A SURFACE-WATER – GROUNDWATER MODEL TO EVALUATE AQUIFER SUSTAINABILITY IN WASHINGTON COUNTY, MINNESOTA

The City of Woodbury is a rapidly growing eastern suburb of St. Paul that relies totally on groundwater for its water supply. Plans by the City to install as many as 13 new high-capacity wells in the Prairie du Chien-Jordan aquifer have called into question the long-term sustainability of groundwater resources in this area. Other nearby communities also rely on groundwater, as does a state-designated trout stream. Washington County, in conjunction with local government units, developed a surface-water – groundwater flow model of the area with funding from the Legislative Commission on Minnesota Resources to address questions of sustainability. The code MIKE SHE, with a MODFLOW-based saturated flow simulator, was used to model groundwater flow in surficial and bedrock aquifers and to simulate many of the surface hydrologic processes. MODFLOW adequately simulates the interplay between recharge, transmissivity, leakage, and withdrawal, which controls the shape of potentiometric surfaces. However, the long-term ability of the aquifer system to supply groundwater for water supplies and base flows to streams is primarily a function of the rate of recharge. MIKE SHE simulates the areal and temporal distribution of recharge using distributed parameters of vegetation, soil type, deeper unsaturated soils, topography, land use, channel flow, seasonal climate, and precipitation. Recharge in MODFLOW was first estimated through an inverse-optimization process using PEST2000 in order to calibrate the saturated flow model to steady-state heads and drawdown from a Woodbury well aquifer test. MIKE SHE was then used to develop a forward simulation of recharge on a daily basis for the period 1975-2003, which included normal, wet, and drought conditions. For many of the simulated years, the annually integrated recharge agreed well with the optimized MODFLOW recharge parameter values (about 4 to 6 inches per year) but during drought years, there was a predicted net loss of water from the saturated zone through evapotranspiration. The results are being applied to monitoring data to discriminate between groundwater level fluctuations due to climatological variations in recharge and the effects of pumping of new water-supply wells. The models will also be a tool for groundwater use and stormwater infiltration planning.