Dearing et al. 2025 — Heavy metals in organic vs non-organic Hawke’s Bay vegetables post-Cyclone Gabrielle
This cross-sectional study assessed heavy metal concentrations in 153 composite vegetable samples from Hawke’s Bay, New Zealand, one year after Severe Tropical Cyclone Gabrielle caused widespread flooding in the region in February 2023. Using ICP-MS in an ISO 17025-accredited laboratory, the study compared organic and non-organic vegetables from flooded and non-flooded growing sites, finding that organic production was independently associated with lower cadmium (p = 0.003, η²p = 0.058) and nickel (p < 0.001, η²p = 0.124) concentrations, while growing on cyclone-flooded land was paradoxically associated with lower rather than higher cadmium (p = 0.030) and nickel (p = 0.024). Mercury was below the LOD (0.01 mg/kg FW) in all 153 samples; arsenic, chromium, and thallium were also below LOD in the overwhelming majority of samples.
Key numbers
All concentrations on a fresh weight (FW) basis. Results given as mean (including sub-LOD values at half-LOD) and, where detectable, mean and max excluding sub-LOD values.
Cadmium (Cd):
- n = 153 (all samples); mean = 0.011 mg/kg FW; median = 0.005 mg/kg FW
- Detectable in 131 of 153 samples; mean (detected) = 0.013 mg/kg FW; median (detected) = 0.007 mg/kg FW; max = 0.093 mg/kg FW (Brassica rapa)
- 1 sample exceeded Codex Alimentarius limit (0.05 mg/kg FW for Brassica): Brassica rapa at 0.093 mg/kg FW
- Genus medians (detected): Allium 0.005; Apium 0.016; Beta 0.015; Brassica 0.006; Lactuca 0.025; Petroselinum 0.013
Lead (Pb):
- n = 153 (all); mean = 0.014 mg/kg FW; median = 0.005 mg/kg FW
- Detectable in only 12 of 153 samples; mean (detected) = 0.123 mg/kg FW; median (detected) = 0.047 mg/kg FW; max = 0.61 mg/kg FW (Lactuca sativa)
- 3 samples exceeded Codex limit (0.1 mg/kg FW): 2 × Lactuca sativa (max 0.61 and 0.37 mg/kg FW) and 1 × Petroselinum crispum
Nickel (Ni):
- n = 153 (all); mean = 0.067 mg/kg FW; median = 0.035 mg/kg FW
- Detectable in 103 of 153 samples; mean (detected) = 0.097 mg/kg FW; median (detected) = 0.055 mg/kg FW; max = 1.5 mg/kg FW (Cucurbita, butternut/grey pumpkin)
Arsenic (total, tAs):
- n = 153 (all); mean = 0.014 mg/kg FW; median = 0.01 mg/kg FW
- Detectable in only 2 of 153 samples; mean (detected) = 0.18 mg/kg FW; max = 0.24 mg/kg FW
Chromium (Cr, total):
- n = 153 (all); mean = 0.02 mg/kg FW; median = 0.01 mg/kg FW
- Detectable in only 4 of 153 samples; mean (detected) = 0.33 mg/kg FW; max = 0.84 mg/kg FW
Mercury (Hg):
- All 153 samples below LOD of 0.01 mg/kg FW; excluded from statistical analysis
Thallium (Tl, not a tracked HMTc analyte):
- Detectable in 10 of 153 samples; mean (detected) = 0.034 mg/kg FW; max = 0.058 mg/kg FW; detected only in Brassica and Lactuca
Organic vs non-organic ANOVA (two-way, log10-transformed):
- Cd: organic significantly lower (F = 9.198, p = 0.003); flooding significantly lower (F = 4.817, p = 0.030); interaction not significant (p = 0.270)
- Ni: organic significantly lower (F = 21.038, p < 0.001); flooding significantly lower (F = 5.164, p = 0.024); interaction significant (F = 6.719, p = 0.010)
Methods (brief)
Cross-sectional survey; samples collected February 2024. 736 individual vegetables combined into 153 composite (representative) samples of approximately 200 g each, one composite per vegetable species per market. Samples washed under running tap water; edible portions only (root vegetables peeled, onions destalked). Dried at 60°C to constant weight, ground, aqua regia digestion, diluted to 2% HNO3 / 1% HCl. ICP-MS analysis in an ISO 17025-accredited laboratory. Sub-LOD values handled as half-LOD for descriptive statistics. Normality tested with D’Agostino-Pearson omnibus; log10-transformation applied; two-way ANOVA with post hoc Tukey used for cadmium and nickel comparisons. Arsenic, chromium, lead, and thallium excluded from ANOVA due to prevalence below LOD.
Reviewers note methodological limitations: composite sampling masks within-sample variability and may underestimate peak exposures; samples were collected at markets rather than directly from fields, introducing uncertainty about true growing location and flood-exposure classification; no pre-cyclone baseline data available; no soil or sediment measurements to substantiate mechanistic flood-effect explanations. Peer review status: 4 reviewers approved with reservations (as of April 2026).
LOD values (mg/kg FW): Hg = 0.01; other metals not explicitly stated but inferrable from the half-LOD substitution and concentration ranges reported.
Implications
Certification: Organic growing status shows a statistically significant but small-to-moderate effect (η²p 0.058–0.124) on cadmium and nickel reduction in this New Zealand regional context. The effect is real but modest and does not eliminate exceedances; cadmium and lead occasionally exceeded Codex limits even in organic samples (the cadmium exceedance was in Brassica rapa, not stratified by organic status in the reported data). This finding supports treating organic certification as a partial but not complete contamination-reduction lever for Cd and Ni in Brassica and leafy vegetables.
Courses: The flooding-reduces-contamination hypothesis is counterintuitive and should be taught as an example of the importance of pre-event baseline data before interpreting post-event surveys. The mechanism proposed (clean sediment dilution of phosphate-fertiliser-accumulated Cd) is plausible but unconfirmed.
App: Hawke’s Bay composite values provide a regional reference for New Zealand vegetables. The very low prevalence of detectable lead, arsenic, chromium, and mercury in this dataset supports low-risk classification for those analytes in NZ market-garden vegetables; cadmium and nickel warrant non-zero estimates, particularly for Brassica and Lactuca genera.