Long-term spatiotemporal evolution of land subsidence in the urban area of Bologna, Italy

Land subsidence in urban areas is a highly significant and globally widespread geological hazard. This type of ground deformation process commonly occurs in rapidly expanding cities due to the combined effects of structural loading from built infrastructures and excessive groundwater withdrawals due to the increasing water demand of growing populations and industries. In this study, we perform a detailed analysis of ongoing subsidence in Bologna (Italy), with respect to historical pumping trends and a 3D geological model of the subsurface. Since the 1960s, the city of Bologna has experienced severe subsidence attributed to the overexploitation of aquifers for civil water use. Ground deformation peaked in the 1970s, with documented maximum rates of approximately 100 mm/year, causing structural and infrastructural damages. Over the years, the subsidence process has been intensively monitored by local authorities, collecting extensive ground displacement measurements employing different and increasingly sophisticated techniques, including topographic levelling and satellite interferometry. Long-term data are essential for a comprehensive understanding of the subsidence process evolution and for calibrating numerical or statistical predictive models. Therefore, we developed a methodology to integrate ground-based and remotely sensed monitoring data and produce cumulative ground displacement time series and maps, capturing the long-term temporal evolution and spatial distribution of the subsidence process, respectively. The long-term deformation field reconstructed consistently aligns with the 3D geological model of the area, and the produced cumulative displacement curves consistently match the pluriannual trends observed in groundwater level and pumping monitoring time series.