Investigation of the Environmental Fate of Tritium in the Atmosphere

INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE
re-evaporation from continental water bodies are similar to those occurring in the original
precipitation, particularly for small water bodies. By comparison, precipitation depositing
on the ocean or on other deep water bodies is rapidly mixed, and greatly diluted, resulting
in lower tritium concentrations in re-evaporated water vapour compared to the levels in the
original precipitation.
Concentrations of tritium in the atmosphere are also known to vary across latitudes and
seasons (e.g., Hauglustaine and Ehhalt, 2002). For example, in addition to the observed
difference in continental water versus oceanic tritium levels, differences also occur as
dictated by the latitudinal movement of air masses. Specifically, tritium concentrations are
typically higher in precipitation originating from air masses with trajectories from the north
and west, compared to those originating from the south and east, likely primarily due to
latitudinal differences in tritium transfer rates from the stratosphere (Brown, 1964).
Vapour exchange processes are thought to account for approximately two-thirds of the tritium
removal into the oceans (Erikkson, 1965), and also account for much of the tritium transfer
to terrestrial soil water. Additional exchange can occur between atmospheric tritium and
precipitation, as dictated by rainfall intensity, as well as the size and corresponding surface
area of precipitation droplets. Precipitation then transfers tritium to the earth’s surface.
In the case of light rainfall, tritium levels in precipitation are thought to be representative
of those occurring at lower heights in the atmosphere, whereas for moderate to heavy rains,
exchange at the lower levels is considered to be insignificant (Bolin, 1961). As noted by
Sejkora (2006), light, long-duration rainfall events can yield higher tritium concentrations in
the ground water, runoff, storm drains and soils than heavy, short-duration events. This may
be important when considering the influence of severe storm events and the potential
influence of climate change on this phenomenon.
Tritium concentrations in precipitation (as HTO) have been found to increase with length
of the travel path through the air mass. For example, comparisons of tritium levels in
precipitation with meteorological factors (such as type of storm, radar height of precipitation
formation, rainfall duration and intensity, and the type and origin of the air mass) have
revealed increases in precipitation tritium levels as the depth of the air mass being traversed
by the precipitation increases (Brown, 1964).
Related to the intensity of the precipitation, the size and surface area of the rain droplets
can also influence tritium exchange between water vapour and rain drops during the process
of film diffusion across the droplets (Chamberlain and Eggleton, 1964). In general, the
tritium specific activity of rain drops that have passed through a plume that is contaminated
with tritium is very dependent upon the size of the droplet, with decreases in tritium
concentrations in larger rain drops due to the smaller surface area-to-volume ratio of
larger droplets. This essentially leads to relatively greater tritium dilution in larger droplets.
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