Vegetation Assessment for Tritium Shows Negligible Environmental Impact

January 29, 1999

Results of an analysis to characterize tritium concentrations in trees in the northeast section of Berkeley Lab - especially those near the National Tritium Labeling Facility (NTLF) - show that all potential tritium doses in the vegetation are well within the safety guidelines established by the Environmental Protection Agency.

These findings are consistent with earlier risk assessments conducted by Berkeley Lab, which indicated that potential radiation doses from the minute amounts of tritium emitted at the NTLF pose negligible public risk.

The study was required by the Department of Energy prior to the removal and off-site disposal of eucalyptus trees as part of a three-year wildfire control project for the East Bay hills. The Laboratory is establishing a mid-canyon firebreak to slow the potential spread of wildland fires.

The project analyzed 171 samples from tree cores and chips, leaves, and "duff" (leaves, twigs and other residual litter on the ground) at distances of approximately 20, 50, 100 and 125 meters from the NTLF emissions stack, as well as from more distant trees in six other targeted groves. Even using the most conservative assumptions, the study found, the highest possible dose would be a small fraction of the EPA's safe-dose threshold.

For example, in tree wood the highest free water tritium concentration was 21 picoCuries per gram and the highest organically bound tritium concentration was 9 picoCuries per gram. If tree wood with these maximum tritium concentrations were converted to mulch and used on residential vegetable gardens, the annual potential dose from eating the vegetables is estimated to be 0.04 mrem.

That dose is about 250 times lower than the 10 millirem-per-year threshold for air exposure established by the EPA - or 2,500 times lower than the 100 millirem threshold established by other federal standards.

The highest organically-bound tritium concentration found (1,280 pico-Curies per gram in one location of duff) would translate to a potential dose of 0.02 millirem if someone were to ingest and handle it weekly for a year. That total is 500 times below the EPA threshold.

By comparison, humans are exposed to radiation doses of 5 millirem on a round-trip cross-country airplane flight or a chest x-ray, 25 millirem for a dental x-ray, and 30 millirem for a mammogram.

Doses were also calculated for prospective disposal of the removed wood, all of which would be less than one-half of one percent of the EPA safe standard of 10 millirem, resulting again in negligible public impact. 

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