Mapeamento de mecanismos hidroquímicos em reservatórios da Bacia do Curu, Ceará, Nordeste do Brasil

Abstract

Monitoring water quality in reservoirs in semi-arid regions is essential to understanding the hydrochemical processes that determine their composition and evolution, especially in areas where climatic variability and anthropogenic pressures strongly influence the availability and potability of water resources. This study investigated, between 2018 and 2023, the Caxitoré, Frios, General Sampaio, Pentecoste, and Tejuçuoca reservoirs, located in the Curu Basin, Ceará, Brazil, with the aim of identifying ionic patterns, assessing stability and agricultural use indices, analyzing the overall water quality, and mapping the predominant hydrochemical mechanisms. Physicochemical and ionic parameters were analyzed, taking into account regional climatic seasonality, in order to understand variations in water quality and the dominant processes of salinization, ion exchange, and chemical stability. The results revealed a predominance of sodium bicarbonate and calcium–magnesium bicarbonate facies, as shown by Piper and Chadha diagrams, with a notable dominance of sodium bicarbonate waters (76.7%) during the rainy season, associated with ion exchange processes between Ca²⁺, Mg²⁺, and Na⁺. The Gibbs diagram indicated that the chemical composition was mainly controlled by rock weathering and, secondarily, by evaporation during the dry season. Multivariate statistical analysis, using the Pearson correlation matrix, identified strong correlations (r ≥ 0.7) between pH, CO₃²⁻, and free CO₂, as well as between Electrical Conductivity (EC), Total Dissolved Solids (TDS), and Cl⁻, reflecting the importance of carbonate equilibrium and salinity in the ionic composition of surface waters. Regarding stability and aggressiveness indices, the Langelier Saturation Index (LSI) indicated water in equilibrium, whereas the Ryznar Stability Index (RSI) and Puckorius Scaling Index (PSI) revealed high levels of corrosivity in all reservoirs. The Aggressiveness Index (AI) classified the waters as non-aggressive, except for Frios reservoir, which showed fluctuations with moderately aggressive values during dry periods. Indices for agricultural use, such as Sodium Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), and Magnesium Ratio (MR), indicated significant restrictions, especially in reservoirs like Caxitoré and Pentecoste, where salinity and hardness conditions reduced suitability for irrigation, particularly in the second semester of each year. The Kelly Index and Permeability Index also showed marked seasonal variations. The Bascarán Water Quality Index (BWQI) values ranged from unsuitable to good quality throughout the years and semesters, reflecting the influence of rainfall conditions and anthropogenic use of the watershed. Persistent high concentrations of dissolved salts during dry periods further increased the challenges for water quality management in the region. This study provides valuable insights into the hydrochemical behavior of the reservoirs, showing them to be heterogeneous and influenced by geological, lithological, climatic, and anthropogenic factors. The integrated application of hydrogeochemical diagrams, ionic indices, and multivariate statistical analysis proved effective in characterizing dominant processes and supporting strategies for the management and monitoring of water resources in semi-arid regions.