title: Modeling variably saturated multispecies reactive groundwater solute transport with MODFLOW-UZF and RT3D.
abstract: A numerical model was developed that is capable of simulating multispecies reactive solute transport in variably saturated porous media. This model consists of a modified version of the reactive transport model RT3D (Reactive Transport in 3 Dimensions) that is linked to the Unsaturated-Zone Flow (UZF1) package and MODFLOW. Referred to as UZF-RT3D, the model is tested against published analytical benchmarks as well as other published contaminant transport models, including HYDRUS-1D, VS2DT, and SUTRA, and the coupled flow and transport modeling system of CATHY and TRAN3D. Comparisons in one-dimensional, two-dimensional, and three-dimensional variably saturated systems are explored. While several test cases are included to verify the correct implementation of variably saturated transport in UZF-RT3D, other cases are included to demonstrate the usefulness of the code in terms of model run-time and handling the reaction kinetics of multiple interacting species in variably saturated subsurface systems. As UZF1 relies on a kinematic-wave approximation for unsaturated flow that neglects the diffusive terms in Richards equation, UZF-RT3D can be used for large-scale aquifer systems for which the UZF1 formulation is reasonable, that is, capillary-pressure gradients can be neglected and soil parameters can be treated as homogeneous. Decreased model run-time and the ability to include site-specific chemical species and chemical reactions make UZF-RT3D an attractive model for efficient simulation of multispecies reactive transport in variably saturated large-scale subsurface systems.
authors.affiliation: United States Geological Survey, 2730 N. Deer Run Road, Carson City, NV 89701. Department of Civil and Environmental Engineering, Colorado State University, 1372 Campus Delivery, Fort Collins, CO 80523-1372.
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Bailey
authors.forename: Eric D
authors.initials: ED
authors.lastname: Morway
authors.forename: Richard G
authors.initials: RG
authors.lastname: Niswonger
authors.forename: Timothy K
authors.initials: TK
authors.lastname: Gates
mesh_terms: D003198:Computer Simulation; D060587:Groundwater / Q000737:chemistry*; D008956:Models, Chemical; D014873:Water Pollutants / Q000737:chemistry*
chemical_list: D014873:Water Pollutants