Li et al. 2021 — National reconnaissance of trace metals in Chinese tea gardens (45 gardens, 15 provinces)
This is the first published national-scale soil and tea plant survey of potentially toxic elements (PTEs) across China’s tea-producing regions, covering 225 paired soil and tea plant samples from 45 tea gardens in 15 provinces spanning East, South, Central, and Southwest China. Collected in May 2019. The overall finding is that Chinese tea garden soils are generally within risk-control standards, but Cd and As have the highest geo-accumulation indices and 2.22% and 4.44% of soils are moderately to heavily contaminated with As and Cd respectively. In tea leaves, Pb concentrations in young leaves (YTL) universally met organic tea thresholds (<2.0 mg/kg); however, 26.7% of mature tea leaf (MTL) samples exceeded the organic threshold for Pb. Cd had the highest Igeo value, making it the priority monitoring element.
Key numbers
Rhizosphere soil (RS) mean concentrations (mg/kg, Table 1):
- As: 20.8; Cd: 0.21; Cr: 74.9; Cu: 26.0; Mn: 419; Pb: 23.6; Zn: 87.5; Ni: 31.9
- Cd: 1.94× Chinese background; As: 1.74× background; Ni: 1.19× background
Young tea leaves (YTL) concentrations (mg/kg, Table 2):
- As: mean 0.08, range 0.00–0.30; Cd: mean 0.06, range 0.02–0.31
- Cr: mean 0.91, range 0.26–5.06; Cu: mean 11.3; Mn: mean 740; Pb: mean 0.66, range 0.07–1.84
- Zn: mean 43.0; Ni: mean 8.57; 90th percentile Pb in YTL: 1.17 mg/kg
Mature tea leaves (MTL) concentrations (mg/kg, Table 2):
- As: mean 0.44; Cd: mean 0.09; Cr: mean 2.59; Cu: mean 6.83; Mn: mean 2830; Pb: mean 13.6; Zn: mean 4.37; Ni: mean 5.98
- 26.7% of MTL Pb samples exceeded organic tea threshold (2.0 mg/kg per CFDA 2017)
- 17.8% of MTL met organic tea Pb threshold; remaining 82.2% approached but did not all exceed
Tea roots (TR) concentrations (mg/kg, Table 2):
- As: mean 1.22; Cd: mean 1.32; Pb: mean 12.51; Mn: mean 364; Ni: mean 8.07
Ecological risk index (RI) across 45 gardens: ranged 18.0 to 292.0, mean 90.0 (low to moderate ecological risk). Cd had highest E(i) value among all metals in RS.
Methods (brief)
ICP-MS (Perkin Elmer NEXION300XX) with detection limits As 0.58, Cd 0.08, Cr 0.14, Cu 0.06, Mn 0.17, Pb 0.07, Zn 0.63, Ni 0.43 µg/L. Microwave digestion (CEM Mars-X500). Soil samples dried, ground, sieved (2mm mesh). Tea samples freeze-dried 72 hrs. At least 15 subsamples per garden; representative composites. Spatial distribution mapped via Arc-GIS 10.2.
Limitations
Cross-sectional single-season (May 2019 only) survey; seasonal variation in metal concentrations not captured. YTL and MTL from same garden in same season — no harvest-timing variation assessed. Tea root data included but roots not consumed, limiting direct food-safety relevance. No speciation for As (total only) or Cr (total only).
Implications
- Certification: Pb in mature tea leaves is a concern — 26.7% of MTL samples in Chinese gardens exceeded the organic tea threshold. Young leaf Pb is uniformly within limits, which is relevant since most commercial tea is made from young leaves. As and Cd in Chinese tea garden soils are significantly above background and point to accumulation risk in older plant material.
- Courses: Key national-scale overview for Chinese tea. Demonstrates geographic heterogeneity across 15 provinces and that “Chinese tea” is not monolithic.
- App: Young leaf Pb mean 0.66 mg/kg and Cd mean 0.06 mg/kg are appropriate reference values for Chinese-origin green tea (young flush). Flag for synthesis pass on tea.