## 1 Introduction  
The development of industry in several major cities in Europe caused groundwater levels to drawdown until they were tens of meters below ground level. As the water tables were so deep, it was often assumed that they were stable and could be safely ignored. Over the last 10-20 years, a reduction in groundwater abstraction has led to a rise in water levels almost everywhere in Europe [1; 2; 3; 4]. Then, since the end of the last century, many European cities have been affected by the groundwater rising phenomenon that is the result in many causes, such as the deindustrialization process, urbanization, and the variation of the aquifers recharge rate because of climate change. These different conditions have led to local rises of the water table, ranging from 3 m to 30-40 m with different consequences [5], with the flooding of underground structures among them. The rising groundwater trends are registered especially in particularly exposed areas, such as the depressions that in the recent past have been the site of lakes, swamps, and peat bogs. On November 2014, the Municipality of Grandate, located 10 km southward of Lake Como, had to deal with a great emergency that was caused by the flooding of numerous undergrounds of buildings and factories that were located in that area. Flooding produced several problems in terms of direct and indirect damages to the economy and the territory. Grandate is located in the Prealps area and in a rather complex geological context. The hydrogeological asset of glacial amphitheatres in the Lombard Prealps has some properties that lead to genesis of stagnation areas [6; 7; 8], caused by the convergence of groundwater flow from the glacial hills (moraines) towards the morphological depressions lying between the morainic arches, that are filled by fluvioglacial and colluvial sediments. Groundwater stagnation areas represent a zero flux and equilibrium points where applied forces are balanced in all directions [9; 10]. The convergence of the flow lines towards these areas makes their drainage difficult, lowering the seepage velocity and generating groundwater stagnation. For these reasons, along the morphological depressions, groundwater level often approaches the ground surface, even generating springs, lakes (Sartirana and Alserio lakes are an example in the Prealps area), and swamps. Analysis and studies about stagnation points and their possible practical applications have been conducted by many researchers [7; 11; 12; 13; 14; 15], providing that, in order to avoid the risks for existing and projected buildings related to this phenomenon, a good knowledge of the system and its behavior is the first requirement. Through the hydrogeological study that was carried out during this work (November 2015-November 2016), the Grandate area has been characterized for the first time--no continuous monitoring data existed in this zone before 2015--and has been recognized as an example of stagnation area. The reconstructed hydrogeological conceptual model has been the base for the implementation of a 3D numerical one that has been run for complete knowledge of the system behavior. Thereafter, the model has been used in order to assess the hydrogeological hazard for the underground structures because of the water table rise. Different scenarios of groundwater system evolution have been defined based on stochastically generated rainfall scenarios. The recharge has been considered as probability distributions and Monte Carlo simulations [16] have been carried out to obtain the probability distributions of the groundwater levels. Therefore, the occurrence probability of groundwater exceeding a defined threshold level has been assessed. Uncertainty is inherent in any model-based risk assessment [17] and this uncertainty is particularly high when analyzing future impact [18; 19].