Geochemical evaluation of arsenic in pleistocene-quaternary ancient fluvio-lacustrine deposits and health risk assessment of groundwater contamination in the Karasu and Morali River sub basins, Van (Türkiye)

Groundwater is the primary source of drinking and irrigation water in the Van Lake Closed Basin, where naturally occurring arsenic threatens water quality and human health. This study evaluates groundwater hydrogeochemistry, arsenic mobilization, and associated health risks in the Pleistocene-Quaternary aquifers of the Karasu and Moralı River sub-basins. Groundwater samples collected during dry and wet seasons were analyzed using hydrochemical characterization, end-member mixing analysis, PCA, Eh-pH and pe-pH diagrams, arsenic speciation, and USEPA health risk assessment. Groundwater chemistry was dominated by Ca-HCO3 water type in the Karasu River Sub-Basin (KRSB) and mixed Ca-HCO3-SO4 water type in the Moralı River Sub-Basin (MRSB). Hydrogeochemical evolution was controlled mainly by silicate weathering, carbonate dissolution, cation exchange, localized evaporite dissolution, and water-rock interaction. PCA explained 92.71-94.54% of the total variance in KRSB and 100% in MRSB, confirming the effects of mineral interactions and redox processes. End-member mixing analysis identified silicate weathering as the dominant process. Arsenic concentrations exceeded the WHO guideline value (0.01 mg/L), particularly in MRSB. Eh-pH and pe-pH diagrams showed predominantly reducing conditions, with 65% of samples within the As(III) stability field. Health risk assessment indicated HQ and CR values above acceptable USEPA limits, with children as the most vulnerable group and risks generally increasing during the wet season. The results demonstrate that arsenic contamination is mainly governed by natural geogenic processes and provide a framework for sustainable groundwater management in the Van Lake Closed Basin.