Abstract

SOLANO-ROJAS, Darío et al. Relation between terrane subsidence InSAR-GPS and depression of the static level in Wells of the Mexico City metropolitan area. Bol. Soc. Geol. Mex [online]. 2015, vol.67, n.2, pp.273-283. ISSN 1405-3322.

The process of land subsidence in the Mexico City Metropolitan Area has been recognized since the beginning of the last century and poses severe challenges for the operation and maintenance of the city's infrastructure. In this work we present land subsidence velocity results from Persistent Scatterers (PSs) through a SqueeSAR interferometric analysis of ENVISAT-ASAR Synthetic Aperture Radar data acquired during the 2003 - 2010 period from nine continuous GPS stations. We then investigated the relationship between the observed subsidence rates and the groundwater level decrease obtained from 180 water well hydrographs distributed throughout the Mexico City Metropolitan Area. Geodetic results indicate differences in land subsidence in the vicinity of the GPS stations, ranging from stable (zero subsidence zones), to slow and rapid subsidence zones. The highest subsidence rates occur in sites with very low local subsidence gradients within the lacustrine sector of the city and vertical GPS velocities of up to -273 mm/yr. Areas with very high local subsidence gradients induce the greatest hazard as they increase the potential for shallow tensile cracks and faulting. The analysis of well hydrographs over the last 20 years indicates groundwater drawdowns of up to 30 m in the most severe cases; groundwater level recovery occurs in very few cases, and only where well locations are within areas of very low subsidence rates. Declining groundwater levels are found throughout the entire range of subsidence values, suggesting that the aquifer-aquitard system is under severe stress due to the extreme groundwater extraction that far exceeds the magnitude of natural recharge. The low correlation between the subsidence rates and the decrease in groundwater levels further suggests that additional variables play an important role in the subsidence process, such as the lithology, the aquitard thickness, water content, the elasto-plastic behavior of the hydrostratigraphy, the drop in pore pressure and groundwater overdraft. A positive correlation is found between land subsidence rates and the thickness of the upper aquitard.

Keywords : Subsidence; groundwater withdrawal; GPS; InSAR; Mexico City.