Urban and peri-urban agriculture (UPA) has become a critical component of food security, income generation, and livelihood support in rapidly expanding cities across sub-Saharan Africa. In Ethiopia, particularly in Addis Ababa, peri-urban vegetable farming supplies fresh produce to urban markets and provides income opportunities for thousands of smallholder farmers (Drechsel, 2010). However, the rapid expansion of UPA has intensified pressures on natural resources, particularly the availability of clean water for irrigation. Due to their limited access to safe and regulated water sources, many peri-urban farmers rely on rivers such as the Akaki, Kebena, and their tributaries, which are heavily polluted with untreated municipal sewage, industrial effluents, and solid waste (Ebissa et al., 2025; Mekuria et al., 2021; Alehegne, 2018; Woldetsadik et al., 2017).
The use of contaminated water for irrigation introduces complex environmental and public health risks. Numerous studies have documented high levels of microbial pathogens – including Escherichia coli, Salmonella, and Shigella – as well as chemical pollutants such as heavy metals (Pb, Cd, Zn), pesticide residues, and hydrocarbons in irrigation water and vegetables cultivated in these systems (Tadesse et al., 2025; Dinede et al., 2023; Alamnie et al., 2020). Leafy vegetables, frequently consumed raw or minimally processed, are particularly vulnerable and serve as direct conduits for foodborne pathogens and chronic toxic exposures. These risks are compounded by the cumulative effects of long-term wastewater irrigation, which can degrade soil quality, increase antimicrobial resistance, and undermine the resilience of local food systems (Belay et al., 2020; Keraita et al., 2008).
Despite the significance of these hazards, food safety surveillance and regulatory oversight in Ethiopia remain weak, and there is no coordinated national strategy for monitoring irrigation water quality in peri-urban farming zones. Informal agricultural practices, low farmer awareness, and inadequate consumer protection frameworks exacerbate these vulnerabilities (Drechsel, 2010). The expansion of UPA in Addis Ababa has undeniably enhanced food availability and livelihoods, yet it has concurrently increased environmental and public health risks, particularly the use of contaminated water sources for irrigating high-risk crops such as cabbage, spinach, and lettuce (Gashaye, 2020; Keraita and Drechsel, 2015).
Empirical studies conducted in Addis Ababa have repeatedly reported alarming contamination levels. Irrigation water and vegetables often contain microbial pathogens and heavy metals exceeding WHO, USEPA, and EU standards (Hiruy et al., 2022; Angello, 2022; Tadesse et al., 2025). Consumption of these contaminated vegetables poses acute and chronic health threats, including gastrointestinal infections, neurological disorders, reproductive toxicity, and carcinogenic effects. Vulnerable populations, especially children and low-income urban residents, face heightened risks due to limited access to clean water, inadequate food safety knowledge, and constrained healthcare resources (Gizaw et al., 2022).
Nevertheless, research findings in this domain remain fragmented, localized, and methodologically inconsistent, limiting their utility for evidence-based policy-making and intervention planning. Many studies focus on isolated contaminants or specific sites, making it challenging to assess spatial-temporal trends, integrate findings across the peri-urban landscape, or inform national food safety strategies (Drechsel, 2010). Moreover, most assessments lack a holistic, One Health perspective that recognizes the interconnectedness of human health, agricultural practices, and environmental sustainability. Without such a comprehensive approach, contaminated irrigation water perpetuates a cycle of food safety risks, environmental degradation, and social vulnerability.
Given these knowledge gaps, there is a pressing need for a systematic synthesis of research conducted between 2010 and 2025 on irrigation water quality, vegetable contamination and associated food safety concerns in peri-urban Addis Ababa. Therefore, the objectives of this review are to: (i) consolidate fragmented empirical evidence on chemical, microbial, and pesticide contamination; (ii) identify key contaminants, exposure pathways, and the most affected crops; (iii) evaluate associated public health risks, with attention to vulnerable populations; and (iv) provide evidence-based guidance for integrated interventions, regulatory policies, and sustainable peri-urban agricultural practices. By addressing these aims, the study seeks to inform policy-makers, urban agriculture planners, and public health authorities, thus contributing to safer food systems and resilient urban environments in Ethiopia.
This study employed a systematic review design guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines (Page et al., 2021). The objective was to identify, synthesize, and analyse peer-reviewed and grey literature published between 2010 and 2025 that examined irrigation water quality, vegetable contamination and food safety risks in the peri-urban areas of Addis Ababa, Ethiopia. The study used a qualitative synthesis with elements of quantitative data extraction where applicable.
