Wei et al. 2025 — Cd and As in milled rice from karst Yunnan, field survey of 125 paired soil-rice samples

This study measured cadmium (Cd) and total arsenic (tAs) concentrations in milled rice and paired paddy soils from 125 field sampling points in the Gejiu-Mengzi area of Yunnan Province, a karst region in southwestern China characterized by naturally elevated geochemical backgrounds for heavy metals. The primary research objective was to derive region-specific soil safety thresholds that perform better than the Chinese national screening values (RSVs) in neutral-to-alkaline karst soils, where the national one-size-fits-all standards produce false-positive misclassification rates of 52.0-66.4%. Rice grain Cd and tAs concentrations from the field survey are the food-matrix occurrence data the threshold derivation rests on.

Key numbers

Sample set: n=125 paired soil-rice samples; stratified random sampling across a 1×1 km to 3×3 km grid during September 2018; milled rice (dehusked, pulverized, sieved at 0.149 mm).

Rice grain concentrations from the four-quadrant regulatory analysis (soil MAC exceedance vs rice MAC exceedance):

• Quadrant I (soil unsafe, rice unsafe; 14.4-17.6% of samples): mean rice Cd = 0.381 mg/kg (381 ppb); mean rice tAs = 1.12 mg/kg (1,120 ppb). These samples cross both the national MACs (Cd: 0.2 mg/kg; As: 0.5 mg/kg in GB 2762-2022) and the RSVs. The methods section oven-dries rice grains before analysis, so these occurrence values should not be labeled wet weight from this source alone.
• The paper does not report a single mean rice Cd or tAs across all 125 samples explicitly in the main text; the quadrant-stratified values above are the primary reported figures.

Soil concentrations (mean across all 125 samples): Cd 2.16 mg/kg; tAs 60.18 mg/kg. Soil pH range 6.56-8.25 (mean 7.65); 74.4% weakly alkaline.

Derived regional thresholds for paddy soil:

• Safety threshold (ST) for Cd: 4.54-7.12 mg/kg (pH-dependent); far above national RSV of 0.6-0.8 mg/kg.
• ST for tAs: 91.43-92.30 mg/kg; far above national RSV of 20-25 mg/kg.
• Hazard threshold (HT) for Cd: 9.66-10.12 mg/kg; 2.5-3.2x the national RIV.
• HT for tAs: 96.90-97.15 mg/kg; slightly below national RIV of 100-120 mg/kg.

Applying the regional ST-HT framework improved soil classification accuracy from 28.1-95.6% to 71.8-100%.

Methods (brief)

Analytical method: the source reports mixed-acid digestion followed by atomic fluorescence spectrometry, and HNO3+HClO4 (4:1 v/v) digestion coupled with ICP-MS (Nexion 350x, PerkinElmer) for Cd and tAs in soil and rice. Quality control: soil standard GBW-07405 and plant standard GBW-10045 inserted every 10 samples with blank runs; recovery rates monitored. Matrix: milled rice (rice grain, dehusked). Metal species: total Cd; total As (tAs; no inorganic/organic speciation performed). Basis: rice grains were oven-dried before dehusking, pulverizing, and sieving; do not relabel the paper’s rice values as wet weight.

Note on speciation: the paper measures total arsenic only; no iAs/DMA speciation was performed. All arsenic values are tAs and must not be interpreted as iAs.

Implications

Certification: The rice Cd and tAs data provide source-level occurrence context for a geologically anomalous karst rice-growing region in Yunnan. The mean rice Cd of 381 ppb and mean rice tAs of 1,120 ppb are quadrant-I values from exceedance-classified sites, not whole-sample means. The finding that soil contamination metrics substantially overpredict actual grain contamination in alkaline karst soils is relevant to interpreting soil-screening evidence alongside rice-grain measurements.

Courses: Illustrates how geochemical background (naturally elevated metals in karst parent rock) interacts with soil chemistry (pH, Fe/Mn oxides) to modulate bioavailability; a clear case study for the gap between soil-total-metal and plant-available-metal framing.

App: Treat the rice-grain Cd and tAs values as region-specific occurrence context for Chinese karst systems. Do not merge the quadrant-I values into global-distribution rice profiles without preserving their exceedance-classified sampling context.

Wiki pages this source may touch

• rice
• cadmium
• arsenic-total

Verification notes

• Cross-vendor audit (Codex, 2026-05-17) corrected matrix terms to broad current vocabulary (rice, soil), removed a non-existent China GB 2762 regulation wikilink, replaced the umbrella arsenic link with arsenic-total, and removed wet-weight labeling that the source methods do not support.
• Strict Part 12 recheck found no sampled-product brand names. Instrument and software vendor names in the methods are retained under Part 12 Exception 2 for scientific reproducibility.
• The reported rice values are not whole-study means; 0.381 mg/kg Cd and 1.12 mg/kg tAs are means for the quadrant-I soil-unsafe/rice-unsafe subset.

Page history

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