Hydrogeologic-hydrogeochemical multidisciplinary study of the confined gravelly aquifer in the coastal Pisan Plain between the Arno River and Scolmatore Canal (Tuscany)

The gravelly horizon of the Pisa plain multilayered system, located at a depth between 50 and 100 m below the sea-level and about 10-20 m thick, is a confined aquifer tapped by a large number of wells. It contains a very important water resource for drinking, industrial and irrigable uses, although in some cases the groundwater is of poor quality. In order to evaluate if in the coastal area between the Arno River and Scolmatore Canal this aquifer is interested by seawater intrusion and to understand the mixing mechanisms with fresh water, a multidisciplinary study was carried out by means of hydrostratigraphic correlations, water level collection and chemical and isotopic analysis. In particular, three on-site surveys were carried out in June and August 2008 and April 2009. In most of the measured points, piezometric values below the sea-level were collected; particularly depressed levels were registered in August 2008 near the coastline in the southern part of the study area (Calambrone zone) and in the internal part to North (S. Piero a Grado zone). Water-wells sampled along the coast, less than 1.5-2 km far from the coastline, show chemical composition and δ18 O‰ values indicative of seawater-fresh water mixing. This phenomenon, in agreement with piezometric conditions, is more evident in the southern zone, toward the Scolmatore Canal, where the fraction of salt water, calculated using the mass balance model of Cl, Br and δ¹⁸ O‰, is about 7-8%. Most of the other samples, collected up to 5 km from the coastline, were not interested by seawater; in these cases, groundwater shows the same characteristics found toward the internal part of the Pisa plain, with relatively low TDS and δ¹⁸O‰ indicative of recharge average altitudes higher than local altitudes. Only two samples, collected in the North-East portion near the Arno River, showed chemical and isotopic characteristics indicative of a seawater presence of about 8-9%. In these cases, δ¹⁸ O‰ values clearly show that the seawater intrusion does not directly occur in the gravelly aquifer, but trough the shallow sandy aquifer, which in this zone is in contact with the gravel.