Skip to main content
ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #414072
Research Project: Science and Technologies for Improving Soil and Water Resources in Agricultural Watersheds

Location: Watershed Physical Processes Research

Title: Groundwater age estimates for the Mississippi River Valley alluvial aquifer based on tracers collected in 2018 through 2020

Author
item GRATZER, MICHAEL - Us Geological Survey (USGS)
item SOLDER, JOHN - Us Geological Survey (USGS)
item KNIERIM, KATHERINE - Us Geological Survey (USGS)
item KINGSBURY, JAMES - Us Geological Survey (USGS)
item O'Reilly, Andrew
item DAVIDSON, GREGG - University Of Mississippi

Submitted to: USGS - Scientific Investigations Report
Publication Type: Government Publication
Publication Acceptance Date: 3/7/2024
Publication Date: 2/13/2025
Citation: Gratzer, M.G., Solder, J.E., Knierim, K.J., Kingsbury, J.A., O’Reilly, A.M., Davidson, G.R., 2025, Groundwater age estimates for the Mississippi River Valley alluvial aquifer based on tracer data collected during 2018–20: U.S. Geological Survey Scientific Investigations Report 2024–5127, p. 1-34, https://doi.org/10.3133/sir20245127
DOI: https://doi.org/10.3133/sir20245127

Interpretive Summary: The Mississippi River Valley alluvial aquifer (MRVAA) is the dominant source of irrigation water in the intensively cultivated Lower Mississippi River Basin, where groundwater flow modeling has found that the amount of water withdrawn from the MRVAA is unsustainable. A better understanding of the timescales of replenishment of the aquifer is needed. Estimates of groundwater age—the length of time water has been in an aquifer—can be used to understand how quickly or slowly an aquifer is recharged. Groundwater samples from 69 MRVAA wells and 19 wells in underlying 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), which are environmental tracers related to groundwater age. Groundwater ages were computed by matching measured tracer concentrations using lumped-parameter models. Mean ages of MRVAA groundwater samples ranged from 12 to 22,000 with a median of 140 years. Mean ages of MEAS groundwater samples ranged from 230 to greater than 52,000 with a median of 13,500 years. Groundwater age results can be combined with other hydrologic data for an aquifer and used by scientists and resource managers to provide information on sources of water to the aquifer, flowpaths within the aquifer, and groundwater travel times through the aquifer.

Technical Abstract: 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 years. Mean ages of MEAS groundwater samples ranged from 230 to 52,000 or more years, with a median of 13,500 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.