Groundwater and Surface Water Quality for Irrigation in the Brazilian Semi-arid Region: Hydrochemical and Microbiological Assessment

Abstract Anthropogenic pressures and climate change are increasing the risk of water quality degradation in semi-arid regions, particularly where water resources are intensively exploited for irrigation. This study evaluated the quality of groundwater (wells tapping sedimentary and crystalline aquifers) and surface water (the Jaguaribe River and the Orós Reservoir) in Ceará, Brazil, through physicochemical and microbiological analyses, irrigation suitability indices, and multivariate statistical approaches. The results revealed pronounced seasonal variability. During the dry season, wells located in the crystalline basement (G8 and G9) exhibited electrical conductivity values exceeding 0.93 dS m⁻ 1 and total dissolved solids above 600 mg L⁻ 1 , characterizing brackish waters, together with elevated concentrations of iron (up to 5.74 mg L⁻ 1 in G4) and manganese (> 1.6 mg L⁻ 1 in G3). The highest pH value (9.6) was recorded at site R7, suggesting eutrophication-related processes. Although the Sodium Adsorption Ratio (SAR) remained low (< 3), the soluble sodium percentage exceeded 50% at R12. In comparison, the magnesium hazard index surpassed 50% at most sites during the dry season, indicating a potential risk of soil structural degradation. In the rainy season, dilution reduced salinity levels but increased microbiological contamination, with Escherichia coli detected in wells (G2 and G3) and river sites (R7 and R12), ranging from 9.3 to 15 CFU. In contrast, total coliform counts reached 46 MPN mL⁻ 1 . Principal Component Analysis distinguished the effects of lithology and evapoconcentration, with the first two components explaining 61.1% of the total variance. These findings provide valuable insights for water monitoring and sustainable management to enhance water and agricultural security in semi-arid regions.