Spatio-temporal Patterns of River Water Quality in the Semiarid Northeastern Brazil

Non-perennial rivers have attracted significant attention due to the progressive increase in water demand and pollution, river engineering side effects and climate change. The present study investigates the interannual, seasonal and spatial variability of nine water quality variables (pH, colour, turbidity, biochemical oxygen demand, total phosphorus, total inorganic nitrogen, chlorophyll-a, total solids and thermotolerant coliforms) from 93 monitoring river sites distributed over 11 watersheds in the highly populated Brazilian semiarid region. Shapiro–Wilk and Kruskal–Wallis tests were applied for assessing data normality and statistical differences, respectively. Additionally, principal component analysis (PCA) was performed for comparison between temporal and spatial data sets, while the Standardized Precipitation Index (SPI) was used to characterize meteorological drought. The results revealed that spatial variability is more evident than temporal variability. In the temporal scale, the interannual variability is more relevant than the seasonal one. The discharge of wastewater seems to attenuate a seasonal hydrological effect on water quality. There is a deterioration of water quality in most watersheds in the drier years, even in the rainy season. This is especially for colour, total phosphorus, total inorganic nitrogen and thermotolerant coliforms. The abrupt transition from dry to wet years also played an important role in changes in river water quality. This study provides new insights for the understanding of the water quality patterns in non-perennial rivers. In addition, we go on to identify important aspects for the management of these water sources, such as the need for wastewater discharge guidelines within scope of each watershed, and restrictions on the use of soil and water in the driest periods.