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

Abstract

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.

1. Introduction

Tea is one of the most important beverages for humans, with over 2 billion people worldwide drinking it [1] and global consumption of 18–20 billion cups per day [2,3]. Drinking tea provides beneficial minerals and compounds for human health, such as zinc, selenium, tea polyphenols, catechins, and caffeine. As a result, drinking tea may prevent cardiovascular and cerebrovascular diseases, reduce blood lipids, and combat cancer [4,5]. However, tea products may also contain harmful elements, such as toxic heavy metals [6], which can enter the human body through tea consumption, thereby posing health risks. Therefore, understanding the current environmental status of heavy metals in tea gardens is crucial for both sustainable tea cultivation and the health of tea drinkers. According to United Nations statistics, China’s average annual tea production has accounted for 35–40% of the world’s total tea production over the past 10 years [7]. In China, over 80% of both tea production and consumption is related to green tea [8]. In 2022, the total area of tea gardens in 18 major tea-producing provinces of China was 3.33 million hectares, and the total output of dry tea was approximately 3.18 million tons [9], with the four provinces of Fujian, Yunnan, Sichuan, and Guizhou being the leading tea producers.

Guizhou, located in the heart of the Yunnan–Guizhou Plateau in Southwest China, is considered to be the birthplace of ancient tea trees around the world. The tea industry plays a vital role in the economic development of Guizhou Province. In 2022, the tea plantation area in Guizhou Province was 0.47 million hectares [10]. Unfortunately, Guizhou is also situated in a low-temperature hydrothermal mineralization area, where the soil mercury (Hg), antimony (Sb), cadmium (Cd), lead (Pb), and thallium (Tl) levels are considerably elevated [11,12,13]. Environmental pollution caused by Tl and Sb has been reported in the mining area of Guizhou, mainly through the consumption of cabbage and other grains and vegetables [14,15], as these plants have an enhanced ability to accumulate heavy metals in their bodies. In addition, there have been a few reports on Hg and Cd pollution in rice [16]. Therefore, as Guizhou Province is a major tea-producing province and an area with high soil heavy metal contamination, it is worth conducting a detailed study on the distribution of harmful heavy metals in the tea produced in this province. Previous research on heavy metals in tea garden soil and tea systems in Guizhou Province mainly focused on a single production area [17,18,19] or a limited number of elements [20,21,22], while studies on multiple areas and elements, like those studying Sb and Tl concentrations in tea leaves, were rarely reported. Tl is a trace element with severe ecotoxicity, and its toxic effects in organisms include gastrointestinal dysfunction and polyneuritis [23]. Sb is recognized as a genotoxic element, and Sb exposure can cause gastrointestinal symptoms, respiratory disease, cardiovascular changes, and reproductive effects [24].

Based on this rationale, this study systematically investigates the contents of eight harmful heavy metals, including Hg, arsenic (As), Pb, Cd, chromium (Cr), nickel (Ni), Sb, and Tl, in the soil and young tea leaves from four major tea production areas in Guizhou and evaluates the potential health risks of drinking tea to assess the heavy metal pollution status of both the soil and tea of Guizhou. This study aims to provide a scientific basis for the distribution of the aforementioned heavy metals in tea gardens and a contamination and health safety evaluation of the tea of Guizhou Province and, finally, to provide the basic data for the green production and management of tea gardens.

2. Materials and Methods

2.1. Sample Collection

This study collected soil and tea leaf samples from four major tea-producing areas in Guizhou Province: Zunyi City (ZY), Qiannan Prefecture (QN), Qiandongnan Prefecture (QDN), and Guiyang City (GY). The sampling locations are shown in Figure 1, and photos of the tea gardens are shown in Figure S1. Specifically, samples 1–11 and 12–21 were collected in Meitan and Fenggang counties in Zunyi City (ZY), respectively; samples 22–28 were collected from Maojian Town in Duyun City and Yunwu Mountain Town in Guiding County in Qiannan Prefecture (QN); samples 29–31 were collected from Leigong Mountain in Leishan County in Qiandongnan Prefecture (QDN); and samples 32–34 and 35–37 were collected from Hefeng town and Nannong town in Kaiyang County of Guiyang City (GY), respectively. A total of 37 pairs of tea leaf and tea garden soil samples were collected. The tea leaves, consisting of one bud and two tender leaves and having germinated less than 2 weeks prior, were collected from a 100 square meter area within a tea garden in early April 2022. The tea tree varieties included Qiancha No. 1, Wuniuzao, and other early tea varieties. Soil samples were collected from 3–5 points in the root zones at a depth of 0–20 cm in the tea growth area to create a composite sample. The soil in Guiyang and Zunyi is calcareous, developing from carbonate rocks and having a texture of clay loam, but it is sandy loam for the sites in Qiannan and Qiandongnan, which developed from sandstone. The soil sample was brought to the laboratory and air-dried, and debris such as stones and branches were removed from the sample. It was then finely ground and sieved through a 100-mesh (0.15 mm) nylon sieve. The tea leaf samples were washed with tap water, rinsed with deionized water, heated in an oven at 180 °C for 7 min, and then finally dried at 80 °C. This preparation process followed the local green tea production process to prevent the tea leaves from turning brown, and the dried leaves were then ground using a plant grinder so that they could pass through a 60-mesh (0.28 mm) sieve.

