Conceptual and numerical modelling of the Versilia coastal aquifer (NW-Tuscany, Italy) for quantitative evaluations on 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 that are deeply altering the hydrology of the Mediterranean regions. In order to protect this precious resource, both in quantitative and qualitative terms, an adequate knowledge of the aquifer system is necessary through the development of conceptual and mathematical hydrogeological models. Based on integrated multidisciplinary approach the conceptual hydrogeological model was defined using stratigraphic, hydrogeological and geochemical data elaboration. Subsequently, groundwater flow mathematical models were created using the ModFlow code and Groundwater Vistas like graphical interface. The models allowed to better understand this aquifer system and to identify and, where possible to quantify, the main processes and groundwater components involved. The most important feeding groundwater component, both in terms of water quantity and quality, is the fan of the Versilia River, mainly fed by the river itself in the foothill zone. Even if, 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, in general, the Versilia fan component seems at present to be able to guarantee relative protection against marine ingression. However, this precarious balance could be disrupted 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 occurred in the Apuan-Versilian region it was possible to make considerations about possible effects of these climate regimes on the aquifer system. These 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.