South-Central Minnesota Groundwater Monitoring of the Mt. Simon Aquifer

The deepest bedrock aquifer of south central/southeastern Minnesota, including the Minneapolis/St. Paul metro area, is the thick (50 to 200 feet) Cambrian sandstone Mt. Simon aquifer. It supplies all or some of the water used by over one million Minnesotans. The few water level measurements available from this aquifer in the Mankato and Minneapolis/St. Paul metro area indicate declining water levels in areas where water is being withdrawn for municipal and industrial use. To better understand the recharge dynamics of the Mt. Simon aquifer the western and northern edge of the Mt. Simon aquifer, where it is not overlain by relatively impermeable Paleozoic shale formations, was considered the most likely area for aquifer recharge. This edge of the Mt. Simon aquifer was investigated and characterized through observation well installations, water level monitoring, groundwater chemical analysis, and aquifer capacity testing to help determine recharge pathways and sustainable limits for this aquifer. Most data collected for this study are derived from the wells installed at 14 locations by contracted drilling companies. The combination of chemical residence time indictors, continuous water level data from nested well locations, and a general knowledge of the regional hydrostratigraphy, show an aquifer with a very slow recharge rate from a large source area located south of the Minnesota River and a smaller source area located in the northern portion of the study area. The younger 14C residence time values of Mt. Simon groundwater (7,000-08,000 years) from this project 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 began 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 suggests a recharge rate of approximately 0.49 cm/yr. The resulting 1.2 billion gallons/year of recharge from the southern source area is less than the amount of groundwater used from the most recent year for which data are available (2009). The results of this project suggest that Mt. Simon aquifer groundwater use in the study area, for the most recent period (2009), may be more than the replacement rate along the Mt. Simon subcrop. Continued monitoring of the observation wells in this region should help determine if more water is used than is being replaced by recharge. A major accomplishment of this project is the creation of a network of observation well nests along the western margin of this aquifer system. Long term water level data and geochemistry from these wells will enable future hydrologists to evaluate the local and regional effects of Mt. Simon groundwater pumping in the region.