Spatial analysis of land subsidence induced by groundwater withdrawal

A comprehensive study of the factors influencing magnitude and distribution of ground settlements observed during the second half of the twentieth century in the area of Bologna (Italy) is presented to derive a unified framework useful for interpreting the observed phenomena and for predicting future scenarios. Information collected over a surface of more than two hundred square kilometres includes previous geological studies, hydro-geological, geotechnical, and topographical investigations carried out with various purposes. The geological features of the whole region have been initially reviewed to figure out the local geological and hydro-geological setup. Then the stratigraphic sequence has been obtained by integrating the results of an extensive campaign carried out over the whole region for water exploitation. The mechanical characterization of the soil has been based on geotechnical tests performed in the area for the construction of new transportation infrastructures. Groundwater levels, periodically recorded on a distributed net of wells, have been interpreted with a two dimensional seepage model capable of back calculating the modification of the groundwater regime induced by water withdrawal. The spatial and temporal distributions of settlements have been derived by combining sequential topographical monitoring campaigns covering a period of about sixty years with satellite records. To simultaneously analyse all information and provide an interpretation of the observed phenomena, all data have been collected in a geographical information system interpolating the measured data with a geostatistical method. In such a way the role of the different factors has been captured, finding a logical correlation between land subsidence, subsoil composition and groundwater withdrawal, and a strategy has been traced which can be exported to the analysis of other similar situations.