TIME LAPSE ELECTRICAL RESISTIVITY TOMOGRAPHY AND DISTRIBUTED TEMPERATURE MEASUREMENTS IN THE HYPORHEIC ZONE OF AN ALPINE RIVER

River beds and the underlying hyporheic zone, considered as the channel bottoms of streams, rivers, and creeks, are fundamental elements to understand the complex links between surface hydrology and groundwater dynamics. They form the physical support system of the overland water flow, and are one of the most important components for hydrological basin characterization. The travel time of water in the catchment compartments (river-channel, hyporheic, riparian and floodplain zones and connected aquifers) can be the unifying concept for upscaling the hydrological, chemical and biophysical processes that control water quality in a catchment. Hydrological models shall be coupled to biogeochemical models at the catchment scale in order to understand the main drivers and stressors influencing the ecological status of water bodies. It is known how flow dynamics and organic materials influence the nature of the stream bottom, how these areas are subject to change and how they react to events such as storms and floods. Despite this importance, local scale dynamics and exchanges between surface and ground water are not well understood due also to the complexity of measurements under riverbeds. Geophysical time-lapse techniques can provide a dynamic monitoring of such a key transition zone. In this contribution we present the first results of the EU FP7 project GLOBAQUA active in the Adige catchment, Northern Italy. We installed an Electrical Resistivity Tomography (ERT) apparatus and optical fibers under the hyporheic zone in the ‘Val di Sole’ valley. The installation was done thanks to the recent horizontal directional drilling technology capable to be remote-guided during perforation. We installed 48 electrodes below the stream, plus 24 electrodes on the top of the riparian zone. Parallel to the ERT cable we also installed raman technology fiber optic cable for the distributed monitoring of the subsoil temperature. We present here the results of the time-lapse measurement surveys of the first year. The under river transition zone evidences an active underflow dynamics that, to be correctly described, needs accurate distributed variably saturated flow and transport modeling.