Seasonal variations in groundwater arsenic occurrence, speciation, and human health risks under contrasting groundwater-level conditions in an arid irrigated aquifer.

Groundwater arsenic contamination in the northern Tianshan Mountains of Xinjiang, China, threatens regional drinking-water safety, yet its seasonal response to groundwater-level fluctuations remains poorly understood. We integrated UNMIX-based hydrochemical factor analysis, hydrogeochemical modeling, and Monte Carlo simulation to investigate arsenic occurrence, mobilization, speciation, and health risks. Results revealed high-arsenic groundwater (>10 μg L-1) in weakly alkaline and reducing confined aquifers at depths of 100-300 m. UNMIX resolved a hydrochemical factor with high loadings of As (87.1%), pH (64.4%), and Eh (54.1%), indicating an association between arsenic enrichment and alkaline, reducing conditions. Along the groundwater flow path, arsenic concentrations and speciation differed between the high water-level period (HWP) and low water-level period (LWP), together with changes in groundwater levels, mineral reactions, hydrogeochemical conditions, organic matter degradation, and pumping. Arsenic was mainly present as As(V) during the HWP, whereas trace As(III) was detected during the LWP, consistent with differences in redox conditions. Probabilistic health-risk assessment based on total arsenic concentration showed that non-carcinogenic risk exceeded the reference level in both age groups, with higher risk in children. Adult carcinogenic risk exceeded 1.0E-04 during both hydrological periods, whereas child risk exceeded this threshold only during the HWP. In children, a C × ED value of 100 μg L-1·yr corresponded to this threshold; in adults, the corresponding C value was 4.10 μg L-1. These findings suggest that seasonal groundwater-level fluctuations are associated with arsenic mobilization, speciation, and health risk in arid irrigated aquifers.