Precipitation samples are gathered when sufficient precipitation occurs to allow for the collection of the minimum sample volume of approximately 20 mL. Samples are taken of monthly composites from Idaho Falls and CFA, and weekly from the EFS. Precipitation samples are analyzed for tritium. Storm events in the third quarter of 2009 produced sufficient precipitation to yield 11 samples.
Tritium was measured above the 3s value in 7 of the 11 samples
collected during the third quarter of 2009. Low levels of tritium exist in
the environment at all times as a result of cosmic ray reactions with water
molecules in the upper atmosphere. The EPA’s RadNet program collects
precipitation samples from across the United States. From 1980 to 2008,
tritium measured in samples from Region 10 (which includes Idaho) averaged
117 pCi/L (EPA 2009). Data for all third quarter precipitation samples
collected by the ESER Program were in the range of this value (averaging 123
pCi/L) and are listed in Table C-5 (Appendix
C).