Soil-to-plant, plant-to-milk and plant-to-meat transferts in ferrallitic soils in Tahiti, French Polynesia
Congress title :IRPA 2006 2nd European IRPA Congress on Radiation Protection
Congress town :Paris
Congress date :15/05/2006
French Polynesia is included in the latitude band 10-30° S. In this band the total déposition of 137Cs is about 1000 Bq.m'2 ; the French tests represent 13 % of this total déposition. The radiologjcal survey of the French Polynesia environment exists since the beginning of the French nuclear program in 1966. It concerns 7 islands : Hiva Oa in the north (10° S), Tubuai in the south (24° S) and, from east to west, Mangareva, Hao, Rangjroa, Tahiti and Maupiti. Tahiti is a récent, high and volcanic island; it is the largest of the French Polynesia as a whole. Under tropical humid climate with heavy rainfall, high summer température and excessive air humidity, the strong relief has been considerably eroded. In the Taravao peninsula of Tahiti a cattle breeding farm of about 400 hectares has been studied since more than 30 years. Local milk is an important contributor to the ingestion dose in Tahiti and also an excellent indicator of the effective decrease of long-lived radionuclides in the environment and characteristic for other foodstuffs, méat for example. During the 1974-1994 period the long-term decrease for milk shows an effective half-live of 14.8 years and an environmental half-live of 24.8 years. In west European zones the effective half-live is about S years. To explain this différence we must mainly consider that the pasture zone in Taravao peninsula is a natural area whereas it is semi-natural in Europe. In the top part of the peninsula it is also possible that a 137Cs stocking zone exists with very humic soils; progressive lixiviation should arise then. For the 1974-1994 period the effective half-live for the beef méat is about 11.5 years against 18.S years for the environmental half live. The différences between effective and environmental, for milk and for méat, can be explained by a greater collection zone for méat than for milk.
The soil-to-plant transfer factors (TF) are about 10 (reporting dry matter) for the genus Sétaria an about 30 for the genus Killingia, two grasses. Thèse values are at least 100 higher than those reported for European silt soils. For this type of soil we suggest that the very low solution K+ concentrations (hik) in the soil favour the Cs transfer to plant. At thèse low K+ concentrations there is little discrimination against Cs+ and the FT is high; the K+ transporter pathway can act. This transfer can be characterized by the concentration factor (CF) ; for Killingia it is about 6000 Lkg"1.
The plant-to-milk transfer factors hâve been calculated for an average situation conceming three types of grasses; the average factor is about 0.005 j.l"1. For the plant-to-meat transfer factors the average factor is about 0.04 jJcg"1 fresh weight. Thèse two kinds of factor are similar than those obtained in European zones. It is not the case for the soil-to-plant transfer factor.