Risk assessment of human exposure to lead and cadmium in maize grains cultivated in soils irrigated either with low-quality water or freshwater

Overview

This Egyptian field-survey study quantifies total and soluble lead (Pb) and cadmium (Cd) in irrigation water, total and DTPA-available Pb and Cd in agricultural soil (0–30 cm), and total Pb and Cd in mature maize (Zea mays L.) grain across nine cultivated sites in three governorates (Giza, Kafr El-Sheikh, El-Beheira), comparing five freshwater-irrigated sites (Nile River water or groundwater) with four sites irrigated with low-quality water (mixed sewage and industrial effluents, or sewage wastewater). Sampling occurred in August 2017; analysis was by ICP-OES (Agilent 5100 SVDV) after acid digestion (soil and water) or dry-ashing with HCl dissolution (grain). Pb and Cd levels in soil and maize grain at low-quality-water sites were 2.4–3.0-fold (soil) and 19–30-fold (grain) higher than at freshwater sites. All maize-grain Pb values at low-quality-water sites exceeded the FAO/WHO permissible limit of 0.20 mg/kg cited by the authors; Cd in maize exceeded the 0.1 mg/kg limit only at Al-Nasiria. Daily intake of metals (DIM) and health risk index (HRI) were computed using a 0.085 fresh-to-dry conversion factor with adult (70.0 kg, 0.345 kg maize/day) and child (32.7 kg, 0.232 kg maize/day) body weight and consumption assumptions, against EPA RfDs of 0.0035 (Pb) and 0.001 (Cd) mg/kg/day; all HRI values were below 1, leading the authors to conclude no potential health risk from maize consumption at the studied sites.

Key numbers

Table 2 — Total and soluble Pb and Cd in irrigation water (mg/L), mean ± SD

< d.l.: below detection limit. Water permissible limits cited in §3.1 (ref [22], FAO Wastewater Quality Guidelines, 2019): Pb = 5.0 mg/L (all samples below); Cd = 0.01 mg/L (all low-quality water samples above; freshwater below except Wadi El-Natron and Al-Hawia, which exceed).

Table 3 — Total and DTPA-available Pb and Cd in agricultural soil (mg/kg), mean ± SD

Soil reference values cited in §3.2 (ref [27], WHO 2006 wastewater-reuse guidelines): permissible total levels are 84 mg/kg (Pb) and 4 mg/kg (Cd); all studied soil values are below these limits. See Verification notes regarding apparent paper-internal discussion–vs–table inconsistencies in the Cd-total column for Kafr-Hakem and Abo-Roash.

Table 4 — Pb in maize grain (mg/kg) and risk metrics

LSD = 0.04. DIM in mg/kg·person·day; HRI dimensionless. Maize-grain Pb permissible limit cited in §3.3 (ref [32], WHO/FAO Codex Alimentarius Codex Stan 193-1995, as available 2016): 0.20 mg/kg. All four low-quality-water sites exceed this limit; all freshwater sites are below detection or below the limit.

Table 5 — Cd in maize grain (mg/kg) and risk metrics

LSD = 0.004. Maize-grain Cd permissible limit cited in §3.3: 0.1 mg/kg. Only Al-Nasiria (0.112 mg/kg) exceeds; the other three low-quality-water sites are below, and the two freshwater sites with detectable Cd (Wadi El-Natron, Al-Hawia) are well below.

Risk-assessment parameters (§2.3, p. 11)

• DIM formula (after Farrag et al. ref [10]): DIM = (C_metal × C_factor × D_food_intake) / B_average_weight
• C_metal: concentration of metal in maize grain (mg/kg)
• C_factor: 0.085 (fresh-to-dry weight conversion)
• D_food_intake: 0.345 kg/person/day (adults), 0.232 kg/person/day (children)
• B_average_weight: 70.0 kg (adults), 32.7 kg (children)
• HRI = DIM / RFD; RfD: 0.001 mg/kg/day (Cd), 0.0035 mg/kg/day (Pb)
• Safety criterion: HRI < 1

Summary headline values reported in §3.3 and Conclusion

• Pb and Cd in low-quality-water maize were 19–30-fold higher than freshwater maize (across sites and both metals).
• Upper acceptable concentrations (back-calculated from HRI = 1, the authors’ “remain below limit” inference): Pb in maize 8.354 (adults) and 5.803 mg/kg (children); Cd in maize 2.387 (adults) and 1.658 mg/kg (children).
• HRI values across all studied sites and both age groups remained < 1.

Methods (brief)

Study design. Cross-sectional field survey, August 2017. Nine cultivated sites across three Egyptian governorates: Giza (Abdel-Rahman, Kafr-Hakem, Abo-Roash), Kafr El-Sheikh (Al-Hawia, Kafr-Dokhmais, Al-Nasiria), and El-Beheira (Wadi El-Natron, El-Nobaria–Abdel-Rakib, El-Nobaria–Abdel-Wahab). Five sites irrigated with freshwater (Nile River water or groundwater); four sites irrigated with low-quality water — defined as freshwater inadvertently contaminated with wastewater (Kafr-Dokhmais, Al-Nasiria) or contaminated with mixed sewage and industrial effluents from Branch 62 of the Kitchener drain (Kafr-Hakem, Abo-Roash, per Table 1 footnote, although text §3.1 also references sewage wastewater for Abo-Roash). Three replicate sampling points per site collected by cluster random sampling. n = 27 per matrix (soil, water, grain).

