Spatio-temporal variability of water quality and trophic state in a semi-arid basin of northeastern Brazil

<title>Abstract</title> <p> Urban semi-arid rivers in developing regions remain understudied regarding water quality, a gap that is critical in the Brazilian semi-arid region, the most densely populated of its kind worldwide. This study evaluated the water quality of the Salgado River, northeastern Brazil, through five sampling campaigns conducted between 2023 and 2024 across rainy and dry hydrological periods. Eleven sites were monitored along the fluvial gradient, and the following parameters were analyzed: pH, temperature, electrical conductivity (EC), total dissolved solids (TDS), total suspended solids (TSS), turbidity, dissolved oxygen (DO), biochemical oxygen demand (BOD), total phosphorus (TP), total nitrogen (TN), and thermotolerant coliforms (TC). pH remained near neutral (7.1 ± 0.3). Temperature was higher in the dry season (29.4 ± 3.1°C) than in the rainy season (27.6 ± 1.5°C). EC increased markedly during the dry period (722 ± 250 µS cm⁻¹), while turbidity was higher in the rainy season (108 ± 150 NTU). DO remained low in both periods (3.3–3.7 mg L⁻¹). BOD was elevated in the dry season (8.7 ± 9.2 mg L⁻¹). TN was higher in the dry period (5.27 ± 6.3 mg L⁻¹), whereas TP showed no significant seasonal variation (1.13–1.81 mg L⁻¹). TC levels were extremely high during the dry season (3.5 × 10⁶ MPN 100 mL⁻¹). The Water Quality Index (WQI) classified the river as fair (39–45), with very poor conditions in urban stretches and relative improvement in rural areas. The Trophic State Index (TSI _{<italic>PT</italic>} ) indicated eutrophic to hypereutrophic conditions (60–77), especially in urban sites. Overall, the Salgado River exhibits selective and seasonal degradation, with critical urban hotspots, partial self-purification reaches, and high eutrophication risk. </p>