Searching strategy and the study protocol

The study has been conducted between 19 March, 2022 and 23 August, 2022. Google Scholar, PubMed, Web of Science, Embase and SCOPUS were mainly used for web-searching of peer-reviewed articles focusing on the distribution and concentration of metals and metalloids in water, sediment and fish in surface waters of Ethiopia. Accordingly, 902 papers from 2005 to 2022 were searched and collected (Figure 1). Important key words mainly “metals,” “trace metals,” “heavy metals,” and “metals in water,” “metals in sediment,” “metals in fish tissue,” and “metals in the aquatic environment” were used in the search engine (databases). The study period of peer-reviewed articles was left close-ended to search for updated papers.

Figure 1.

A personal creation shows the flowchart representing the methodology of the review.

Inclusion and exclusion criteria

For the inclusion of peer-reviewed papers in the study, basically, the following considerations were taken such as all factors related to metals and metalloids in surface water of Ethiopia. The focus of the study was on published and peer-reviewed papers. However, books, reviewed papers, conference articles, and master’s and PhD thesis were excluded from the study. Throughout the review, all searches may have a limitation according to the following considerations: (1) Published literatures may be omitted due to a lack of a linkage with the important keywords, (2) all used literatures were only in the English versions, (3) some may not available due to closed access, (4) others were not cataloged in the electronic databases, and (5) all included studies are cross-sectional studies.

Because of the variation of metals and metalloids concentration and their multitude in different surface water of the region, the collected data was analyzed from numerous points of view including the spatial distribution of metals and metalloids in the aquatic environment. All the units of concentration are expressed as µg/L and mg/kg for metal and metalloid levels in water, sediment and fish tissue respectively. Concentrations that were below the limit of quantification (detection) (<LOQ/LOD), were substituted with a value of LOQ/2.^23,24

Bioaccumulation -and biota-sediment accumulation factor

The most common types of edible fish species found in many studies were Nile tilapia (Oreochromis niloticus), African sharp tooth catfish (Clarias gariepinus), and African big barb (Barbus intermedius). Following, the bioaccumulation-and Biota-Sediment Accumulation factor (BAF and BSAF) of metals and metalloids were calculated as follows using equations.¹⁶

Where: BAF = bioaccumulation factor (L/kg), BSAF = biota-sediment accumulation factor (mg/kg/dw), C_b = concentration of metal in fish (µg/g dw), C_s = concentration of metal in sediment (µg/g dw), and C_w = concentration of metal in water (mg/L).

Ecological risk assessment of metals and metalloids in water and sediment

To determine whether the measured dissolved metals and metalloids in the surface waters pose adverse effects on the benthic aquatic fauna, the average concentrations of metals and metalloids were compared to the water quality standards of the Ethiopian Environmental Protection Authority (EEPA)²⁵ and the European Union water quality standard guidelines for inland surface freshwater bodies.²⁶ Likewise, to determine whether the detected metals and metalloids in the lake sediment have potential adverse effects on the benthic fauna, the investigated concentrations were compared to the United States Consensus-Based Sediment Quality Guidelines (SQGs) for freshwater. These values represent Threshold/Tolerable Effect Concentration (TEC) and Probable Effect Concentration (PEC) values.²⁷

Human health risk assessment

To estimate the potential non-carcinogenic human health risk associated with metals and metalloids in the surface waters of Ethiopia, the detected metals and metalloids concentrations were compared with water quality standards for drinking water by the World Health Organization (WHO)²⁸. Likewise, to estimate human health risks associated with metal or metalloid contaminated fish consumption, estimated daily intake (EDI in mg/kg/day), average daily intake (ADI in mg/kg/day), and hazard quotient (HQ) values for metals and metalloids were calculated using the corresponding reference dose (RfD in mg/kg/day).²⁹ A 60 kg body weight person (ie, an average body weight of Ethiopian adults) and a national fish consumption of 0.19 kg/week (~0.027 kg/day)³⁰ were used for the calculations. The maximum edible amount (MEA in kg/day) of fish consumption per person per day, without causing a potential human health risk, was calculated using the reference dose (the maximum tolerable daily intake of a metal or metalloid that does not pose any adverse health effect) of metals and metalloids established by United States Environmental Protection Authority,³¹ Agency for Toxic Substances and Disease Registry,³² World Health Organization,³³ and United States Food and Drug Administration.³⁴ The used concentrations of metals and metalloids in fish muscle were the average of the measured concentrations in each edible fish species in Ethiopia. EDI, HQ, and MEA were calculated using the following equations^29,35:

