ASSESSMENT OF WATER QUALITY IN THE CAIMA AND MAU RIVER BASINS (PORTUGAL) USING GEOCHEMICAL AND BIOLOGICAL INDICES M. L. NUNES 1 , E.FERREIRA DA SILVA 1∗ andS. F. P. DE ALMEIDA 2 1 Department of Geosciences, University of Aveiro, Aveiro, Portugal; 2 Department of Biology, University of Aveiro, Aveiro, Portugal ( ∗ author for correspondence, e-mail: eafsilva @geo.ua.pt, Fax: +351 234370605) (Received 7 May 2003; accepted 21 May 2003) Abstract. In order to study the anthropogenic impact of trace metals in the Caima and Mau River basins (north-central Portugal), 415 stream sediment samples were randomly collected in the main streams of these rivers and the minus 80-mesh fraction was analysed by Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES). The presence of abandoned mines was identified as the main source of Cu, Pb, Cd and Ni in the area. A strong pair-wise correlation between Pb, Zn, Cd and Ni was found. In this study, the geochemical indices (Contamination Index (CI) and Mul- tiple Pollution Index (MPI)), and the diatom indices (Specific Pollution Index (SPI) and Biological Diatom Index (BDI)) are used to define the Caima and Mau River basins environmental quality. The estimation of the CI and MPI indices shows that the sediments near Coval da Mó area are enriched with Pb, Cd, Zn Ni and Co suggesting an anthropogenic origin of metal contamination in this zone. In order to evaluate the biological effects caused by the trace metals, 17 sites were selected from the two River basins. Surface water and epilithic diatom communities were sampled during the summer of 2001. In general, the water and diatom data showed medium to good water quality. However, samples collected near the mining areas (Coval da Mó) revealed low abundance and diversity of diatom taxa indicating a stressed environment. Some diatoms showed morphologically abnormal valves that may be related to the high level of metal pollution in this area. Keywords: Caima and Mau Rivers, epilithic diatoms, heavy metal pollution, indices, stream sedi- ments, surface water quality 1. Introduction The quality of river water in the natural environment as reflected by different types and origins of dissolved and suspended compounds is largely determined by the geology of the catchment area and the climate acting upon it. In populated areas, river water quality is determined by human activities: deforestation, farming, min- ing, industrial and domestic sewage discharge, which causes changes in colour, suspended solids, pH, temperature, nutrients and run-off characteristics. Geochemical indices are commonly used in the detection and distribution of contamination. Numerous studies have aimed at the elucidation of the effect of trace element contamination on soils, plants, waters and sediments in various coun- tries (Thornton, 1980; Fuge et al., 1989; Merrington and Alloway, 1994; Pestana Water, Air, and Soil Pollution 149: 227–250, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.