Coupling unsaturated and saturated zone reactive transport : Development and benchmarking of the MTHP tool
Authors: Leterme Bertrand , Jacques Diederik , Neyens Cas 
Published: April 2021

&lt;p&gt;MTHP stands for &lt;strong&gt;M&lt;/strong&gt;odflow &lt;strong&gt;T&lt;/strong&gt;ransport &lt;strong&gt;H&lt;/strong&gt;ydrus &lt;strong&gt;P&lt;/strong&gt;hreeqC and aims to provide an effective coupling tool for simulating reactive transport in the unsaturated and saturated zones. It builds upon the existing codes HYDRUS-1D (&amp;#352;im&amp;#367;nek et al., 2013) / HP1 (Jacques et al., 2018), MODFLOW (Harbaugh, 2005), MT3D-USGS (Bedekar et al., 2016) and PhreeqC (Parkhurst and Appelo, 2013).&lt;/p&gt;
&lt;p&gt;Two-way coupling between HYDRUS 1-D and MT3D-USGS has been implemented. HYDRUS-1D provides a mass flux of solute to the topmost saturated cell in MT3D-USGS. After one time step of solute transport has been solved in groundwater, the resulting solute concentration profile in the saturated zone is updated in HYDRUS. The code has been benchmarked against HYDRUS for a 1-D case but still requires to be adapted for 2 and 3-D cases when solute concentrations change in the unsaturated zone following lateral transport in groundwater.&lt;/p&gt;
&lt;p&gt;Coupling PhreeqC to HYDRUS 1-D was already implemented within HP1. Simulating geochemical reactions in the aquifer required coupling MT3D-USGS to PhreeqC. This has been implemented by adding a new module MCP (&lt;strong&gt;M&lt;/strong&gt;ulti&lt;strong&gt;C&lt;/strong&gt;omponent &lt;strong&gt;P&lt;/strong&gt;ackage) to the MT3D-USGS code using a similar versatile approach as in HPx. MCP has been successfully benchmarked against examples from the similar PHT3D code (Prommer and Post, 2010).&lt;/p&gt;
&lt;p&gt;An application of this new module MCP for the simulation of redox plume development from a landfill, is presented. In this case study, reactive transport in the unsaturated zone is not included (i.e. only the MT3D-USGS &amp;#8211; PhreeqC coupling is used), as the contamination source is suitably conceptualized to be at the water table surface. Kinetic degradation of dissolved organic carbon (DOC) in the presence of several electron acceptors is simulated. Observations of ion concentrations at different points in space and time are used to calibrate the MTHP simulations and investigate what is the acceptable level of process and parameter simplification.&lt;/p&gt;
&lt;p&gt;This research is part of the RESPONSE project, funded by the Belgian Science Policy within the framework of the BRAIN-be programme (contract BR/165/A2/RESPONSE).&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Bedekar, V., Morway, E.D., Langevin, C.D., and Tonkin, M., 2016, &lt;em&gt;MT3D-USGS version 1: A U.S. Geological Survey release of MT3DMS updated with new and expanded transport capabilities for use with MODFLOW&lt;/em&gt;: U.S. Geological Survey Techniques and Methods 6-A53, 69 p.&lt;/p&gt;
&lt;p&gt;Harbaugh, A.W., 2005, &lt;em&gt;MODFLOW-2005, The U.S. Geological Survey Modular Ground-Water Model &amp;#8212; the Ground-Water Flow Process&lt;/em&gt;, U.S. Geological Survey Techniques and Methods.&lt;/p&gt;
&lt;p&gt;Jacques, D., &amp;#352;im&amp;#367;nek, J., Mallants, D., and van Genuchten, M. T., 2018, The HPx software for multicomponent reactive transport during variably-saturated flow: Recent developments and applications. &lt;em&gt;J. Hydrol. Hydromech&lt;/em&gt;, 66(2), 211-226.&lt;/p&gt;
&lt;p&gt;Parkhurst, D.L. and Appelo, C.A.J., 2013, &lt;em&gt;Description of input and examples for PHREEQC version 3--A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations&lt;/em&gt;: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p.&lt;/p&gt;
&lt;p&gt;Prommer, H., and Post, V., 2010, &lt;em&gt;PHT3D: A Reactive Multicomponent Transport Model for Saturated Porous Media&lt;/em&gt;, Version 2.10 User&amp;#8217;s Manual.&lt;/p&gt;
&lt;p&gt;&amp;#352;im&amp;#367;nek, J., &amp;#352;ejna, M., Saito, H., Sakai, M., and van Genuchten, M. Th., 2013, &lt;em&gt;The Hydrus-1D Software Package for Simulating the Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media, Version 4.17&lt;/em&gt;, HYDRUS Software Series 3, Department of Environmental Sciences, University of California Riverside, Riverside, California, USA, 342 p.&amp;#160;&lt;/p&gt;