Gatto, L.

This study was conducted in western Sicily, in the Marsala and Castelvetrano-Campobello di Mazara coastal plain area. The economy of the area is mainly based on intense farming of specific agricultural crops (citrus, grapes, olives and others in greenhouses). These activities require a substantial and growing amount of water drawn, essentially, from local aquifers. In summers, water demand increases due to high population density. In order to satisfy this demand, numerous wells were drilled throughout the area, in some cases with a density of 10 wells / km2. The uncontrolled groundwater exploitation in the last 10 years has resulted in a drastic decrease in water well levels, consequently allowing extensive seawater intrusion. The main aquifer is a sandy-calcarenitic, plio-pleistocenic complex, interbedded with clayey-marly layers in the lower layers. Multi-layered aquifers are present with the upper layers unconfined. Related studies are also being conducted in this area including monitoring of over 100 wells to better define potential groundwater resources. More specifically, the authors of this study, who are part of the U.O. 4.17 of the Gruppo Nazionale Difesa dalle Catastrofi Idrogeologiche (G.N.D.C.I) (National Defense Group for Hydrogeologic Catastrophes) of the Consiglio Nazionale delle Ricerche (C.N.R.) (National Research Council), are completing a “Pollution Vulnerability Map” of all aquifers in south-western Sicily (1:50.000 scale) using the zoning method for homogeneous areas (G.N.D.C.I.-C.N.R. method). This map is a valid instrument for all authorities governing the management and protection of groundwater resources in the area.

The main results of a hydrogeochemical survey carried out during 2002–2003 along the coast of the south-eastern Sicily, which aimed at geochemical characterization of both groundwater chemistry and submarine groundwater discharge (SGD) in the area are presented. A general frame of chemical processes affecting the studied groundwater and SGD point out that most samples fall within the calcite-anhydrite-dolomite field (CAD). The chemical composition of the samples within the CAD triangle is essentially controlled by calcite, dolomite and gypsum dissolution, which are the main minerals of the carbonate rocks hosting the aquifers. An additional process evidenced in this study is a mixing with seawater. Nitrate is the most typical ion significantly disturbed in the groundwater chemistry influenced by agricultural activities. The strong correlation with SO4 2 indicates that the use of ammonium sulphate fertilisers is widespread in the study area. The K+ vs. NO3 diagram evidences a correlation occurring at lower and higher concentrations, and implies that there is not a common source of both nitrate and potassium, at least on a regional scale. High-phosphate concentration is found in submarine springs along the coast, specifically in the Donnalucata and Avola areas, while its content in inland wells is generally lower. Phosphate is also associated with high-bicarbonate contents in the Donnalucata area, suggesting its possible origin is phosphate-rich carbonate rocks, which are commonly outcropping in the area.

A complex approach in characterisation of submarine groundwater discharge (SGD) off south-eastern Sicily comprising applications of radioactive and non-radioactive tracers, direct seepage measurements, geophysical surveys and a numerical modelling is presented. SGD fluxes in the Donnalucata boat basin were estimated by direct seepage measurements to be from 4 to 12 L s 1, which are comparable with the total SGD flux in the basin of 17 L s 1 obtained from radon measurements. The integrated SGD flux over the Donnalucata coast estimated on the basis of Ra isotopes was around 60 m3 s 1 per km of the coast. Spatial variations of SGD were observed in the Donnalucata boat basin, the average 222Rn activity concentration in seawater varied fromw0.1 kBq m 3 to 3.7 kBq m 3 showing an inverse relationship with salinity. The continuous monitoring carried out at the site closest to the coast has revealed an inverse relationship of 222Rn activity concentration on the tide. The 222Rn concentrations in seawater varied from 2.3 kBq m 3 during high tides to 4.8 kBq m 3 during low tides, thus confirming an influence of the tide on submarine groundwater discharge. Stable isotopes (d2H and d18O) showed that SGD samples consist up to 50% of groundwater. Geo-electrical measurements showed a spatial variability of the salt/ fresh water interface and its complex transformation in the coastal zone. The presented results imply that in the studied Donnalucata site there are at least two different sources of SGD, one superficial, represented by mixed fresh water and seawater, and the second one which originates in a deeper limestone aquifer.