Quantification of the interception and initial retention of radioactive contaminants deposited on pasture grass by simulated rain

Abstract Simulated rain containing both soluble radionuclides and insoluble particles labeled with radionuclides was applied to pasture-type vegetation under conditions similar to those found during convective storms. The fraction of material in rain intercepted by vegetation and initially retained was determined for three sizes of insoluble polystyrene microspheres (3, 9 and 25 μm), soluble 7Be2+ and soluble 131I as periodate or iodide, over a range of rainfall amounts of both moderate- and high-intensity precipitation. Values for the interception and initial retention by vegetation (interception fractions) for soluble forms of 131I in simulated rain are much less than those for insoluble particles and the reactive cation 7Be2+. The interception fraction for soluble 131I is an inverse function of rain amount, varying from about 0.3 at 1 mm rain to 0.006 at 30 mm. The mass interception factor (the interception fraction normalized for biomass) of 131I is almost solely dependent on the amount of rain, with values from about 2.5 m2 kg−1 at 1 mm to less than 0.1 m2 kg−1 at 30 mm; the 131I vegetation-to-rain concentration ratio is relatively constant at approximately 2.6 l kg−1. For 7Be2+ and the insoluble particles, the interception fractions range from 0.1 to 0.6 with geometric means of approximately 0.3. For these materials there is a greater dependence on biomass than on rain amount; the geometric means of the mass interception factors for these substances range from 0.99 to 2.4 m2 kg−1, with no single variable being a major controlling factor. These results indicate that anionic 131I is essentially removed with the water once the vegetation surface becomes saturated and that the 7Be cation and the in s oluble particles are adsorbed to or settle out on the plant surface. The discrepancy between the behavior of the anionic and the cationic species is consistent with a negative charge on the plant surface.