The area under study is located near Estarreja, in central Portugal where chemical industries are the main source of pollution. Until 1975, liquid effluents produced by these industrial units were discharged directly into several nearby streams. These effluents contained many different types of contaminants, such as aniline, benzene, monochlorobenzene, mononitrobenzene, As, Hg, Zn and Pb, amongst others. The industrial units also dumped a large amount of solid waste, which was stockpiled, directly on the surrounding land. This solid waste contains pyrite-rich ashes, dusts and sludge containing Hg and calcium hydroxide. A geochemical soil survey was conducted in a 49-km2 area where a total of 985 soil samples were collected during 1998 and 1999, on a regular grid of 200 × 200 m. The soil samples were analysed for Cu, Pb, Zn, Fe, Mn, Co, Ni, Cr and As, following an HCl–HNO3–HF digestion.
To identify the main soil contaminants and to map the most important contaminated areas, Principal Component Analysis (PCA) and geostatistical techniques (Variography and Kriging map) were used. The results of PCA data analysis, compared to the available information in pollution indices (Contamination Index – Cidx), outline one important area contaminated by Cu, Pb, Zn and As as a result of effluent dumping by the chemical industry.