{
  "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"
}