Avaliação da qualidade da água no trecho urbano do rio Purus no município de Lábrea - AM

Abstract

Water, a constantly renewing natural resource, is indispensable for sustaining life. Brazil has several laws aimed at guaranteeing the quality and availability of water resources nationwide. In this context, this study aimed to evaluate the seasonality of water quality in the Purus River, in the municipality of Lábrea/AM, during the dry and rainy seasons. An on-site visit was conducted to verify the sampling points, defining three specific points: 1 – upstream, 2 – riverbed, 3 – downstream. After this definition, the points were mapped and georeferenced. All of them are located on the edge of the urban area of the municipality of Lábrea/AM. Physical parameters (water temperature, turbidity, and total dissolved solids), chemical parameters (dissolved oxygen, biochemical oxygen demand, total phosphorus, total nitrogen, and pH), and microbiological parameters (thermotolerant coliforms) were analyzed. During monitoring campaigns conducted on the Purus River during the rainy and dry seasons, it was observed that water quality varied according to the hydrological regime, showing better conditions during the dry season. During the rainy season, high concentrations of thermotolerant coliforms were observed, ranging from 1,900 to 250,000 MPN/100 mL, values that significantly exceed the maximum limit of 1,000 MPN/100 mL established by CONAMA Resolution No. 357/2005, indicating significant fecal contamination. Total phosphorus levels also showed high values, with 0.84 mg/L at point P1 and 0.74 mg/L at P3, exceeding the limit of 0.1 mg/L. During the dry season, a significant reduction in the values of these parameters was observed, along with stability in dissolved oxygen values, which ranged between 6.27 and 6.54 mg/L. Thermotolerant coliforms decreased to 1.1 MPN/100 mL, reflecting lower microbiological contamination. BOD₅ varied from 5.23 mg/L (P2) to 25.3 mg/L (P3), with an increase at the latter point, indicating a higher load of biodegradable organic matter and localized anthropogenic influence. The Water Quality Index (WQI) assessment confirmed these trends, classifying the waters as having "good" to "fair" quality for both periods. Overall, the Purus River presents good environmental quality, although specific points reveal localized anthropogenic impacts, highlighting the importance of continuous monitoring for water quality conservation.