2.2. Chemical Analysis

The mercury in the soil and tea samples was measured using a DMA-80 direct mercury analyzer (Milestone Inc., Milan, Italy), while the contents of Sb and As were determined using acid digestion (HNO₃) coupled with the cold atomic fluorescence method (Beijing Haiguang Model 9530, Beijing, China). Other heavy metals were digested using a mixed acid containing nitric acid and hydrofluoric acid and measured using an inductively coupled plasma spectrometer (ICP-MS, model Thermo Fisher X2, Thermo Fisher Scientific, Waltham, MA, USA). The quality control of the analysis process was performed using standard substances and duplicate samples, as well as blanks. The plant leaf standard substances were tea (GBW10016a), green tea (GBW10052a), tobacco leaves (GBW10236), and citrus leaves (GBW10020). The soil standard substances used were GBW07405, GBW07427, and GBW07430. These standard substances provide environmental media similar to tea garden soil and tea leaves and cover a wide range of harmful element levels. The average recovery of the eight elements in the plant leaf standard samples was as follows: Hg = 100.4 ± 2.1%; As = 93.3 ± 3.2%; Pb = 112.4 ± 6.7%; Cd = 105.1 ± 3.3%; Cr = 110.2 ± 8.3%; Ni = 114.3 ± 16.3%; Sb = 108.7 ± 10.6%; and Tl = 105.7 ± 8.7%. The average recovery of eight elements in the soil standard samples was as follows: Hg = 100.5 ± 2.8%; As = 86.2 ± 0.9%; Pb = 106.0 ± 2.6%; Cd = 110.7 ± 7.1%; Cr = 104.0 ± 1.5%; Ni = 104.7 ± 15.4%; Sb = 110.6 ± 10.5%; and Tl = 111.6 ± 2.5%. The relative deviation of the duplicate samples was less than 5%. Additionally, the soil organic matter (SOM) and pH were also measured. The SOM was measured using the sulfuric acid–potassium dichromate method [25], whereas the pH was determined using the water extraction electrode method (with a soil-to-water ratio of 1:2.5) (SX836 pH meter, Shanghai Sanxin Instrument Factory, Shanghai, China).

2.3. Soil Pollution Assessment and Tea Bioconcentration Factor Calculation

The geo-accumulation index method and bioconcentration factors (BCFs) were used to comprehensively evaluate the heavy metal contamination or enrichment in both the tea garden soil and tea leaves in this study. In addition, Chinese standards for agricultural soil environment or tea plantations, such as the environmental standard for organic tea plantations (NY5199-2002) and Chinese national standards for harmful elements in tea (NY 659-2003, GB 2762-2022), were also referenced to evaluate both the soil environmental quality and tea quality in this study.

2.3.1. Geo-Accumulation Index Method

The geo-accumulation index method, proposed by Muller in 1969, is used to quantitatively evaluate the pollution levels of individual heavy metals in soil [26]. The formula used to calculate the geo-accumulation index is as follows:

$${\mathrm{I}}_{\mathrm{geo}}={\mathrm{Iog}}_2{\displaystyle \frac{Cn}{K\times \mathrm{B}\mathrm{n}}}$$

(1)

where C_n is the measured concentration of an element in the sample (mg/kg) and Bn is the background reference value (mg/kg) of that element in soil. The background values for the soil Hg, As, Pb, Cd, Cr, Ni, Tl, and Sb in Guizhou Province were taken to be 0.13, 13.48, 33.57, 0.4, 39.3, 98.98, 0.68, and 2.24 mg/kg, respectively, based on a recent study [27]. K is an adjustment constant, which is usually 1.5. The I_geo index results classify heavy metal pollution into seven levels: I_geo ≤ 0, meaning no pollution; 0 < I_geo ≤ 1, meaning mild pollution; 1 < I_geo ≤ 2, meaning moderate pollution; 2 < I_geo ≤ 3, meaning heavy pollution; 3 < I_geo ≤ 4, meaning severe pollution; 4 < I_geo ≤ 5, meaning severe pollution; and I_geo > 5, meaning extremely serious pollution.

2.3.2. Bioconcentration Factor (BCF) of Trace Metals in Tea Leaves

The bioconcentration factor (BCF) is an important indicator of crop pollution. Since the concentration of toxic elements varies between plant species, the BCF is commonly used to estimate the quantity of metals transferred from soil to plants. Specifically, the BCF refers to the ratio of the heavy metal concentration in a plant to that in soil [28]. In this study, the concentrations of toxic elements in the tea leaves were compared to those in soil with the following calculation formula:

BCF = C_leave/C_soil

(2)

where C_leave and C_soil represent the concentrations of toxic elements in tea leaves (on a dry weight basis) and the corresponding soil (mg/kg), respectively.

