Long-term water quality conditions and trends in 12 tropical coastal rivers in Northeast Brazil

The water quality and trends in 12 tropical rivers in northeastern Brazil over a 27-year period (1990-2016; N = 39,008 samples) were evaluated. The analyzed parameters included temperature, conductivity, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), nitrogen (NH4+), total phosphorus (P), and fecal coliforms. Densely populated basins (> 1000 inhab km(-2)) presented lower DO values (average 3.4 mg l(-1); 43% DO saturation), while those with low demographic density (< 100 inhab km(-2)) presented values that aligned well with the recommendations of environmental legislation (average 5.8 mg l(-1); 75% DO saturation). The NH4+ and P compound concentrations were typical of water bodies affected by urban inputs. The average p values were above the allowable limit (< 0.1 mg l(-1)) at all stations. The NH4+ values were high at the stations showing low DO concentrations, which suggested that due to reducing conditions after NH4+ accumulation was favored in those aquatic systems. In densely populated basins, the average fecal coliform concentrations were> 40,000 MPN 100 ml(-1), indicating the input of improperly treated domestic/industrial liquid wastes. For the period from 1990 to 2016, 45% of the stations (N = 19) showed a rate of DO reduction that ranged from 0.01 to 0.17 mg l(-1).O-2 year(-1). An increase in NH4+ concentrations was observed in 33% of the stations (N = 14), with an estimated average increase rate from 0.013 to 1.8 mg l(-1) NH4+ year(-1). These results demonstrated that the rates of increase in anthropogenic factors were significant (p < 0.05), while the natural factors remained constant.

Automated summary

This study evaluated the water quality and trends in 12 tropical rivers in northeastern Brazil over a 27-year period. It found that densely populated areas had lower DO values, while areas with low demographic density aligned well with environmental legislation. The concentrations of NH4+ and P were affected by urban inputs, with high fecal coliform concentrations in densely populated basins. Additionally, some stations showed a reduction in DO levels and an increase in NH4+ concentrations over time.