MODFLOW No-Delay Flow Simulation for Low Permeable Confining Units

MODFLOW is the U.S. Geological Survey modular finite-difference flow model, which is a computer code that solves the groundwater flow equation. The program is used by hydrogeologists over the world to simulate the flow of groundwater through aquifer systems.Compressible low permeable sediments in aquifer systems may occur as discontinuous interbeds or aquitards within aquifers or as extensive confining units adjacent to aquifers. Flow in the extensive confining units can be simulated by MODFLOW through taking the units as no-delay flow model layers. Flow delay was found by Terzaghi in 1925 through his 1D analytical consolidation solution. For a double-drainage confining unit adjacent to aquifers, consolidation degree reaches 99.4% while time factor value reaches 2. A 99.4% consolidation degree during one time step is employed in this paper to denote a no-delay flow model layer in MODFLOW simulation. An additional delay coefficient compared to Terzaghi’s flow delay in a low permeable confining unit was found in this paper to be in a range of 87.5 to 29 which depend on model layer number of the confining unit in MODFLOW simulation. No-delay option requires that the minimum time step used in MODFLOW is larger than a delay time length determined by a production of time factor of 2, time constant and additional delay coefficient of a confining unit. It is not appropriate to apply no-delay option to simulating flow in a confining unit if the minimum time step is less than a determined delay time length with a selected sub model layer number and drainage length of the unit.No-delay option can be only applied to very thin confining units in aquifer systems.