2.4. Health Risk Assessment

2.4.1. Calculation of Estimated Daily Intake

Drinking tea may pose health risks if the tea contains high levels of toxic elements. Therefore, the estimated daily intake (EDI) of each element is calculated using the following equation:

EDI = (C × F_IR × T_R)/(W_AB × 1000)

(3)

where C is the concentration (mg/kg) of each metal in the tea leaves. In this study, C is taken as the upper limit of the 95% confidence interval for the mean ( upper confidence limit (UCL)) of each studied element to yield an estimate of the “reasonable maximum exposure”. F_IR is the daily tea intake rate (gram/person/day). In this study, we take the 95% upper confidence interval of the tea consumption rate to assess the reasonable maximum exposure, and the 95% upper confidence interval of the tea consumption rate is 9.5 g/person/day based on two large-scale surveys on the daily tea drinking habits of Chinese people [29,30]. One of the surveys was conducted in five provinces in Southwest China, including Guizhou [30]. T_R is the transfer rate of elements in the tea leaves into the tea infusion during the brewing process. In this study, the transfer rates of different heavy metals are as follows: Pb = 7.11%; Hg = 32.83%; As = 23.83%; Cd = 14.18%; Cr = 11.45%; Ni = 67.71%; Sb = 11.78%; and Tl = 33.1%. These transfer rates were based on previous studies [17,22,31,32]. Finally, W_AB is the average body weight, with 60 kg used for Chinses adults [33].

2.4.2. Calculation of Target Hazard Quotient

The target hazard quotient of one specific heavy metal (THQ_elementi) to quantify the potential non-carcinogenic effects of a single metal can be evaluated using the following equation [33]:

THQelementi = EDI/RfD_i

(4)

where RfD is the oral reference dose (mg/kg/d) of element i and EDI is the daily average exposure dose of each metal (mg/kg/d). In this study, the oral reference dose (RfD) for different elements was set as follows: Cd = 5.0 × 10⁻⁴ mg/kg/d; Pb = 1.5 × 10⁻³ mg/kg/d; Hg = 3.0 × 10⁻⁴ mg/kg/d; As = 3.0 × 10⁻⁴ mg/kg/d; Cr = 1.5 mg/kg/d; Ni = 0.02 mg/kg/d; Tl = 1.0 × 10⁻⁵ mg/kg/d; and Sb = 4 × 10⁻⁴ mg/kg/d [34,35].

If a THQ value for one element is less than one, then this means there is no significant risk of non-carcinogenic effects on the exposed population for this element, but if the THQ value rises, then the non-carcinogenic effects would also increase.

2.4.3. Calculation of Hazard Index of Drinking Tea

The hazard index (HI) was adopted to assess the total non-carcinogenic health risks arising from exposure to multiple metals [17], and it is the sum of the target hazard quotient of all studied elements based on the assumption of the toxic effects of heavy metals in tea on human health being additive rather than synergistic or antagonistic. Hence, HI is calculated as follows in this study:

HI = THQ_Hg + THQ_As + THQ_Pb + THQ_Cd + THQ_Cr + THQ_Ni + THQ_Sb + THQ_Tl

(5)

Therefore, if the calculated HI is less than one, then this indicates the exposure dose being below the threshold for adverse effects, and there is no non-carcinogenic risk, while if HI is greater than one, then this means an exposure dose exceeding the threshold for adverse effects and heavy metals and being highly likely to negatively impact human health.

2.5. Data Processing and Software

Statistical analysis, correlation analysis, one-way analysis of variance (ANOVA), and principal component analysis were performed using the software IBM SPSS 22.0 (International Business Machines Corporation, Amonk, New York, USA). Figures were created using the software Origin 8.6 (OriginLab, Northampton, MA, USA) and ArcGIS 10.8 (Environmental Systems Research Institute Inc., Redlands, CA, USA).

3. Results and Discussion

3.1. Physicochemical Properties of and Heavy Metals in Tea Garden Soil

The range of soil organic matter (SOM) in the surface soil was between 1.11% and 9.18%, with an average value of 3.84% (

Table 1. Statistics of heavy metal concentrations (mg/kg) and physical and chemical parameters of tea garden soil in Guizhou Province.

