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An environmental monitoring program was carried out to determine the impact of eight pesticides on the surface and groundwater quality of agricultural areas within the Ebro, Duero and Mino river basins. Three triazines and their desethyl degradation products, metolachlor and metalaxyl, were monitored during 18 months in 63 sites. Solid-phase extraction (SPE) using OASIS HLB 60mg cartridges and gas chromatography-mass spectrometry (GC-EI-MS) provided good analytical quality parameters and limits of quantification of 0.01 mu g/L. Environmental data were assessed using descriptive statistical analysis and multivariate data analysis with principal component analysis (PCA) to elucidate the relevant contamination patterns and provide a description of their seasonal trends, according to the pesticide application timing. Duero was the site with the highest frequency of detection and highest concentration levels, followed by the Ebro and Mino basins. The frequency of detection of the studied compounds, considering all surface and groundwater samples, was atrazine > desethylatrazine > simazine > desethylsimazine > metolachlor > desethylterbuthylazine > terbuthylazine > metalaxyl. Over all results, and taking into consideration the European Union (EU) maximum residual limit of pesticides in groundwater, only 12% of the results exceeded the 0.1 mu/L limit. However, sporadic high levels up to 2.46 mu g/L in groundwater and 0.63 mu g/L in surface water were detected. PCA permitted to state that Duero and Ebro river basins were especially affected by a contamination pattern dominated by atrazine, the Ebro river basin being occasionally affected by a contamination pattern dominated by simazine. Only trace levels were rarely detected in the Mino river basin. Groundwater levels were higher than surface water levels for the studied pesticides. (c) 2008 Elsevier Ltd. All rights reserved.