SIMULATING GROUNDWATER/SURFACE-WATER INTERACTIONS NEAR INDUSTRIAL SAND MINES

Over the past several years, more than a dozen industrial sand mines have been permitted to operate on approximately 2,900 acres in western Chippewa County, Wisconsin. Potential effects of industrial sand mining on the hydrologic cycle stem from the use of groundwater to meet operational water needs and from changes to land use and soil structure that could affect infiltration and recharge. This presentation focuses on methods to evaluate groundwater/surface-water interaction in an area characterized by headwater streams that dissect layered sandstone aquifers.

A steady-state groundwater-flow model was developed using the USGS code MODFLOW-NWT to simulate effects on headwater streamflows caused by variations in recharge and pumping from wells supporting sand mines, irrigated agriculture, and municipal supply. Spatially variable recharge generated by a soil-water balance model was applied to the MODFLOW model and adjusted to improve simulation of observed base flow in streams at gages and synoptic measurement locations. The model structure was developed from hydrostratigraphic layering produced through manual contouring and GIS-based interpolation of lithologic descriptions from geologic logs and borehole geophysical data. Shale facies within the Eau Claire and Mount Simon Formations were the basis on which model layers were divided because of their expected spatial influence on base-flow gains and potential influence on the propagation of pumping stresses. The location and depth of high-capacity wells are expected to influence base-flow response to pumping. High-capacity industrial sand wells are generally closer to hydrologic divides and cased into the Mount Simon aquifer, whereas irrigation wells are generally shallow and commonly located in valleys near streams.

Scenarios of estimated pumping and recharge for pre-mining, active mining, and reclamation phases will be used to simulate potential effects of irrigation and mining on area streams and rivers. Insight gained from this analysis can help to guide the siting of pumping wells and inform best-management practices for mined and reclaimed areas.