Phase difference between groundwater storage changes and groundwater level fluctuations due to compaction of an aquifer-aquitard system

Drainage and deformation of intercalated confining layers due to internal stress change (discharge/recharge cycle) in an aquifer-aquitard system not only can have great effects on the groundwater storage (GWS), but also can cause unsynchronized water level (WL) fluctuations. The phase relationship between GWS and WL is crucial for the accuracy and attribution of GWS changes. We identify the dominating episodic components of GWS and WL through the Singular Spectrum Analysis (SSA) analysis and Harmonic analysis. First, we analyzed a generic aquifer-aquitard system using numerical simulations which showed that the dissipation of overpressure from the aquitard and flow from it is the inherent cause of a phase shift between the GWS and WL observations. Water released from confining layers with compaction time constant of zero (no-delay) to century time scale results in detected phase shift between total GWS and WL in the aquifer. Then, we analyzed the complex and varied phase relationship between GWS derived using the Gravity Recovery and Climate Experiment (GRACE) data and measured WL time series in the subsiding North China Plain (NCP) aquifer. The spatially varied phase relationship between GWS and measured WL is reasonably related to varied land subsidence development features and may be related with the compaction time constant and thickness of the confining layers. Results of the generic numerical model and the NCP observations suggest that elastic/inelastic response need to be considered in the interpretation and correction of GWS changes for a compacted aquifer-aquitard system.