SOUTH-CENTRAL MINNESOTA GROUNDWATER MONITORING OF THE MT. SIMON AQUIFER

The deepest bedrock aquifer of southeastern Minnesota is the thick (50 to >200 feet) Cambrian Mt. Simon sandstone aquifer. This aquifer supplies water to over one million Minnesotans. Water levels in this aquifer have been declining in the Mankato area (locally) and Minneapolis/St. Paul metro area (locally and regionally) from municipal and industrial use. To better understand the regional recharge of the Mt. Simon aquifer in south-central Minnesota, the western edge was investigated where it was not likely to be overlain by impermeable Paleozoic shale. This investigation included observation well installations at 14 sites, water level monitoring, groundwater chemical analysis, and aquifer capacity testing to help determine recharge pathways and sustainable limits for this aquifer.

The combination of chemical residence time indicators, continuous water level data from nested well locations, and a general knowledge of the regional hydrostratigraphy, shows the Mt. Simon aquifer is very slowly recharged from a large source area located south of the Minnesota River and a smaller source area north of the river. The younger ¹⁴C residence times of Mt. Simon aquifer groundwater (7,000-8,000 years) roughly correspond to a time after the last ice sheet had receded from southern Minnesota, suggesting groundwater in the Mt. Simon aquifer in this region originated as precipitation that infiltrated during the post-glacial period. The stable isotope data of oxygen and hydrogen support this conclusion. A recharge estimate of the Mt. Simon aquifer south of the Minnesota River based on these minimum residence time data suggest an infiltration rate of approximately 2 cm/year. The resulting 5 billion gallons/year of recharge to the aquifer from the southern source area is approximately equal to permitted appropriations. At current pumping rates the water levels in the Mt. Simon aquifer appear to be in a steady state on a regional scale. Increased pumping in south-central Minnesota from the Mt. Simon aquifer may cause declining water levels. If long term water-level data acquisition is continued from the wells installed by this investigation these data will enable the next generation of hydrologists to evaluate the local and regional effects of any increase of Mt. Simon aquifer groundwater pumping beyond current volumes.