With C is the concentration of metals or metalloids in µg/g ww, DFC is fish consumption in kg/day, BW is adult Ethiopian body weight (60 kg), ED is the exposure duration (30 years for an adult),³⁶ EF is the exposure frequency (365 days/year),³⁷ AT is the average exposure time for non-carcinogens (365 days/year × ED),³⁷ and RfD is the reference dose for Cd (0.00001 mg/kg/day), Cu (0.01 mg/kg/day), Pb (0.004 mg/kg/day), Cr (0.0009 mg/kg/day), Co (0.01 mg/kg/day), As (0.00003 mg/kg/day),³² Ni (0.02 mg/kg/day), Zn (0.3 mg/kg/day),³¹ Se (0.17 mg/kg/day), and Hg (0.00023 mg/kg/day).³³ EDI, HQ, and MEA values were also calculated based on the maximum concentrations detected in each edible fish species in Ethiopia, to determine the worst-case scenario.

Data accessibility

After searching, 902 peer-reviewed articles focusing on the distribution and accumulation pattern of metals and metalloids; that is Arsenic (As), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Copper (Cu), Mercury (Hg), Lead (Pb), Nickel (Ni), Selenium (Se), and Zinc (Zn) in water, sediment, and fish muscle from 2005 to 2022, 15 articles were used for the analysis of the result. After screening all the reports in the Ethiopian aquatic environment, a majority (73.68%) of reports were from the Rift Valley Lakes (RVLs). For instance, from the RVLs, more (47.37%) was done in Lake Hawassa followed by Lake Ziway (26.32%).

Detection method of metals and metalloids

Observing the collected data, different methods (instrumental analysis) were used for the detection of metals and metalloids in water, sediment and fish. The most frequently used detection method (Figure 2) for metals and metalloids analysis was Atomic Absorption Spectrophotometer (AAS = 66.7%) followed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (26.7%). However, this was unexpected, even though both are the most common detection methods of metals and metalloids, AAS detects metals and metalloids serially while ICP-MS did concurrently. Moreover, ICP-MS is faster and has cheaper processing costs and better detection limits (LOD) compared to ASS.³⁸

Figure 2.

Instrumental analysis of metals and metalloids in water, sediment, and fish tissue in Ethiopian surface waters from 2005 to 2022.

Metals and metalloids in water, sediment, and fish muscle

The concentration of 10 metals and metalloids (ie, As, Cd, Cr, Co, Cu, Hg, Pb, Ni, Se, and Zn) were collected from previously done literatures. An overview of the concentration of metals and metalloids in water (µg/L), sediment (mg/kg dw), and fish muscle (mg/kg dw) of Ethiopian surface waters are presented in Table A1 to A3 (see in Appendix). Zn has the highest concentration (575.90 µg/L) of metals and metalloids in water (Figure A1 in Appendix) which has shown a similar pattern as compared to previous studies.^{17,18,39–41} The average concentration of Cu, Cd, Pb, Ni, Cr, Co, Hg, As, and Se in water were 176.43, 66.60, 59.07, 191.84, 35.32, 109.04, 0.32, 2.10, and 0.82 µg/L. In terms of specific location, more data were collected from Rift Valley Lakes, Lake Hawassa, with the highest concentration of Zn 1059.76 µg/L and was similar with other studies.^17,18,39,41 In sediment, Figure A2 (see in Appendix) the concentration of Cr (4529.21 mg/kg dw) and Ni (4422.56 mg/kg dw) was higher when compared with other metals and metalloids which is similar compared to Kassaye et al⁴¹ in Lake Ziway and Hawassa. The concentration of Cu, Cd, Pb, Zn, Co, Hg, As, and Se were 32.93, 24.49, 16.10, 259.30, 23.17, 8.15, 2.08, and 0.18 mg/kg dw. The highest concentration of Cr in sediment may be ingested by fish and later accumulated. Overall, the concentration of metals and metalloids in sediment was higher than in water.

Analysis of the results also revealed metals and metalloids levels in fish muscle (Table A3). For instance, in all fish species, the concentration of Zn was highest which was 15.40, 18.48, and 31.00 mg/kg dw in C. gariepinus, O. niloticus, and B. intermedius respectively. This shows a similar pattern with previous studies,^17,19,22 which may be due to the essentiality of Zn for physiological role and uptake from the aquatic environment by fish preferentially. Overall, similar to previous studies,^17,19,22 the finding of this study also revealed that the concentration of most metals and metalloids were relatively higher in carnivorous fish than in herbivorous fish.

Bioaccumulation-and biota sediment accumulation factors

The average concentration of detected metals and metalloids in water (µg/L), sediment (mg/kg dw), fish muscle (mg/kg dw), and their BAF and BSAF are summarized in Table 1. For instance, the BAF of Hg in O. niloticus and B. intermedius and As in O. niloticus was greater than 1. The BSAF of Se in all fish species and As for O. niloticus and Co for B. intermedius was also greater than 1. Moreover, BSAF of As and Se for O. niloticus was greater than 2. In contrast, previous studies have shown a higher BAF for Cd⁴⁰ and Cr and Hg¹⁷ and BSAF of all metals or metalloids was also less than 1.¹⁷ Overall, BAF and BSAF value greater than 1 indicates that metal or metalloid may be accumulated in fish muscle, while a value less than 1 indicates that a metal or metalloid is not accumulated in fish tissue directly from water and is associated with sediment respectively.⁴² In addition, an organism tissue with BSAF > 2, 1 < BSAF < 2, and BSAF < 1 can be considered as a macro-concentrator, micro-concentrator, and de-concentrator respectively.⁴³ Therefore, the results of this study illustrated that O. niloticus is a bio-concentrator of As and Se. While, authors like Arnot and Gobas⁴⁴ suggest that BAF values <1000, 1000 < BAF < 5000, and >5000 are also considered as having no probability of accumulation, bio-accumulative, and extremely accumulative respectively.

Table 1.

The average concentration of detected metals and metalloids in water (µg/L), sediment (mg/kg dw), fish tissue (O. niloticus, C. gariepinus, and B. intermedius) (mg/kg dw)), and respective BAF and BSAF values.

Ecological risk assessment

Based on data analysis, in surface waters of Ethiopia, the dissolved concentration of Cu, Cd, Pb, and Ni were higher compared to the water quality standards of the Ethiopian Environmental Protection Authority²⁵ and the European Union water quality standard guidelines for inland surface freshwater bodies²⁶ (Table A1). Likewise, in sediment of surface waters (Table A2), Cu, Pb, Ni, Zn, and Cr were above TEC values and Cd, Ni, and Cr were above PEC levels when compared to the United States Consensus-Based Sediment Quality Guidelines (SQGs) for freshwater which represent Threshold/Tolerable Effect Concentration (TEC) and Probable Effect Concentration (PEC) values,²⁷ and this was similar to a previous study¹⁷ in Lake Hawassa. This indicates that these detected metals in water and sediment may pose risk to aquatic organisms or benthic fauna.

Human health risk assessment

The concentration of detected metals and metalloids in water is not associated with any disease. Fish muscle is consumed by people in most countries.^17,45 Analysis of human health risk (hazard quotient of metals and metalloids) associated with metal or metalloid-contaminated fish consumption (Table A4), HQ for all metals and metalloids for all edible fish species was <1. This was as expected because previous studies found, except Hg, metals, and metalloids were not hazardous to human consumers.^17,21,22 A HQ > 1 is considered to be hazardous and a HQ < 1 is non-hazardous.^29,46,47 Similarly, the MEA of most metals and metalloids in all edible fish species was higher than the average daily national fish consumption of Ethiopians. This indicates that a consumer (a 60 kg body weight person) is not at risk or will not be poisoned by the measured metals or metalloids. Accordingly to Dsikowitzky et al,²² a regional fish consumption of 1.05 kg/week was recorded. Therefore, local consumers who live close to each freshwater ecosystem may be more exposed to health hazards.