Seasonal and Land Use Interaction on Water Quality in the Campo Alegre Stream, Goiano Cerrado: Evidence of Urban Impact

Freshwater ecosystems in the Brazilian Cerrado face escalating degradation from agricultural expansion and urban growth. Understanding the synergistic interaction between land use and seasonal hydrological pulses is critical for water resource management. This study evaluated the spatial-temporal dynamics of water quality and potential ecotoxicological risks in the Campo Alegre Stream basin (Santa Helena de Goiás, Brazil). Water samples were collected across three land-use zones - Conservation Unit, Agricultural Area, and Urban Area - during the Rainy Season and Dry Season of 2023. Twenty-six physicochemical, optical, metallic, and microbiological parameters were analyzed according to standard methodologies. Statistical evaluations included two-way ANOVA with Tukey's post-hoc test, Pearson correlation, Hierarchical Cluster Analysis (HCA), and the calculation of the Water Quality Index (WQI). Compliance was assessed against CONAMA Resolution No. 357/2005 Class 2 standards. Seasonal rainfall-driven runoff degraded water clarity and elevated metal concentrations during the Rainy Season, with Turbidity (mean of 189.33 NTU), Aluminium (mean of 0.15 mg L⁻¹), and Manganese (mean of 0.10 mg L⁻¹) exceeding regulatory thresholds. The Urban Area exhibited chronic microbial and organic contamination, consistently breaching limits for Escherichia coli (up to 4.41 log₁₀ MPN 100 mL⁻¹) and Biochemical Oxygen Demand (BOD, mean of 4.70 mg L⁻¹) across both seasons. Total Hardness, Total Dissolved Solids (TDS), Electrical Conductivity (EC), and Nitrate increased substantially during the Dry Season due to reduced dilution. Pearson correlation and HCA grouped samples primarily by seasonality and secondarily by land use. The WQI classified the Conservation Unit and Agricultural Area from "Good" (62.5–69.8) in the Rainy Season to "Good"/"Excellent" (77.2–79.2) in the Dry Season, whereas the Urban Area remained deteriorated at the "Good/Moderate" border (~52.0) year-round. Stormwater runoff from highly weathered Cerrado soils coupled with urban surface washing drives seasonal spikes in particulate matter and bioactive metals, posing toxicological risks to aquatic biota. Persistent urban non-compliance highlights untreated domestic sewage discharge that overrides natural river self-purification. Low Phosphorus levels (<0.03 mg L⁻¹) currently prevent severe eutrophication despite high Nitrate availability. Water quality in the Campo Alegre basin is governed by the interplay between seasonal hydrological pulses and chronic urban pollution. Urban pressures continuously degrade the stream beyond regulatory limits for Class 2 waters, emphasizing the urgent need for expanded municipal wastewater treatment and agricultural soil conservation practices.