SIMULATING GROUNDWATER RECHARGE AT 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 changes to land use and soil structure that could affect infiltration and recharge. A combination of tools was used to evaluate recharge over time and across the landscape as mines are developed, operated, and eventually reclaimed.

We applied the soil water balance model (SWB), developed by the U.S. Geological Survey and the Wisconsin Geological and Natural History Survey, which estimates deep infiltration, or recharge, by tracking precipitation, interception, runoff, evapotranspiration, and soil moisture capacity. The methodology incorporates daily rainfall and temperature records, soil type, and land use to produce spatially and temporally varying recharge estimates. Based on a 63-year record of precipitation, spatially-averaged annual recharge across this region varies from 2.4 inches in dry years to up to 14.1 inches in wet years, with an average rate of 8.1 inches. Average recharge prior to mining at the permitted sites is about 10 inches per year, higher than the regional average due to the presence of very permeable soils. Additional work with the SWB model includes simulating conditions during mining and in reclaimed areas. Actively mined areas are simulated with increased runoff potential, reflecting the bare rock that is exposed at the surface during these periods. Simulation of reclaimed areas is challenging because of uncertainty about soil properties following reclamation. A limited number of infiltration rates measured at reclaimed sites in Wisconsin indicate that infiltration increases over time at these locations, which we attribute to formation of macropores in the soil structure.

A regional groundwater-flow model developed for this area will incorporate recharge estimates for pre-mining, active mining, and reclaimed areas. The SWB and flow models are designed to simulate potential effects of water use, mining, and changing land use on area streams and rivers. The insight gained from this analysis can inform best-management practices for mined and reclaimed areas.