1. Andersen, S., Odegard, S., Vogt, R. D. and Seip, H. M.: 1994, ‘Background levels of heavy-metals in Polish Forest soils’, Ecol. Eng. 3, 245–253.
2. Angelone, M. and Bini, C.: 1992, ‘Trace Elements Concentrations in Soils and Plants of Western Europe’, in: D. C. Adriano (eds.), Biogeochemistry of Trace Elements, Lewis Publishers, Boca Raton, FL, pp. 10–60.
3. Bachman, G., Freier, R. and Konietzka, R.: 1995, ‘Soil Levels Based on the German Soil Protection Bill’, in: W. J. van der Brink, R. Bosman and F. Arendt, (eds.), Contaminated Soil’95, Kluwer Academic Publishers, pp. 711–719.
4. Bech, J.: 2000, Heavy Metals and Trace Elements in Soils of Catalonia (Spain): Several Examples, Mem. Real Ac. Cienc. y Artes de Barcelona, 3 epoc. No. 967, Vol. LIX(2), pp. 43–79.
5. Bech, J.: 1991, Estudi del Backgrounds en Metals Pesants als sols de Catalunya, University of Barcelona, Unpublished Report.