Data for this systematic review were drawn from a diverse range of academic and institutional sources to ensure comprehensive coverage of the topic. Academic databases such as Scopus, Web of Science, PubMed, AGRICOLA, Google Scholar, African Journals Online (AJOL) and ScienceDirect were systematically searched for peer-reviewed articles. In addition, institutional repositories including the Ethiopian Public Health Institute (EPHI), Addis Ababa University Digital Library, the Ministry of Agriculture (MoA), and the International Water Management Institute (IWMI) were consulted to access relevant research outputs and policy documents. To capture contextual and region-specific insights, grey literature was also included comprising technical reports, theses, dissertations, and publications by government bodies and NGOs focusing on irrigation, urban agriculture, and food safety in Ethiopia.
The literature search was conducted using a combination of Boolean operators, Medical Subject Headings (MeSH) terms, and relevant keywords to ensure a comprehensive and systematic retrieval of studies. The following keyword string was adapted to suit each database: (“irrigation water quality” OR “wastewater irrigation” OR “polluted water”) AND (“vegetable contamination” OR “heavy metals” OR “microbial contamination” OR “pesticide residues”) AND (“food safety” OR “public health risk”) AND (“Addis Ababa” OR “peri-urban Ethiopia”) AND (2010:2025). The search was restricted to documents published in English and those specifically relevant to the Ethiopian context. In addition to database searches, citation chaining and manual screening of reference lists from relevant articles were employed to identify additional studies that may not have appeared in the initial search results.
A comprehensive literature search was conducted across multiple electronic databases, including PubMed, Scopus, African Journals Online (AJOL) and Google Scholar, to capture peer-reviewed studies relevant to waste-water irrigation, vegetable contamination, and food safety in peri-urban agricultural systems. This search was supplemented by grey literature sources, such as institutional reports, policy documents, conference proceedings and government publications, with the aim being to ensure inclusion of relevant non-peer-reviewed information. The initial search yielded 404 records from both database and supplementary sources. The titles and abstracts of these records were screened for relevance, and studies that did not focus on wastewater irrigation, peri-urban agriculture, vegetable contamination or associated public health risks were excluded. Full-text assessment was then performed on the remaining studies to confirm eligibility based on predefined inclusion criteria such as relevance to heavy metal contamination, microbial hazards, pesticide residues, and environmental or health outcomes. Following this evaluation, 55 documents met the inclusion criteria and were retained for the final review and synthesis. The use of a PRISMA-guided selection process ensured systematic, transparent, and rigorous identification of studies, providing a robust evidence base for synthesizing contamination patterns, spatial and seasonal variability, and the public health risks associated with wastewater-irrigated peri-urban agriculture.
A structured data extraction form was employed to systematically collect key information from each included study. Extracted data included study details such as the following: authors, year of publication, location, and study design; type and source of irrigation water; type of vegetables analysed; contaminants assessed, including microbial, chemical, and heavy metal pollutants; sampling and analytical methods used; key findings and contaminant concentration levels; reported food safety or health risks; and any policy or regulatory recommendations provided. To ensure consistency and accuracy, careful reviews were carried out to extract the data.
A thematic synthesis approach was employed to identify key patterns and thematic categories across the included studies. Extracted data were organized and coded to facilitate systematic analysis. The synthesis followed a narrative approach, with results grouped into key thematic categories: sources and types of contamination, contaminated crops and exposure pathways, levels and trends of contaminants, identified health risks, and policy and institutional responses. This approach facilitated a comprehensive understanding of the multifaceted issues surrounding irrigation water quality and vegetable contamination. Where applicable, quantitative findings such as contaminant concentrations were tabulated and summarized using descriptive statistics, including means, ranges, and standard deviations, to provide a clearer picture of the extent and variation of contamination levels reported across studies.
The findings reveal a highly interconnected and multi-dimensional pattern of heavy metal contamination across irrigation water, soils, sediments, and vegetables in peri-urban agricultural systems (Table 1). This continuum of pollution demonstrates how contaminants are transferred systematically from water to soil and ultimately to crops, thus presenting a persistent environmental and public health challenge. The dominance of metals in soils, in the order Mn > Ni > Pb > Cu > Zn, with concentrations exceeding the permissible limits set by WHO, USEPA, and EU standards, underscores systemic contamination largely driven by anthropogenic activities. These results are consistent with earlier studies in Nigeria (Adamu and Nganje, 2010), emphasizing that wastewater-irrigated agricultural systems across developing countries face similar contamination challenges.
