Hydrogeological model of a complex coastal aquifers: the case of Sibari Plain (Southern Italy)

Abstract

The increasing overexploitation of water resources is observed on a global scale in the previous decades; this trend involves the coastal regions of Mediterranean Basin (Van Beynen et alii, 2012). As an effect of increasing groundwater discharge from coastal aquifers, the phenomenon of seawater intrusion is becoming a serious problem for most of the coastal aquifers, especially in the Mediterranean area (COST, 2003; COST, 2005). The aim of this paper is to present the modeling of a coastal porous aquifer located in the complex plain of Sibari (Southern Italy), a plain deeply modified as an effect of reclamation works realized after thirties. The model was implemented using piezometric historical data (from thirties) to establish the effect of seawater intrusion when the well discharge was negligible (natural conditions), the modification in subsequent decades, to be used for forecasting purpose and for evaluate the evolution of groundwater resource. The area actually represents a landscape with anthropic equilibrium based on the works of a land reclamation project (1960s-1990s). Study area is about 365 km2 for a coastline of about 35 km, about which an hydrogeological conceptual model was defined. The area can be conceptualized into three hydrogeological complexes (from the top): sand complex, a clay complex and a sand and conglomerate complex, constituting the deep aquifer, the bottom of which is not well-defined. Shallow aquifer is predominantly fed by direct rainwater infiltration. Deep aquifer is fed by outflows of the mountainous aquifers as the case of limestone aquifer of Pollino Mount,and of shallow granitic aquifer of the Sila massif (Guerricchio and Melidoro, 1975). The maximum piezometric levels of the deep aquifer are equivalent to approximately 40 m asl, so in some areas it presents artesian feature. The computer codes selected for numerical groundwater modelling were MODFLOW (McDonald and Harbaugh, 1998) and SEAWAT (Guo and Langevin, 2002). This groundwater flow modelling is based on the concept of a equivalent homogeneous porous medium by which it is assumed that the real heterogeneous aquifer can be simulated as homogeneous porous media within cells or elements (Anderson, 2002). The modeled aquifer area was uniformly discretized, using Peclet number, into a finite difference grid of 97,735 cells of 240 m x 350 m. For the vertical discretization, model was divided into five layers of variable thicknesses, defined on the basis of a multi-methodological geological survey. Climatic, hydrological and agricultural data were processed to defines inputs for the numerical model based on the variable-density flow. An hydrological balance using monthly and annual of 13 thermo-pluviometric stations falling in the study area, in the time period 1930-1975, was done (Polemio and Casarano, 2008, Polemio et alii, 2013). The numerical model was calibrated with PEST code with a correlation coefficient equal to 0,90. Preliminary results of steady flow and of groundwater salinity spatial are now available and shows it is not sustainable in the case of shallow aquifer and it can be improved in the case of deep aquifer.