Leaking recharge mechanism in the multi-layer aquifer system of a typical land subsidence area in Beijing

Abstract. It is of great significance to reveal the mechanism of land subsidence to further find out the mechanism of leakage flow in multi-layer aquifer systems and the water-release compression of weak aquifers. In this paper, conditions of groundwater leakage flow are expounded, and the initial hydraulic gradient calculation formula in the aquitards are deduced. The data of Tianzhu Land Subsidence Monitoring Station are selected to preliminarily discuss the leakage flow mechanism of the multi-layer groundwater system and water-release compression of weak aquifers. The results show that, firstly the weak aquifer layers in the shallow strata above 91.32 m in Tianzhu Land Subsidence Station are all in the state of unidirectional drainage consolidation, and the water heads in the upper strata are higher than those in the lower strata. However, the hydraulic gradient between the two adjacent aquifers is smaller than the initial hydraulic gradient in the weak aquifer layer, so no leakage recharge effect is generated. Secondly, the water level of the two adjacent aquifers in the deep formation below 91.32 m shows a continuous downward trend, among which the weak aquifer is drained and consolidated on both sides, and the pore water head in the middle of the weak aquifer is the lowest. Although the strata has a large amount of compression, no leakage recharge phenomenon occurs. Thirdly, in a multi-layer aquifer system, when the head difference between adjacent aquifers is small and there is a thick viscous layer between them, it is difficult for the upper aquifer to overcome the shear strength of the bound water in the weak aquifer below, leaking recharge barely occurs.