Modelling tools for quantitative evaluations on the Versilia coastal aquifer system (Tuscany, Italy) in terms of groundwater components and possible effects of climate extreme events

The Versilia coastal plain hosts an important and strategic aquifer for water supply. Like all coastal aquifers, it is particularly vulnerable to the saltwater intrusion, which can be amplified not only by fresh water over-exploitation, but also by the effects of climate change, including the increase of extreme events. For an optimal management of this precious resource and for its protection both in quantitative and qualitative terms, an adequate knowledge of the aquifer system is necessary through the development of conceptual and mathematical hydrogeological models. The conceptual hydrogeological model was defined on the base of an integrated multidisciplinary approach with the elaboration of stratigraphic, hydrogeological and geochemical-isotopic data. Subsequently, groundwater flow mathematical models were created using the ModFlow code and Groundwater Vistas like graphical interface, subsequently transferred to the Freewat open platform. The models enabled acquiring further knowledge about this aquifer system and to identify and, where possible to quantify, the main processes and groundwater components involved, including the seawater ingression. An important groundwater component, both in terms of water quantity and quality, resulted widespread in the fan of the Versilia River and mainly fed by the river itself in the foothill zone. Although this component seems to be able to guarantee relative protection against marine ingression, in the summer season some piezometric depressions tied to groundwater exploitation tend to expand and move towards the coast, thus favouring the seawater intrusion process. These issues can be amplified by the extreme rainy events that frequently occur in the Apuan Alps region. The huge quantity of water that quickly flows by the river up to the sea during extreme events represents a lack of feeding respect to the aquifer, and consequently the mitigation role of the fan component towards seawater intrusion can be significantly weakened. Thanks to the water budget achieved by numerical model and considering real extreme events recently occurred in the Apuan-Versilian region it was possible to make considerations about possible effects these climate regimes on the aquifer system. As outcomes, we concluded that extreme events as those occurred in the area in the past, and awaited more frequently in the future, represent a concrete threat for the coastal aquifer system that over next decades could suffer more and more seawater intrusion. Given the reliance of local human activities on groundwater, far-sighted actions of water management (e.g. managed aquifer recharge) are recommended for mitigating such as climate effects.