{"accessLevel": "public", "bureauCode": ["010:12"], "contactPoint": {"@type": "vcard:Contact", "fn": "Bruce D. Lindsey", "hasEmail": "mailto:blindsey@usgs.gov"}, "description": "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. \nThe 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. \nThis 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.", "distribution": [{"@type": "dcat:Distribution", "accessURL": "https://doi.org/10.5066/P9DU94RV", "description": "Landing page for access to the data", "format": "XML", "mediaType": "application/http", "title": "Digital Data"}, {"@type": "dcat:Distribution", "description": "The metadata original format", "downloadURL": "https://data.usgs.gov/datacatalog/metadata/USGS.5bfff281e4b0815414cad45b.xml", "format": "XML", "mediaType": "text/xml", "title": "Original Metadata"}], "identifier": "http://datainventory.doi.gov/id/dataset/USGS_5bfff281e4b0815414cad45b", "keyword": ["Basin and Range aquifers", "Basin and Range carbonate-rock aquifers", "Biscayne aquifer", "Cambrian-Ordovician aquifer system", "Central Valley", "Coastal lowlands aquifer system", "Colorado Plateaus aquifers", "Columbia Plateau basin-fill and basaltic-rock aquifers", "Conterminous United States", "Edwards-Trinity aquifer system", "Floridan aquifer system", "Glacial aquifer system", "Groundwater age", "High Plains aquifer", "Iowa", "Mississippi Embayment-Texas Coastal Uplands aquifer system", "National Water-Quality Assessment project", "Nevada", "New England", "Northern Atlantic Coastal Plain aquifer system", "Ozark Plateaus aquifer system", "Piedmont and Blue Ridge crystalline-rock aquifers", "Recharge", "Rio Grande aquifer system", "Tritium", "USGS:5bfff281e4b0815414cad45b", "Utah", "Valley and Ridge and Piedmont and Blue Ridge carbonate-rock aquifers", "Water quality", "Wisconsin"], "modified": "2020-08-26T00:00:00Z", "publisher": {"@type": "org:Organization", "name": "U.S. Geological Survey"}, "spatial": "-125.0, 25.0, -65.0, 49.0", "theme": ["geospatial"], "title": "Data for Tritium as an Indicator of Modern, Mixed and Premodern Groundwater Age"}