EVALUATING THE SUSTAINABILITY OF GROUNDWATER WITHDRAWALS FROM CONFINED GLACIAL AQUIFERS

Confined (or buried) aquifers overlain by till confining units provide drinking water to thousands of people in Minnesota. Confined aquifers are thought to be protected from surface contamination, but may recharge very slowly and may be prone to unsustainable groundwater withdrawals. Till hydraulic properties, confined aquifer extents, and connections between confined aquifers are often poorly characterized, but necessary for evaluations of sustainable water withdrawals. A series of steady-state interpretive MODFLOW models was used to (1) investigate a range of long-term surface-water and groundwater responses to pumping from confined aquifers in a variety of hydrogeologic settings and (2) complete a sensitivity analysis to quantify the effects that variations in model parameter values have on the simulated source of water to buried aquifers. The MODFLOW model geometries and hydraulic conductivity parameter ranges were based on observations made in sediment cores and well nests at two well-characterized confined aquifer sites in Minnesota: Cromwell (Superior lobe deposits) and Litchfield (Des Moines lobe deposits). A series of model scenarios demonstrated that pumping-induced hydraulic gradients can be established through tills above confined aquifers. These gradients induced downward flow through 80 feet of low hydraulic conductivity tills (Kv = 0.001 feet per day) which in turn affected the fluxes of groundwater to overlying surface-water features. A second series of model scenarios demonstrated that the relative amounts of water reaching a buried aquifer from above and laterally is highly dependent on the hydrogeologic setting. Variations in the overlying till vertical hydraulic conductivity, the confined aquifer’s lateral connectivity to adjacent till and aquifers, and the areal extent of the confined aquifer resulted in large changes in the proportion of water entering the aquifer from above. This proportion was less sensitive to the thickness of the upper till, the total pumping rate, and the confined aquifer’s horizontal hydraulic conductivity. This interpretive modeling gives insight to the relative importance of hydrogeologic properties for evaluating the sustainability of pumping water from confined aquifers.