Simulating the impact of coastal pumping for fish farming purposes on the quality of groundwater: a case study in southern Tuscany, Italy

The Ansedonia promontory (southern Tuscany, Italy) is characterised by the presence of fish farms that pump 48•106 m3/year of thermal (21-24 °C) saline groundwater in order to guarantee the continuity and quality of production (1.7•103 tons/year); the water is extracted from a carbonate aquifer with high permeability due to fracturing and karstification (cavernous limestone), which is also exploited for irrigation purposes (2•106 m3/year) and domestic use (7•106 m3/year). Such exploitation has led to the degradation of groundwater quality, and remedial actions are required to improve the management of local freshwater resources. Geological, hydrogeological and hydrogeochemical studies reveal the extensive and complex circulation of waters within the aquifer system that allow mixing in variable proportions of fresh and salt waters, both superficial (recent) and deep (old); the flow along the freshwater/saltwater interface may even derive from considerable depths, with the upwelling of waters of different age, salinity and temperature. The conceptual model has allowed the development of a 3D finite element flow and transport numerical model using the FEFLOW code. The freshwater budget was negative (9∙106 m3) in the hydrologic year 2004-2005, indicating that the aquifer was heavily overexploited especially due to the quantity of freshwater extracted by fish farms (26 % of the total). Results suggest that fish farming wells should be extended to deeper portions of the aquifer stably occupied by marine waters in order to limit the environmental impact of facilities; this solution, which is being implemented, will also increase our knowledge of aquifer characteristics and zone mixing, thereby improving the model’s ability to reproduce the behaviour of the aquifer under different stress conditions. The model has also allowed a preliminary forecast of the quantitative and qualitative evolution of resources over time, checking the effects of differing hypotheses of management.