GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.4 217 second Word War, the urban area of Bologna city expanded, and, as a consequence of the industrial and agricultural development, the groundwater withdrawal increased dramatically, driving in the area a land subsidence up to 20 mm/y (e.g. Stramondo et al. 2007). In order to monitor the water levels of aquifers the regional agency for environmental protection (ARPA Emilia Romagna) have installed a network of water wells in the Bologna city area (Fig. 1). The present work focus on the water level and vertical ground movement changes occurred during the period 2006-2019. We consider a network of 57 water wells; four of which have provided an hourly water level measurement, and in the remaining 53 wells the water level has been measured with an interval of some months. For these reasons, we deal with a set of wells with different time span and sampling of the measurements. The well times series characterized with the highest sampling rate as well as the ones with a short time span are not considered in the set used in our study. In order to obtain the same sampling rate in the data set we have developed a procedure with a linear interpolation method to resampling all the time series. To the resampled water level time series a principal component analysis (PCA) is applied to extract common seasonal and nonperiodic signals. The analysis shows a significative and sudden variation on the water levels probably correlated to the anthropic activities. With a procedure similar to the one described in Cenni et al. (2012, 2013, 2015) we study the position time series of the continuous GNSS station (CGNSS) located in the Bologna urban city area and its surroundings, to estimate the long term velocity value, amplitude and periods of seasonal signals and noise. In this procedure we estimate average velocity values on the entire time series by a weighted least square method. Differently from Cenni et al. (2012, 2013, 2015) in Fig. 1 - Study area and distribution of the water wells and continuous GNSS stations used in this work. The yellow and red dots represent the automatic and manual water wells, respectively. Diamonds indicate the position of the Continuous GNSS stations.