Emerging Contaminants xxx (xxxx) xxx
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Distribution of total mercury in surface sediments and African catfish (Clarias gariepinus) from Akaki River catchment and Aba Samuel Reservoir, downstream to the mega-city Addis Ababa, Ethiopia Alemnew Berhanu Kassegne a, b, *, Jonathan O. Okonkwo c, Tarekegn Berhanu Esho d, Kebede Nigussie Mekonnen e, f, Tshia Malehase c, Seyoum Leta Asfaw a a
Center for Environmental Science, Addis Ababa University, P. O. Box 1176, Addis Ababa, Ethiopia Department of Chemistry, Debre Berhan University, P. O. Box 445, Debre Berhan, Ethiopia Department of Environmental, Water & Earth Sciences, Tshwane University of Technology, 175 Nelson Mandela Drive, Private Bag X680, Arcadia, Pretoria, South Africa d Central Research Laboratory, Addis Ababa Science and Technology University, P. O. Box 16417, Addis, Ababa, Ethiopia e Department of Chemistry, Tshwane University of Technology, P. O. Box 56208, Arcadia, 0007, South Africa f Department of Chemistry, College of Natural and Computational Sciences, Mekelle University, P. O. Box 231, Mekelle, Ethiopia b c
a r t i c l e i n f o Article history: Received 10 July 2018 Received in revised form 15 October 2018 Accepted 23 October 2018 Available online xxx
1. Introduction Mercury (Hg) and its compounds are ubiquitous in the environment and global pollutants of concern for human and ecosystem health [1]. It occupies special concern among heavy metals due to its toxicity, persistence and bio-accumulative nature and association with historic and current events of contamination in aquatic environments [2]. The Hg pollution can occur in three types of emissions: natural sources, global Hg cycling and anthropogenic sources [1]. It exists in the environment in different chemical forms: elemental (Hg0), inorganic (Hgþ and Hg2þ), and organic mercury. However, most often Hg present in the inorganic Hg2þ throughout the Earth's crust and fast inter-conversion occurs to more toxic organic mercury form in the aquatic environment [3]. The concern about the health effects of Hg pollution, increased since the mass poisoning incident that occurred in Minamata, Japan in the 1950s, after the Hg containing waste was released into the nearby Bay by the Chisso Corporation Company. This led to Hg in
* Corresponding author. Center for Environmental Science, Addis Ababa University, P. O. Box 1176, Addis Ababa, Ethiopia E-mail address:
[email protected] (A.B. Kassegne). Peer review under responsibility of KeAi Communications Co., Ltd.
bottom sediments being methylated into methylmercury (MeHg) which is the most toxic form of Hg, resulting in subsequent bioaccumulation in the food chain. Fish consumption was the main pathway of methyl mercury (MeHg) into human body organs and subsequent accumulation. This resulted in devastating event where thousands of the population who relied on fish diet suffered from the symptoms of Hg poisoning [4]. As a result of this and other related incidences, the United Nations through the United Nations Environmental Program (UNDP) endorsed the Minamata convention on mercury, which aims at identifying, monitoring and eliminating sources of Hg in the environment. Anthropogenic activities such as fossil fuel burning, amalgam gold mining processes, wastewater and sludge, incineration and the use and disposal of consumables such as batteries, fluorescent lamps, thermometers, thermostats, dental amalgam, paints, pesticides and chlor-alkali industry are potential sources of Hg into the environment, leading to a significant increase in the levels of Hg [5e7]. Furthermore, biomass burnings and soil erosion are estimated to be the major sources of Hg for the lake and reservoir ecosystem in developing countries [8]. In Africa, emissions of Hg concentrated in West Africa, East African rift valley region, and South Africa, with different emission sources [1,9]. East Africa is home to fast growing countries in the continent and there is a high
https://doi.org/10.1016/j.emcon.2018.10.003 2405-6650/Copyright © 2018, KeAi Communications Co., Ltd. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: A.B. Kassegne et al., Distribution of total mercury in surface sediments and African catfish (Clarias gariepinus) from Akaki River catchment and Aba Samuel Reservoir, downstream to the mega-city Addis Ababa, Ethiopia, Emerging Contaminants, https:// doi.org/10.1016/j.emcon.2018.10.003
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A.B. Kassegne et al. / Emerging Contaminants xxx (xxxx) xxx
