Residues of organochlorine pesticides in aquatic ecosystems of Southwestern Ethiopia: implications for ecological risks

Organochlorine pesticides (OCPs) remain a critical environmental concern due to their persistence and bioaccumulative potential. This study investigated OCP residues across three matrices-water, sediment, and macroinvertebrates-within the Awetu, Kito, and Boye aquatic systems in southwestern Ethiopia. Analysis using Gas Chromatography with Electron Capture Detector (GC-ECD) identified nine OCPs, with p, p'-DDT and its metabolite p, p'-DDE as the dominant contaminants. Concentrations varied across the three matrices (water: ng/L; sediment and macroinvertebrates: ng/g), and values are not directly comparable due to differences in units, matrix properties, and analytical methodologies. The screening-level Ecological Risk Assessment (ERA) indicated potential high ecological risk to aquatic biota. Risk Quotients (RQ) for macroinvertebrates showed extreme risk from heptachlor epoxide (RQ = 738.8) and high risk from dieldrin (RQ = 1.53), demonstrating that these legacy pollutants bioaccumulate to levels that may cause secondary poisoning to higher trophic levels. Statistical analysis confirmed significant variation in pesticide distribution across the three matrices (p < 0.001), with a contamination hierarchy of sediment > macroinvertebrates > water. This pattern is primarily driven by the high lipophilicity and environmental persistence of OCPs, which favor adsorption to organic-rich sediments and subsequent bioaccumulation in aquatic biota. In contrast, the low concentrations in the water column reflect the poor aqueous solubility of these hydrophobic compounds. These findings highlight the persistence of legacy OCPs in Ethiopian aquatic ecosystems despite existing bans, with the Boye-Awetu-Kito system acting as a long-term reservoir. The extreme ecological risk observed underscores the urgent need for targeted remediation of the Boye wetland and stronger regulatory enforcement to mitigate trophic transfer and protect the regional aquatic ecosystem.