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, wet weight); mean rice tAs = 1.12 mg/kg (1,120 ppb, wet weight). 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 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: ICP-MS (Nexion 350x, PerkinElmer) for Cd and tAs in both soil and rice after HNO3+HClO4 (4:1 v/v) digestion. Atomic fluorescence spectrometry (AFS) used for total Cd/As as a cross-check. 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: dry weight implied for grain after oven-drying, but the paper does not explicitly state wet vs dry weight for grain; values are compared against Chinese MACs (which apply to the food product as-sold, typically on a wet-weight basis for milled rice).
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 from this geologically anomalous region lie well above typical commercial rice ranges (mean rice Cd 381 ppb and rice tAs 1,120 ppb in exceedance-classified sites are elevated relative to most global surveys). HMT&C sourcing guidance for rice should flag geographic provenance from karst mining regions in Yunnan as a higher-risk origin. The finding that soil contamination metrics substantially overpredict actual grain contamination in alkaline karst soils has practical relevance for supply-chain screening.
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: Contributes to tAs and Cd contamination profile for rice from Chinese karst regions; note these values are from a geologically anomalous area and should be flagged separately from global-distribution rice provenance data.