rate of Hg emission from uncontrolled discharge of industrial and municipal wastes into the nearby watercourses and land, often causes water and soil pollution. According to IPEN-BDI (2013) report, electronic waste dumping sites in Ethiopia are the main hot spots of mercury pollution in the country. Most discarded consumer electronics shipped to Ethiopia end up in landfills causing serious public health and environmental hazards [10]. Sediments and fish are often exposed to toxic substances such as Hg. Thus, quantification of Hg in sediment and fish is extremely important [3,11]. Sediments provide important information about the long-term trends in geochemical and ecological conditions of aquatic systems and their catchment areas [12]. The Hg originating from natural sources, water course runoff and atmospheric deposition adheres to suspended particles which later settles at the bottom of water bodies and may also be deposited directly into the sediments of the water course. Sediments provide favorable conditions for biogeochemical methylation and cycling and bioaccumulation of MeHg. Particularly because it is the primary feeding habitat for micro and macro organisms, which are later prayed by larger upper sphere organism such as fish. Thus, Hg in sediments has increased solubility, mobility, bioavailability, bioaccumulation, and biomagnification [2,12e14]. Hence, sediments characterize the degree of environmental contamination by metals like Hg and therefore are suitable target for pollution studies [13]. Fish attracted a lot of attention as bioindicator for monitoring of aquatic pollution, due to their relatively large body size, long life cycle, the position in the aquatic food chain and their use for human consumption [15e17]. The African catfish, Clarias gariepinus is distributed in freshwater habitats across Turkey, the Middle East, and the African continent [18]. It is an important part of many commercial and subsistence fisheries and a major source of protein in many people across Africa [19]. It is also regarded as one of the most suitable species for aquaculture due to its fast growth [20], its ability to change its diet depending on food availability [21,22], and its hardiness to withstand extreme environmental conditions [16,23,24]. In the current study site, Aba Samuel Reservoir, a significant percentage of Clarias gariepinus is captured and consumed by the local dwellers without information on the levels of Hg. Previous studies on Hg accumulation in sediment and fish are hardly sufficient in Ethiopia in general and in the catchment area in particular. Most studies in Akaki River catchment focused on the determination of metals other than Hg in vegetables, water, soil and sediments [25,26]. Ittana [27], Fitamo et al. [28] and Mekonnen et al. [12] determined Hg in vegetables, soils and sediments in Akaki River catchment respectively, and not on Aba Samuel reservoir. Furthermore, information on Hg concentration in Clarias gariepinus from the study area is unavailable. Therefore, this study was aimed at determining the concentrations and investigating seasonal variations of total Hg in sediment and Clarias gariepinus at Aba Samuel Reservoir and Akaki River catchment. This work will also provide an insight on the potential risk of Hg to population that consumes the fish. With this regard, twenty two surface sediment samples and thirty six fish samples were collected and analyzed for their THg content. 2. Materials and methods 2.1. Study area and sampling stations Addis Ababa, the capital city of Ethiopia, is one of the fast expanding cities in the country. It is geographically located between 8 490 55.92900 and 9 50 53.85300 N and 38 380 16.55500 and 38 54019.54700 E between 2200 and 2500 m above sea level. It is the country's commercial, manufacturing and cultural center. Addis Ababa has uncontrolled urbanization, industrialization, waste
disposal from different sources and poor sanitation. Greater Akaki River (GAR) (locally known as Tiliku Akaki River) and Little Akaki River (LAR) (locally known as Tinishu Akaki River), with their tributaries, drain the city from north to south. These river systems are the dumping sites of solid and liquid wastes and thus, the environment of the city is threatened by severe pollution of toxic chemicals [29]. The LAR, in which most of the industries are established, is more polluted than GAR [25]. The confluence point of the two river systems is Aba Samuel Reservoir (37 km south-west of Addis Ababa) which serves as pollutant sink from the upstream Addis Ababa city. It was the first hydropower station in Ethiopia built in 1939, while it was abandoned because of long years out of repair in the 1970s [30]. It was rebuilt and come to life again in 2016. The local people around the resevior use the river system for irrigation, drinking water for cattle, washing cloths, fishing and other domestic needs without strict water