Ecotoxicological risk assessment of organochlorine pesticides in surface sediments from South Coast Estuaries, Kenya

Organochlorine pesticides (OCPs) remain contaminants of global concern in aquatic ecosystems because of their persistence, hydrophobicity, toxicity, and potential for long-term accumulation in sediments. In many tropical estuaries, weak monitoring systems and continued catchment disturbances allow these legacy contaminants to persist undetected in benthic habitats. This study assessed the occurrence and ecotoxicological risk of OCPs in surface sediments from selected estuaries along the South Coast of Kenya. Sediment samples were collected monthly for 12 months from 12 stations distributed across five estuarine systems and analyzed using standard extraction, clean-up, and gas chromatographic procedures. Sixteen target OCPs were detected, with mean concentrations generally ranging from 1.45 to 1.91 µg/kg. Hexachlorobenzene (HCB; 1.91 µg/kg), p,p′-DDE (1.81 µg/kg), and heptachlor (1.79 µg/kg) recorded the highest mean concentrations. Sediment contamination and ecological risk were evaluated using contamination factor (Cf), geo-accumulation index (Igeo), pollution load index (PLI), potential ecological risk index (RI), Nemerow pollution index (PN), and mean effect range-median quotient (ERM-Q). All compounds showed substantial enrichment relative to baseline values, while the composite indices indicated heavy sediment pollution and elevated ecological risk. The cumulative RI revealed very high ecological risk, and the PN value confirmed severe pollution status across the study area. Although ERM-Q suggested only a moderate probability of adverse biological effects, the combined index-based assessment demonstrated that the estuarine sediments act as important reservoirs of legacy OCP contamination. These findings highlight the need for routine sediment quality monitoring, source tracking, and management interventions to reduce persistent pesticide burdens in tropical estuarine ecosystems.