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 (IRWQI(SC) = 36.4) than groundwater (IRWQI(GC) = 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 (IRWQI(ST) = 55.3) than in groundwater (IRWQI(GT) = 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 (C-d) 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.