Date Published: October 4, 2018
Publisher: Public Library of Science
Author(s): Niguse Bekele Dirbaba, Sen Li, Hongjuan Wu, Xue Yan, Jun Wang, Chon-Lin Lee.
This study was initiated to document information on the levels of sediment contamination with organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). Moreover, it was intended to identify compounds which impose major ecological risks to aquatic organisms. Surficial sediments were collected from 46 locations within the streams and rivers of the Awash River Basin. In total 30 compounds were included in this study: 16 OCPs, 7 PCBs and 7 PBDEs. The total concentrations of OCPs, PCBs, and PBDEs ranged from 6.63 to 206.13 ng g-1- dry weight (dw), 0.85 to 26.56 ng g-1-dw and 3.71 to 18.95 ng g-1-dw respectively. Out of all the tested OCPs, heptachlor, heptachlor epoxide, p,p′-dichlorodiphenyltrichloroethane (p,p′-DDT) and β-hexachlorocyclohexane (β-HCH) were the most abundant in the study area. The ratio of (β-HCH/∑HCHs) indicated that HCHs were originally from earlier usage of HCH in the area whereas the ratio of (p,p’-DDT/p,p’-DDE) showed that the majority of DDT components were recently introduced into most of the sampling locations. Even though there were relatively low concentrations of PBDEs and PCBs across the sampling sites, substantial amounts of PCBs were observed in Addis Ababa City. According to the established ecological risk indices, p,p’-DDT and γ-HCH are the major concerns for potential adverse ecological impacts. This study provided the first comprehensive information on organohalogenated compounds’ (OCs’) occurrences, spatial distributions, and ecological risks in sediments of the Awash River Basin. Thus, the report will be very useful background information for further studies on sediment contamination with OCs’ in the region. It also adds important first-hand data to the field of fresh water ecology and provides useful empirical evidence for setting pollution control priorities for an ecologically important, yet largely understudied region.
Organohalogenated compounds (OCs) such as organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) were listed under persistent organic pollutants (POPs) in Stockholm Convention, 2001[1, 2]. These compounds were manufactured and used for different purposes (e.g. as pesticides, flame retardant additives, transformer dielectric fluids) before they were legally banned [3, 4]. Even though these products were produced for specific uses, they were also known for their adverse effects on the natural environment. It was clearly ratified that OCs are “poisonous to human beings, animals, plants and overall food webs” . Exposure to OCs can cause hormone-dependent cancers, impair reproductive fitness and decrease the serum concentration of thyroid hormones in human beings and other animals [3, 6, 7]. Even at low concentrations, OCs can be a threat to natural ecosystems and human health as they are lipophilic, have low biodegradability, can easily accumulate in tissues and gradually cause chronic toxicities [8–10].
This study provides the first comprehensive information on occurrence, spatial distribution and ecological risk of POPs in sediments of the Awash River Basin. Even though low concentrations of PBDEs and PCBs were detected in the sediments, the prevalence of high concentrations of OCPs indicates that sediment pollution with POPs is a major ecological issue in the Awash River Basin. Among OCPs, heptachlor was the most abundant contaminant. This points out that heptachlor was widely used as an insecticide. β-HCH and p,p’-DDT were the dominant components of HCHs and DDTs, respectively. Therefore, it is likely that, HCHs were sustained in the sediments from aged deposit while the technical DDT was recently introduced. The distributions of all tested OCs were weakly influenced by the TOC content of the river sediments. Even though the ecological risk assessment guidelines are chemical specific and do not establish causality where chemical mixtures occur, adverse ecological impacts are expected to occur mainly due to sediment contamination with p,p’–DDT and γ-HCH. Therefore, the government should set priority to control the use of DDTs and lindane in the Awash River Basin to prevent further contaminations.