Data for Tritium as an Indicator of Modern, Mixed and Premodern Groundwater Age

Categorical classification of groundwater age based on concentrations of tritium (3H) in groundwater can provide useful information for the assessment and understanding of groundwater resources. These data present a three-part groundwater age classification system for the continental United States based on tritium thresholds that vary in space and time: modern (recharged after 1952), if the measured value is larger than an upper threshold; premodern (recharged prior to 1953) if the measured value is smaller than a lower threshold; or mixed if the measured value is between the two thresholds. Inclusion of spatially-varying that vary geographically on the basis of the location of the sample rather than a single threshold accounts for the observed systematic variation in 3H deposition across the U.S. Inclusion of time-varying thresholds rather than a single threshold accounts for the date of sampling given the radioactive decay of 3H. The efficacy of the three-part classification system was evaluated at national and regional scales. The system was evaluated at a national-scale by classifying samples from 1,788 public supply wells distributed across 19 Principal Aquifers (Tritium_PublicSupply_CONUS.csv and Tritium_CentralValley_CA.csv) and comparing those results with expectations based on hydrogeologic principles. The regional-scale data are from five paired networks of shallow and deep wells (287 wells; Tritium_PairedNetworks_CONUS.csv). As expected, modern groundwater is more prevalent in shallower wells than in deeper wells; in fractured-rock and carbonate aquifers as compared to clastic aquifers; in unconfined areas as compared to confined areas; and in humid climates as compared to arid climates. The results from the three-part classification system were also compared to results for groundwater ages from previously published studies. For 14 previous age-dating studies available for comparison, results from this method compare favorably with the previous studies. This work improves on previous work by developing methods: for choosing post-1952 3H thresholds that minimize the misclassification of modern samples as mixed; choosing a pre-1953 threshold to estimate pre-bomb background concentrations; and adds a mixed category to classify samples that are clearly neither entirely modern nor entirely premodern. As with any tritium-based approach, it can fail when the 3H record in precipitation does not accurately reflect the record of 3H in recharge.