Heavy Metal Contamination of Guizhou Tea Gardens: Soil Enrichment, Low Bioavailability, and Consumption Risks

The content and health impact of harmful heavy metals in agricultural products from strong geological background concentration areas have received increasing attention. To investigate the impact of soil heavy metal contamination on the tea plantation gardens of Guizhou Province, a major tea-producing area with strong geological background concentrations in China, a total of 37 paired soil–tender tea leaf samples (containing one bud and two leaves) were collected and analyzed for eight harmful heavy metals. The results showed that the average contents of Hg, As, Pb, Cd, Cr, Ni, Sb, and Tl in the surface soil (0–20 cm) were 0.26, 23.9, 37.9, 0.29, 75.9, 37, 2.78, and 0.84 mg/kg, respectively. The majority of the soil Hg, As, Pb, Sb, and Tl levels exceeded their background values for cultivated land soil in Guizhou Province to some extent. The geo-accumulation index revealed that Sb and As are the main pollutants of tea garden soil. The average contents of Hg, As, Pb, Cd, Cr, Ni, Sb, and Tl in the tea leaves were 4, 49, 310, 55, 717, 12,100, 30, and 20 μg/kg (on a dry weight basis), respectively, all of which were significantly lower than their national recommended limits for tea. The bioconcentration factors of these eight heavy metals in tea leaves were relatively low when compared with those in soil, ranging between 0.003 (for As) and 0.603 (for Ni). The health risk assessment indicated that the total hazard quotient (THQ) due to drinking tea was in the order of Tl > Ni > As > Pb > Cd >Sb > Hg > Cr, with both the THQ for each heavy metal and the health risk index (HI) being less than 0.29, indicating that the risk of exposure to these heavy metals through drinking Guizhou green tea is low. Although some harmful heavy metals are present in the tea garden soil of Guizhou, their bioavailability for young tea leaves is extremely low. This may be related to the physical and chemical properties of the soil, such as the high proportion of organic matter (up to 9%) which strongly binds with these elements.