Seasonal assessment of surface and groundwater quality, heavy metals, and water quality indices (case study: Dehloran watershed, Ilam, Iran)

Abstract This study evaluated physicochemical parameters and heavy metal contamination in the Dehloran watershed during the dry (September 2020) and wet (March 2021) seasons. Surface water samples (n = 62) were selected based on natural conditions and river accessibility, and groundwater samples (n = 10) were collected from low-lying areas. Physicochemical parameters covering organic load, ionic composition, nutrients, and salinity (e.g., BOD, COD, DO, EC, pH, TDS, major cations–anions, and turbidity) and heavy metals (As, Cd, Cr, Cu, Fe, Zn) were analyzed using standard methods and ICP‑MS. Paired t-test revealed significant (p < 0.05) seasonal contrasts: EC, TDS, Ca, Cl, SO4, Cu, Cr, and As were elevated in the dry season, whereas DO, TH, HCO3, and Mg increased during the wet season. Surface water had higher quality (IRWQISC = 36.4) than groundwater (IRWQIGC = 15.5), likely reflecting agricultural and anthropogenic impacts, with better quality observed upstream than downstream. Toxic parameter values were also slightly higher in surface water (IRWQIST = 55.3) than in groundwater (IRWQIGT = 54.3). PCA extracted four principal components in both seasons, dominated by salinity hardness and heavy metal factors. Piper plots revealed Ca–SO4 facies associated with gypsum rich Gachsaran formation and regional sulfur and bituminous springs. Wilcox classification showed over 60% of samples as highly saline, limiting irrigation suitability. Results from the heavy metal evaluation index (HEI) and contamination index (Cd) consistently indicated overall low levels of heavy metal contamination. These findings provide quantitative evidence supporting targeted mitigation and sustainable water resource management in semi-arid basins.