Soil, grain and water chemistry and human selenium imbalances in Enshi District, Hubei Province, China. (WC/96/054)
This report outlines the results of a collaborative study undertaken by the British Geological Survey (BGS), the Institute of Rock and Mineral Analysis (IRMA), Beijing and the Enshi Public Health Department to investigate Se-related disease incidence in Enshi District, Hubei Province.
The current study was designed to address knowledge gaps into why some villages underlain by carbonaceous strata had several cases of Se toxicosis whereas other villages sited in apparently similar geological and geochemical environments had no recorded cases of human or animal toxicosis and to use the new information on geochemical controls to identify strategies which may help enhance the selenium status of residents in selenium deficient areas and reduce the risk of selenium toxicity to people living in the high selenium areas of Enshi District.
During the present study, fifteen villages from three Se environments in Enshi District were investigated. Soil, grain, drinking water and human hair samples were collected from five Low-Se-Keshan-Disease villages (LK), five High-Se-No-toxicity (HN) villages, and five High-Se-Toxicity (HT) villages. This sampling strategy was designed to establish the relationship between the human Se status of communities (represented by hair samples) and their dietary Se intake (represented by grain and drinking water samples) to the local geochemical environment characterised by cultivated soils. Where possible, soil, grain and hair samples were collected from the same farm. Grain (maizecorn) samples were collected from village grain stores therefore it was not possible to relate the grain samples directly to the soil samples.
Total and water soluble (bioavailable) Se in soil, together with total Se in water, grain and hair samples indicate that Se levels in the LK villages are 17 to 690 times lower than in the HN and HT villages (Table i). Low levels of Se are associated with Jurassic siltstones and sandstones which outcrop on the north-west of Enshi District whereas high environmental Se occurs in areas underlain by Permian carbonaceous strata. These results confirm the fundamental geological control on Se distribution in Enshi District and the link between environmental Se levels and the Se status of the local population.
The results of the present study demonstrate that information on soil water soluble Se, Ca, Fe, P and organic matter contents and soil pH are required to define more clearly, specific villages and populations at risk, on the basis of geochemical data. This method of assessing risk involves a number of analyses and careful interpretation of the results. Alternatively, grain total Se levels give a good indication of the likely human Se status and it is therefore recommended that grain total Se levels provide the simplest means of assessing potential risk at the village scale. Within villages in the Enshi high- Se region, Se levels vary markedly. In order to asses the risk of Se toxicity to individuals, detailed sampling of soil and crops from specific fields would be required.
The present study has defined more clearly the controls on Se availability in the environment of Enshi District and the relationships between environmental Se concentrations and human Se status at the village level. The results confirm the general conclusions of previous studies carried out as part of the present project in the Sedeficient Zhangjiakou area of China (Johnson et al., 1996). These results also have broader application to other Se deficient and excessive environments in China and other countries where Se-deficiency may have a negative impact on human health, such as Sri Lanka and Zaire were Se-deficiency has been implicated as a causative factor in iodine deficiency disorders (IDD).
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Fordyce, F.M.; Zhang, G.; Green, K.; Liu, X. Soil, grain and water chemistry and human selenium imbalances in Enshi District, Hubei Province, China. (WC/96/054). (1996)