Ecological risk and source apportionment of PAHs and OCPs in surface sediments of the Upper Gulf of Thailand

This study investigates the occurrence, distribution, and ecological risks of polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs) in surface sediments collected from 17 stations in the Upper Gulf of Thailand (uGoT). Total PAH concentrations ranged from 6.6 to 744.6 ng g -1 dw, with the highest concentrations recorded at the Chao Phraya river mouth, primarily driven by pyrogenic sources and localized petrogenic inputs from maritime activities. In contrast, total OCP concentrations (ranging from 159.0–1,657.7 ng g -1 dw) exhibited an offshore maximum, suggesting that the central gulf serves as a critical sink for agricultural residues transported via complex hydrodynamic processes. Source diagnostic ratios revealed 4,4'-DDE as the predominant OCP congener across all sites; elevated (4,4'-DDE+4,4'-DDD) / ∑DDT ratios indicate that these residues represent legacy contaminants from historical pesticide use rather than recent inputs. Based on sediment quality guidelines, PAHs generally fell below acute toxicity thresholds, whereas OCPs exhibited severe exceedances. Specifically, 4,4'-DDE, 4,4'-DDD, 4,4'-DDT, and ΣDDT surpassed the Probable Effect Level at over 80% of sites. Incremental Lifetime Cancer Risk (ILCR) assessments showed that PAH-related risks remained below safe thresholds, driven primarily by dermal contact. In contrast, OCP-related risks for adults exceeded acceptable carcinogenic limits at major hotspots (peaking at 1.2 × 10 -6 ), with oral ingestion as the primary exposure pathway. These findings underscore the long-term persistence of legacy pesticides in tropical, organic-rich sediments and highlight the need for spatially targeted interventions to safeguard regional public health and marine biodiversity.