Inter-species reactions

[cneyens]

Is your feature request related to a problem? Please describe.

Inter-species reactions occur at a variety of groundwater contamination sites. For example, chlorinated hydrocarbon pollutions are widespread and undergo sequential or parallel parent-daughter decay reactions, and the fate of BTEX contamination is governed by microbially-mediated redox processes. These types of reactions have extensively been studied and are the subject of many case-studies and modelling efforts in the field of hydrogeology. More recently, studies have shown that long-term PFAS leaching into soil- and groundwater is controlled by the transformation of precursors, which can be simulated using first-order decay reactions.

Although the MODFLOW 6 framework allows for multiple GWT models in the same simulation to simulate multi-species transport, reactions between the species are not available at the moment. MODFLOW 6 can therefore not be used out-of-the-box to simulate these kind of cases. Earlier MODFLOW-based transport codes such as MT3D-USGS or RT3D do have this capability.

Describe the solution you'd like
Incorporating inter-species reactions in MODFLOW 6 similar to those currently available in MT3D-USGS. In the GWT documentation it is alluded that inter-species reactions may become available through a GWT-GWT exchange in the future. Perhaps a separate package could be added for multi-species reactions which couples the corresponding GWT models at the matrix level within the same solution. Immobile species such as solid phases could be represented by GWT models without ADV and DSP.

I realize this is no small feat, and resources may be limited. If this feature request is not feasible or not within the scope of MODFLOW 6, I'm happy to explore other options.

Describe alternatives you've considered
Creating a new package through the API, using an approach similar to the CTS package, or using MT3D-USGS with MODFLOW 6, although that does not take advantage of the full capabilities of MODFLOW 6.

[aprovost-usgs]

@cneyens Adding reactive transport to MODFLOW 6 is something we have considered, but we (the USGS) currently have no plans to implement that. As you said, it would be a substantial undertaking. As you also pointed out, the API offers an opportunity for contributors from the broader MODFLOW community to add such capabilities or to couple MODFLOW closely with existing codes. I'll convert this to a discussion in case there are members of the community who may be working on something along these lines or might be interested in doing so in the future.