Naturally occurring thallium: a hidden geoenvironmental health hazard? Tangfu Xiao a,b, * , Jayanta Guha b , Dan Boyle c,1 , Cong-Qiang Liu a , Baoshan Zheng a , Graham C. Wilson d , Alain Rouleau b , Jingan Chen a a State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China b Sciences de la Terre/Centre d’E ´ tudes sur les Ressources Mine ´rales, Universite ´ du Que ´bec a ` Chicoutimi, Quebec, Canada G7H 2B1 c Division of Applied Geochemistry, Geological Survey of Canada, Ottawa, Canada K1A 0E8 d Turnstone Geological Services Limited, PO Box 130, Station ‘‘B,’’ Toronto, Canada M5T 2T3 Received 15 July 2003; accepted 13 October 2003 Abstract This paper illustrates a real environmental concern and draws attention to the fact that natural processes can mobilize thallium (Tl), a highly toxic metal, which may enter the food chain as a ‘‘hidden health killer’’ with severe health impacts on local human population. Natural processes may be exacerbated by human activities such as mining and farming, and may cause enrichment of Tl in the environment. In geochemically anomalous areas with concentrated levels of Tl in the surface environment (bedrocks, waters, soils, and crops), such as the Lanmuchang area in southwestern Guizhou Province, China, it is essential to establish base-level values and to pay heed to the geological context of ‘‘natural contamination,’’ as high concentrations of Tl in bedrocks/ores (6–35,000 mg/kg) can lead to enrichment of Tl in the aquatic system (0.005 – 1100 Ag/l in groundwaters and 0.07 – 31 Ag/l in surface waters) and soil layers (1.5 – 124 mg/kg). In sensitive areas such as the Yanshang area of southwestern Guizhou, elevated natural levels of Tl from bedrocks may also cause higher concentrations of Tl in the surface environment, and thus more attention must be paid to geoenvironmental management of human activities if socio-economic catastrophes are to be avoided. Due to high uptake of Tl by crops, Tl can be transferred from soils to crops and remarkably concentrated in food crops. Concentrations of 1 – 500 mg/kg Tl based on dry weight (DW) were determined in many food crops growing on Tl-contaminated arable soils from the Lanmuchang area. The daily intake of 1.9 mg of Tl from consumed food crops was estimated for the local adult inhabitant of Lanmuchang. Thus, Tl is regarded as a latent health hazard with potential risk of toxicity in humans within areas of ‘‘natural’’ contamination by Tl. D 2003 Elsevier Ltd. All rights reserved. Keywords: Thallium; Geoenvironment; Health hazard; Contamination; Chronic poisoning; Guizhou; China 1. Introduction The fact that ‘‘nature’’ contributes to pollution is often overlooked or ignored when searching for the source of problems involving metal pollution. Human activities can accelerate natural processes. Thus, mining operations in sensitive areas can generate toxic pollution, by either the liberation of natural elements within the rocks, or by addition of artificial compounds such as cyanide leachates or hydrocarbons. To provide reliable identification of the cause of pollution, it is important to ask what kinds of base- level data exist concerning the natural migration of toxic metals prior to the onset of mining activities. Such data are essential if the impact of pollution related to metal disper- sion induced by human activities is to be adequately measured. This involves the concept of geoenvironment. The term geoenvironment is defined here as a geoscien- tific regime, which can play a role, in conjunction with other parameters [such as mineral resources, soil, topography, (sub)surface water, crops, socio-economic development, etc.], in defining environmental impacts (Guha, 2003). These impacts can be chemical or physical, and they will greatly influence the choice of parameters required to make a clear diagnosis of assessment criteria needed for proper management. The scientific and medical literature documents negative effects of numerous mildly to highly toxic metals on human health (Christensen, 1995; Nixon and Moyer, 1996; Janssen 0160-4120/$ - see front matter D 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.envint.2003.10.004 * Corresponding author. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China. Tel.: +86-851-888-1768; fax: +86-851-589-1609. E-mail address: tfxiao@mail.gyig.ac.cn (T. Xiao). 1 Deceased. www.elsevier.com/locate/envint Environment International 30 (2004) 501 – 507