Influence mechanism of underground structures on groundwater head and retention time

According to the definition of the barrier effect of underground structures, the concept of equivalent barrier depth was introduced. Analytical solutions for the groundwater head, groundwater retention time distribution, and average retention time in the urban confined aquifer under three boundary conditions of constant flow rate, constant head, and diffuse recharge were derived. Numerical experiments were set up to verify the results, and the results show that the analytical solution is in good agreement with the numerical solution. The results demonstrate that the analytical solution is in strong agreement with the numerical solution. However, the fitting performance deteriorates due to hydrodynamic dispersion when the dimensions of the underground structure are relatively large. Moreover, the analytical solutions of the average retention time agree well with the numerical results in various simulation cases (with a relative error of no more than 5.94%). Even in the three-dimensional flow field, the derived analytical solutions can be used to estimate the average retention time of confined aquifers. Finally, the analytical solution is applied to the confined aquifer flow field before and after the construction of four subway stations in Chengdu Metro Line 2. The calculation results show that compared to the head, the retention time is more sensitive to the parameters of underground structures.