___

• Adriano, D.C., 1986. Trace elements in terrestrial environments: Biogeochemistry, bioavailability, and risks of metals. Springer-Verlag, New York. 867p.
• Alloway, B. J., 1995. Heavy Metals in Soils. Chapman and Hall, London, UK, 372p.
• Atanasov, I., 2003. Proposal for utmost permissible concentrations of heavy metals and metalloids in soils in dependence on different manners of land use and objects of protection. The final report on contract № 3327/369 signed on 25.11.2002 between the Bulgarian Ministry of Environment and Water and contractor leaded by prof. Ivan Atanasov, Sofia, Bulgaria. 181p. [in Bulgarian].
• Barbante, C., Boutron, C., Morel, C., Ferrari, C., Jaffrezo, J.L., Cozzi, G., Gaspari, V., Cescon, P., 2003. Seasonal variations of heavy metals in central Greenland snow deposited from 1991 to 1995. Journal of Environmental Monitoring 5(2): 328–335.
• Becker, D.E., Smith, S.E., 1951. The level of cobalt tolerance in yearling sheep. Journal of Animal Science 10(1): 266-271.
• Boikat, U., Fink, A., Bleck-Neuhaus, J., 1985. Cesium and cobalt transfer from soil to vegetation on permanent pastures. Radiation and Environmental Biophysics 24(4): 287-301.
• Cole, C.J., Carson, B.L., 1981. Cobalt in the food chain. In: Trace metals in the environments. Volume 6, Cobalt. Smith, I.C., Carson, B.L. (Eds.). Ann Arbor Science Publishers Inc., Ann Arbor, Ml., pp. 777-924.
• Condie, K. C., 1976. Plate tectonics and crustal evolution. Butterworth Heinemann. Elsevier Science, Oxford, UK. 288 p.
• Dinev, N., 2012. Environmental monitoring and remediation strategies for contaminated soils with heavy metals. Doctor of Science Thesis, Institute of Soil Science, Agrotechnologies and Plant Protection “N. Poushkarov”, Department of Soil Chemistry, Sofia, 275 p.. [in Bulgarian].
• Dinev, N., Banov, М., Nikova, I., 2008. Monitoring and risk assessment of contaminated soils. General and Applied Physiology 34(3-4): 389-396 [in Bulgarian].
• Filcheva, E.G., Tsadilas, C.D.,. 2002. Influence of clinoptilolite and compost on soil properties. Communications of Soil Science and Plant Analysis 33(3-4): 595-607.
• Gong, H., Rose, A.W., Suhr, N.H., 1977. The geochemistry of cadmium in some sedimentary rocks. Geochimica et Cosmochimica Acta 41(12): 1687- 1692.
• ISO 11466:2010. Soil quality - Extraction of trace elements soluble in aqua regia. International Organization for Standardization.
• ISO 22036:2012. Soil quality - Determination of trace elements in extracts of soil by inductively coupled plasma - atomic emission spectrometry (ICP - AES). International Organization for Standardization.
• ISO 10390:2005. Soil quality - Determination of pH. International Organization for Standardization.
• Hristova, M., 2013. Content and availability of microelements metals in technosols. PhD Thesis. Institute of Soil Science, Agrotechnologies and Plant Protection “N. Poushkarov”, Department of Soil Genesis, Classification and Geography, Sofia Bulgaria. 143p. [in Bulgarian].
• Hutton, M., 1982. Cadmium in the European Community: A prospective assessment of sources, human exposure and environmental impact. Monitoring and Assessment Research Centre Technical Report No. 26. University of London, UK.
• Jurlik, J., 2001. Soil inorganic contamination in Slovak Republic and some environmental consequences. Assessment of the quality of contaminated soils and sites in Central and Eastern European Countries (CEEC) and new independent states (NIS): International conference on contaminated soils and sites, Sofia, Bulgaria, 30 September - 3 October. Book of Proceeding. pp. 114-121.
• Jeleva, E., 2008. Analysis of the ecological conditions in the area of Bobov dol mines Ltd. in relation to the reclamation of the damaged terrains. International Scientific Conference on "Remediation, balance and perspective in environmental protection". 14-15 May 2008. Belgrade, Serbia.