Sampling Area	Parameters	SOM (%)	pH	Hg	As	Pb	Cd	Cr	Ni	Sb	Tl
Zunyi (ZY, N = 21)	Mean	3.00c	4.23a	0.17b	26ab	39ab	0.28a	89a	41ab	1.87b	0.78b
	SD	0.91	0.36	0.09	12	14	0.14	27	17	0.53	0.32
Guiyang (GY, N = 6)	Mean	3.60bc	4.69a	0.73a	33a	53a	0.39a	74ab	66a	4.68a	1.62a
	SD	0.99	0.27	0.64	17	16	0.22	16	51	3.61	1.05
Qiandongnan (QDN, N = 3)	Mean	5.25ab	4.69a	0.13b	12b	30b	0.22a	40c	14b	5.26a	0.55b
	SD	2.34	0.61	0.04	4	6	0.03	4	5	3.21	0.05
Qiannan (QN, N = 7)	Mean	6.00a	4.33a	0.19b	14b	26b	0.25a	55bc	9b	2.84ab	0.48b
	SD	2.11	0.50	0.08	4	8	0.12	14	5	0.92	0.17
Total (N = 37)	Min. Max.	1.11 9.18	3.64 5.54	0.05 1.61	7 54	14 79	0.10 0.72	33 165	3 164	1.05 11.29	0.15 3.05
	Mean	3.84	4.36	0.26	24	38	0.29	76	37	2.78	0.84
	SD	1.83	0.45	0.34	14	16	0.15	28	31	2.23	0.62
Background of surface soil in Guizhou province [27]		3.07	6.09	0.13	13	34	0.40	99	39	1.35	0.68
Background of surface soil in China [37]		3.10	6.70	0.065	11	26	0.10	61	27	1.21	0.62
Environmental standard for organic tea production in China (NY 5199-2002)		--	4.0–6.5	0.150	40	50	0.20	90	--	--

Note: Different letters indicate the same index with a significant difference between different areas (LSD test) (p < 0.05). The same applies below.

). This value is higher than the background soil value in China (3.10) and Guizhou (3.07%), likely due to the accumulation of organic matter in the tea garden resulting from dead branches, fallen leaves, and pruned branches. The SOM values in Zunyi (3.0%) and Guiyang (3.6%) were lower than those in the other two areas (5.2–6.0%), which may be attributed to the difference in management of the tea gardens. The soil pH in the tea gardens across Guizhou ranged from 3.64 to 5.54, with an average value of 4.36. This is significantly lower than the background value of Chinese soil (6.70) and Guizhou agricultural soil (6.10) by approximately two units. Moreover, this figure is also lower than the national average pH (4.68) of tea garden soil in China [36]. Tea trees are acid-loving plants that can only grow normally in acidic soil. The suitable soil pH for tea cultivation ranges from 4.0 to 6.5, with an optimal range of 4.5–5.5 [37]. In this study, about 20% of the tea garden soil had a pH that was too low (e.g., pH < 4.0) and not conducive to the growth of tea trees. The acidification of tea garden soil is mainly caused by a series of factors. The first factor is the growth and metabolism of tea trees, which absorb salt ions in the soil during growth and release free hydrogen ions (H⁺) through metabolic processes. Additionally, the root metabolism of tea trees and the decomposition of dead branches and leaves can also exacerbate soil acidification. The second factor is the use of chemical fertilizers. The excessive use of acidic fertilizers and the imbalanced application of nitrogen fertilizers can exacerbate soil acidification [37]. The soil total nitrogen in the tea gardens in this study was in the range of 2.11–3.51 g/kg, with an average value of 2.38 g/kg. This indicates a significantly high nitrogen level and the extensive use of nitrogen fertilizers. The third factor is the deposition of atmospheric acid rain. In recent decades, particularly from the 1980s to the 2000s, Guizhou experienced severe acid rain due to the combustion of high-sulfur coal, with the precipitation pH sometimes becoming as low as 4.0 [38].

In this study, the soil concentrations of different harmful trace metals were as follows (Table 1). The Hg concentration ranged from 0.05 to 1.61 mg/kg, with an average concentration of 0.26 mg/kg; the As concentration ranged from 7.01 to 54.13 mg/kg, with an average concentration of 23.89 mg/kg; the Cd concentration ranged from 0.10 to 0.66 mg/kg, with an average concentration of 0.29 mg/kg; the Pb concentration ranged from 14.11 to 79.03 mg/kg, with an average concentration of 37.90 mg/kg; the Ni concentration ranged from 3.2 to 163.7 mg/kg, with an average concentration of 37.0 mg/kg; the Tl concentration ranged from 0.15 to 3.05 mg/kg, with an average concentration of 0.84 mg/kg; the Cr concentration ranged from 32.68 to 164.85 mg/kg, with an average concentration of 75.93 mg/kg; and the Sb concentration ranged from 0.05 to 11.29 mg/kg, with an average concentration of 2.75 mg/kg. Compared with the background values of cultivated land in Guizhou ([27,39]; Table 1), the average soil contents of Hg, As, Pb, Sb, and Tl were 2.00, 1.78, 1.13, 1.23, and 1.24 times higher in this study, respectively. Meanwhile, the average content of Ni was similar to that of the soil background in Guizhou (Table 1). Notably, the depletion of soil Cd and Cr compared with the provincial soil background reached 27% and 23%, respectively. This was probably due to the severe soil acidification that accelerates the leaching of these elements from soil, since the migration (or the exchangeable fractions) of soil Cd and Cr is pH-sensitive and increases sharply under acidic conditions [40,41]. For other elements, the high proportion of soil organic matter (up to 9%) might effectively promote their adsorption, inhibiting their leaching out and promoting their accumulation in the rhizosphere soil. Nevertheless, the agriculture soil in Guizhou was generally rich in Hg, Cd, Pb, and Cr when compared with the national average (Table 1). According to the environmental standard for organic tea production in China (NY 5199-2002), 49% of the tea garden soil in this study exceeded the prescribed limit for the Hg content (0.15 mg/kg), and 57% exceeded the prescribed limit for the Cd content (0.20 mg/kg). Additionally, the average contents of Pb, As, and Cr in the soil were lower than the reference values of organic tea gardens (Table 1).

