Baseline concentration of heavy metals in agricultural soils

Rueda-Saa and colleagues measured total metals in agricultural soils from 489 sites across 13 Colombian watersheds. Concentrations differed substantially by watershed, with the highest mean nickel concentration in Pescador. These are upstream soil measurements: the study did not measure harvested crops or establish soil-to-crop transfer factors.

Key numbers

The sample frame comprised 489 sites; 44% of samples came from sugarcane cultivation and 56% from other crops (p. 3). Laboratory analyses occurred in 2017–2019; collection dates and separate annual results are not stated. All concentrations below retain the paper’s mg/kg unit. The paper does not explicitly identify a dry- or wet-weight basis.

Overall soil summaries

Table 1, p. 4. Zeros and sub-detection values are source-reported entries, not proof of absence. The table footnote identifies starred values as below detection; the substitution rule used to calculate summaries is not stated.

Geographic breakdown

Mean concentrations by watershed, Table 2, pp. 6–7. RUT means Roldanillo–Unión–Toro. Every watershed is kept separate; no product concentration is inferred from these soil values.

Table 2 additionally provides medians, minima and maxima for all seven analytes in every watershed. For example, Pescador nickel has median 121.00, minimum 23.00 and maximum 226.00 mg/kg; San Pedro cadmium has median 0.19, minimum 0.00 and maximum 2.77 mg/kg. All 364 watershed statistic cells, 63 overall statistic cells, 84 soil-order statistic cells and seven detection limits are preserved in the structured extraction.

Methods (brief)

The authors used simple random sampling across mapped agricultural areas. Disturbed composite soil samples were homogenized from approximately 3 cm thickness, 20 cm width and 30 cm depth. Methods lists HCl–HNO3 wet digestion and graphite-furnace atomic absorption for cadmium (Cd), nickel (Ni), lead (Pb) and Co. It supplies a chromium (Cr) detection limit but omits Cr from that method’s analyte enumeration, so a complete Cr method is not independently specified. arsenic (As) and mercury (Hg) are described as hydride-generation atomic absorption measurements (p. 3); no arsenic, mercury or chromium speciation is reported.

Detection limits in mg/kg were Cd 0.02, Cr 1, Ni 1, Pb 0.75, Co 0.10, As 0.10 and Hg 0.10. The paper reports neither a clear moisture-basis statement nor a full recovery/reference-material quality-control record. Source comparisons to background ranges are literature comparisons, not verified regulatory instruments.

Implications

Evidence Fitness: EF-4 for product-occurrence use. The study directly supports geographic variation in agricultural-soil total metals and can inform investigations of upstream contamination. It cannot establish edible-crop concentrations, transfer factors, metal species concentrations, year-on-year trends, or attribution to a particular polluter. Missing crop measurements do not invalidate its supply-chain evidence.

Related evidence

• Soil-to-plant transfer of heavy metals
• Arsenic, Total
• Mercury, Total
• Cadmium
• Chromium
• Nickel
• Lead
• Cobalt

Update history

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