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MT3DMS concentration in MODFLOW-NWT dry cells

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This question involves two related simulations I am reviewing (not creating) that both use MODFLOW-NWT and MT3DMS (i.e., NOT MT3D-USGS).  In the results of the first simulation (steady-state flow), cells in the upper two layers are saturated and have finite concentration values, as calculated by MT3DMS.  These concentration results are then used as the initial concentrations for the second simulation.  In the second simulation, the water table is lowered below the upper two layers (also steady-state flow).  These cells are now above the water table and are "inactive," but the results show non-zero concentrations.  The unsaturated zone is not being simulated.  I understand that water can flow through such "dry" cells in NWT.  However, because the original MT3DMS is used I would have thought that the concentration in the dry cells would be zero, i.e., MT3DMS would mark these dry cells as inactive.  But when viewing the results in GMS, cells that are clearly marked as dry cells have finite concentration values.  I would think that these dry cells would have zero concentration even if MT3D-USGS was being used.  If anyone can help me understand this I would appreciate it.  Thanks.

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Hello,

I think that MTD3MS was released long before MODFLOW-NWT. MODFLOW-NWT does not change IBOUND value to 0 if cell goes dry and this is probably the cause of problems you are facing. I think MT3D-USGS has improved handling of such situation:

MT3D-USGS also includes the capability to route a solute through dry 
cells that may occur in the Newton-Raphson formulation of MODFLOW (that is,
MODFLOW-NWT).
Edited by Michal

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