Long-term transient groundwater pressure and deep infiltration in Alpine mountain slopes (Poschiavo Valley, Switzerland)

Bedrock aquifers in alpine catchments are important regional sources of freshwater. Data regarding bedrock groundwater-recharge processes are scarce and governing dynamics are poorly understood. The main datasets used to constrain regional groundwater recharge and flow, so far, have been based on indirect methods (environmental isotopes, river discharge rates, tunnel inflows). Here, a unique dataset is presented of long-term pore-water-pressure measurements from five deep boreholes situated in the upper reaches of a mountain slope at 1,500–2,300 m above sea level. In addition to multilevel pore pressure records, a detailed analysis of the hydrogeological conditions in the Alpine catchment is provided, along with the results from investigations of groundwater recharge mechanisms in response to variations in climatic conditions and hydraulic rock-mass properties. The recorded pressure data show annual pressure variations with amplitudes of 5–45 m and responses within a few days to summer rainstorms in the available depth range of 45–277 m below ground surface. One-dimensional analytical pore-pressure diffusion models and numerical infiltration models were applied to investigate pore-pressure dynamics and water-table variations. The model results reproduced the following parameters for the uppermost 100-m-thick layer: the observed amplitudes, rates and delays of pressure increase with porosities of 0.05–0.1%, specific storage of 5E-5 to 5E-7 m^-1, and hydraulic diffusivities of 1E-1 to 1E-3 m²/s. Boreholes located in high-diffusivity areas (strongly fractured bedrock below coarse slope debris) had the strongest pressure variations and were most sensitive to weather conditions.