New insights into soda water in shallow groundwater of the North China Plain

Soda water in shallow aquifers represents a unique hydrochemical type, often enriched in arsenic (As), Fluorine (F), Iodine (I), and other components, while also acting as a critical driver of soil salinization. However, existing studies have failed to effectively distinguish between salinization (characterized by soluble salt accumulation) and alkalization (characterized by soda-alkali enrichment). The "new insights" of this study do not rely on new data but derive from an in-depth excavation and interpretation of the 2006–2009 National Groundwater Pollution Survey dataset—the only authoritative background dataset covering the entire North China Plain. Focusing on shallow groundwater in the North China Plain, this study refines the identification criteria for soda water based on existing concepts, analyzes its spatial distribution characteristics, delineates typical zones, and conducts a preliminary investigation into the genetic differences across regions. Results show that when using the criterion—"HCO₃⁻ + CO₃²⁻ as dominant anions with [(HCO₃⁻ + CO₃²⁻) - (Ca²⁺ + Mg²⁺)] > 0 meq%"—combined with hydrochemical cluster analysis, soda water is primarily concentrated in two zones: The mountain-front discharge zone (Area A) and the runoff-ancient Yellow River channel zone (Area B). These two zones account for 88.48% of all soda water samples and exhibit distinct hydrochemical features. In Area A, groundwater has a simple anion composition dominated by HCO₃⁻, a median Total Dissolved Solids (TDS) content of 501.15 mg/L, and elevated concentrations of F⁻ and NO₃⁻. In contrast, Area B is characterized by diverse anions (HCO₃⁻, SO₄²⁻, and Cl⁻), a higher median TDS (863.56 mg/L), and enrichment of reductive components including As, F⁻, I⁻, Fe, and Mn. Genetic analysis reveals that soda water in Area A forms through the combined effects of mineral weathering, dissolution, and calcite-dolomite precipitation. In contrast, groundwater in Area B evolves under calcite-dolomite precipitation controlled by evaporative concentration, with further modifications by microbial geochemical processes and agricultural activities. This study clarifies the spatial distribution patterns and genetic mechanisms of soda water in the North China Plain, laying a foundation for further research on its formation processes.