Correlação entre índice de qualidade da água e usos múltiplos na bacia hidrográfica do igarapé da Prata, Capitão Poço/PA

Abstract

Water quality indicators are important tools for the summary and generation of solid foundations about water resources degradation. The Water Quality Index of the Environmental Company of the State of São Paulo (WQI-CETESB) is the main water quality index used in Brazil. In relation between water quality and degradation in drainage basins, it is common to associate multiple uses of soil and water. This research correlates the WQI-CETESB to multiple uses of water and soil at three collection sites in the Prata catchment, located in the municipality of Capitão Poço / PA, about 154 km east of Belém / PA. It is a rural area in northeast Pará with intense agricultural activities. It was carried out eighteen campaigns with twenty-three collections, being that are twelve ones in the rainy season and eleven ones in the less rainy season. The index was calculated from the analysis of its nine parameters: turbidity, dissolved oxygen (DO), biochemical oxygen demand (BOD), total nitrogen, total phosphorus, total solids, pH, temperature and thermotolerant coliforms. The descriptive statistics was applied to synthesize the obtained data. The morphometry of the drainage areas of the collection sites was analyzed in order to understand the influence of these physical characteristics on the water quantity and quality. Soil use and coverage were mapped using georeferencing tools. The association between WQI-CETESB results and multiple uses was verified by checking the variables with the greatest influence on the value obtained for the index. Based on the WQI-CETESB, the water quality in the Prata catchment ranges between the categories Fair and Good. The values obtained in the majority did not present significant variations for the different periods (rainy and less rainy). The descriptive statistics proved to be efficient for the analysis of the results obtained from WQI-CETESB. The morphometric analysis of the drainage areas showed that the shape and the topography are not factors that impact the water quantity and quality in the catchment. The mapping of soil use and coverage and activities related to multiple uses of water, showed to be efficient as an aid in the visualization of local dynamics. The variables that most influenced the results were thermotolerant coliforms, pH, DO, BOD and total phosphorus, with representativeness associated to the three collection sites studied. The main sources of pollution raised by the mapping are strongly related to the results obtained. Targets for quality improvement and mitigation measures should be established with the aim of reducing local water degradation.