Sources, types of contamination, and primary human exposure pathways
| Environmental medium | Type of contamination | Key contaminants | Major sources of contamination | Primary exposure pathways to humans |
|---|---|---|---|---|
| Irrigation water | Heavy metals | Pb, Mn, Ni, Cu, Zn, Cd | Industrial effluents, untreated wastewater, urban runoff | Consumption of irrigated vegetables; drinking contaminated water; dermal contact |
| Microbial | E. coli, fecal coliforms | Sewage discharge, open defecation | Consumption of raw vegetables; direct ingestion | |
| Parasitic | Ascaris, Giardia, Cryptosporidium | Raw manure, untreated wastewater | Consumption of raw/poorly washed vegetables | |
| Chemical residues | Antibiotics, agrochemicals | Livestock waste, agricultural inputs | Indirect ingestion through crops | |
| Soil (agricultural land) | Heavy metals | Mn, Ni, Pb, Cu, Zn, Cr, Cd, As | Wastewater irrigation, industrial discharge, waste dumping | Crop uptake → human consumption |
| Chemical contamination | Fertilizers, pesticides | Agricultural practices | Food chain transfer; occupational exposure | |
| Vegetables | Heavy metals (bioaccumulated) | Ni, Pb, Mn, Cu, Zn, Fe, As | Uptake from soil and water | Direct consumption (especially raw) |
| Microbial contamination | E. coli, coliforms | Contaminated irrigation water | Consumption of raw vegetables | |
| Parasitic contamination | Helminths, protozoa | Wastewater irrigation | Consumption of raw/uncooked vegetables | |
| Sediments | Heavy metals | Cr, Ni, Zn, As, Pb | Industrial discharge, runoff, erosion | Release into water → irrigation → food chain |
Source: synthesized based on literature review.
The elevated heavy metal concentrations in irrigated lands compared to wetlands and forest soils strongly implicate wastewater irrigation as a major driver of soil degradation. In Addis Ababa, the peri-urban farming systems heavily rely on untreated or poorly treated municipal wastewater, which is due to insufficient safe water sources and high urban vegetable demand (Drechsel, 2010; Keraita and Drechsel, 2015). Municipal wastewater, including domestic sewage and greywater, often discharges directly into rivers such as the Akaki River as a consequence of the overloaded or nonfunctional sewage infrastructure (Tarekegn and Weldekidan, 2022). Industrial zones, particularly Akaki-Kality and Bole-Lemi, further exacerbate this contamination by releasing effluents rich in Pb, Cd, Cr, and Zn into irrigation sources (Tadesse et al., 2025). The combination of municipal and industrial discharges, along with runoff from urban areas, creates cumulative contamination that propagates through the water–soil–plant continuum.
The spatial and seasonal variation in contamination highlights the dynamic nature of environmental and anthropogenic influences. Statistically significant differences observed across land-use types and seasons indicate that environmental processes, such as hydrological dynamics, interact with human activities to modulate contamination levels. For instance, higher metal concentrations during the dry season are likely driven by reduced dilution and increased evapoconcentration, consistent with observations from Bangladesh (Ahmed et al., 2019). This temporal variability underscores the importance of seasonally adaptive risk management strategies.
The high metal concentrations in river water, particularly in middle and downstream sections, indicate that aquatic systems act both as reservoirs and transport pathways for contamination. The marked increase in Pb and Mn along the river course exemplifies how urban and industrial discharges amplify environmental pollution, a pattern also documented in Ghana, India, and Pakistan (Bempah and Ewusi, 2016; Ghosh et al., 2012; Waseem et al., 2014). Consequently, irrigation with such contaminated water extends pollution into soils and crops, establishing a direct pathway to the human food chain.
Strong positive correlations among heavy metals in soils and vegetables indicate shared pollution sources and similar geochemical behaviour, therefore reinforcing the systemic nature of contamination. The observed negative correlation of Zn with Cu, Mn, and Pb highlights differential mobility and uptake mechanisms, which are influenced by soil chemistry, competitive absorption, and plant physiology. These findings suggest that soil remediation and management strategies must account for metal-specific dynamics rather than treating contamination as homogeneous.
Vegetables, particularly leafy types such as lettuce and cabbage, emerge as critical bioaccumulators, with Ni and Pb concentrations frequently exceeding international safety thresholds. High bioconcentration factors (BCF > 1) for Cu in lettuce demonstrate efficient metal transfer from soil to plant, confirming vegetables as key conduits for human exposure (Aschale et al., 2015b; Gebeyehu and Bayissa, 2020; Lente et al., 2014). These results align with prior studies in Addis Ababa, where vegetables irrigated with polluted river water exhibited elevated Pb, Mn, and other metal levels (Itanna, 1998, 2002). The spatial distribution of contamination, with hotspots in industrially influenced areas such as Kera, Akaki, and Gofa, reflects cumulative pollution from urban runoff, industrial effluents, and wastewater irrigation (Weldegebriel et al., 2012; Aschale et al., 2015a).
