MONITORING AND MODELLING GROUNDWATER FLOW SYSTEMS IN HARD ROCK AQUIFERS: EXPERIENCES FROM NORTHERN APENNINES (ITALY)

In Northern Apennines, the northern sector of the Italian “backbone”, hard rock aquifers prevail on karst and porous aquifers. A big portion of the chain is formed by siliciclastic and calcareous turbidites, arenites and ophiolites, both effusive like basalts or intrusive like peridotites. These geological units constitute the most valuable groundwater reservoir of the area in contrast with the overexploited and locally contaminated and salinized alluvial and coastal aquifers of nearby Po plain. A plenty of springs discharge from these huge outcrops of hard rock aquifers; these springs originate from a variety of groundwater flow systems (GFS), connected to hydro-structures of different complexity and depth: from local shallow GFS developed along slopes inside the regolith or the detensioned upper mantle of the hard rock, to regional systems related to main extensional post-orogenic fault zones. In some instances GFS develop inside a sort of plateaus made by a permeable lithology completely surrounded and underlayed by clayey aquiclude complexes. Mean spring flow rate in such a hydrogeological framework is low (VII-VI class of Meinzer), due to the dominant low-medium permeability of the rock mass; the GFS discharge is mainly stream-focused and represents the basis of surface waters environmental flows. Locally, in proper conditions, bigger springs occur, attaining discharges of 10-20 L/s during low flow conditions and so capable to sustain public water supply of local community. A research group formed by the authors is conducting hydrogeological characterization and springs/upreach streams monitoring activity on different test sites, in order to evaluate groundwater resources and to rank different GFS in terms of yield, base flow discharge, recession coefficient and hydrochemistry. The aim of the research follows two main routes: perform an hydrogeological mapping of groundwater resources in Northern Apennines, correlating discharge regime and base flow hydrological behavior with lithology and geological structure, and define a methodology to locate spring head protection areas (this last one with the aid of numerical modeling too). The presented paper reports, in a synthetic way, the main results obtained up to now. Particular emphasis is put in: hydrologic recession analysis in evaluating yield of sub-basins; need to couple springs and stream monitoring to evaluate properly groundwater resources as a whole; necessity of the accomplishment of hydrogeological study with a detailed geological reconstruction of the local hydrostructure; importance of numerical modeling, properly calibrated by continuous discharge monitoring, in help to overcome the limits associated to define recharge areas.