An analytical method to estimate groundwater depletion from a confining layer

An analytical method was proposed to estimate depletion of groundwater in the confining layers through their entire exploitation history and also for limited periods, especially when data on the drawdown history of the pumped aquifer were sparse. The method was based on the solution for drawdown distribution in the confining layer under the condition that the water level was not under equilibrium conditions but resulted from the delayed drainage phenomenon at the outset. First, the method was used on a hypothetical confined regional aquifer system. It suggested that the calculation error of depletion was generally small and that the cumulative released water from the confining layer can be calculated as being closer to the actual total depletion (2.88 km3) than an existing method. Then, the method was applied to estimate the groundwater quantity released from confining layers in the Su-Xi-Chang region of China, where there is a paucity of groundwater level data. The depletion of groundwater in confining layers represented approximately 54 to 82 % of total withdrawals (9.2 km3) from 1965 to 2000. Therefore, the confining layers are the main consolidation layers in Su-Xi-Chang area. Uncertainty in the properties of the confining layers made the depletion calculation subject to error.