BALWOIS 2012 - Ohrid, Republic of Macedonia - 28 May, 2 June 2012 1 Salinization and Freshening Processes in Coastal Aquifer of Ferrara (Italy) Beatrice M.S. Giambastiani 1 , Micòl Mastrocicco 1 , Nicolò Colombani 1 , Paolo Severi 2 1 Earth Sciences Department, University of Ferrara, Ferrara, Italy 2 Geological, Seismic and Soil Survey, Emilia-Romagna Region, Bologna, Italy mtm@unife.it Abstract Salinization due to seawater intrusion is one of the major threats to coastal aquifers. Understanding origin and dynamics of the fresh-saltwater interface is a prerequisite for effective management of available water resources. This study identifies salinization and freshening processes on short-term monitoring in the coastal aquifer of Ferrara, Emilia-Romagna Region (Italy). A detailed vertical characterization of the aquifer along a flow line was performed by straddle packer multilevel sampling (MSL) technique. Water types and geochemical processes occurring in the coastal system were described. The base exchange index for application to dolomitic systems (BEX D ) was used to characterize the status of the coastal aquifer. The aqueous geochemical modelling code PHREEQC was used to calculate saturation indexes of calcite and dolomite for water samples collected. Results emphasize that the salinization affects almost the entire coastal aquifer with only shallow zones of freshening in proximity of both irrigation canals and topographic heights, such as palaeodunes, which constitute recharge zone where infiltrating precipitation produces freshening process. The aquifer displays a transition zone of brackish and brackish-salt water type (Cl - from 0.3 to 10 g/l) with a thickness ranging from 2 to 5 m. Saline (Cl - from 10 to 20 g/l) to hyperhaline (Cl - >20 g/l) water type is present in the remaining thickness of the aquifer. The main water types identified in the aquifer are brackish-salt and high-very high alkaline water, and hyperhaline and high-rather extreme alkaline water, dominated by Na + and Cl - and salinization processes. Hyperhaline water is likely due to fossil seawater, relic of Holocene transgression, rather than modern seawater intrusion. The chemical composition of groundwater in this lowland coastal aquifer is mainly controlled by dissolution of evaporite salts formed during the Holocene transgression, cation-exchange processes, and calcite and dolomite mineral dissolution. Keywords: Base Exchange Index; coastal aquifer; freshening; groundwater, salinization, water type Introduction Coastal areas constitute vulnerable ecosystems subject to severe anthropogenic pressure and natural hazards such as sea-level rise, land subsidence, coastal erosion, flooding and salinization (1). Groundwater salinization of coastal aquifers has been objective of research for many years and hydrogeologists have long known that hydrogeochemical tool are needed for on-time identification of spatial and temporal changes in the position of the fresh-saltwater interface for preventing further deterioration of groundwater resources. Coastal areas are often characterized by a complex distribution of freshwater and saltwater, or, in general, of different water types. Understanding origin and dynamics of the fresh-saltwater interface is a prerequisite for effective management of available water resources (2, 3). Generally salinization of coastal aquifers is explained in terms of lateral seawater intrusion in the aquifer, but other processes may be involved, such as inland encroachment of seawater due to overexploitation (4, 5), anthropogenic contamination due to agriculture return flow (6), or presence of fossil saltwater (7, 8).