These elements exhibited some spatial differences, with the average concentrations of all elements in Guiyang and Zunyi being significantly higher than those in the other two regions (Qiandongnan and Qiannian), except for Sb, which was highest in Qiandongnan (Table 1). Specifically, the highest values of soil Hg, Pb, Ni, Tl, Sb, and As were found in the tea gardens of Nannong Township, Kaiyang County, Guiyang City, while the highest concentrations of Cd and Cr were found in the tea gardens of Zunyi (Figure 2). In our previous study on the planting plots of Houttuynia cordata in different townships of Kaiyang County, Guiyang City [42], we found that the Hg content in the soil of Nannong Township was significantly higher than those in other townships, reaching 2.2 mg/kg, which is consistent with the findings of this study. As a mercury mining area of Guizhou Province [11,42], Kaiyang County has a strong geological background for soil Hg, as well as other elements in soil, such as Hg, Pb, Cd, Tl, As, and Sb (Figure 2), since these elements are often associated with mercury during the mineralization process [43,44]. The content of heavy metals in soil is primarily influenced by the parent rock and human activities or pollution [45,46], and these tea gardens are generally located far from human activities or pollution sources. Therefore, the heavy metal content in soil primarily reflects differences in the trace elements in the parent rocks. As a karst region, Guizhou’s soil is predominantly formed from carbonate rocks, while in parts of Qiannan, Qiandongnan, and the former northwest regions, soil is formed from the weathering of sandstone [47]. Thus, there may be significant differences in soil element composition among different regions due to variations in the parent rocks [12,27].

A correlation analysis revealed that there was almost no significant correlation between the SOM or pH and these elements in the soil (Table S1). Additionally, except for Cr, all other elements exhibited significant positive correlations (p < 0.05), while Cr showed significant positive correlations only with Cd and Ni. This suggests that the source of Cr in the soil differs slightly from that of other elements, as evidenced by the spatial distribution patterns of different elements (Figure 2). The principal component analysis of all elements across different sampling points (Figure 3) revealed that the soil sampling points in Zunyi (mainly located in the first and second quadrants) differed significantly from those in Qiandongnan Prefecture and Qiannan Prefecture (mainly located in the fourth quadrant), while the three sampling points in Nannong Township, Kaiyang County, Guiyang City were located in positions (third quadrant) significantly different from all other sampling points. The soil of the remaining three sampling points in Kaiyang County (Hefeng Township) was more similar to the soil in Zunyi City. Therefore, it was further confirmed that, due to the geological background, significant differences in the compositions of trace elements in tea garden soil exist between the different regions of Guizhou Province.

3.2. Heavy Metal Contamination Levels in Tea Garden Soil

The geo-accumulation index method was employed to assess the heavy metal pollution in the tea garden soil. The I_geo values of eight heavy metals in the soil are presented in Figure 4. The I_geo value range of Hg in the soil was the broadest, being from −1.85 to 3.05 and spanning from unpolluted to heavily polluted. The maximum I_geo values of Sb (2.48), Ni (1.48), As (1.47), and Tl (1.58) in the soil were in the range of 1–3, indicating levels of being moderately polluted. Among these elements, over half of the sampling sites were slightly to mildly polluted with Sb, Hg and As, suggesting that the tea garden soil of Guizhou is primarily contaminated by Sb, Hg and As. The maximum I_geo values of the heavy metals Pb (0.63), Cd (0.26), and Cr (0.15) in the soil were below 1, indicating pollution levels ranging from no pollution to slight pollution. Among them, the maximum I_geo values of Cd and Cr in the soil were less than 0.5, and the I_geo values of most sites were less than 0, indicating that Cd and Cr pollution in Guizhou’s tea garden soil is minimal. The average I_geo values of these eight elements in the tea garden soil, from highest to lowest, were as follows: Sb (0.18) > As (0.06) > Hg (−1.0) > Pb = Tl (−0.55) > Cr (−1.06) > Ni (−1.12) > Cd (−1.24). This indicates that Sb and As pollution is worthy of concern in Guizhou tea gardens as a whole. Furthermore, in some tea gardens, there is multi-element compound pollution (such as sampling points 35–37 at Kaiyang County in Guiyang City, where the I_geo values of Hg, Tl, and Sb exceeded one or even three) (Figure 4). The overall contamination of Cr, Ni, and Cd in tea garden soil in Guizhou Province was relatively slight or minimal, while there was significant fluctuation in the contamination levels of the other five elements among different tea gardens. This mainly reflects the influence of parent materials or bedrock, as discussed in the aforementioned section.