Beyond heavy metals, peri-urban agriculture in Addis Ababa is exposed to a wide array of contaminants. Pesticide residues (organophosphates and organochlorines) from overuse and improper handling, organic pollutants from solid waste leachates, pharmaceutical residues from medical waste, and microbial pathogens from animal husbandry and greywater all converge within the soil-plant system (Tadesse et al., 2025; Loha et al., 2020; Hiruy et al., 2022; Gizaw et al., 2022; Hailu et al., 2024). These co-occurring contaminants not only pose chronic health risks, including neurotoxicity, renal impairment, endocrine disruption, and antimicrobial resistance, but also interact synergistically, amplifying the overall environmental and health burden.
From a health perspective, the implications are substantial. The hazard index (HI), target hazard quotient (THQ), and health risk index (HRI) analyses that have been conducted indicate that certain population groups– particularly children–are more vulnerable to non-carcinogenic risks, mainly from Pb and Ni exposure. This aligns with global findings (Zhu et al., 2016; Uddin et al., 2023), where children consistently exhibit higher risk levels due to their lower body weight and higher intake-to-body-mass ratios. Moreover, documented associations between heavy metal exposure and health outcomes, including gastrointestinal cancers and reproductive disorders (Dutta et al., 2022; Parida and Patel, 2023), reinforce the seriousness of long-term exposure even when short-term indices appear acceptable.
The findings also reveal an important contradiction: while some irrigation water samples meet permissible limits, soils and vegetables still exhibit elevated contamination levels. This suggests that cumulative effects, long-term irrigation, and sediment interactions play a more critical role than instantaneous water quality. Consequently, relying solely on water quality standards may underestimate actual risks within the food system.
The findings from this study reveal a complex contamination profile in irrigation water, soils and vegetables in the peri-urban areas of Addis Ababa, reflecting both microbial and heavy metal hazards. Irrigation water and vegetables were highly contaminated with microbial pathogens, exceeding WHO standards for E. coli and faecal coliforms (Table 2). Parasitic organisms such as Ascaris, Giardia, and Cryptosporidium were also detected, highlighting serious public health risks (Tomass and Kidane, 2012). Seasonal variation amplified contamination, with higher microbial loads during the dry season due to reduced dilution of wastewater, a pattern observed in Bangladesh and Pakistan (Ahmed et al., 2019; Yasmin et al., 2023; Zohra et al., 2021). These results align with previous Ethiopian studies in the Akaki River basin, Jimma, and other peri-urban areas, which similarly documented high microbial loads in wastewater-irrigated vegetables (Weldesilassie et al., 2011; Delesa, 2017). The evidence clearly indicates that the use of untreated wastewater, coupled with open defecation and unregulated sewage discharge, is a primary driver of microbial contamination in both irrigation water and crops.
Microbial, parasitic, and heavy metal contamination in wastewater-irrigated vegetables, soils, and irrigation water
| Sample type | Key findings | Seasonal variation | Standards / guidelines | Key sources of contamination | Trend / observation |
|---|---|---|---|---|---|
| Irrigation water | Low contamination: Pb 0.001 mg/L; Mn 0.03 mg/L; Cu 0.002 mg/L; Zn 0.005 mg/L; E. coli 1.0 × 103 CFU/10 mL | Slight increase in dry season | WHO (2006) ≤1000 FC/100 mL; USEPA heavy metal limits | Minimal industrial or domestic discharge | Safe; dilution effect observed |
| Elevated contamination: Pb 79.8–82 mg/L; Mn 4.8–5.9 mg/L; Cu 0.04–0.05 mg/L; Zn 1.1–1.2 mg/L; E. coli 2.7 × 103 CFU/10 mL | Higher in dry season | WHO (2006); USEPA | Industrial effluents, municipal wastewater, open drainage | High contamination; Pb and Mn highest; cumulative industrial impact | |
| Persistent contamination: Pb 60–61 mg/L; Mn 4.3–4.5 mg/L; Cu 0.02–0.03 mg/L; Zn 0.95–1.0 mg/L; E. coli 3.0 × 103 CFU/10 mL | Dry season concentration higher | WHO (2006); USEPA | Industrial discharge, urban runoff | Contamination persists along river; risk to irrigated crops | |
