hydraulic conductivity values are due to fissure flow systems using preferential dissolution along discontinuities and zones of weakness, such as bedding phases and tectonic fractures. In the chalk matrix, however, the hydraulic conductivity value is much lower although the total porosity can be very high. A morphostructural study associated to geophysical prospection results provided information on the main fracturation axis where mostly advective transport was expected. Results of multi-tracer tests, interpretation and modelling in each of these particular sites have allowed to characterize three solute transport behaviours in the chalk aquifer, illustrated by three kinds of typical breakthrough curves. (1) Transport with a dominant advective component: narrow and symmetrical observed breakthrough curves showing maximum velocity of tracer included between 10 and 110 mh-1 (for distances between 5 and 130 m and for fluorescent dyes as well as ionic tracers). The concentration tailing is also rapid as nearly no retardation (adsorption/desorption) and nearly no diffusion into the porous chalk matrix are observed. Groundwater flow occurs mainly in the preferential fissured (or slightly karstified) channels which were most often previously detected by morphostructural analysis and shallow geoelectrical prospection. (2) Transport with advective and dispersive components: more spread-out breakthrough curves are observed with maximum velocity of the tracer from 1 to 10 mh-1. Retardation effects can affect the breakthrough curves creating a non-symmetrical trend. This type of transport behaviour occurs in micro-fissured/ fractured zones of the chalk matrix. (3) Transport with a dominant dispersive component, advection remains but can be considered slow in comparison with (1) and (2): the breakthrough curve is flat and the maximum recorded velocities are lower than 1 mh-1. Retardation and immobile water effects induced a low decrease of the concentration after the peak. This transport behaviour can be considered as typical for the chalk matrix.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting