51. Determination of U and Th in Soil and Plants Obtained from High Natural Radiation Area in China Using ICP-MS

Masae Yukawa, Yoshito Watanabe, Yoshikazu Nishimura,Yicao Guo, Yongru Zha Hui-min Lu, Wei Zhang, Luxin Wei and Zufan Tao

Keywords: U, Th, daily dietary intake, high natural radiation, internal dose, annual effective dose

There are high natural radiation areas of China, where the high radiation is caused by ²³⁸U, ²³²Th and their decay products that are present in soil at higher levels than elsewhere. Area inhabitants ingest these nuclides through food grown in the soil. The internal radiation doses of these radionuclides have not been sufficiently evaluated. We collected soil and foods in a high natural radiation area (High Background Radiation Area: HBRA) and a control area (CA) in Guangdong Province of China in summer 1997 and winter 1998, and determined U and Th in the samples using ICP-MS (Inductively Coupled Plasma Mass Spectrometry)

Three hamlets from HBRA and one hamlet from CA were selected as sampling areas, and three families from each hamlet were chosen for this study. We interviewed the heads of the selected families in each hamlet concerning their eating habits to get informa tion about the food consumption of one person in the hamlets. In all hamlets, the major food is rice and the second one is sweet potato. The main vegetable is water spinach in summer and cabbage in winter Soil samples from rice and sweet potato fields, and summer vegetables, cereals and drinking water in each family were collected for the analysis in 1997 Total diet samples and winter vegetables were obtained in 1998 from the same families as the summer1997.

About 2Omg of dried soil was digested with a mixture of ultra-pure nitric acid, hydrogen peroxide and hydrofluoric acid using a microwave digester Elemental concentrations in t.he solution were deter mined by ICP-MS. U and Th in plants were also determined by ICP-IVYS in almost the same way as in soils. Sample weights of dried leaf and rice grain were about 1OOmg and 1g, respectively. In the case of animal food, about 20Omg of the ash were used for ICP-MS analysis to determine U and Th. We checked accuracy and precision of the results by ana lyzing standard reference materials, repeating the analysis and comparing to the results of INAA (Instrumental Neutron Activation Analysis) and spectrometry. The accuracy and precision of ICP MS are good enough for the determination of U and Th in soil and food samples obtained from HBRA and CA.

Table 5 shows the concentrations of U and Th in major plant food and some animal meat with their daily consumption in HBRA and CA. The daily dietary intake of the element shown in the table was calculated by summing up the products of concentra tion of the element in each food and the daily consumption. The effective dose coefficients of ²³²Th and ²³⁸U are given as 0.22Sv/Bq and 0.044 Sv/Bq respectively in ICRP68. The annual effective doses derived from these data are 0.302 p Sv/y for ²³⁸U and 1.86 p Sv/y for ²³²Th in average in HBRA, and 0.0101 p Sv/y for ²³⁸U and 0.177Sv/y for ²³²Th in CA. An earlier study reported that the whole annual effective dose is 2.1 mSv/y in HBRA and 0.68 mSv/ y in CA. The contribution rate of internal dose from these two radionuelides ingested into the human body is approximately 0.l% in HBRA and less than 0.1% in CA.

Table 5. Concentrations of Th and U in in Food Group and Their Daily Intakes