• Jones, A.S.G., 1973. The concentration of copper, lead, zinc and cadmium in shallow marine sediments, Cardigan Bay (Wales). Marine Geology 14(2): M1-M9.
• Kabata-Pendias, A., Mukherjee, A.B., 2007. Trace elements from soil to human. Springer-Verlag Berlin Heidelberg, 550p.
• Kabata-Pendias, A., Pendias, H., 1984. Trace elements in soils and plants. CRC Press. 315p.
• Keener, H.A., Percival, G.P., Morrow, K.S., Ellis, G.H., 1949. Cobalt tolerance in young dairy cattle. Journal of Dairy Science 32(6): 527-533.
• Ketris, M.P., Yudovich, Y.E., 2009. Estimations of clarkes for carbonaceous biolithes: World averages for trace element contents in black shales and coal. International Journal of Coal Geology 78(2): 135–148.
• Koljonen, T., 1992. The Geochemical Atlas of Finland, Part 2: Till. Geological Survey of Finland, Espoo, Finland, 218 p.
• Kononova, M. M., 1963. Soil organic matter. Its nature, properties and methods of the study. ANSSSR, Moscow, Russia. 314p. [in Russian].
• Kortenski, J., 2011. Elements impurities and ash-forming elements in the Bulgarian coal. University Mining Geology “St. Ivan Rilski”, Sofia, Bulgaria, 248 p. [in Bulgarian].
• Krauskopf, K.B., 1956. Factors controlling the concentrations of thirteen rare metals in sea-water. Geochimica et Cosmochimica Acta 9(1-2): A32-B32.
• Kuikin S., Atanasov I., Hristova Y., Hristov D., 2001. Background content of heavy metals and arsenic in soil parent rocks of Bulgaria. Assessment of the quality of contaminated soils and sites in Central and Eastern European Countries (CEEC) and new independent states (NIS): International conference on contaminated soils and sites, Sofia, Bulgaria, 30 September - 3 October. Book of Proceeding. p.121-131.
• Lalor, G.C., 1995. Chromium distribution in Jamaican soils. In: A geochemical atlas of Jamaica, Canoe Press, Kingston, Jamaica. p. 24-25,
• Lee, Y., Tebo, B.M., 1994. Cobalt(II) oxidation by the marine manganese(ii)-oxidizing Bacillus sp. strain SG-1. Applied and Environmental Microbiology 60(8): 2949-2957.
• Lee, K., Hur, S. D., Hou, S., Hong, S., Qin, X., Ren, J., Liu, Y., Rosman, K.J.R., Barbante, C., Boutron, C.F., 2008. Atmospheric pollution for trace elements in the remote high-altitude atmosphere in central Asia as recorded in snow from Mt. Qomolangma (Everest) of the Himalayas. Science of The Total Environment 404(1): 171–181.
• Lund, L.J., Betty, E.E., Page, A.L., Elliot, R.A. 1981. Occurrence of naturally high cadmium levels in soils and its accumulation by vegetation. Journal of Environmental Quality 10(4): 551-556.
• Marinkina, C., 1999. Study and opportunities for reclamation of sulphide containing material obtained from open mining. PhD Thesis. Institute of Soil Science, Agrotechnologies and Plant Protection “N. Poushkarov”, Department of Soil Genesis, Classification and Geography, Sofia., Bulgaria. 271 p. [in Bulgarian].
• McBride, M.B., 1994. Environmental chemistry of soils. Oxford University Press Inc. New York, USA. 406p.
• McLennan, S.M., Murray, R.W., 1999. Geochemistry of sediments. In: Encyclopedia of geochemistry. Marshall, C.P., Fairbridge, R.W. (Eds), Kluwer Academic Publishers, Dordrecht, The Netherlands. pp. 282-292.
• Mielke, J.E., 1979. Composition of the Earth's crust and distribution of the elements. In: Review of Research on Modern Problems in Geochemistry. F.R. Siegel (Ed.), UNESCO Report, Paris, pp. 13–37.
• Minkina, T., Mandzhieva, S., Motusova, G., Burachevskaya, M., Nazarenko, O., Sushkova, S., Kızılkaya, R., 2014. Heavy metal compounds in a soil of technogenic zone as indicate of its ecological state. Eurasian Journal of Soil Science 3(2): 144-151.
