Groundwater Hydrology

Oral Abstract

Simulation of an aquifer system changing from homogeneous to heterogeneous one due to head-dependent conductivity

Wednesday, January 4
10:30 AM - 12:30 PM
Location: Chu Feng Hall

In most groundwater flow models, both soil compressibility and hydraulic conductivity are assumed to be constant for convenience. Apparently, the assumptions may oversimplify an aquifer system that has stress-dependent parameters such as the specific storage coefficient Ss and hydraulic conductivity K, because sedimentary material is not linearly compressible and deformable. In this paper, an exponential model is introduced for hydraulic conductivity to investigate how the aquifer system responds changes in hydraulic head due to groundwater withdrawal. As a result, the head-dependent K(h) leads to an nonlinear flow equation, and a new package NONK was developed for MODFLOW to run the numerical simulation. The considered conceptual model is a five-layer aquifer system comprising three aquifers and two aquitards as well as a pumping well fully penetrating into the second confined aquifer. In the conceptual model, the upper aquitard is sandwiched with an overlying unconfined aquifer and an underlying confined aquifer and the lower aquitard is sandwiched with two confined aquifers. To understand how an aquifer system is initially homogenous and later changes to be heterogeneous one after turning on the pump, the steady-state flow was concentrated. Comparison was made based on the results of 3D groundwater flow simulation for two different cases. One was modeled using MODLFOW with constant conductivity K0, and the other with the head-dependent K(h) using the modified MODFLOW with the NONK package. Simulation results showed that the applied stress due to pumping changed initially homogeneous layers to be heterogeneous ones. The magnitude of such changes significantly depends on the ratio RC (=Cc/Ck) of two compression indices Cc and Ck that are related to effective volume stress and hydraulic conductivity, respectively. Moreover, comparison of results also indicated that drawdowns in the aquifers were overestimated by using constant hydraulic conductivity or a linear poroelastic stress-strain relation.

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