Analysis of tea quality and assessment of heavy metal content status in tea plantations of southern Shaanxi Province, China

Abstract: Accumulation of heavy metals in agricultural soils tends to increase crop uptake of heavy metals, and can adversely impact human health through food chains. In this study, we assessed the tea quality, heavy metal status, and the potential ecological risk of tea plantation soils and leaves in Hanzhong city, southern Shaanxi province, China. A total of 330 topsoil samples and 33 leaves samples were collected to analyze the concentrations of seven heavy metals (Pb, Zn, Cu, Cr, Hg, As, and Cd) and the pH level. Also, the tea quality and the content of heavy metals in tea leaves and leachate in 33 leaves samples were determined and analyzed. Results showed that the contents of tea polyphenols ranged from 22.23% to 27.55%, and the mean content of polyphenols was 25.58%. The contents of catechins ranged from 18.48% to 22.37%, the mean content of catechins was 20.74%. The contents of free amino acids ranged from 3.37% to 6.42%, and the mean amount of free amino acids was 4.86%. The contents of theanine ranged from 1.70% to 3.18%, and the mean content of theanine was 2.45%. The contents of heavy metals in tea leaves ranged from 0.02 mg/kg to 0.19 mg/kg for Cd, 3.18 mg/kg to 20.16 mg/kg for Cu, 0.04 mg/kg to 2.65 mg/kg for Pb, 0.09 mg/kg to 4.89 mg/kg for Cr, 0.02 mg/kg to 0.30 mg/kg for Hg, 0.66 mg/kg to 1.16 mg/kg for As, 34.82 mg/kg to 55.38 mg/kg for Zn. The heavy metals in tea leave samples almost conform to the relevant standards of pollution-free food tea except that the content of Hg in one sample exceeded the relevant standards of pollution-free food tea of 1.66%. The content of Cu was conformed to the relevant standards of organic tea. Among the contents of seven heavy metals, only Zn, Cu, and Cr were detected in the leaching solution, and the average leaching rates were 32.65%, 24.96%, and 10.50%, respectively. The soil analysis showed that the contents of heavy metals in soils ranged 0.03-0.32 mg/kg for Cd, 7.48-29.03 mg/kg for Cu, 2.45-29.16 mg/kg for Pb, 0.83 -27.76 mg/kg for Cr, 0.08-0.38 mg/kg for Hg, 2.31-13.97 mg/kg for As, 25.73-146.94 mg/kg for Zn. The averaged contents of Cd, Cu, Pb, Cr, Hg, As, and Zn were 0.11,16.42, 10.03, 12.38, 0.20, 6.89, and 87.61 mg/kg, respectively. The coefficient of variation of the different elements ranged from 29.16% to 52.24% and was 49.30% for Cd and 48.55% for Hg. In 5.76% of the samples, Cd concentrations exceeded the risk screening value of the environmental quality agricultural land soil pollution risk control standard (GB 15618-2018). The average single pollution index of Pb, Zn, Cu, Cr, Hg, As, and Cd were 2.32, 0.83, 4.26, 0.41, 44.46, 9.37, and 33.89, only the average single pollution index of Hg was over 40. The comprehensive potential ecological risk index ranged from 23.58 to 226.90, and the average potential ecological risk index was 95.4, the total samples at a low level of potential ecological risk. Thus, the tea plantations were in a low potential ecological risk level and good quality of tea leaves for 33 samples.