Simulation of the regional groundwater-flow system in the St. Louis River basin, Minnesota

Abstract

The St. Louis River Basin (SLRB) covers 3,600 square miles in northeastern Minnesota, with headwaters in the Mesabi Range and extensive wetlands and lakes throughout the basin. To better understand the regional groundwater system in the SLRB, a two-dimensional, steady-state groundwater-flow model of the SLRB was developed by the U.S. Geological Survey, in cooperation with the Minnesota Ojibwe Bands, using the analytic-element computer code GFLOW. The parameter-estimation software suite PEST was used to obtain a best fit of the modeled to measured groundwater levels and streamflows. The calibrated regional model was locally refined to create a smaller version of the model, the central SLRB model, that was used to evaluate hydrologic effects from extensive ditching in wetlands of the central SLRB. The refinements included adding ditches that were not represented in the regional model and modifying the aquifer base elevation to be more representative of the localized area. The central SLRB model was recalibrated to better match the distribution of mapped wetlands. Two scenarios were run of the central SLRB model: one with ditches and one without ditches. The model results were compared between the two scenarios to assess the effect of ditching on the groundwater system and potential changes to hydrologic conditions that support wetlands. Calibration of the regional SRLB model resulted in average horizontal hydraulic conductivity values of 6–39 feet per day for the glacial deposits and 3–4 feet per day for the uppermost fractured bedrock in the Biwabik Iron-Formation on the Mesabi Range. Average recharge across the calibrated model was 5.9 inches per year. Linesink resistance for the routed stream network was calibrated by using resistance categories based on the mapped soil hydrologic groups. The modeled regional groundwater-flow direction was generally to the south near the Mesabi Range topographic high and south or southwest across the rest of the basin. The updated calibration of the central SLRB model resulted in average horizontal hydraulic conductivity values of 5–36 feet per day for the glacial deposits and 3 feet per day for the uppermost fractured Biwabik Iron-Formation of the Mesabi Range. Average recharge across the ditch scenarios was 4.1 inches per year. Comparison of the preditch and postditch model scenarios showed that ditching reduced the area where the modeled water table was within 1 foot of the land surface (a wetland hydrology indicator) in as much as 40,000 acres, or 37 percent, of mapped permanent wetlands in the SLRB. An increase in the depth to the water table in wetland areas has the potential to degrade wetland persistence or function.