Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 57

Fig. 3.4 Schematic diagram of the production, deposition, and use of 210 Pb and 7 Be as sediment
tracers in drainage basins

subjected to no erosion will decline through time as a result of the cessation of
fallout and radioactive decay after the 1970s/1980s, the continuous flux of 210 Pb ex to
the soil creates a steady-state condition in which atmospheric deposition is bal-
anced by radioactive decay (Zhang et al. 2006 ). Spatially, atmospheric deposi-
tional rates exhibit considerable variability; on a global scale fluxes are typically
lower where air masses have traversed oceanic bodies and have had little chance of
accumulating 222 Rn from soils and other geological materials. Higher depositional
rates are associated with circulating air masses that have traveled over continen-
tal areas and have had an opportunity to accumulate 222 Rn. Locally, depositional
rates are strongly influenced by geographical patterns in the amount and intensity of
rainfall.
As was the case of

210 Pb ex is assumed to be strongly and irreversibly
attached to soil particles upon deposition from the atmosphere (Dickin 1997 ). Thus,
210 Pb ex exhibits a vertical profile very similar to that of 137 Cs where the highest
concentrations are located at the ground surface and decrease exponentially with
depth (Fig. 3.3 c). A zone of uniform or lower concentration immediately below the
ground surface does not occur as a result of the continuous and current flux of 210 Pb
to the soil. In agricultural areas, 210 Pb ex concentrations exhibit semi-uniform values
through the plough zone as a result of mixing of the upper horizons (Fig. 3.3 d).
Beryllium-7 is a naturally produced, cosmogenic radionuclide created by cosmic
ray bombardment of atmospheric N and O (Fig. 3.4 ). Thus, the production of 7 Be is

137 Cs

,