Pollution signatures of potentially toxic elements in rivers discharging to the Marmara Sea (Türkiye): ecotoxicological insights and Monte Carlo-based health risk assessment.

Rivers discharging into the Marmara Sea integrate contaminant inputs from industrial, urban, agricultural, mining, and geogenic sources. Dissolved potentially toxic elements (PTEs) and phosphorus were measured in spring 2024 at 60 freshwater river channels immediately upstream of their discharge into the Marmara Sea. Concentrations showed substantial spatial heterogeneity. The WHO drinking-water guideline values for boron (2400 µg/L) and arsenic (10.00 µg/L) were exceeded at 2/60 and 10/60 stations, respectively. Dilovası Creek in the İzmit Gulf represented the principal industrial-metal hotspot, with the basin-wide maxima for Ni (119.2 µg/L), Al (368.4 µg/L), Cr (19.2 µg/L), Zn (142.3 µg/L), and Co (1.99 µg/L). Çapraz Stream in the Susurluk Basin had the highest B (3522.8 µg/L) and As (36.8 µg/L) concentrations and the poorest index-based water-quality status (WQI = 93.826; HPI = 232.391), whereas Şerefli Creek in the Tekirdağ Region showed the maximum dissolved-P concentration (6656.3 µg/L). Multivariate analysis and positive matrix factorization distinguished industrial-metal, B-As-enriched, and nutrient-related chemical profiles. Arsenic was the principal toxicological concern: Monte Carlo 95th-percentile HI values were 1.28 for adults and 1.60 for children, while corresponding CR values were 5.81E-04 and 7.29E-04. These screening results identify B-As-enriched southwestern sub-basins and industrialized eastern sites as priorities for targeted monitoring and source-specific mitigation.