Groundwater age categories based on tritium concentrations in samples collected from the Mississippi River Valley alluvial aquifer and aquifers of the Mississippi embayment principal aquifer system

The Mississippi River Valley alluvial aquifer (MRVA) overlies and is bounded by several regional aquifers that make up the Mississippi embayment aquifer system (MEAS) in the central United States. The MRVA, which consists of Quaternary alluvium, is one of the most heavily pumped aquifers in the nation and is a major source of groundwater for irrigation. Large groundwater-level declines in portions of the aquifer have raised concerns about sustainable use of this important resource. An aquifer-scale assessment of groundwater-age categories based on tritium concentrations was completed to better understand groundwater availability and susceptibility. The presence of tritium, a radioactive isotope of hydrogen, in a groundwater sample is indicative of some component of modern groundwater (recharged after 1953). Tritium data for samples collected from 1988 to 2019 were acquired from the U.S. Geological Survey National Water Information System (NWIS) for the MRVA (n = 178), Pleistocene terrace deposits (n = 77), and aquifers of the MEAS (n=325). The focus of the age categorization was on MRVA groundwater but including samples from the MEAS provides context for groundwater age where water from the younger and older aquifers may be mixing. Samples were grouped into three qualitative age categories (modern, premodern, and mixed) based on tritium concentration, well location, and collection date (Lindsey and others, 2019). Modern groundwater consists mostly of water that was recharged in or after 1953, premodern water consists mostly of water that was recharged before 1953, and mixed water is a combination of premodern and modern water. For some censored samples, the tritium detection limit was greater than the tritium threshold concentrations for that location and sample date, so the precise qualitative age category could not be determined. If the detection limit was greater than the modern threshold, groundwater age could not be categorized and the qualitative age is “undetermined.” If the detection limit was greater than the premodern threshold, the sample was categorized as “premodern/mixed” because the sample may be either category. Many (n = 137) of the groundwater samples from the MRVA and Pleistocene terrace deposits are categorized as modern, but 116 samples in the premodern, mixed, and premodern/mixed categories are also present. The premodern and mixed samples in the MRVA indicate either lower recharge rates or longer residence times in portions of the surficial aquifer. The samples collected from the underlying MEAS are primarily premodern (n = 181). The mixed (n = 78) and modern (n = 52) samples in the MEAS tended to fall outside the extent of the MRVA in the recharge areas for MEAS units. Samples with high detection limits (> 30 picoCuries per liter) were not assigned a groundwater-age category (n = 9).