Enrichment of potentially toxic elements in areas underlain by black shales and slates in Korea Jin-Soo Lee 1 , Hyo-Taek Chon 1 *, Jong-Shin Kim 1 , Kyoung-Woong Kim 2 and Hi-Soo Moon 3 1 Department of Mineral and Petroleum Engineering, College of Engineering, Seoul National University, Seoul 151^742, Korea 2 Department of Environmental Science and Engineering, Kwangju Institute of Science & Technology, Kwangju 506^712, Korea 3 Department of Geology, Yonsei University, Seoul 120^749, Korea The Okchon black shale in Korea provides an important example of natural geological materials containing toxic elements; the Chung-Joo, Duk-Pyung, Geum-Kwan, I-Won and Chu-Bu areas are underlain by these black shales and slates of the Guryongsan Formation. This formation is part of the Okchon Group which is found in the central part of Korea. Geochemical surveys were undertaken in these five study areas in the Ok- chon Zone in order to examine the level of enrichment and dispersion patterns of potentially toxic elements in rocks and soils. After appropriate preparation, samples were analysed by instrumental neutron activation analysis and inductively coupled plasma atomic emission spectrometry for a range of elements. Arsenic, Cu, Mo, V, U and Zn are highly enriched in the Okchon black shales and their mean concentrations are signific- antly higher than those in black slates. These elements are closely associated with one another from a geo- chemical viewpoint and may be enriched simultaneously. Mean concentrations of As, Mo and U in soils derived from black shales occurring in the Duk-Pyung and Chu-Bu areas are higher than the permissible le- vel suggested by Kloke (1979), and the enrichment index decreases in the order of Duk-Pyung > Chu-Bu > Chung-Joo > Geum- Kwan  I-Won areas. Uranium-bearing minerals such as uraninite and brannerite have been identified in black shales from the Chung-Joo area by electron probe micro-analysis. Uraninite grains are closely associated with monazite and pyrite with a grain size ranging from 2m to 10m whereas bran- nerite grains occur as a euhedral form 50 m in diameter. Keywords: Toxic elements, Okchon black shale, enrichment index, U-bearing mineral Introduction The degree and extent of heavy-metal pollution as a result of human activities has been one of the main topics studied in environmental geochemistry (Plant and Raiswell, 1983). In addition to anthropogenic metal inputs, high levels of potentially toxic elements of concern to man and animals are present in natural geochemical materials, such as Cd and Mo in some black shales (Lakin, 1972; Lund et al., 1981; Thorn- ton, 1983). Many workers have reported data con- cerning enriched values of trace elements in black shales throughout the world (Atkinson, 1967; Fletcher, 1968; Vine and Tourtelot, 1970; Armands, 1972; Coveney and Glascock, 1989; Kim, 1989). Black shales with high concentrations of trace ele- ments are significant in environmental geochemistry and the Okchon uraniferous black shale in Korea may provide an important example of this (Kim, 1989). The trace elements, Cu, Mo, Ni, Pb, U, V and Zn are highly enriched in the Okchon uranifer- ous black shale in Korea (Kim and Thornton, 1993; Chon et al., 1996). Here black shales are members of the assemblage of strata which are the part of the Guryongsan Formation or the Changri Formation, being found in the Chung-Joo, Duk-Pyung and Chu- Bu areas. The Guryongsan Formation also consists of black slates which are distributed in the Geum- Kwan and I-Won areas (Lee and Kim, 1972). Soils derived from these parent materials tend to reflect their extreme geochemical composition (Bowie and Thornton, 1984) and may influence human health by affecting the elemental composition of crop plants (Foth, 1978). In this study, five areas with similar bedrock geo- chemistry were selected, (1) to investigate the concen- tration of potentially toxic elements in the rock-soil system from areas underlain by black shales and slates; (2) to evaluate the level of enrichment of potentially toxic elements in soils derived from black shales and slates; and (3) to identify uranium-bearing minerals in black shales by electron probe micro- analysis. *To whom correspondence should be addressed. 0269±4042 # 1998 Chapman & Hall Environmental Geochemistry and Health (1998), 20, 135±147