Adding reactive transport capabilities to the 2DSOIL model with the integration of PhreeqcRM 

Process-based crop models are often coupled with soil models to compute the soil water and nutrient status in the root zone. The integration of a geochemical module with existing soil models can enhance their accuracy and capability to simulate additional key bio-geochemical processes. 2DSOIL is a legacy soil model integrated with several prominent process based crop models such as those for maize (MAIZSIM), cotton (GOSSYM), soybean (GLYCIM) and potato (SPUDSIM). However, this soil model lacks a dedicated geochemical component. This study addresses this limitation by integrating the prominent geochemical model, PhreeqcRM, with 2DSOIL using the operator splitting approach, resulting in an improved reactive transport model named ‘2DSOIL-PhreeqcRM’. This new model was validated with two exercises: (i) benchmarking simulated reactive transport against the standard analytical solutions; and (ii) inter-model comparison between cation-exchange simulations from 2DSOIL-PhreeqcRM versus PHREEQC’s built-in transport module. 2DSOIL-PhreeqcRM performed well in both exercises, with a mean absolute percentage error less than 4.75 % and RMSE less than 0.015 mol/l. This research establishes the accuracy and robustness of the 2DSOIL-PhreeqcRM, paving the way for its future use in simulating complex agro-bio-geochemical processes such as the nutrient transformations, precipitation and dissolution of minerals, effect of the addition of lime, ammonia and urea etc.