Soil and plants content of 40K, 137Cs and 226Ra in a seminatural grassland in central Argentina

JURI AYUB, J.; VELASCO, R. H.; RIZZOTTO, M.; QUINTANA, E. E.; AGUIAR, J. C.

Buzios, Brasil.

Simposio; 8th International Symposium on the Natural Radiation Environment; 2007

Activity concentrations of gamma emitting radionuclides have been determined in soil and plant samples, collected in permanent grassland in central Argentina. Two near sites (S1 and S2) under field condition with soil undisturbed at least in the last four decades were selected. The soils are a Sandy type, sand content ranged 40-80% and clay content ranged 1-7%. The dominant vegetation in these areas consists of herbaceous gramineous, which is used for the feeding of cattle. In S1 were made 2 transects with 4 sampling points each one and in S2 were made 3 transects with 3 sampling points each one. Grass samples were taken from each sampling point; washed (distilled water), oven - dried and ground. Soil profiles were sampled down to 0-25cm depth, in 5 cm-sliced and dried and homogenized. Total inventories for 40K are similar in S1 (173±6 kBqm-2) and S2 (177±29 kBqm-2) and vertical profile is homogeneous, but superficial soil layers in S2 (0-5 cm) are impoverished of potassium. In 137Cs case, the inventories was 0.56±0.06 and 0.52±0.2 kBqm-2, for S1 and S2, respectively. Vertical profile of 137Cs showed in S1, an increase to 5-10cm depth and decreases to deeper layers. The other site (S2) showed 137Cs activity concentration maxima to 10-15cm depth, but the activity levels were equal to those of S1. In soil layer 5-10cm, S1 showed 137Cs activity concentration greater than S2; but to deeper layers (>10cm) these two sites showed similar distribution and concentration. We have calculated the diffusion coefficients (Ds) and convection velocity (vs) with a convection-diffusion model with constant parameters. Ds values are in the range reported by bibliography. However, vs values are one order of magnitude higher. After 40 years the greater amount of of 137Cs fallout still is to a depth of 10-15cm. The great penetration of 137Cs (25cm) in these soils can be result of the high sand content and the low content in fine materials. For 226Ra, the inventory in S1 (28±2 kBqm-2) was lower than in S2 (38±3 kBqm-2), but the distribution through the soil profile was homogenous in both sites. 137Cs and 226Ra were not detected in grass samples. Activity concentration of 40K in vegetal samples ranged from 116 to 613 Bqkg-1 and the TF ranged from 0.05 to 0.42. The low levels of 137Cs activity in roots zone make expectable that the activity in plants is below detection limits.