Recent developments in the HPx-codes for coupled reactive transport in porous media

Coupled reactive transport codes are indispensable tools for simulating the fate of solutes in porous media for both environmental and engineering applications. HP1 and HP2/3 are some of the most versatile tools for coupled processes of variably-saturated water flow, multicomponent solute transport, heat transfer, and equilibrium-kinetic chemical reaction networks (Jacques et al., 2018). To date, multiple extensions are included that significantly increase the flexibility of the HPx codes. In addition to the default PHREEQC geochemical solver, HPx provides alternatives for the geochemical step: the geochemical solver ORCHESTRA (Meeussen, 2003) or direct scripting. The ORCHESTRA solver is relatively small and efficient and comes with a large set of user definable adsorption models, including the NICA-Donnan model. The choice of the scripting language has been extended from the classical BASIC scripting language to the structured, prototype-based programming variant of BASIC and Python. The latter gives the possibility to include several libraries of Python immediately in the HPx based models. For example, machine learning techniques can replace computationally expensive geochemical calculations to speed up the calculations. The HPx code is also coupled to the MT3D-USGS code, the groundwater solute transport simulator for MODFLOW. Via the MODFLOW-HYDRUS1D integration, soil flow and transport processes can be integrated as an unsaturated zone component into MODFLOW and MT3D-USGS. The last change is the updated graphical user interface (GUI) for the geochemical model input and post-processing output, incorporated in the standard HYDRUS GUI. Besides, a stand-alone GUI version is available as an advanced interface for geochemical calculations with PHREEQC.

Jacques, D., J. Simunek, D. Mallants and M. T. van Genuchten (2018).  JOURNAL OF HYDROLOGY AND HYDROMECHANICS 66(2): 211-226.

Meeussen, J. C. L. (2003). Environmental Science & Technology 37(6): 1175-1182.