Abstract

Groundwater for a total discharge of about 0.5 m <sup>3</sup> s outflows at Castellammare di Stabia, 30 km south of Naples. Groundwater outflow occurs from several springs located near the coastline, along a stretch of about 1 km, at the base of the carbonate relief of Mount Faito. The 28 springs differ in discharge rate and chemical composition of water. Previous studies have explained the differences of the chemical compositions of the springs as: a) the consequence of shallow and active groundwater circulation in the carbonate aquifer, as regards the main spring of Fontana Grande (discharge of about 0.3 m <sup>3</sup> /s) and b) the consequence of deep groundwater circulation interacting with salt water, as regards the mineral springs of Terme di Stabia (discharge of about 0.1 m <sup>3</sup> /s). New hydrogeological and hydrogeochemical investigation has been carried out to examine closely the causes of the variety of the rate and quality of the Castellammare Springs. These investigations included: i) analysis of the fracturing degree of the carbonate rocks; ii) hydrogeological survey of the area of groundwater outflow; iii) examination of the data of the discharge and piezometric head of the main springs; iv) reconstruction of the water morphology of the area around the springs; v) new chemical analyses of spring water. Results show that an upconing of the fresh-salt water interface occurs in the coastal carbonate aquifer, this upconing is due to the lowering of piezometric head in the aquifer around the main springs. The rise in the interface is seen to be small below the major spring of Fontana Grande, characterised by low temperature, poorly mineralised bicarbonate-calcium water. The rise in the interface is seen to be high below the mineral springs of Terme di Stabia, characterised by low temperature; highly mineralised chlorine-sodium water with considerable gas content (CO <sub>2</sub> and H <sub>2</sub> S). The difference in upconing below the two spring areas is due to the hydraulic heterogeneity of the carbonate aquifer, the geometry and permeability of the aquifers present below the springs, differences in the discharge rate along the stretch and the presence of a deep faulted zone. It is through the faulted zone that deep fluid may rise, which is responsible for the gas content of the mineralised springs. The conceptual hydrogeological model outlined and the variations in chemical composition of water (highlighted by the comparison between previous and present research) provide the starting point to plan the safeguarding of groundwater resources outflowing at Castellammare di Stabia.

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