John E. Solder Katherine J. Knierim James A. Kingsbury Andrew M. O’Reilly Gregg R. Davidson Michael C. Gratzer 2025 <p>This study characterized groundwater age across the Mississippi River Valley alluvial aquifer (MRVA). Groundwater samples from 69 MRVA wells and 19 wells in Tertiary units of the Mississippi embayment aquifer system (MEAS) were analyzed for sulfur hexafluoride (SF₆), tritium (³H), helium (He), and (or) carbon-14 of dissolved inorganic carbon (¹⁴C). The age distributions of 89 samples were estimated by fitting lumped parameter models to processed tracer concentrations with the U.S. Geological Survey software TracerLPM. Mean ages of MRVA groundwater samples ranged from 12 to 22,000 years, with a median of 140&nbsp;years. Mean ages of MEAS groundwater samples ranged from 230 to 52,000 or more years, with a median of 13,500&nbsp;years. The spatial distribution of MRVA groundwater ages was found to be influenced by depth, inflow of groundwater from deeper units, and soil saturated hydraulic conductivity. In parts of the MRVA, the spatial distribution of MRVA groundwater ages was found to be influenced by annual recharge and (or) annual groundwater pumpage.<br></p> application/pdf 10.3133/sir20245127 en U.S. Geological Survey Groundwater age estimates for the Mississippi River Valley alluvial aquifer based on tracer data collected during 2018–20 reports