Abstract
In this study, a stress-dependent groundwater model, MODFLOW-SD, has been developed and coupled with the nonlinear subsidence model, NDIS, to predict vertical deformation occurring in basins with highly compressible deposits. The MODFLOW-SD is a modified version of MODFLOW (the USGS Modular Three-Dimensional Groundwater Flow Model) with two new packages, NONK and NONS, to update hydraulic conductivity and skeletal specific storage due to change in effective stress. The NDIS package was developed based on Darcy–Gersevanov Law and bulk flux to model land subsidence. Results of sample simulations run for a conceptual model showed that hydraulic heads calculated by MODFLOW significantly overestimated for confining units and slightly underestimated for aquifer ones. Moreover, it showed that applied stress due to pumping changed initially homogeneous layers to be heterogeneous ones. Comparison of vertical deformations calculated by NDIS and MODFLOW-SUB showed that neglecting horizontal strain and stress-dependency of aquifer parameters can overestimate future subsidence. Furthermore, compared to the SUB (Subsidence and Aquifer-System Compaction) package, NDIS is more likely to provide a more accurate compaction model for a complex aquifer system with vertically variable compression (Cc), recompression (Cr), and hydraulic conductivity change (Ck) indices.
Highlights
Groundwater models have been used to understand the mechanics of an aquifer system and to predict the system response to future extraction scenarios
Land subsidence associated with groundwater extraction is the outcome of the compaction of susceptible geologic layers or units caused by long-term groundwater extraction and under a transient flow condition
If C = Ck, the hydraulic diffusivity becomes a constant that results in a special case, which means that both hydraulic conductivity and the specific storage coefficient are functions of effective volume stress, their ratio can be a constant that allows that a solution can be found using a linear flow equation for a case of material nonlinearity
Summary
Groundwater models have been used to understand the mechanics of an aquifer system and to predict the system response to future extraction scenarios. Subsidence and Aquifer-System Compaction (SUB) [15], have been developed to model land subsidence and compaction of deformable layers based on hydraulic head distribution calculated by MODFLOW. These packages can produce vertical land movement models using constant aquifer parameters, including hydraulic conductivity and specific storage [15]. Soil properties including permeability and compressibility vary significantly with changes in effective stress This fact needs to be considered and appropriately implemented in the modeling process to simulate groundwater flow and land subsidence more accurately [20,21,22,23].
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