GROUNDWATER AVAILABILITY AND FLOW PROCESSES IN THE WILLISTON AND POWDER RIVER BASINS, UNITED STATES AND CANADA

The recent oil and gas development in the Williston structural basin and the Powder River structural basin in the Northern Great Plains, provides an opportunity to examine the groundwater and energy nexus in this region. A substantial amount of water is needed for energy development in these basins, and the primary groundwater sources are glacial sand and gravel aquifers and the lower Tertiary and Upper Cretaceous aquifer systems. The U.S. Geological Survey is in year 3 of a 4-year groundwater availability study of these regional aquifer systems, which includes conceptual and numerical models of groundwater flow. The conceptual model has been completed and includes a three-dimensional hydrogeologic framework, potentiometric surfaces, a description of groundwater flow processes, and quantification of recharge and discharge components. The conceptual model is currently being used to develop inputs for the numerical model of groundwater flow for the Williston structural basin, which include initial and boundary conditions, aquifer geometries, and calibration targets. The numerical model was constructed in MODFLOW-NWT modeling software consisting of seven to nine vertical layers with a horizontal grid spacing of one mile. Recharge from precipitation was estimated on the basis of a soil-water-balance model. The Streamflow-Routing Package will be used to simulate the exchange of water between streams and aquifers in the numerical model. A calibrated steady-state model will be used to estimate initial hydraulic properties and conditions for a transient simulation spanning 1960-2005. The transient simulation will be calibrated to hydraulic-head measurements and stream base flows for the same time period. The transient model will be used to simulate aquifer responses to increases in groundwater withdrawals and different climatic scenarios. This study will provide an assessment of how the groundwater resources have changed over time, an estimation of groundwater-flow directions and inter-aquifer connection, and an estimation of the effects of potential future environmental and anthropogenic stresses on groundwater in the Northern Great Plains.

Project web site: http://mt.water.usgs.gov/projects/WaPR/