Overview
The authors analyzed soils, irrigation water and nine vegetables from one urban farm in Sekondi-Takoradi, Ghana, irrigated with shallow wells and a stream dominated by municipal waste. Cadmium, iron, mercury and zinc were measured by AAS on a fresh-weight basis; chromium and nickel were below detection in every vegetable. Lead and arsenic were not measured. All vegetable means were below the cited guideline limits.
Key numbers
- Cadmium (mg/kg fresh weight): carrot 0.035 ± 0.010, Corchorus 0.024, cabbage (Brassica oleracea leaf) 0.018, Brassica rapa leaf 0.008 (Table 2).
- Mercury: Corchorus 0.013, carrot 0.011, Brassica oleracea leaf 0.001 mg/kg fresh weight (Table 2).
- Chromium and nickel < 0.001 mg/kg in all vegetables (Table 2).
Methods (brief)
Edible parts were washed, freeze-dried and acid-digested; mercury was measured by cold-vapor AAS and the other metals by flame AAS, with results reported on fresh weight. Detection limits were 0.001 to 0.010 mg/kg. Spike recoveries were 70 to 102%, and NIST reference materials are listed. Table 2 also prints a fitted 95th percentile for each mean.
Evidence Fitness
One farm with a contaminated irrigation source, so the results describe a specific growing setting, not retail supply. Results are triplicate means with fitted percentiles; several printed values are internally inconsistent (an amaranth cadmium mean of 0.00027 against a 95th percentile of 0.027, mercury 95th percentiles ten times their means, and a mercury overall mean above the maximum). Lead was not measured. Brassica oleracea and Brassica rapa leaves are close Brassica proxies for collard-greens but are not collards; the single-species results are a proxy for vegetables-mixed.