Coupled processes of groundwater dynamics and land subsidence in Tianjin, China after the South-to-North Water Transfer Project

Over the past six decades, the groundwater became the main source of water following a serious shortage in surface water in the North China Plain (NCP). This resulted in a large area of groundwater level (GWL) depression and land subsidence cones. To address this crisis, the Chinese government implemented the largest water transfer project in human historythe South-to-North Water Transfer Project (SNWTP), the middle route of which was completed and put into operation in 2014. In this context, Tianjin, one of the main beneficiaries of this project, has been relieved from water shortages and begun to implement Groundwater Management Plans (GMP) such as water source conversion and ecological water replenishment for rivers and lakes since 2018, which undoubtedly have a significant effect on the groundwater recovery. Meanwhile, this provides a good case for studying the coupled process of ground settlement and groundwater dynamics, especially the soil deformation pattern driven by groundwater level (GWL) rebound. To analyze these issues in detail, field well data was collected to depict groundwater flow field. Moreover, geodetic data was also collated, including leveling, GPS, and InSAR, so that a vertical deformation field with high spatiotemporal resolution could be generated. The results reveal that the GWL of the third confined aquifer which is the main exploitation layer in Tianjin recovered significantly since 2018 with a rate of 2.1 m/yr. The area of GWL depression cones with a depth greater than 70 m has decreased by 85%. The dynamic deformation patterns indicate that the area of land subsidence cones in Tianjin has reduced significantly, accompanied by a sharply declining subsidence rate (decreased from -32.2 mm/yr to -4.5 mm/yr). Particularly, a significant poroelastic rebound has occurred in the Wuqing and Beichen districts since 2020, with the uplift rates in some areas exceeding 10 mm/yr. Furthermore, due to the delayed pore pressure dissipation in the aquitard, we find a time delay of 0.35.5 years between land subsidence and GWL time series, which is far less than that estimated by hydrogeological parameters, as the latter ignored the recharge and recovery capacity of the aquifer system. Finally, a evolution model in Tianjin was presented to illustrate interactive process among the deformation, pore pressure, and hydraulic head. In general, the SNWDP and the GMP has restored the pore pressure of aquifer, reduced the land subsidence, and alleviated the groundwater storage depletion of Tianjin, China.