This chapter describes the functionality of iMOD to perform calculations for density-dependent flow and reactive transport. This functionality comes as a stand-alone feature. More specifically, density-dependent flow and reactive transport calculations are performed with a separate executable,
iMOD-WQ_V5_1_SVN342_X64R.exe. The functionality is called iMOD-WQ (iMOD-Water Quality).
iMOD-WQ is a combination of three codes:
• Seawat v4: the USGS code for density-dependent flow and transport ([Langevin et al.(2008)]);
• MT3DMS v5.3: the University of Alabama code for 3D multispecies reactive transport ([Zheng and Wang(1999), Zheng(2010)]);
• RT3D v2.5: the Pacific Northwest National Laboratory code for 3D multispecies reactive transport for more complex reactions ([Clement(1997), Clement and Johnson(2003)]).
Although iMOD-WQ combines all three codes into one, the original codes themselves are already very much intertwined. Seawat is a combination of MODFLOW and MT3DMS; MODFLOW takes care of the flow calculation whereas MT3DMS calculates the transport of density-influencing compounds (e.g. salts), on its turn influencing flow. RT3D is also based on MT3DMS, but has additional functionality. Its implicit solver allows for the incorporation of more complex reactions (e.g. intra-species reactions, reactions chains, chemical and microbial kinetics, user-defined reactions).
By bringing the three abovementioned codes in the iMOD family, iMOD-WQ allows the user to build and use Seawat, MT3DMS and RT3D models with I/O in iMOD formats and extends most of iMOD’s clipping and scaling functionality to such models. iMOD-WQ models can be invoked using a runfile. Thus, all advantages that come from using a database in combination with a runfile to construct and run a model now also apply to density-dependent flow and reactive transport modelling. The iMOD-WQ runfile, however, has a design that is different from the iMODFLOW runfile. Section 2 describes in detail the design of the iMOD-WQ runfile.
Seawat v4 is based on MODFLOW 2000. This is a different version from the version on which iMODFLOW is based, which is MODFLOW-2005. The discrepancy in MODFLOW versions is the main reason why the iMOD-WQ functionality comes as a standalone feature. Integration of Seawat functionality into MODFLOW-2005 was not opted for, considering the developments of MODFLOW 6, which will also support density-dependent flow.