Sample collection. Soil 0–30 cm with auger. Irrigation water 20 cm below surface in plastic bottles containing 2 mL concentrated HNO₃ (pH < 2). Maize grain collected from field at maturity.

Soil digestion. Page et al. [16] procedure: 250 mL acid mixture HNO₃ + HClO₄ + H₂SO₄ on 1 g sample; filter Whatman ashless; dilute to volume with distilled water (total Pb and Cd). DTPA (diethylene triamine pentaacetic acid) extraction by Soriano-Disla [17] for available Pb and Cd. APHA [18] method-reference for measurement protocol.

Water digestion. 250 mL water sample heated with 5 mL HNO₃ on hot plate at 80–85 °C to final volume 10–20 mL; digestion repeated twice. Beaker walls rinsed with deionized water; transferred to 25 mL volumetric flask; filtered (ashless filter paper). Available heavy metals (Cd, Pb) measured directly after filtration without digestion.

Maize digestion. ~5 g maize grain weighed into crucible; dried; dry-ashed in muffle furnace. Ash dissolved in 1 mL concentrated HCl on crucible walls, transferred to 25 mL volumetric flask, filtered through ashless filter paper, brought to volume with de-ionized water (AOAC ref [20]). Final solution refrigerated until ICP analysis.

Instrumentation. Agilent 5100 Synchronous Vertical Dual View (SVDV) ICP-OES (Inductively Coupled Plasma — Optical Emission Spectrometry), per APHA [18].

Speciation. Total Pb and total Cd only in all three matrices. DTPA-extractable Pb and Cd separately reported for soils (operationally defined “available” fraction). No arsenic (As)/mercury (Hg)/chromium (Cr) panel.

Risk assessment. DIM and HRI computed per Farrag et al. [10] formulas (parameters above). C_factor 0.085 converts fresh maize weight to dry; D_food_intake 0.345 kg/day adults, 0.232 kg/day children; B_average_weight 70.0 kg adults, 32.7 kg children; RfDs 0.0035 (Pb), 0.001 (Cd) mg/kg/day. HRI < 1 = “safe.”

Statistics. One-way ANOVA at α = 0.05 via SAS [21]. Means with different superscript letters within a column differ at 5 % level (Duncan-style multiple comparison). LSD reported in tables 2–5.

Implications

This source contributes Egyptian field-survey occurrence data for total and soluble Pb and Cd in irrigation water, total and DTPA-available Pb and Cd in agricultural soil, and total Pb and Cd in maize grain across a freshwater-vs-low-quality-water irrigation contrast. Its principal contributions to the wiki evidence pool:

• Bulk maize-grain Pb occurrence at four wastewater-irrigated Egyptian sites (Kafr-Dokhmais 0.55, Al-Nasiria 0.52, Abo-Roash 0.39, Kafr-Hakem 0.26 mg/kg) all exceeding the FAO/WHO Codex 0.20 mg/kg cereal-grain Pb ceiling cited by the authors. Freshwater-irrigated maize Pb was below detection or ≤ 0.04 mg/kg at all five freshwater sites — a 19–30-fold contrast.
• Bulk maize-grain Cd occurrence at the same sites: Al-Nasiria 0.112 mg/kg exceeds the 0.1 mg/kg cereal-grain Cd ceiling cited by the authors; the other three low-quality-water sites are below (0.038–0.076 mg/kg) but still 4–11× higher than the freshwater detectable values (0.009–0.010 mg/kg).
• Pathway evidence. The study triangulates irrigation water composition, soil total and DTPA-extractable burden, and grain uptake at the same nine sites; the 2.4–3.0-fold soil and 19–30-fold grain contrasts under shared geography but different irrigation-water grade isolate irrigation-water contamination as the dominant pathway (with the caveat that the four low-quality sites are not geographically interleaved with the five freshwater sites, so confounding by atmospheric deposition, fertilizer regime, or soil parent material is not ruled out by the study design).
• Risk-assessment parameter set. The paper documents a complete set of Egyptian-context DIM/HRI assumptions (0.085 fresh-to-dry, 0.345/0.232 kg/day adult/child consumption, 70.0/32.7 kg body weight, 0.0035/0.001 RfD) drawn from Farrag et al. [10]. Useful as a reference parameterization for downstream Egyptian-maize exposure synthesis. Note that the 0.345 kg/day adult maize consumption is a high-end value reflecting maize-as-staple Egyptian village contexts and is not transferable to settings where maize is a non-staple commodity.
• Speciation limitation. Total Pb and total Cd only; DTPA-extractable fraction is operationally defined “available” in the soil chemistry sense, not a chemical-speciation result. No As, Hg, Cr, or other-metal panel. The source metadata metals: reflects this ([Pb, Cd]).

Sample size is small (n = 27 per matrix; 9 sites × 3 replicates), but ICP-OES with replicate digestions delivers tight per-site precision (SD typically 1–5 % of mean). The geographic footprint covers three governorates in the Nile Delta and adjacent areas, supporting B-tier generalization to wastewater-irrigated Egyptian agriculture but not to other Egyptian regions or to non-wastewater-irrigation contamination pathways. The paper does not isolate the wastewater source contributions (Branch 62 industrial vs. domestic sewage) and does not measure other crops grown on the same sites, limiting cross-commodity comparison from this study alone.

Related evidence

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