MF6PQC v1.0: coupling MODFLOW 6 and PhreeqcRM for variable-density reactive transport with evolving porosity and hydraulic conductivity

Abstract. Geochemical reactions can alter groundwater flow by changing solution density and porous-medium properties. Representing these feedbacks requires consistent exchange of chemical states and physical properties between reaction, flow, and transport calculations. We present MF6PQC v1.0, a Python framework coupling groundwater flow and transport in MODFLOW 6 with geochemical reactions in PhreeqcRM. MF6PQC updates solution density and converts mineral-volume changes into changes in porosity, hydraulic conductivity, and diffusion coefficients. It supports three schemes for coupling transport and reactions while retaining existing flow and geochemical model-building workflows. Comparisons with two established reactive transport models showed agreement in component transport, mineral reactions, and the evolution of porous-medium properties. An idealized evaporating salt-lake model reproduced density-driven circulation and evaporite zonation. In a heterogeneous evaporite-leaching aquifer, dissolution concentrated flow along preferential pathways, and density feedback strengthened this redistribution. MF6PQC provides configurable coupling schemes and property relationships for examining these interactions in groundwater reactive transport.