Tritium deposition in precipitation in the United States, 1953–2012
Tritium is a radioactive isotope of hydrogen (half-life is equal to 12.32 years). Since it is part of the water molecule, tritium can be used to track and date groundwater and surface water when the history of tritium in precipitation and recharge is known. To facilitate that effort, tritium concentrations in precipitation were reconstructed from measurements and correlations for 10 precipitation stations in the continental United States. Using these data, and other data sets and correlations, estimates of tritium concentrations in precipitation from 1953 through 2012 and total tritium deposition from 1953 through 1983 were derived for 2 degrees latitude by 5 degrees longitude quadrangles of the continental United States. For August 1953 through December 1987 correlations were derived from the data of the Ottawa, Canada, station; for estimates after 1987, the Vienna, Austria, station was used. For quadrangles where no precipitation station records are available from measurements or correlations, concentrations were interpolated. The International Atomic Energy Agency has proposed that a correlation with the Vienna, Austria, station, which has been in operation since 1960, be used instead of the Ottawa, Canada, station for the period after 1987. Linear correlations calculated for all stations with the Vienna data are reported. Correlations also have been calculated for each latitude-longitude quadrangle using the estimated concentrations and the measured Vienna data for 1960–87. Because the Vienna, Austria, station is on a different continent, and not subject to the same seasonal weather patterns that affect North American stations, a correlation was obtained for each month individually. This reflects the fact that the North American stations were strongly affected by the input of low-tritium moisture from the south during the summer. The correlated station values were then used with measured Vienna data to obtain estimates of tritium concentrations in precipitation for the period 1988–2012 at each precipitation station and latitude-longitude quadrangles. This approach has two major advantages: (1) it blends the Ottawa correlation with the Vienna correlation to cover the period 1953 to present, and (2) it yields correlation coefficients for each quadrangle that can be used in future years when more data become available from the Vienna, Austria, station. The data, the estimated tritium concentrations derived from the correlations, and the correlation coefficients are provided as spreadsheets and associated comma delimited files in a data release that accompanies this report.