| Soil | Mn 1440–1489 mg/kg; Ni 51–53 mg/kg; Pb 61–63 mg/kg, Cu 32–33 mg/kg; Zn 79–82 mg/kg; high microbial load | Slight decrease in wet season | WHO / USEPA / EU | Irrigation with contaminated water, urban runoff, manure | Highest contamination; metals order: Mn > Ni > Pb > Cu > Zn; bioaccumulation potential |
| Mn 1150–1200 mg/kg, Ni 47–48 mg/kg; Pb 53–55 mg/kg; Cu 29–30 mg/kg; Zn 68–70 mg/kg | Slight reduction in wet season | WHO / USEPA / EU | Runoff, minor anthropogenic input | Moderate contamination; land-use dependent | |
| Mn 820–840 mg/kg; Ni 34–35 mg/kg; Pb 36–38 mg/kg; Cu 24–25 mg/kg; Zn 49–50 mg/kg | Minimal seasonal change | WHO / USEPA / EU | Background soil, limited human influence | Lowest contamination among land-use types | |
| Vegetables | Ni 29–30 mg/kg; Pb 1.38–1.40 mg/kg; Mn 19–20 mg/kg; Cu 11–12 mg/kg; Zn 8.9–9.0 mg/kg; total coliforms, fecal coliforms above limits | Slight increase in dry season | WHO / USEPA; WHO (2006); ICMSF (1998) | Contaminated irrigation water, soil, manure | Highest accumulation of Ni; metals decreasing: Ni > Pb > Mn > Cu > Zn; leafy vegetables most susceptible |
| Ni 22–24 mg/kg; Pb 1.18–1.20 mg/kg; Mn 17–18 mg/kg; Cu 10–10.5 mg/kg; Zn 8.4–8.5 mg/kg; microbial load elevated | Dry season higher | WHO / USEPA; WHO (2006); ICMSF (1998) | Irrigation, contaminated soil | Slightly lower than lettuce; significant BCF for Pb and Cu | |
| Ni 20–21 mg/kg; Pb 1.12–1.15 mg/kg; Mn 17–17.5 mg/kg; Cu 9.8–10 mg/kg; Zn 8.1–8.2 mg/kg | Minimal seasonal variation | WHO / USEPA | Irrigation, soil | Lowest heavy metal accumulation among vegetables | |
| Ni 1.18–1.20 mg/kg; Pb 0.48– 0.50 mg/kg; Mn 1.08–1.10 mg/kg; Cu 0.50–0.52 mg/kg; Zn 0.48–0.50 mg/kg; microbial load low | Minimal variation | WHO / USEPA; WHO (2006) | Background soil; minimal irrigation contamination | BCF < 1; minimal contamination |
Source: synthesized based on a literature review.
Regarding heavy metals, irrigation water generally contained concentrations below FAO limits, suggesting a low immediate risk (Table 2). In contrast, soils and vegetables exhibited significant localized accumulation of Pb, Ni, and Mn. Soil concentrations followed the order Mn > Ni > Pb > Cu > Zn, with irrigated lands showing the highest levels, a point consistent with observations from Mojo, Akaki, Kera, and Addis Ababa (Gebeyehu and Bayissa, 2020; Itanna, 1998, 2002). Seasonal rainfall slightly reduced soil metal concentrations during the wet season due to dilution, mirroring trends reported in Bangladesh and Pakistan (Ahmed et al., 2019; Waseem et al., 2014). Land-use type also influenced metal accumulation, with irrigated farms showing the highest levels, wetlands intermediate, and forest or garden soils the lowest (Hu et al., 2020), reflecting the combined impacts of irrigation with contaminated water and urban runoff (Gebeyehu and Bayissa, 2020; Aschale et al., 2015b).
Vegetables displayed bioaccumulation trends of Ni > Pb > Mn > Cu > Zn, with leafy vegetables such as lettuce and cabbage showing the highest levels. These results corroborate findings from Ghana, Pakistan, and Bangladesh, where wastewater-irrigated leafy vegetables often exceeded safe limits (Bempah and Ewusi, 2016; Waseem et al., 2014; Ahmed et al., 2019). Bio-concentration factor (BCF) analysis confirmed that Swiss chard, lettuce, and cabbage act as hyperaccumulators for metals including Cu, Ni, Pb, and Zn, which highlights both the potential risks to human health and the opportunities for phytoremediation (Cooper et al., 2020; Arthur et al., 2022). Soil-to-plant transfer factors (TF) further illustrated variability among metals, with Cd, Cu, and Zn showing higher mobility, while Cr and Fe remained largely immobile (Kabata-Pendias and Skibniewska, 2010). These dynamics suggest that metal uptake depends not only on soil concentration but also on soil properties (pH, organic matter, and texture), plant species, and seasonal conditions (Hooda and Alloway, 1994; Luo and Rimmer, 1995). Importantly, even when soil concentrations remain below international guidelines, vegetable contamination can exceed safe consumption levels, emphasizing the limitations of current standards that do not fully account for bioavailability and crop-specific uptake.
