Abstract: Aquitard is an important part of the subsurface aquifer system. Hydraulic parameters of an aquitard, including the vertical hydraulic conductivity Kv and elastic specific storage Sske, are of great importance for the investigation of land subsidence and groundwater resources evaluation. An analytical solution for the flow in an overconsolidated aquitard is derived in dimensionless form under the condition of drawdown in adjacent aquifers fluctuating cyclically with time. The theoretical curve of cumulative deformation of the aquitard versus time is further derived in dimensionless form as well. Due to the low permeability of the aquitard, its deformation lags behind water level changes in adjacent aquifers. The relationship between the time delay in deformation and half cycle of aquifer drawdown is derived. Then a new method is proposed for estimating Kv and Sske. This method can be applied to an overconsolidated aquitard demonstrating elastic deformation. Before using the new method, the half cycle of cyclical aquifer drawdown and the time delay in aquitard deformation should be determined according to observed extensometer and hydrograph data. The new method is applied to obtain Kv and Sske of the second aquitard at the f10-7 extensometer site in Shanghai area. Field test results demonstrated that Kv and Sske are, respectively, 4.12×10-10 m/s and 1.07×10-4 m-1. The results are verified to be reliable.

Key words: overconsolidated aquitard, vertical hydraulic conductivity, elastic specific storage, time lag deformation