Seasonal and spatial evaluation of the surface water quality in the Longá river watershed, Piauí, Brazil

Abstract This article evaluates the quality of the surface waters in the Longá River watershed in the State of Piauí, Brazil. Seven points were monitored, based on their proximity to meteorological stations and accessibility, to obtain better spatial and land-use characterizations of the watershed. Water samples were collected monthly (01/2015-12/2015). The variables temperature, pH, turbidity, electrical conductivity, total solids, total phosphorus, nitrate, dissolved oxygen, biochemical oxygen demand (BOD) and Escherichia coli (E. coli) were measured. The results were evaluated individually and compared to Brazilian standards for Class 2 fresh waters, according to the National Environment Council’s (CONAMA) Resolution 357/2005. The data were evaluated using multivariate analyses (clustering and principal component analysis - PCA). E. coli, pH, turbidity, nitrate, BOD, and electrical conductivity varied between the two periods analyzed (dry and rainy). The BOD and E. coli variants were noncompliant with the CONAMA standards at urban points P2, P3, and P5, because of nonpoint source pollution (NPS) during the rainy season. Clustering showed a spatial trend, presenting two distinct groups (rural and urban). PCA identified four main components that explained a 58.64% change in the data. Component 1 (CP1) reflected a contribution of pollutants related to NPS from agricultural and urban areas, in addition to environmental factors. CP2 was strongly associated with pollution, especially by untreated sewage releases. The other CPs also reflect pressures from anthropogenic activities on the water bodies.