The combination of microbial and heavy metal contamination presents significant public health concerns.
Hazard index (HI) and target hazard quotient (THQ) analyses indicate that children are particularly vulnerable due to lower body weight and higher relative vegetable consumption (Ishak et al., 2016). Values exceeding 1 for Pb and Ni in certain leafy vegetables during both wet and dry seasons signal potential non-carcinogenic risks (Uddin et al., 2023; Zhu et al., 2015). Chronic exposure, even at lower concentrations, may result in long-term health effects, including organ damage, reproductive complications, and neurological disorders (Parida and Patel, 2023; Cooper et al., 2020). The dual nature of contamination–immediate microbial threats and cumulative heavy metal exposure–underscores the urgency of interventions.
Comparisons with other regions highlight how wastewater-irrigated agriculture is a global challenge. Studies in Ethiopia, Pakistan, Bangladesh, India, and Ghana reveal similar patterns, with untreated waste-water irrigation, industrial effluents, and urban runoff leading to metal concentrations in leafy vegetables exceeding safe limits (Ahmed et al., 2019; Waseem et al., 2014; Bempah and Ewusi, 2016). Regional variations in soil type, climate, and irrigation practices modulate metal bioaccumulation, suggesting that risk mitigation strategies must be locally tailored.
In conclusion, this study demonstrates that waste-water-irrigated systems in peri-urban Addis Ababa face a dual contamination challenge: microbial pathogens present immediate health hazards, while heavy metals pose long-term, cumulative risks. The findings emphasize the need for integrated interventions, including wastewater treatment infrastructure, industrial effluent regulation, soil remediation practices, informed crop selection to reduce metal uptake, and community education on safe vegetable consumption. Only a coordinated, multi-pronged approach can mitigate the compounded public health risks associated with these agricultural systems.
Spatial distribution of heavy metals, microbial loads, and pesticide residues across farming zones The assessment of heavy metals, microbial load, and pesticide residues in peri-urban farming zones of Addis Ababa highlights significant spatial variability in contamination, reflecting both environmental and anthropogenic drivers. Akaki-Kality recorded the highest concentrations of Pb (0.85 mg/kg), Ni (30.4 mg/kg), and Mn (1,489 mg/kg), alongside elevated E. coli counts (3.0 × 103 CFU/g) and the highest percentage of pesticide exceedance (48%). These findings are consistent with prior studies emphasizing the influence of industrial effluents and urban wastewater on agricultural zones proximal to the Akaki River (Gebeyehu and Bayissa, 2020; Weldegebriel et al., 2012). The elevated levels of metals and microbial contamination in this zone suggest that crops, particularly leafy vegetables such as lettuce and cabbage, are highly exposed to both chemical and biological hazards.
Similarly, Bole Bulbula exhibited substantial contamination, with Pb, Ni, and Mn concentrations slightly lower than Akaki-Kality (0.71, 28.2, and 1,200 mg/kg, respectively), high microbial counts (2.7 × 103 CFU/g), and pesticide exceedance of 42%. The combination of intensive farming practices, frequent pesticide application, and exposure to untreated water sources explains these elevated values, aligning with observations by Aschale et al. (2015b), Mengesha et al. (2021), and Aschale et al. (2019). Crops cultivated in this zone, such as spinach and tomato, are thus at moderate to high risk of accumulating both heavy metals and pesticide residues.
In contrast, Yeka and Kolfe Keranio recorded relatively lower contamination levels. Yeka reported Pb at 0.56 mg/kg, Ni at 25.6 mg/kg, Mn at 950 mg/kg, E. coli at 1.9 × 103 CFU/g, and 30% pesticide exceedance, while Kolfe Keranio measured Pb at 0.49 mg/kg, Ni at 24.0 mg/kg, Mn at 900 mg/kg, E. coli at 1.5 × 103 CFU/g, and 25% pesticide exceedance (Gebeyehu and Bayissa, 2020; Weldegebriel et al., 2012; Mohammed et al., 2023). The reduced contamination in these zones is likely due to lower industrial influence and the use of relatively cleaner water sources for irrigation. Dominant crops, including cabbage, green pepper, kale, and tomato, still face exposure to contaminants, albeit at lower levels than Akaki-Kality and Bole Bulbula.