quality monitoring [25,26]. The existence of the historical Aba Samuel monastery near the reservoir and the attractive topography of the reservoir, make the area one of the potential ecotourism site. 2.2. Sample collection and pretreatment Twenty two (22) sediment samples were collected in August 2016 (rainy season) and January 2017 (dry seasons) using Ekman bottom Grab sampler according to the standard procedure described by the USEPA (1994) [31]. Sediment samples were collected from eleven sampling stations: GAR at Entoto Kidanemihiret Monastery (S1, control site 1), GAR at Tirunesh Beijing Hospital (S2), GAR below Akaki Town (S3), LAR above Geferesa Reservoir (S4, control site 2), LAR at Lafto Bridge (S5), LAR at Jugan Kebele, boundary of Addis Ababa and Oromia Special Zone (S6), Aba Samuel Reservoir below the confluence point of GAR and LAR (S7), Aba Samuel Reservoir at the midpoint (S8), Aba Samuel Reservoir above the Dam (S9), downstream about 50 m from the Reservoir (S10) and downstream about 1000 m from the Reservoir (S11) (Fig. 1). Four samples were taken from each sampling site, pooled, homogenized, placed in clean dark-polyethylene bags, labeled, stored on ice-cooled container and transported to the laboratory. Subsequently, the sediment samples were air dried at ambient temperature in a shade, grounded, homogenized, sub-sampled and passed through a stainless steel sieve of different mesh size (50045 mm) and stored until further treatment. Fish samples (n ¼ 36) were collected in August 2016 (rainy season) and January 2017 (dry season) from the Reservoir from Site S8 and S9 (Fig. 1) from the same locality and duration of time where the sediment samples were collected. The species of fish was identified at the sampling site by the expert from Limnology Department, Addis Ababa University, Ethiopia. The fish samples were put in sterile polyethylene bags and taken in icebox to the laboratory where they were washed with running tap water to remove dirt. Total length (in cm) and weight (in gm) were measured immediately after sampling. A sample from the back, dorso-lateral muscle was removed from each individual fish using sterile scissors. Tissue samples were placed in aluminum foil, labeled, and then frozen at 20 C in deep-freezer until further pretreatment and analysis. Three fish samples were pooled to form a composite sample of homogenized muscle tissue based on length. A portion of the pooled samples was oven dried to constant weight at 105 C for water content determination (average of 82.5% water content was obtained). The wet sub-samples were freeze-dried for 72 h, powdered and homogenized using an agate mortar and pestle before acid digestion procedure. Finally, the processed sediment and fish samples were transported to the Laboratory of Environmental, Water and Earth Sciences, Tshwane University of Technology, Pretoria, South Africa for digestion and then for the determination of THg.
Please cite this article as: A.B. Kassegne et al., Distribution of total mercury in surface sediments and African catfish (Clarias gariepinus) from Akaki River catchment and Aba Samuel Reservoir, downstream to the mega-city Addis Ababa, Ethiopia, Emerging Contaminants, https:// doi.org/10.1016/j.emcon.2018.10.003
A.B. Kassegne et al. / Emerging Contaminants xxx (xxxx) xxx
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Fig. 1. Map of Ethiopia (top right), Oromia regional state (middle right) and Aba Samuel Reservoir, Little Akaki and Greater Akaki Rivers (left) showing the sampling sites.
2.3. Standards and reagents All the reagents used were Hg free and were supplied by SigmaAldrich (Germany). Hydrochloric acid (37%), nitric acid (70%), potassium permanganate (>99%, low in Hg), sodium chloride (>99%), tin(II) chloride di-hydrate (98%) and hydroxylamine sulphate (99%) were used throughout the experiment. Antifoaming agent, 1octanol (97%) was supplied by Merck (South Africa). Mercury (II) stock standard solution, 1000 mg/L was supplied by Fluka Analytical (Switzerland) and mercury stream sediment certified reference material (CRM) was supplied by Industrial Analytical Pty (China). Ultra-pure water prepared in our laboratory using SG Series Compact purchased at Evoqua Water Technologies (United Kingdom) was used throughout the experiment. 2.4. Sample digestion and instrumental analysis For the digestion of the sediment and fish samples, previously developed acid digestion procedures for solid and semi-solid waste and tissue samples, respectively (USEPA Method 7471B, Revision 2 and USEPA/SW-846 Methods 7000A/7470A/7471A) [32,33], was used with minor modifications and THg was determined using cold vapor atomic absorption spectrometer (CV-AAS) technique. Briefly, about 1g of sediment (