• Misheva, L., Tsolova, V., Banov, M., Poynarova, M., Hadjijanakiev, Y., Zlatev, A., Kojouharova, S., Zhelyazkova, S., Vasileva, H., 2007. Potential for land use and land contaminated with radionuclides and heavy metals. Institute of Soil Science, Agrotechnologies and Plant Protection “N. Poushkarov”, Sofia, Bulgaria. p. 39. [in Bulgarian].
• Nicollet, C., Andriambololona, D.R., 1980. Distribution of transition elements in crustal metabasic igneous rocks. Chemical Geology 28: 79-90.
• Nikova, I., Hristov, B., Zdravkov, A.., Ruskov, K., Petrov, D., Bakardjiev, D., 2013. Monitoring of soil chemical characteristics in the area of "Bobov Dol". Soil Science Agrochemistry and Ecology 3: 55-63. [in Bulgarian].
• Petrova, R., 2009. Assessment of soil contamination on the area of “Cumerio-Copper". Forestry Ideas 15(2): 59-67.
• Planchon, F.M., Boutron, C.F., Barbante, C., Cozzi, G., Gaspari, V., Wolff, E.V., Ferrari, C.P., Cescon, P., 2002. Short-term variations in the occurrence of heavy metals in Antarctic snow from Coats Land since the 1920s. Science of The Total Environment 300(1-3): 129–142.
• Qian, Y., Li, G., Chen, W., Li, B., Jin, X., 1998. Synthesis and crystal structure of tetrahydrofurfurylcyclopentadienyl titanium trichloride and its derivatives. Journal of Organometallic Chemistry 373(2):185-191.
• Reimann, C., Caritat, P., 1998. Chemical elements in the environment - factsheets for the geochemist and environmental scientist. Springer-Verlag Berlin Heidelberg, 398p.
• Schroeder, W.H., Dobson, M., Kane, D.M., Johnson, N.D., 1987. Toxic trace elements associated with airborne particulate matter: A review. Journal of the Air & Waste Management Association 37(11): 1267-1285.
• Salminen, R., Batista, M.J., Bidovec, M., Demetriades, A., de Vivo, B., de Vos, W., Duris, M., Gilucis, A., Gregorauskiene, V., Halamic, J., Heitzmann, P., Lima, A., Jordan, G., Klaver, G., Klein, P., Lis, J., Locutura, J., Marsina, K., Mazreku, A., O'Connor, P.J., Olsson, S.Å., Ottesen, R.T., Petersell, V., Plant, J.A., Reeder, S., Salpeteur, I., Sandström, H., Siewers, U., Steenfelt, A., Tarvainen, T., 2005. Geochemical Atlas of Europe. Part 1 - Background Information, Methodology and Maps. FOREGS. Available at: http://weppi.gtk.fi/publ/foregsatlas/ [access date: 06.07.2015].
• Ure, A.M., Berrow, M.L., 1982. The chemical constituents of soils. In: Environmental chemistry. Bowen, H.J.M. (Ed.). Royal Society of Chemistry, Burlington House, London, UK. pp. 94-202.
• Van de Velde, K., Boutron, C.F., Ferrari, C.P., Moreau, A.L., Delmas, R.J., Barbante, C., Bellomi, T., Capodaglio, G., Cescon, P., 2000. A two hundred years record of atmospheric cadmium, copper and zinc concentrations in high altitude snow and ice from the French-Italian Alps. Geophysical Research Letters 27(2): 249–252.
• Vassilev, S.V., Vassileva, C.G., 1997. Geochemistry of coal, coal ashes and combustion wastes from coal-fired power stations. Fuel Processing Technology 51(1-2): 19–45.
• Vine, J.D., Tourtelot, E.B., 1970. Geochemistry of black shales – a summary report. Economic Geology 65(3): 253-272.
• Waldron, H. A., 1980. Metals in the environment. Academic Press Inc., London. UK. 333p.
• Wedepohl, K.K., 1978. Handbook of geochemistry. Volume 2, Part 5. Springer-Verlag Berlin Heidelberg, 1546 p.
• WRBSR, 2014. World reference base for soil resources 2014. International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports No. 106. Food and Agriculture Organization of the United Nations (FAO), Rome. Italy.
• Yudovitch, J., Ketris, M., 2005. Toxic trace elements in fossil coal. Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia.