1 Introduction 

1.1 Background 

Urbanization is increasing, particularly in developing countries and Ethiopia is amongst of the developing countries that urbanization is increasing alarmingly. Addis Ababa, the capital of Ethiopia and one of the largest cities of Africa, is located at the edge of the East Africa Rift Valley. Since its establishment 130 years ago, the city has grown from a sparse and scattered settlement to 530 km2. Over the last 50 years, the population of Addis Ababa has grown from 100,000 to 3.5 million inhabitants in 2010 with a yearly growth rate of 3.8%. On the contrary, provision of sanitation and wastewater infrastructure has not kept pace with population and urbanization growth (CSA, 2007).

Currently, Addis Ababa city with a population of approximately 5 million,hosts a large number of industries whose wastewaters are discharged into the small river network, most often untreated. According to CSA, (2013), in 2010, there were 887 large and medium scale manufacturing industries operating in Addis Ababa. Except for some very old enterprises, most of these large and medium industries are located in industrial zones where Akaki River is crossing down. Accordingly, old as well as new factories, commercial, public and domestic utilities release untreated wastes into nearby rivers and streams, causing main challenge to surface water quality in Addis Ababa (Alemayehu, 2006).

Among the wide range of pollutants affecting water resources, heavy metal contamination is one of the major water quality issues, and elevated occurrences in waters and biota usually indicate the presence of anthropogenic sources. The contamination of surface water by heavy metals is a serious ecological problem as many heavy metals such as Hg, As, Pb, Sb, Ni, Sr and Cd are toxic even at low concentrations. This is attributed to their non-biodegradable nature; long half-lives and high bioaccumulation potential. Though some metals such as Cu, Fe, Mn and Zn are essential as micronutrients for living organisms, they can be detrimental to their physiology at higher concentrations (Kar et al., 2008). 

 In Addis Ababa, large volume of this untreated wastewater is used for irrigation for vegetable production which is a highly prevailing practice in the city.About, 60% of the city’s vegetable consumption, particularly leafy vegetables, is supplied by urban farmers who irrigate their crops using polluted river water (Weldegebriel et al., 2012). 

Since the ultimate repository of industrial discharge in Addis Ababa is the aquatic environment, there is an immense concern on the impact of pollutants to water, soil, vegetable and fish as it can pose serious health risks within the urban food system. These risks range from occupational hazards of exposure to toxic elements while farming, handling and distributing food crops, to the short and long-term effects of consuming foods contaminated by heavy metals.  

Within this context this experimental study tried to assess the distribution and transportation of toxic metals in the Akaki river system (irrigation water, soil, vegetables and fish species of Aba-Samuel dam and Lake Koka).

1.2 Statement of the Problem 

As rivers are invaluable sources of drinking water, recreational and commercial opportunities, keeping tabs on quality is of paramount importance for sustaining their many uses. Water quality oversight can only be actualized through reliable monitoring, which can provide a sufficient amount of data to pinpoint any changes that appear in water bodies over a period of time and implement predictive models (IWC, 2017).

Nevertheless, in Ethiopia so far there is no well-organized water quality monitoring system and centralized water quality database. Except the drinking water quality guidelines, there are no prepared standards for the purpose of controlling effluent discharge, irrigation and industrial water qualities. Furthermore, there is no wellestablished water pollution monitoring system (Alemayehu, 2006).

Few studies conducted on Akaki River have revealed that the river is highly polluted with a range of pollution sources being discharged into the river without any efficient treatment. Though such studies are available, currently there is an impending interest for national pollution monitoring system requiring for robust surveillance system, which can enhance the capacity of pollution monitoring and abatement efforts. 

Therefore, identifying/documenting the distribution and transportation of toxic compounds across the environmental gradient is essential. When said transportation, it is the translocation of the toxic compounds, for instance metals across the river water, soil, vegetables, and living organisms. In addition, it is compelling to summarize Akaki river water quality analysis results into a single representative value to calculate the river water quality index (WQI).  

Moreover, such data can also contribute to efforts related to human health risk assessment and trend analysis. Previously, Yard et al., (2015), conducted comprehensive study on the status of exposure of Akaki population to hazardous toxic chemicals, solely heavy metals. The short coming of this study was that, the sampling technique did not include population groups (farmers) that are believed to be receivers of the maximum effect of the pollution. For this reason, risk assessment focusing on this part of the community is required to provide preliminary information on the health risk that farmers are facing with the consumption of vegetables that they are growing and exposure through occupational inhalation of heavy metals while working on the field.

With regard to bioaccumulation, a notable phenomenon is the presence of fish species in the lower catchment of the Akaki River, Aba Samuel reservoir and Koka reservoir (artificial lake) found in central Ethiopia. However, to the best of our knowledge, there is a paucity of reports on level of heavy metals in fishes dwelling in these two reservoirs. Thus monitoring the level of heavy metals in fishes can enhance our understanding about the distribution of these pollutants. 

The sample sizes, the comprehensiveness of the study are special features of this research project; perhaps this can be changed into periodic surveillance system.

1.3 Objectives of the Study

       1.3.1    General Objective

The overall arching objective of this study was to assess the distribution and translocation of toxic metals in the Akaki River system and their bioaccumulation in fish species of Aba-Samuel reservoir and Koka Lake.

       1.3.2    Specific Objectives

  • To quantify the level of selected trace metals in Akaki river and soil matrices; 
  • To assess the bioaccumulation of trace metals in Ethiopian kale, Lettuce, and Swiss chard vegetables irrigated with polluted  Akaki river;  
  • To determine the bioaccumulation of selected trace metals indifferent tissues of Clarias gariepinus (Cat fish) fish species of Aba-Samuel and Koka Lake; 
  • To determine both cancerous and non-cancerous health risks, amongst farmers working on agricultural fields, that extends along the Akaki river catchment. 

2 Literature Review 

2.1 Metal contamination of river irrigation water, soil, and vegetables

Even though the natural levels of potentially toxic elements are recognized, those arising from aquatic ecosystems have increased considerably with industrial and urban development. Moreover, the situation is aggravated by the ability of these metals to accumulate in the soil and food chain (Mekonnen et al., 2014). 

A number of studies from developing countries have reported heavy metal contamination in wastewater and wastewater irrigated soil (Cao & Hu, 2000; Singh et al., 2004;

Mapanda et al., 2005; Tiwari et al., 2011). In manydeveloping countries including

Ethiopia, farmers are irrigating their crop plants with contaminated river water (Itanna, 1998, 2002; Kaushik et al., 2005; Abbas et al., 2007; Tiwari et al., 2011; Weldegebriel et al., 2012) having high level of toxic metals. Market proximity, high opportunities for income generation, reliable and free irrigation water supply, and minimum artificial fertilizer requirement are often cited benefits of irrigation within cities (Qadir et al., 2010).

Several researches have been documented on the use of diluted industrial effluents as growth and productivity enhancing effects on crop plants (Abbas et al., 2007). However, loading of heavy metals often leads to degradation of soil health and contamination of food chain mainly through the vegetables grown on such soils (Rattan et al., 2005).

Depending on the biochemical reactions; the potential impact of toxic metals varies from poisoning, allergy, asthma, nervous system disturbance, cancer, and many other abnormalities. The main characteristic of food-borne heavy metal poisoning is often chronic in nature with its impact being more systematic and long lasting (Itanna, 2002).

According to CSA, (2013), there are over 887 registered industries in Addis Ababa (40 % of all industries in the country). There is a countable of research that records the chemical pollution that these industries are imparting on the river Akaki. For instance, Aschale et al., (2016), quantified the amount of Cr, Mn, Sb, B and Pb in Little Akaki river which exceeded the permissible limits of the Ethiopian, European Community and WHO for drinking water quality guidelines. The concentration of Cr, Zn, Cu and Pb exceeded the annual average thresholds for surface waters set in SI 272 of 2009. Levels of Cr, Mn and Sr were also higher than the international guidelines value for irrigation water. Mekonnen et al., (2014), further strengthens that Little Akaki River, Lake Hawassa, and Lake Ziway are highly loaded with Cr, Mn, Co, Ni, Ag, Hg and Pb which are believed to be released from domestic and industrial establishments.