Multi-Technique groundwater flow system analysis and dating of deep aquifers in Alessandria Basin (Piedmont-IT)

Abstract

The aim of the study was to set up a protection system from pollution of the deep aquifer of the Alessandria basin, by redefining the recharge areas, focused on this portion of Piedmont territory, and therefore by creating some reserve areas of deep groundwater, to be preserved for future human drinking purposes. In addition to the classical hydrodynamic and geochemical monitoring techniques, the groundwaters were analyzed with reference to a monitoring network of 25 selected wells with deep screens (80-300 m below ground surface) combining radiochemical dating (14C and δ13C of dissolved inorganic carbon-DIC) with anthropogenic tracers (CFCs, SF6) as indicators of recent recharge/ mixing. Stable isotope composition (δ2H and δ18O-H2O) was assessed during a 1-year sampling of snow-rain precipitations gauges distributed in altitude in order to define Local Meteoric Water Lines; the isotopic composition was also seasonally measured for the main rivers and in selected deep-wells. The 3D numerical model was implemented in FEflow platform and calibrated on the basis of the available monitoring data; it was used as a support tool in the delimitation of the recharge areas, starting from the analysis of the distribution of flows. This model was crucial for delimiting the “reserve areas“, since it was able to simulate groundwater flows using both purely advective transport conditions (particle tracking technique), and more realistic conditions of advective and dispersive transport, by introducing dispersive parameters and using the Life Time Expectancy (LTE) reservoir distribution. The integrated use of “traditional“ techniques of regional groundwater flow system monitoring (hydrochemistry, stable isotopic composition) and of dating techniques based on radioactive isotopes and anthropogenic tracers, provided a reliable support to the validation of flow and transport simulation model, oriented to identify recharge areas and “reserve areas“ of future extraction of deep groundwater for drinking purpose.