MODFLOW-6 model of variable-density groundwater flow and brine discharge to the Dolores River in the Paradox Valley, Colorado

MODFLOW-6 with the Groundwater Flow (GWF) and Groundwater Transport (GWT) models was used to simulate the geohydrology of the aquifer system of the Paradox Valley in western Colorado. Salinity of the Colorado River is a major concern in both the United States and the Republic of Mexico. The Colorado River and its tributaries supply water for about 40 million people and irrigation of about 5.5 million acres in the United States, and for about 3 million people and irrigation of about 500,000 acres in Mexico. In Paradox Valley, groundwater transports salt dissolved from buried halite to the Dolores River and is a major natural contributor to river salinity in the Upper Colorado River Basin. Since July 1996, the Bureau of Reclamation has utilized extraction wells adjacent to the Dolores River, known as the Paradox Valley Unit (PVU), to extract saline groundwater and reduce the discharge of salt into the Dolores River. The extracted groundwater has been disposed of into a15,000-feet deep injection well completed in limestone of the Leadville Formation. After more than twenty years of injection, increased fluid pressure has likely induced seismicity and limited future brine disposal into this portion of the Leadville Formation. Alternative strategies to manage the natural discharge of brine into the Dolores River are being considered. A variable-density groundwater flow and transport model was developed to assist evaluation of alternative strategies to manage the natural discharge of brine into the Dolores River. The model focuses on the shallow alluvial aquifer system that is connected to the Dolores River in Paradox Valley. The GWF model simulates variable-density flow with the Buoyancy Package, with density calculated as a function of the concentrations simulated by the linked GWT model. For the simulated hydrogeologic-unit properties, a 1000-year simulation was used to calculate a groundwater concentration and density distribution in reasonable equilibrium with groundwater-flow field simulated with average boundary stresses. The concentration and head distributions simulated at the end of the 1000-year simulation defined the initial conditions for the subsequent 33-year transient simulation, in which boundary stresses varied monthly. The 33-year simulation was calibrated to streamflow and total dissolved solids (TDS) concentrations in the Dolores River from 1987 through 2020 and periodic water-level measurements in observation wells. For the 33-year simulation, TDS flux from the collapse breccia into the model uses a variable rate dependent on precipitation. The 33-year simulation was run with two other configurations, the first setting TDS flux from the collapse breccia to a constant rate equal to the average flux rate in the 33-year simulation and the second setting it to a constant rate equal to the average estimated extraction rate from the PVU extraction wells. Each management scenario simulates 5 years (2021 to 2025) after the end of the 33-year simulation and utilizes monthly average boundary stresses. Scenario 1 simulates no pumping from the Paradox Valley Unit (PVU) extraction wells, Scenarios 2 and 3 simulate different options for reduced PVU pumping, and Scenario 4 simulates reduced irrigation-return flow. Scenario 5 simulates a 5-year drought with no PVU pumping. This U.S. Geological Survey (USGS) data release contains all the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20245038).