GNGTS 2014 - Atti del 33° Convegno Nazionale

Time-lapse monitoring of the hyporheic zone of an alpine river using non-invasive methodologies L. Busato, J. Boaga, M.T. Perri, G. Cassiani Dipartimento di Geoscienze, Università degli Studi di Padova, Italy Introduction. The hyporheic zone (HZ) is the area located beneath and adjacent to rivers and streams, where the interactions between surface water and groundwater take place (Reidy and Clinton, 2004; Westhoff et al. , 2011). Therefore, this domain allows the transport of several substances (e.g., water, nutrients, and pollution) (Boulton et al. , 1998) from a stream to the unconfined aquifer below, and vice versa, thus playing a fundamental role in the river ecosystem. The importance of the hyporheic zone in such a complex environment makes its characterization a goal shared by several disciplines, which range from applied geophysics to biogeochemistry, from hydraulics to ecology (Bridge, 2005): Regardless of the field of study, the main aim is always to completely describe the structures and the processes that distinguish this zone. Furthermore, flow and transport models are nowadays key instruments to efficiently characterize the HZ, given their ability of simulating surface water-groundwater exchange phenomena at a local scale (Constantz, 1998; Bianchin et al. , 2010). In order to achieve this common purposes, almost all these disciplines offer many invasive techniques that permit punctual in situ surveys and/or sample analysis (Bridge, 2005); on the other hand, applied geophysics supplies a few non-invasive methodologies (e.g., Electrical Resistivity Tomography – ERT – and Distributed Temperature Sensing – DTS), which allow a high resolution characterization of the hyporheic zone, overcoming the critical problem of measurements under riverbeds. In fact, ERT is a state-of-the-art technique for this kind of surveys, although it is commonly applied in a cross-well configuration or with a superficial electrodes deployment (Acworth and Dasey, 2003; Crook et. al , 2008); conversely, the DTS usage in hydrogeophysics has been developing since the last decade, revealing a wide applicability to the typical issues of this field of study: Based on Raman scattering, this methodology employs heat as tracer and uses a fiber-optic cable to acquire temperature (T) values (Boulton et. al 1998; Anderson, 2005; Selker et al. , 2006; Lane et al., 2008). In this work, we present the preliminary results (first year) regarding the characterization of the hyporheic zone of an alpine river (Vermigliana creek), obtained combining ERT and DTS time-lapse measurements. The typical ERT superficial data collection benefits from an innovative instrumentation deployment, which consists of both an ERT multicore cable and a DTS fiber-optic located in two separated boreholes drilled under the watercourse and perpendicular to it. These acquisition schemes led to high quality data capable to highlight some of the dynamics taking place in the HZ, which, however, still need to be coupled with a flow and transport model, in order to completely describe the domain of interest. The site and the results here described are part of the EU FP7 CLIMB (Climate Induced Changes on the Hydrology of Mediterranean Basins) project. Creek and site description. The Vermigliana creek is the main watercourse of the Upper Val di Sole, Northern Italy, originates from the Presena Glacier (Presanella Group) and is one of the main tributaries of the Noce creek. Being the Vermigliana an alpine river, it presents a nivo-glacial regime, which entails higher flow rates during the summer (due to snow and glacial melting) and lower flow rates during the winter season. A variation in terms of discharges takes place also daily, since morning flow rates are on average higher than those in the late afternoon. The whole valley bottom, where the Vermigliana flows, is entirely filled with heterogeneous glacial till and quaternary slope deposits, made up of material whose granulometry ranges from clays to boulders. Our site is located near the small village of Vermiglio (TN), 1165 meters above mean sea level. GNGTS 2014 S essione 3.2 129