The health risk associated with the consumption of selected leafy vegetables grown on wastewater-irrigated farming sites was evaluated using the Target Hazard Quotient (THQ) method, which is widely applied to assess non-carcinogenic risks from dietary exposure to heavy metals (Zheng et al., 2007; Zhuang et al., 2009). According to the established guidelines, THQ values below 1 indicate negligible health risks (USEPA, 2007). In the study by Woldetsadik et al. (2017), THQ values ranged from 0.042–0.108 (Cd), 0.005–0.014 (Co), 0.0002–0.0004 (Cr), 0.048–0.078 (Cu), 0.015–0.037 (Ni), 0.194–0.298 (Pb), and 0.026–0.037 (Zn) (Table 3), indicating that individual metal exposure through vegetable consumption did not exceed the acceptable risk thresholds.
Contamination by farming zone (Peri-Urban Addis Ababa)
| Zone | Pb (mg/kg) | Ni (mg/kg) | Mn (mg/kg) | E. coli (CFU/g) | Pesticide Exceedance (%) | Dominant Crops | Source(s) |
|---|---|---|---|---|---|---|---|
| Akaki-Kality | 0.85 | 30.4 | 1,489 | 3.0 × 103 | 48% | Lettuce, cabbage | Gebeyehu and Bayissa, 2020; Weldegebriel et al., 2012 |
| Bole Bulbula | 0.71 | 28.2 | 1,200 | 2.7 × 103 | 42% | Spinach, tomato | Aschale et al., 2015a; Mengesha et al., 2021; Aschale et al., 2019 |
| Yeka | 0.56 | 25.6 | 950 | 1.9 × 103 | 30% | Cabbage, green pepper | Gebeyehu and Bayissa, 2020; Mohammed et al., 2023 |
| Kolfe Keranio | 0.49 | 24.0 | 900 | 1.5 × 103 | 25% | Kale, tomato | Weldegebriel et al., 2012; Mohammed et al., 2023 |
Source: synthesized based on literature review.
Spatial variability in THQ values across sub-city administrative areas highlights the role of localized contamination sources in shaping exposure risk (Table 3). The highest Pb-related THQ values were observed in Chirkos (0.298), followed by Nefas Silk Lafto (0.244), Bole (0.243), Akaki Kaliti (0.197), and Kolfe Keraniyo (0.194), indicating the uneven distribution of contamination risk across urban agricultural zones (Woldetsadik et al., 2017). Similar spatial patterns have been documented in industrially affected agricultural systems, where Pb and Cd are identified as dominant contributors to cumulative exposure risk due to their persistence and bioaccumulative properties (Zheng et al., 2007; Zhuang et al., 2009).
The cumulative exposure risk, assessed using the Total Target Hazard Quotient (TTHQ), ranged from 0.33 to 0.53, remaining below the safety threshold of 1 across all study locations. The decreasing trend in cumulative risk (Chirkos > Nefas Silk Lafto > Akaki Kaliti > Bole > Kolfe Keraniyo) suggests spatial differences in contamination intensity across administrative areas. The identification of Pb and Cd as major contributors to TTHQ values in the study by Woldetsadik et al. (2017) is consistent with previous studies conducted near industrial and mining sites, where these metals were reported as dominant contributors to dietary exposure risks (Zheng et al., 2007; Zhuang et al., 2009).
Although TTHQ values remained below the critical level, the persistence of Pb and Cd as dominant contributors highlights the importance of continued monitoring. Comparable studies in emerging economies have reported higher cumulative risk levels exceeding the recommended limits, indicating that wastewater-irrigated systems can present significant long-term exposure risks when contamination sources are not adequately controlled (Abbasi et al., 2013; Qureshi et al., 2016).
Furthermore, previous studies on wastewater-irrigated systems have emphasized that multiple contaminants may interact within food systems, potentially influencing cumulative exposure risks beyond individual metal assessments (Zheng et al., 2007; Zhuang et al., 2009). This highlights the importance of considering cumulative exposure when evaluating food safety risks.
From a management perspective, the observed spatial differences in THQ and TTHQ values emphasize the need for targeted risk mitigation strategies focused on high-risk areas. Strengthening industrial effluent regulation, improving wastewater management, and enhancing routine environmental monitoring have been widely recommended as effective measures to reduce heavy metal exposure in wastewater-irrigated agricultural systems (Abbasi et al., 2013; Qureshi et al., 2016). Continuous surveillance of contamination hotspots remains essential to minimize long-term public health risks and support safe urban agriculture.
