Hydrochemical Responses to Land Use and Seasonality in a Semiarid Watershed Under Anthropogenic Pressure

Anthropogenic activities and hydroclimatic variability have intensified hydrochemical changes and water-quality degradation in semiarid watersheds, increasing the risks of salinization and eutrophication. This study evaluated the influence of land use and seasonal variability on the hydrochemical and microbiological characteristics of surface waters in the lower Piranhas-Açu River basin, northeastern Brazil. Water samples were collected at seven monitoring points downstream of the Armando Ribeiro Gonçalves Reservoir during dry and rainy periods between 2019 and 2021. Physical, chemical, and microbiological parameters, including pH, electrical conductivity (EC), Na⁺, K⁺, Ca2⁺, Mg2⁺, Cl⁻, CO₃2⁻, HCO₃⁻, alkalinity, nitrogen, phosphorus, thermotolerant coliforms, and chlorophyll-a, were evaluated using Pearson correlation, principal component analysis, cluster analysis, and factor analysis. Multivariate analyses identified three distinct water-quality groups, while factor analysis retained three factors explaining 71.93% of the total variance and representing trophic conditions, ionic enrichment, and carbonate–bicarbonate dynamics. Dry periods were associated with greater salinity, alkalinity, phosphorus concentrations, and phytoplankton response, whereas rainy periods showed higher N, K⁺, and HCO₃⁻ levels and lower overall mineralization. Urbanized areas were mainly associated with phosphorus enrichment and eutrophic indicators, while agricultural activities and shrimp farming coincided with higher dissolved-ion concentrations, EC, and alkalinity. Overall, land-use pressures and hydroclimatic seasonality increased spatial and temporal water-quality heterogeneity, supporting multivariate approaches for integrated water-resource management in semiarid regions. • Multivariate analyses revealed distinct spatial and temporal water-quality patterns. • Factor analysis identified three hydrochemical factors explaining 71.93% of total variance. • Rainfall reduced P and chlorophyll-a by 95.10% and 78.90%, respectively. • Dry-season waters showed greater ionic enrichment and alkalinity than rainy-season waters. • Contrasting land uses coincided with distinct hydrochemical and nutrient-related spatial patterns.