Numerical simulation of the Rhade effect in layered aquifer systems due to groundwater pumping shutoff

A series of numerical simulations using a fully coupled poroelastic numerical model is performed to analyze the so-called Rhade effect. A three-layer aquifer system composed of two aquifers separated by an aquitard, and a corresponding single-layer aquifer system composed of an equivalent lumped material are simulated for the purpose of comparison. In the numerical simulation of the layered aquifer system, the Rhade effect is observed in the aquitard and upper aquifer immediately after the stop of groundwater pumping from the lower aquifer. In contrast, the numerical simulation results of the lumped aquifer system do not show such Rhade effect throughout the entire domain during the groundwater pumping shutoff period. These numerical simulation results strongly suggest that hydraulically less permeable and mechanically more deformable aquitards that generally exist in layered aquifer systems particularly play an important role in causing the Rhade effect at the end of groundwater pumping. The Rhade effect is caused by two mechanisms: a slower hydraulic propagation (head recovery) of the unpumping stress than its mechanical propagation (extension) from the pumped aquifer into the adjacent aquitard and unpumped aquifer due to the relatively lower hydraulic conductivity of the aquitard, and an amplification of the faster mechanical propagation (excessive extension) in the lower part of the aquitard due to its relatively higher deformability. However, the unpumping stress is evenly distributed throughout the entire domain of the lumped aquifer system over time without such hydrogeomechanical mechanisms since it does not have an aquitard and hence is hydraulically and mechanically homogeneous.