3.3. Heavy Metal Contents in Tea Leaves

The statistical data for the eight heavy metals in Guizhou tea leaves are presented in

Table 2. Heavy metal contents of tea leaves from different sampling areas of this study (mg/kg, on a dry weight basis) and recommended limits for heavy metals in tea for China.

Sampling Area	Parameters	Hg	As	Pb	Cd	Cr	Ni	Sb	Tl
Zunyi (ZY, N = 21)	Mean	0.003b *	0.057a	0.31a	0.056a	0.62b	12.8a	0.024c	0.025a
	SD	0.001	0.014	0.12	0.030	0.24	4.5	0.005	0.027
Guiyang (GY, N = 6)	Mean	0.006a	0.039ab	0.33a	0.068a	0.53b	9.6a	0.025c	0.014a
	SD	0.003	0.010	0.17	0.025	0.19	4.6	0.005	0.011
Qiandongnan (QDN, N = 3)	Mean	0.004ab	0.041ab	0.32a	0.054a	0.62b	11.7a	0.063a	0.008a
	SD	0.001	0.007	0.08	0.037	0.39	0.9	0.024	0.001
Qiannan (QN, N = 7)	Mean	0.004b	0.034b	0.21a	0.035a	1.18a	12.3a	0.040b	0.013a
	SD	0.001	0.011	0.08	0.011	0.22	4.3	0.014	0.008
Total (N = 37)	Min.	0.002	0.020	0.10	0.009	0.25	5.9	0.013	0.004
	Max.	0.009	0.101	0.60	0.133	1.47	21.5	0.078	0.136
	Mean	0.004	0.048	0.30	0.054	0.71	12.1	0.030	0.019
	SD	0.002	0.016	0.13	0.028	0.32	4.3	0.015	0.021
	95%UCL **	0.004	0.054	0.35	0.064	0.81	13.5	0.035	0.027
Limit values of trace elements in tea (NY 659-2003, GB 2762-2022)		0.3	2	5	1	5	-	-	-

Note: * Different letters indicate the same index with a significant difference between different areas (LSD test) (p < 0.05). The same applies below. ** The 95% UCL is the upper limit of the 95% confidence interval for the mean.

. The average concentrations of heavy metals, from the highest to the lowest concentrations, were as follows: Ni (12.1 mg/kg) > Cr (0.71 mg/kg) > Pb (0.30 mg/kg) > Cd (0.054 mg/kg) > As (0.048 mg/kg) > Sb (0.030 mg/kg) > Tl (0.019 mg/kg) > Hg (0.004 mg/kg). There was no significant difference in the concentrations of harmful heavy metals such as Pb, Cd, Ni, Tl, and Hg in the tea leaves across the four regions, but the As content was the highest in the Zunyi area, the Cr content was the highest in Qiannan Prefecture, and the Sb content was significantly higher in Qiandongnan Prefecture and Qiannan Prefecture than in Zunyi City and Guiyang City (Table 2 and Figure 2). According to the Chinese national standard for trace harmful elements in food and tea safety (NY659-2003 and GB2762-2022), the maximum allowable concentrations for Hg, As, Pb, Cd, and Cr in tea are 0.3, 2, 5, 1, and 5 mg/kg, respectively (Table 2 and Figure 2). These standards do not specify the recommended limits for Ni, Sb, and Tl in tea. Therefore, this study does not address whether these elements exceeded the specified limits. The results indicate that the concentrations of Hg, As, Pb, Cd, and Cr in the tender tea leaves were well below the recommended limits for tea, hence fully complying with the national standards. The low concentrations of these elements in tender tea leaves may be related to two reasons. One is the short absorption time (less than two weeks) of these elements from the surrounding environment (such as the soil and atmosphere). Generally, the harmful element content in mature or old tea leaves is higher than that in tender tea leaves [12], and these elements gradually accumulate in the tea leaves as a tea tree grows. On the other hand, the special physical and chemical properties of the soil result in a lower proportion of harmful elements absorbed from the soil or the low bioavailability of heavy metals in the soil, although some elements in soil are moderately contaminated, and this will be discussed in detail in a later section.

Table 3. Comparison of heavy metal contents in tender tea leaves or green tea products of different studies (unit: mg/kg).

