Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Gratzer II M.C., Knierim K.J., Solder J.E., Kingsbury J.A., O'Reilly A.M.

Lumped parameter models (LPM) were used to estimate groundwater ages from age tracer concentrations in samples collected from 89 wells in the Mississippi River Valley alluvial aquifer (MRVA) and Mississippi Embayment aquifer system (MEAS). The type of LPM for each well was chosen based on confinement, likely sources of water, and observed age-tracer concentrations. Ages for 47 MRVA wells were estimated with LPMs representing unconfined conditions, and ages for 25 MRVA and all 17 MEAS wells were estimated with LPMs representing confined conditions. Ages for 55 MRVA wells and 14 MEAS wells were estimated with LPMs representative of distributed recharge (as opposed to localized recharge) within the areal extent of the MRVA. Age dates for 17 MRVA wells and 3 MEAS wells were modeled as binary mixtures of distributed recharge and localized recharge or water from an adjacent aquifer. Mean ages of MRVA samples (n=72) range from 12 to 24,000 years, with more than 70% of the ages being less than 830 years. Mean ages of MEAS samples (n=17) range from 350 to >50,000 years, with more than 70% of the ages being greater than 8,800 years. This presentation will discuss relations between estimated ages and various types of related hydrologic data, including water chemistry data, airborne electromagnetic resistivity surveys of the Mississippi Alluvial Plain, and numerically modeled groundwater flow paths. Improved knowledge of the distribution of groundwater age in the MRVA will refine groundwater flow models and improve understanding of groundwater chemical evolution in the aquifer, ultimately helping water resource managers decide how best to use and protect one of the most heavily used aquifers in the United States.