Interception and retention of technetium by vegetation and soil

Field experiments were performed to quantify the initial interception and retention of technetium (Tc) by herbaceous vegetation as a consequence of direct deposition and uptake from soil. A simulated rain containing a solution of 95/sup m/TcO4- was applied to plots of bare soil and plots with standing vegetation. Vegetation emerging in the baresoil plots obtained Tc predominantly from root uptake. Vegetation standing during the initial application obtained Tc from both direct foliar interception and root uptake. For the plots with standing vegetation, the estimated initial interception fraction (r) ranged from 0.079 to 0.17. The mass interception factor (r/Y/sub v/) ranged from 0.65 to 1.1 m2/kg. The retention of Tc by vegetation receiving direct foliar contamination varied with environmental half-times (T/sub w/) ranging from 15.9 to 18.7 days. Adjusting for the effects of growth dilution increased the values of T/sub w/ (range 21.6-28.7 days). For vegetation emerging from the plots of initial bare soil, retention was equivalent to a T/sub w/ of 43 days. These data indicated that uptake of Tc from soil diminished with time and that Tc was removed from plant tissue. In soil, a downward movement was evident, because Tc in the top 2 cm decreased with time and concentrations at lower soil depths (6-8 and 14-15 cm) and increased with time. The application of these data to predict steady-state concentrations of Tc in vegetation resulting from continuous deposition does not differ substantially from predictions based on the use of generic-default parameter values recommend in Regulatory Guide 1.109 of the U. S. Nuclear Regulatory Commission