Metodologie non invasive per lo studio delle zone iporeica e ripariale.

The hyporheic and riparian zones are the regions surrounding and underlying a stream. Their characterization can be carried out through different approaches depending on the discipline involved, for instance applied geophysics, biogeochemistry, geotechnics and so on: regardless of the field of study, the main aim is always to describe the structures and the processes that distinguish this zones. Some non invasive geophysical methods are available to obtain information about this domains. The most commonly applied technique is electrical resistivity tomography (ERT), which can be carried out both from the ground surface or in a cross--well configuration. This methodology is often employed with saline tracers, which help understand the exchanges that take place between surface water and groundwater. Although ERT is nowadays state of the art in the characterization of the hyporheic and riparian zones, literature lacks in examples of electrodes placed under the riverbed: the use of this new geometry of in situ acquisition, together with ground surface configuration, can lead to high resolution datasets that allow the representation of both subriverbed and riparian structures and phenomena. In this work we present the example of the Vermigliana creek, whose catchment has been studied as part of the EU FP7 CLIMB (Climate Induced Changes on the Hydrology of Mediterranean Basins) project. We used 72 electrodes, whereof 24 of them are positioned on the river levees (12 on each side) and 48 lay in a bore drilled under the stream and perpendicular to it, in the hyporheic zone. The ERT acquisitions took place at three different times, approximately once a month (02/07/2013, 30/07/2013 and 27/08/2013). Thus, we obtained three high--resolution datasets that allowed two different approaches: the first consists of three absolute resistivity sections, which represent the state of the hyporheic and riparian zones at the measuring time, while the latter is a time--lapse inversion used to highlight the resistivity modifications that took place during the summer in the domains of interest. As results, we discovered a high--conductivity zone located in the sub-riverbed of Vermigliana creek. The existence of this area is established also by two other ERT acquisitions, carried out putting the multicore cable on the bottom of the streambed. In our opinion this high--conductivity domain can be explained in two different ways, i.e. with the presence of certain bacteria's species or with a high clay fraction, due to the sediment transport put into effect by the Vermigliana creek from the Presanella glacier. On the other hand, time--lapse inversions helped in showing the seepage process that took place in the hyporheic and riparian zones: although the stream was characterized by high flow rates, this datas represented a phenomenon slower then expected, probably because of the presence of a high clay fraction that reduces the hydraulic conductivity. In the end we developed a preliminary hydrological model to describe the seepage process under the Vermigliana creek. Even if it is in an early state, this model well represents the exchanges between surface water and groundwater.