A study on surface water quality in the Acaraú Basin, in the North of the state of Ceará, Brazil was performed. Qualitative dynamics of water flowing to the Acaraú River that supplies water to the irrigation project in the area was evaluated. Multivariate Factor Analysis/Principal Component Analysis was used for evaluation of water quality in order to develop a water quality index (WQI) that reflects soil salinity and sodicity risks and water toxicity to plants. From April/2003 to September/2005 water were sampled from ten sampling sites covering the basin, where physical and chemical parameters that contribute to the WQI were evaluated. The results showed that the use of water for irrigation in the Acaraú basin, according to the proposed WQI, are potentially prone to cause toxicity (crop cycle) and sodicity problem in the long run, if the soil-water-plant is not carefully monitored. Um estudo foi realizado sobre a qualidade da água de superfície na Bacia do Acaraú, porção Norte do estado do Ceará, Brasil. Foram avaliados os parâmetros qualitativos da água do Rio Acaraú que abastece o projeto de irrigação nessa área. Análise da Componente Principal/Análise do Fator Multivariada foi usada na avaliação da qualidade da água a fim de desenvolver um índice de qualidade de água (IQA) que refletisse os riscos de salinidade e sodicidade do solo bem como a toxicidade por íons específicos às plantas. As amostras de água foram coletadas em dez locais cobrindo toda a bacia no período de abril/2003 e setembro/2005, em que os parâmetros físicos e químicos que contribuem para o IQA foram avaliados. Os resultados mostraram que a águas utilizadas para irrigação na bacia do Acaraú, de acordo com o IQA proposto, são potencialmente propensas a causar problemas de sodicidade e toxicidade ao longo do tempo, se a relação solo-água-planta não for rigorosamente manejada

To ensure a safe drinking water supply, it is necessary to protect water quality. To classify the suitability of the Orós Reservoir (Northeast of Brazil) water for human consumption, a Water Quality Index (WQI) was enhanced and refined through a Principal Component Analysis (PCA). Samples were collected bi-monthly at seven points (P1 – P7) from July 2009 to July 2011. Samples were analysed for 29 physico-chemical attributes and 4 macroinvertebrate metrics associated with the macrophytes Pistia stratiotes and Eichhornia crassipes. PCA allowed us to reduce the number of attributes from 33 to 12, and 85.32 % of the variance was explained in five dimensions (C1 – C5). Components C1 and C3 were related to water-soluble salts and reflect the weathering process, while C2 was related to surface runoff. C4 was associated with macroinvertebrate diversity, represented by ten pollution-resistant families. C5 was related to the nutrient phosphorus, an indicator of the degree of eutrophication. The mean values for the WQIs ranged from 49 to 65 (rated as fair), indicating that water can be used for human consumption after treatment. The lowest values for the WQI were recorded at the entry points to the reservoir (P3, P1, P5, and P4), while the best WQIs were recorded at the exit points (P6 and P7), highlighting the reservoir’s purification ability. The proposed WQI adequately expressed water quality, and can be used for monitoring surface water quality.