Spatial and Seasonal Hydrochemical Variability and Water-Type Classification of Al-Dalmaj Marsh and Temporary Ponds in Iraq

Inland saline waters are sensitive to seasonal hydrological forcing and evaporation, yet their spatial and seasonal hydrochemical variability remains insufficiently characterized. This study investigated chemical composition and hydrochemical classification of semi-static and astatic inland saline waters in Iraq. Water samples were collected from six stations in Al-Dalmaj Marsh (ADM) and three temporary ponds in Babylon Province. pH ranged from 7.0 to 9.7, and electrical conductivity ranged from 5.7 to 471 mS cm−1. Salinity in ADM ranged from 4 to 12 practical salinity units (PSU), whereas the highest reported sodium-chloride-equivalent salt concentration in the temporary ponds was 328,100 mg L−1. Total dissolved solids (TDS) in the ponds reached 315,100 mg L−1. Sodium dominated pond cation composition, whereas magnesium was relatively more important in ADM. Equivalent-percentage classification identified ADM and Pond 3 as saline, while Ponds 1 and 2 showed soda-saline characteristics. Mineral saturation-index (SI) calculations indicated supersaturation with respect to calcite, dolomite, and gypsum, whereas halite remained undersaturated across all sites, suggesting mineral-specific thermodynamic controls on water chemistry. Seasonal analyses showed significant changes in carbonate-related variables, total organic carbon (TOC), and phosphate; warmer periods were generally associated with higher carbonate-related values in ADM, whereas pond carbonate concentrations differed mainly between autumn and winter. Durov diagrams, Gibbs plots, Pearson correlations, and generalized additive model (GAM) results indicated that hydrological setting, evaporative concentration, Na-rich pond chemistry, chloride salinity, and carbonate-bicarbonate dynamics contributed to differentiation between marshes and ponds. TOC and nitrate were associated with carbonate variability, suggesting links with organic-matter processing and nutrient conditions rather than direct causation.