## 5 Interpretation and Discussion
For the remaining period, the groundwater level is relatively stable, with short-term fluctuations reflecting seasonal effects and/or the effects of local groundwater extractions. Piezometer Car Wash 2000 is measuring in the Quaternary aquifer in the centre of the Senne river valley. Here too, the evolution of the groundwater level does not show a clear trend and appears to be mainly influenced by seasonal effects and/or local groundwater extractions. Considering the evolution of groundwater recharge in the deep aquifers below the Kortrijk Formation aquitard, increased pore water pressures and elastic rebound of the area seem to provide a plausible mechanism for the uplift observed during the ERS and Envisat timespan (1992-2010, Figure 5A,B). The centre of the uplift is migrating from the city centre towards northern Brussels during the Envisat timespan. This evolution could be linked either to remaining groundwater pumping activities or either to an increased thickness of the Cretaceous layer or a combination of both. It is well known from drillings data that the Cretaceous is thinner in the SW and thicker towards the NE  
Figure 8: Evolution of the water table in the selected piezometers generally for the time interval ranging between 1988 and 2012 (depending on available data) and for the different aquifers. along the Senne River (Figure 7). The increased thickness implies a delay in the groundwater recharge due to a higher volume of the unsaturated aquifer. The TerraSAR-X data (2011-2014, Figure 5C) indicate a decrease in both spatial extent and rate of uplift, which corresponds with the reduced rates of groundwater recharge observed after 2006 (notably in the Elis and Wash Express piezometers). The lateral migration to the west of the uplift should be associated to groundwater recharge, which cannot be proved because this area suffers from a lack of piezometers. As for the Sentinel 1 data (2014-2017, Figure 5D), the uplift has virtually vanished inside the city centre, and has even reversed within the Senne river valley. This declining trend in uplift rates is further exemplified by averaged time series of PS points located in a 50 m buffer around the Tour & Taxis site, NW of the city centre (Figures 1 and 5A). The PS points show a steady uplift throughout the ERS and Envisat periods, which gradually declines during the TerraSAR-X timespan and reaching more stable fluctuations during Sentinel 1 timespan (Figure 9). Another possibility is that the aquifer recharge has ended and the slight negative values of the SAR data are reflecting only a natural compaction process that has been observed in alluvial plains on recent Quaternary sediments. This natural subsidence effect was never highlighted along the Senne River because it was hidden by stronger positive values related to the aquifer recharge. A comparison between the time series of PS located in a 50 m buffer around the piezometers Elis and Wash Express (Figure 10) was realized. With respect to the scale change between the two types of measurements, it shows that the deformation trends match during the recharge period observed in the piezometers. During the TerraSAR-X period, a decrease of the velocity is visible and corresponds to the reduction of the recharge rate visible in Wash express. This decrease of the recharge rate induced the reduction of the PS velocity but it delayed over time, which is a signal that there is an offset between the recharge and its effect at the surface. The Sentinel 1 PS data show negative velocities of about \(-3\) to \(-5\) mm/year for both piezometers. However, this subsidence trend recorded by Sentinel-1 in the vicinity of the piezometer does not follow the schema of a recharge of the Paleozoic-Cretaceous aquifer recharge. The PS velocity trend in the city centre for the Envisat period of time and TerraSAR-X is characterized by a reduction of the rates towards stable ground movements.