A Systematic Review of Irrigation Water Quality, Vegetable Contamination, and Food Safety Concerns in Peri-Urban Addis Ababa, Ethiopia

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
Soil (agricultural land)	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

[i] Source: synthesized based on literature review.

Table 2.

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

[i] Source: synthesized based on a literature review.

Table 3.

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