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

Abstract

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 3.4mg 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 5.8mg 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 (<0.1mg 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 100ml-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 0.17mg l-1.O2 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 1.8mg 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.