Data for groundwater age estimates for the Mississippi River Valley alluvial aquifer based on tracer data collected during 2018–20

This data release contains tracer data and estimated age distributions of groundwater samples collected as part of a water availability study for the Mississippi alluvial plain (MAP). The MAP is the floodplain of the lower Mississippi River and its main tributaries and distributaries. The soils, climate, and irrigation water resources of the MAP have made it a highly productive agricultural region for the nation. The surficial aquifer of the MAP, the Mississippi River Valley alluvial aquifer (MRVA) supplies most of the irrigation water for the region. Parts of the MRVA overlie and may exchange water with the Mississippi embayment aquifer system (MEAS) (Kresse and others, 2014). The MRVA has shown signs of severe decline in water quantity, motivating research on the functioning of this aquifer. An aquifer-scale characterization of groundwater age was initiated to provide information on the timescales of MRVA groundwater replenishment by recharge. Age distributions were estimated for 71 MRVA groundwater samples, one sample from an adjacent terrace aquifer, 16 samples from the underlying middle Claiborne aquifer (MCAQ), and two samples from the underlying Wilcox Group. The age distribution of each sample was estimated using a lumped parameter model (LPM) fit to measured tracer concentrations, with the U.S. Geological Survey (USGS) software TracerLPM (Jurgens and others, 2012). Samples were analyzed for one to four of the following age tracers: tritium (³H), sulfur hexafluoride (SF₆), carbon-14 (¹⁴C), and helium (He) isotopes (helium-3 [³He] and helium-4 [⁴He]). Mean ages of MRVA groundwater samples ranged from 12 years to 22,000 years. Mean ages of MEAS samples ranged from 230 years to greater than or equal to 52,000 years. A scientific investigations report (SIR-20245127) describes the methods used for this study and the modeling results. Water quality results in this data release are available on the USGS National Water Information System (NWIS) online database (U.S. Geological Survey, 2021). This data release contains 8 data tables and a data dictionary table. Table 1 (Table1_Wells.shp) is a shapefile containing coordinates, aquifer codes, and screened intervals of the wells included in this data release. In addition to the wells sampled for age tracers for this study, Table 1 contains 422 wells sampled for ³H in the Mississippi embayment for previous studies such as Gonthier (2003) and Solder (2020). Data from these additional wells were used in Gratzer and others (2025) to provide context to the results and findings of this study. For five samples, more than one possible age distribution was reported, because more than one conceptual model of the physical hydrology affecting the sample was probable or because the sample had more than one corrected ¹⁴C, SF₆, or tritiogenic helium-3 (³Hetrit) concentration to which an appropriate LPM could be fit. Table 2 (Table2_MeanAgeSummary.csv) summarizes the one or more mean ages estimated for each site. Table 3 (Table3_tritium.csv) contains age categories assigned to samples based on ³H concentrations, using the methods of Lindsey and others (2019). Table 4 (Table4_C14.csv) contains measured and corrected ¹⁴C activities (a¹⁴C); corrections were made using the Revised Fontes and Garnier open- and closed-system ¹⁴C correction models to help account for chemical processes that decrease a¹⁴C and bias age estimates old (Han and Plummer, 2013; Gratzer and others, 2025). Table 5 (Table5_N2Ar.csv) contains measured concentrations of dissolved nitrogen (N₂), argon (Ar), oxygen (O₂), carbon dioxide (CO₂), and methane (CH₄). Table 5 also contains recharge temperatures, excess air concentrations, and excess N₂ concentrations estimated for samples based on their N₂ and Ar concentrations, using the USGS software DGMETA (Jurgens and others, 2020). Table 6 (Table6_SF6.csv) contains groundwater concentrations of dissolved SF₆ (expressed in fmol/kg) and atmospheric mixing ratios of SF₆ (expressed in pptv). The mixing ratios are corrected for excess air concentrations estimated by the analyzing entity (USGS Groundwater Dating Lab) based on models fit to measured N₂ and Ar concentrations. Table 7 (Table7_NobleGases.csv) contains measured ³He/⁴He ratios and concentrations of dissolved He, neon (Ne), Ar, krypton (Kr), and xenon (Xe), as well as ³Hetrit abundances, terrigenic ⁴He abundances, and corrected SF₆ mixing ratios based on models fit to noble gas concentrations. Table 8 (Table8_LPMs.csv) contains the input and output of each LPM used to estimate the age distribution of each sample. Table 9 (Table9_DataDictionary.csv) describes the fields in Tables 1 through 8.