Hydrogeology and groundwater geochemistry of two glacigenic aquitard/aquifer systems in north-central and south-central Minnesota
Buried glacial aquifers in Minnesota contribute drinking water to many communities throughout the state. Vertical leakage through overlying till aquitards is largely unknown, causing uncertainty in predicting aquifer sustainability. As part of a study of confined aquifer sustainability, the U.S. Geological Survey, Iowa State University, and Minnesota agencies have investigated vertical leakage through till aquitards deposited by glacial advances in Minnesota. Although estimating leakage involves assessment of aquitard hydraulic properties, previous studies show that groundwater geochemistry data in till aquitards can corroborate estimated vertical travel times.
In Phase II (2017-2019) of the study, a rotary-sonic rig was used to install 12 small-diameter piezometers in nests at the town of Olivia, MN (depths from 13 to 210 ft) and at the MN Hydrogeology Field Camp (HFC; depths from 115 to 200 ft) near Akeley, MN. Transducers were emplaced to gather evidence of hydraulic head fluctuations, and groundwater samples and pore water samples from till cores were analyzed for major and trace elements, anions, nutrients, enriched 3H, δ18O, and δ2H. Slug tests were performed to estimate Kh in the till and aquifer tests were performed to estimate the bulk Kv of the aquitards.
Hydraulic data at both sites suggest that groundwater flow is vertically downward. Geometric mean Kh values from slug tests were 4x10-3 ft/d in till at Olivia and 3x10-2 ft/d in till at HFC. The aquifer test analyses of HFC estimated a minimum Kv of 1x10-2 ft/d and a maximum of 3x10-2 ft/d, while the Olivia site aquifer tests were not able to be analyzed due to reverse water level fluctuations in the till. Travel times of groundwater through the till were estimated from the slug test data to be about 216 years and 56 years for the Olivia site and HFC site, respectively. The aquifer test results for the HFC site suggest a travel time of about 77 years.
Stable isotope data supports these travel times and suggest that there is no glacial-age groundwater in the till at either of the sites. A possible 3H bomb peak (21.4 TU) in the till at the HFC site supports shorter travel times there. Modern 3H concentrations at the Olivia site are present in the groundwater only in the shallow part of the till (about 20 ft below land surface). Chloride and Cl/Br mass ratio data suggest vertical penetration of anthropogenic contaminants at the Olivia site, but they do not corroborate the Darcy's Law travel times through the till. Redox species suggest that the till aquitards at both sites support reducing conditions, and that there are stronger reducing conditions at the Olivia site, perhaps because of longer travel times there. Analysis of water chemistry type using Piper and Stiff plots show possible evolution from Ca-Mg-HCO3-type to Na-HCO3-type groundwater at the Olivia site. Groundwater is primarily a Ca-HCO3-type groundwater at the HFC site, with the exception of one sample which showed Na-HCO3-type groundwater – possibly a result of bentonite contamination.
Overall, the research suggests that a vertical recharge/leakage through aquitards occurs to underlying sand and gravel (glacial) aquifers, but it is possible that the vertical flux is not high enough at the Olivia site to sustain pumping of the aquifer into the future. The study also showed that the hydraulics and groundwater geochemistry of the aquitards are quite complex and that further studies are needed to understand groundwater flow dynamics in these systems.