The study reveals that peri-urban vegetable farming systems in Addis Ababa are exposed to significant and multi-dimensional contamination involving heavy metals, microbial pathogens, and pesticide residues. Soils in irrigated areas show high levels of Mn, Ni, and Pb, which are bioaccumulated in leafy vegetables such as lettuce and cabbage, thus posing direct risks to human health. Microbial contamination, including E. coli, faecal coliforms, and parasites like Ascaris and Giardia, is widespread, with higher loads observed during the dry season due to reduced dilution of wastewater. Spatial analysis indicates notable heterogeneity, with contamination hotspots in Akaki-Kality and Bole Bulbula linked to the proximity to industrial discharges, intensive farming, and the use of untreated wastewater for irrigation. Children are particularly vulnerable to non-carcinogenic risks from Pb and Ni exposure, while chronic exposure among the general population may result in organ damage, neurological disorders, and reproductive complications. The study also confirms that even when irrigation water meets permissible limits, soils and vegetables may still contain contaminants above safe thresholds, highlighting cumulative effects and limitations of current standards.
To reduce pollution from untreated municipal sewage and industrial effluents, city authorities and environmental regulators should expand both centralized and decentralized wastewater treatment facilities along major urban rivers, including the Akaki, Kebena, and Little Akaki. This includes upgrading existing sewage treatment plants, enforcing effluent quality standards, and imposing penalties on industries that discharge untreated wastewater. Prioritizing access to treated water for peri-urban farms will enhance irrigation water quality, reduce downstream contamination, and safeguard aquatic ecosystems and public health. By 2030, all major river segments in Addis Ababa should be covered by treatment facilities, supplemented by decentralized treatment systems in key farming zones.
City administrations should designate safe urban agriculture zones equipped with infrastructure for treated wastewater reuse, including sedimentation tanks, storage facilities, and drip irrigation systems. Agricultural extension services, local NGOs, and cooperatives should train farmers on safe irrigation practices, rainwater harvesting, proper irrigation scheduling, and crop selection strategies that reduce heavy metal uptake. Farmers should also adopt soil management practices, such as applying organic amendments, rotating crops, and selecting crops with lower bioaccumulation potential. Incentives–including technical support, market access, and certification–should be provided to encourage adherence to safety standards. Regular farm registration, routine inspections, and enforceable penalties for unsafe practices will strengthen compliance, while mobile surveillance units and enhanced laboratory infrastructure can improve monitoring efficiency.
Urban agriculture should be formally embedded within broader green infrastructure, land-use planning, and river rehabilitation projects. This integration can improve food and nutritional security, create employment, stabilize soils, recycle organic waste, and contribute to climate regulation. Aligning urban agriculture with sustainable development goals–particularly SDG 2 (Zero Hunger), SDG 3 (Good Health), and SDG 11 (Sustainable Cities)–will enhance public health and environmental resilience. Municipalities should adopt health-conscious, food-safe, and climate-resilient designs for urban farming that link river restoration with productive agricultural use.
A city-wide food safety monitoring system should be established to regularly test vegetables, irrigation water and soils for microbial and chemical contaminants. Health agencies, working in collaboration with research institutions, should implement rapid response protocols for foodborne disease outbreaks, with special attention to vulnerable populations, including children. Integrating surveillance within a One Health framework ensures coherence across human, animal, and environmental health. Real-time monitoring with decentralized reporting, early warning mechanisms, and targeted inspections of informal markets should be operational to reduce exposure to contaminated produce.
To address the fragmented responsibilities between water, agriculture, health, and environmental agencies, Ethiopia should establish a National Integrated Water, Agriculture, and Health Coordination Platform (NIWAHCP) under a legally binding framework. This platform will facilitate inter-ministerial coordination, joint planning, and accountability among the EPA, Ministry of Agriculture, Ministry of Health, and Ministry of Water and Energy. Strengthened enforcement of effluent standards, regular monitoring, transparent reporting, and centralized data sharing will support evidence-based decision-making, harmonize urban sanitation and agricultural policies, and close compliance gaps.
Ethiopia should adopt a One Health approach to integrate human, animal, and environmental health for food safety and environmental management. Pilot interventions should include farmer training on safe irrigation and integrated pest management, improvements in post-harvest handling, and community-based risk reduction programmes. International partners can support these initiatives, which should be scaled nationally by 2030 with sustainable funding streams and strong institutional backing. Embedding One Health principles in policy frameworks will reduce foodborne illness, mitigate environmental contamination, and create safer, more resilient urban agriculture systems.
Research institutions and universities should establish routine monitoring programmes in contamination hotspots such as Akaki-Kality and Bole Bulbula. Regular testing of irrigation water, soils, and crops, combined with geospatial mapping, will help identify priority areas and evaluate the effectiveness of interventions. These data should inform adaptive policies, guide farmer training, and support public awareness campaigns. Community health agencies should lead awareness programs on proper vegetable handling, washing, peeling, and cooking to minimize microbial and chemical exposure. By assigning clear roles and implementing these practical measures, stakeholders can effectively reduce the compounded risks of microbial and heavy metal contamination in Addis Ababa and peri-urban Addis Ababa.