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.
