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Potentially toxic metals in irrigation water, soil, and vegetables and their health risks using Monte Carlo models

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Overview

Orosun and coauthors measured As, Cd, Cr, and Pb in irrigation water, soil, and two farm-grown leafy vegetables (cabbage and a plant labelled “spinach (Amaranthushybridus)”) in Ilorin, Kwara State, Nigeria, and modeled non-cancer and cancer health risks using Monte Carlo simulation. The leafy-green species the paper calls spinach is Amaranthus hybridus (an African leafy green, locally called spinach), not Spinacia oleracea; this source routes to the leafy-vegetables pages rather than the spinach pages. The paper provides field/context evidence tied to contaminated irrigation and soil along the lower Niger River basin.

Key numbers

  • Mean soil concentrations were Cr 81.77, Pb 19.91, As 13.23, and Cd 3.25 mg/kg (Table 2).
  • Mean irrigation-water concentrations were Cr 1.87, Pb 1.65, As 0.85, and Cd 0.20 mg/L (Table 4).
  • Cabbage (Brassica oleracea, n=60 individual heads → 12 composites) and the Amaranthus leafy-green (n=95 plants → 19 composites) mean concentrations respectively were Cr 5.37 and 5.88 mg/kg, Pb 3.57 and 4.33 mg/kg, As 1.09 and 1.67 mg/kg, and Cd 0.48 and 1.04 mg/kg (Table 5, mean ± SD).
  • Bioaccumulation factors (vegetable/soil) for the Amaranthus leafy-green were higher than for cabbage across all four metals (Cd BAF 0.32 vs 0.15 the most pronounced); the Amaranthus leafy-green carried higher concentrations than cabbage for every metal measured.
  • Modified Pollution Index for soil Cd was 9.88 (Table 6), close to the “extreme contamination” threshold of 10; soil Pb was 0.81 (minimal); As 2.33 and Cr 1.46 (moderate/slight).
  • Hazard Index for the Amaranthus leafy-green ranged 4.44–11.62 (mean 9.10) and for cabbage 3.90–8.38 (mean 6.43); both exceed USEPA’s HI = 1 threshold (Table 8).
  • Incremental lifetime cancer risk (Table 9) mean was 5.46×10⁻³ for the Amaranthus leafy-green and 4.21×10⁻³ for cabbage, both above the 1×10⁻⁴ “high risk” threshold.

Methods (brief)

Surface soil (0–15 cm) and edible-leaf vegetable samples were collected in March 2021 from 31 sampling sites along the lower Niger River basin around Ilorin in an X-shape composite design (five subsamples per site). Cabbage and the Amaranthus leafy-green were air-dried then oven-dried at 70 °C to constant weight. Aqua regia digestion (3 mL HNO₃ + 9 mL HCl per 1 g sample, heated to fume cessation) was used for soil, vegetable, and filtered (0.45 µm) irrigation-water samples following prEN16174. As, Cd, Cr, and Pb were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES); detection limits were 0.50 (Pb), 0.50 (Cr), 0.05 (As), and 0.07 (Cd) mg/L. The instrument recovered ≥90% on blanks and gave linear calibrations (R² = 0.999). No speciation was performed (total As, total Cr; no separation of inorganic As or Cr-VI). Health-risk modeling used Average Daily Intake, Target Hazard Quotient, Hazard Index, and Incremental Lifetime Cancer Risk against USEPA reference doses and slope factors, with 10,000-trial Monte Carlo simulation in ORACLE Crystal Ball software v11.1.2.4.850.

Implications

Provides Nigerian contaminated-irrigation occurrence data for an Amaranthus leafy-green and cabbage; values reflect a wastewater-irrigated farming context and are not general-market baselines. Adds field-context evidence for the leafy-vegetables and cabbage ingredient pages and the leafy-vegetables-other product page. Useful for exposure-modeling worked examples in course material; the Monte Carlo distributions on Hazard Index and ILCR illustrate how site-specific variance shapes risk estimates.

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