Region	Hg	As	Pb	Cd	Cr	Ni	Sb	Tl	Reference
Guizhou, China	0.004	0.048	0.300	0.054	0.71	12.1	0.030	0.019	This study
Jiangxi, China	0.042	0.200	0.800	0.038	0.67	7.71	---	---	[48,49]
Jiangsu, China	---	0.080	0.890	0.047	0.66	13.2	---	---	[50]
Anhui, China	0.020	0.053	0.470	0.023	0.38	8.97	---	---	[50,51]
Zhejiang, China	---	0.110	0.650	0.030	0.88	11.9	---	---	[50]
Henan, China	---	0.067	0.843	0.040	0.47	6.13	---	---	[50]
Hunan, China	---	0.070	0.440	0.055	0.37	9.23	---	---	[52]
Hubei, China	---	---	0.410	5.010	0.42	13.9	---	---	[53]
Guangdong, China	---	---	0.760	4.810	0.62	3.17	---	---	[53]
Gansu, China	0.007	0.105	0.972	0.045	0.71	---	0.043	---	[54]
Shandong, China	0.011	0.058	0.373	0.048	---	---	---	---	[55]
Hainan, China	0.002	0.050	0.080	0.020	0.59	---	---	---	[56]
India	---	---	1.000	0.030	1.00	6.00	---	---	[57]
Japan	---	---	8.400	0.060	1.60	5.20	---	---	[57]
Sri Lanka	---	---	0.003	0.003	0.37	3.10	---	---	[58]
Nepal	---	---	0.002	0.004	0.67	4.07	---	---	[58]
South Korea	---	---	0.031	0.005	0.15	18.0	---	---	[58]
USA	---	0.380	---	0.060	---	---	---	---	[59]

presents the concentrations of eight heavy metals in green tea from different regions, both locally and internationally. Compared with other regions of China, the concentrations of Hg, As, Pb, and Sb in tea from Guizhou Province were relatively low, and the contents of Cd, Cr, and Ni were moderate. Compared with other countries, the concentration of As in tea in Guizhou Province was relatively low, while the concentrations of Pb, Cd, Cr, and Ni were moderate.

3.4. Factors Influencing Heavy Metals in Tea Leaves

The heavy metal concentrations in tea are closely related to their concentrations in soil, the physicochemical properties of tea garden soil, and the assimilation (bioavailability) of these heavy metals from the soil to tea plants [40]. A correlation analysis was performed on the heavy metal concentrations in tea leaves and the soil physicochemical properties (Figure 5). The results indicated that the mechanism of accumulation of heavy metals in tea is relatively complex. In addition to the influence of soil heavy metal concentrations (such as Ni and Sb), the soil physicochemical properties (such as organic matter) also affect the absorption of certain elements (such as Sb). However, the soil pH and organic matter did not significantly affect the absorption of other studied elements in tea leaves. The concentrations of As, Pb, Cd, and Tl in tea leaves were positively correlated with the corresponding metal content in the soil, but they did not reach a significant level, which may be related to the available fractions of these elements in soil. The Hg content in tea leaves is thought to primarily originate from the atmosphere [22]. In this study, the sampling points with high soil mercury concentrations (such as Kaiyang and Guiyang) still exhibited low Hg concentrations in tea (Figure 2), indicating that the atmospheric Hg level at these sites was also low. In recent years, the atmospheric mercury concentrations in China have significantly decreased [60]. Therefore, the Hg concentrations in tea leaves were relatively low.

3.5. Enrichment of Heavy Metals in Tea Leaves

Heavy metals in the soil primarily enter a tea plant through its roots. The BCF, which is the ratio of a heavy metal concentration in a tea plant to that in soil, is used to determine the migration ability and enrichment degree of heavy metals in tea. Based on

Table 4. Bio-accumulation factors of heavy metals in tender tea leaves.

Sampling Area	Parameters	Hg	As	Pb	Cd	Cr	Ni	Sb	Tl
Zunyi (ZY, N = 21)	Mean	0.023a	0.003a	0.009a	0.222a	0.008b	0.343c	0.036a	0.033a
	SD	0.012	0.001	0.005	0.115	0.004	0.132	0.005	0.005
Guiyang (GY, N = 6)	Mean	0.025a	0.002a	0.007a	0.211a	0.007b	0.172c	0.008a	0.010a
	SD	0.037	0.001	0.005	0.170	0.002	0.061	0.007	0.004
Qiandongnan (QDN, N = 3)	Mean	0.042a	0.004a	0.011a	0.242a	0.017a	0.959b	0.015a	0.014a
	SD	0.022	0.001	0.001	0.116	0.010	0.423	0.006	0.001
Qiannan (QN, N = 7)	Mean	0.023a	0.003a	0.009a	0.164a	0.023a	1.725a	0.015a	0.032a
	SD	0.011	0.001	0.004	0.059	0.010	0.796	0.006	0.021
Total (N = 37)	Mean	0.025	0.003	0.009	0.211	0.011	0.603	0.025	0.027
	SD	0.021	0.001	0.005	0.124	0.008	0.655	0.074	0.021

Note: Different letters indicate the same index with a significant difference between different areas (LSD test) (p < 0.05). The same applies below.

