SUPHRE: A Reactive Transport Model With Unsaturated and Density-Dependent Flow

Although unsaturated and density-dependent flow affect solute fate in groundwater, they are rarely both included in reactive transport models. Using the operator-splitting method, a new reactive transport model (SUPHRE) was developed by combining a variable-saturation and variable-density multiple-component solute transport model (SUTRA-MS) with a geochemical reaction module (PhreeqcRM). In contrast to existing reactive transport models, SUPHRE accounts for both unsaturated and density-dependent flow. Model setup for SUPHRE is convenient, as only one setup file is required in addition to the usual input files of SUTRA-MS and PhreeqcRM. By further implementing a time-variant boundary condition into SUTRA-MS, SUPHRE can simulate multi-component reactive transport in tidally influenced coastal unconfined aquifers where unsaturated and density-dependent flow prevail. Two examples were used to validate the new reactive transport model, including single-species decay and sorption within a one-dimensional soil column and a four-species decay chain in a two-dimensional aquifer. Following validation, SUPHRE was adopted to reveal unsaturated flow effects on oxygen consumption and nitrate reduction in tidally influenced coastal unconfined aquifers. Whether for simulating oxygen consumption or nitrate reduction, there were visible deviations between numerical results without and with unsaturated flow, highlighting that unsaturated flow can affect reactive solute transport and transformation.