1. Alloway, B. J. (1995). Introduction. In B. J. Alloway (Ed.), Heavy metals in soils (2nd ed., pp. 3–10). London: Blackie Academic and Professional.
2. Bullock, J. H., Cathcart, J. D., & Betterton, W. J. (2003). Analytical methods utilized by the United States Geological Survey for the analysis of coal and coal combustion by products. Open-file report 02–389. Denver: U.S. Geological Survey.
3. Callender, E. (2005). Heavy metals in the environmental–historical trends. In B. S. Lollar (Ed.), Environmental geochemistry (pp. 67–105). Amsterdam: Elsevier.
4. Carney, J. N., Aldiss, D. T., & Lock, N. P. (1994). The geology of Botswana. Bulletin 37. Botswana: Geological Survey Department.
5. Casas, J. M., Rosas, H., Solé, M., & Lao, C. (2003). Heavy metals and metalloids in sediments from the Llobregat basin, Spain. Environmental Geology, 44, 325–332. doi: 10.1007/s00254-003-0765-6 .