, the bioconcentration coefficients of all heavy metals in tea leaves from different tea gardens were <1. The average BCF values were ranked as follows: Ni (0.603) > Cd (0.211) > Tl (0.027) > Hg = Sb (0.025) > Cr (0.011) ≈ Pb (0.009) > As (0.003). Overall, the BCF values of Ni and Cd in tea were one order of magnitude higher than those of other heavy metals. The BCF value of other metals were quite low, and this low BCF value may have been a result of the high SOM (up to 9%), soil iron (an average value of 3.73% in this study), and soil sulfur content (an average value of 434 mg/kg in this study), with strongly adsorbed heavy metals in the soil or insoluble precipitates forming, thereby reducing their bioavailability and resulting in a lower heavy metal content in tea leaves [61]. The additional reason why As had the lowest BCF value is that polyphenols in tea efficiently inhibit the absorption of inorganic arsenic compounds from soil [62]. An appropriate concentration of Ni in soil is beneficial for plant growth through the synthesis of the enzyme urease [63]. Acidic soil results in an increase in Cd bioavailability in soil, and Cd is then absorbed by plants. Therefore, the BCF values of Ni and Cd were the highest. Overall, although tea garden soil has a high concentration of heavy metals (such as Cd, Tl, and Sb), the concentrations of heavy metals in the tender tea leaves were not high (Figure 2), indicating that the absorption process of soil elements by tea leaves is limited and complicated, and the heavy metal content in tea leaves cannot be predicted based on the total soil content of these elements.

3.6. Health Risk Assessment of Drinking Tea

Table S2 shows the estimated daily intake (EDI) values of heavy metals based on tea consumption in Guizhou Province. Health risk assessment is conducted only for adults, as children rarely drink tea. For adults residing in Guizhou Province, the EDI values of different heavy metals from tea, from high to low, were as follows: Ni > Cr > Pb > As > Cd > Tl > Sb > Hg. Ni (1.45 × 10⁻³ mg/kg bw/day) contributed the most to the daily intake of heavy metals. Figure 6 and Table S3 show the THQ and HI values of the eight heavy metals present in Guizhou due to drinking this tea. The mean THQ values of eight heavy metals in Guizhou tea for adults had the following order, from highest to lowest: Tl (1.37 × 10⁻¹) > Ni (7.25 × 10⁻²) > As (6.74 × 10⁻³) >Pb (2.63 × 10⁻³) >Cd (2.86 × 10⁻³) >Sb (1.62 × 10⁻³) >Hg (7.32 × 10⁻⁴) > Cr (9.94 × 10⁻⁶). The THQ value of Tl was higher than those of the other heavy metals in the study, contributing 61.2% to the HI on average, which could be related to the extremely low RfD value of Tl (1.0 × 10⁻⁵ mg kg⁻¹ bw d⁻¹). The second highest contributor to the HI was Ni (32.3%), while other elements each contributed less than 4% to the HI (Figure 6) The THQ values of the eight heavy metals present in Guizhou tea for adults were considerably lower than 1, with the maximum value of 0.19 being for Tl in Zunyi (Figure 6 and Table S3), indicating that a single heavy metal being present in Guizhou tea poses a slight threat to human health. The HI values of heavy metals due to drinking Guizhou tea in the four study areas were in the range of 0.16–0.29, which is below 1, indicating that drinking Guizhou green tea has low carcinogenic risk. However, other research showed the hazard indexes (HIs) of heavy metal exposure for adults from consuming rice in nine cities (HI = 2.6 [64]) and vegetables in a Hg mining area (HI = 2.3 [65]) in Guizhou to be at higher risk levels, while consuming corn induced a relatively low HI of 0.2 [64], and the risk for potatoes [66]) and pork [65] was low (with HIs less than 0.1). The health risk of drinking tea produced in other parts of Guizhou (such as Puan in Southwest Guizhou [17] and Baihuahu in Central Guizhou [19]) was also relatively low (HI = <0.1–0.27). Based on this, green tea made from young tea leaves in Guizhou has a low health risk with respect to heavy metals.

4. Conclusions

Based on this study, we can conclude that (1) the contents of heavy metals such as Hg, As, Sb, Pb, and Tl in the majority of the tea garden soils of Guizhou Province slightly to moderately exceeded the background values of these heavy metals in the agricultural soil of this region; (2) the geo-accumulation index results indicate that the soil was mainly polluted with Sb and As, and some tea gardens are facing compound pollution from multiple heavy metals, while soil Cd, Cr, and Ni pollution in most tea gardens was minimal; (3) the heavy metal content of the Guizhou tender tea leaves was relatively low and did not exceed the safety limits for tea; and (4) the overall bioconcentration factors, except for Ni and Cd, were low, and the health risk of heavy metal intake by drinking tea was minimal. Nevertheless, due to the continuous accumulation and enrichment of harmful elements in tea leaves, we recommend brewing only tender tea leaves that grow in geologically rich background areas. In this circumstance, the risk of drinking tea leaves is safe. However, further research is needed on the mechanism of heavy metal enrichment in tea garden soils and the reason for low accumulation of these elements in tea leaves, namely whether it results from the influence of soil’s physicochemical properties (e.g., organic matter or pH), the interaction between other elements (such as iron, sulfur, and other heavy metals), or the special physiological function of tea trees. In addition, the contents of heavy metals in mature or old tea leaves or their products (such as black tea) in rich geological background areas and the associated health risks of drinking these teas need to be further evaluated in the future.