Ali et al. 2022 — Meta-analysis of lead in wastewater-irrigated soil and crops
This systematic review and meta-analysis synthesises 24 studies across 13 countries to quantify the accumulation of lead in municipal wastewater, wastewater-irrigated soil, and edible crops, and to assess associated public health risks. The studies are concentrated in South and East Asia (India, Pakistan, Iran, China), where large-scale untreated wastewater irrigation is documented. The review includes 44 crops, of which 38 are leafy and non-leafy vegetables. Key finding: in all studied crops produced using untreated wastewater, lead concentrations in edible parts exceeded WHO limits for edibility, and health risk indices (HRI) calculated for multiple countries (India, Iraq, Morocco, Egypt) exceed 1, indicating unacceptable risk.
Key numbers
- Pb in irrigation wastewater: mean concentration ranged from 0.0196 ± 0.01 mg/L to 52.4 ± 0.02 mg/L in untreated wastewater; 0.001–0.88 ± 0.13 mg/L in treated wastewater.
- 50% of wastewater values exceeded the WHO/FAO standard for irrigation water (0.5 mg/L Pb).
- Pb in irrigated soil: 0.04 ± 2.3 mg/kg (Ethiopia) to 441 ± 19.8 mg/kg (Iran).
- 44 crops studied across 24 studies; most frequently studied: spinach, cabbage, lettuce, wheat, corn, broad beans.
- In all crops produced with untreated wastewater, Pb levels in edible parts exceeded WHO edibility limits.
- HRI > 1 for adult consumption of vegetables in India, Iraq, Morocco, and Egypt.
- Bioconcentration factors (BCF) and plant concentration factors (PCF) indicate significant root-to-edible-tissue transfer.
- Detection method: AAS in 19 of 24 studies; GF-AAS in 2 studies; FAS in 3 studies.
- Meta-analysis heterogeneity: I² = 98%, indicating very high between-study variability; effect size used was Hedges’ g (random-effects model).
Methods (brief)
Systematic review following PRISMA 2009. Search databases: PubMed, ScienceDirect, Google Scholar. Inclusion criteria: original research reporting Pb concentrations (mean ± SD) in irrigation wastewater, irrigated soil, and edible plant parts, with a control group and clearly stated wastewater type. 5,268 articles identified; 24 met final inclusion criteria. Meta-analysis conducted in Stata v16.0 and Cochrane RevMan 5. Health risk assessment using health risk index (HRI = DIM/RfD), bioconcentration factor (BCF), pollution load index (PLI), and soil accumulation factor (SAF). RfD for Pb (adults): 0.001 mg/kg/day.
Implications
Certification: Directly relevant to ingredient sourcing geography; supply chains using produce from South Asian or Middle Eastern regions irrigated with untreated municipal wastewater face documented Pb risk exceeding WHO edibility limits. HMT&C sourcing specifications for high-risk vegetables (spinach, leafy greens) should flag irrigation water source as a risk factor. Courses: Core evidence for wastewater irrigation as a Pb transfer pathway; illustrates HRI methodology and BCF concept for QA and supply-chain training. App: Leafy vegetables (spinach, cabbage, lettuce) from wastewater-irrigated origins carry elevated Pb; HRI exceedance documented across multiple countries; supply-chain origin flag warranted. Microbiome: Not directly applicable.