3.1 The distribution characteristics and pollution evaluation of total mercury in soil of tea garden.
In the renowned tea production region, the total mercury content ranged from 0.025 to 0.296 mg/kg, with an average of 0.112 mg/kg (Fig. 1), surpassing the background mercury content in Chinese soil of approximately 0.038 mg/kg 30. However, these levels are lower than the mercury concentrations in soils from cropland and forest areas near the Jinsha TPP in China, which were 0.70 and 0.30 mg Hg/kg, respectively 31. The results indicated that 48.3% of the soil samples had Hg concentrations below 0.10 mg/kg, while 90.5% of the samples exceeded the average Hg concentration of 0.0672 mg/kg reported for Spanish soil 32. By using Kriging interpolation, the results shown in Fig. 2 were derived based on the average mercury content of various township samples. Upon further evaluation of the geoaccumulation index (Table 1), it was found that the mean geoaccumulation index values for Yongchun County, Anxi County, Dehua County, and Nanan City were − 0.22, -0.09, -0.20, and − 0.48, respectively, all of which were less than zero. This indicates that the tea garden soil in these areas was uncontaminated and had a low overall total mercury concentration. However, it is important to note that the maximum value in Yongchun County exceeded 1, reaching a moderate pollution level of 1.29. The statistics in Table 2 provided detailed information on pollution levels, indicating that 67.81% of the 146 samples were free from pollution, 31.51% showed no to moderate pollution, and only 0.68% were at a moderate pollution level. Overall, the mercury pollution in tea garden soil was relatively satisfactory. However, there were still some samples with moderate pollution levels that required attention. It is also important to focus on the cumulative effect of mercury pollution to prevent its larger impact on the environment.
Table 1
Evaluation result of soil total mercury accumulation index in tea garden
Area | Samples | Min | Max | Ave |
YC | 49 | -1.06 | 1.29 | -0.22 |
AX | 49 | -1.19 | 0.88 | -0.09 |
DH | 18 | -2.28 | 0.94 | -0.20 |
NA | 30 | -1.75 | 0.47 | -0.48 |
Table 2
Total mercury pollution in soil of tea garden
Classification | Pollution degree | Cumulative index | Samples | Proportion(%) |
Level 1 | pollution-free | I ≤ 0 | 99 | 67.81 |
Level 2 | No pollution to medium pollution | 0 < I ≤ 1 | 46 | 31.51 |
Level 3 | medium pollution | 1 < I ≤ 2 | 1 | 0.68 |
Understanding the distribution patterns of soil mercury and the factors that influence it is crucial for preventing and managing mercury pollution. Research in karst catchments in the southwestern region showed that soil mercury content varied significantly among different land-use types 33. Agricultural areas had higher mercury levels compared to forest and grassland areas. In mountainous regions of southwestern China, it was observed that soil mercury content decreased with increasing altitude 34. This decrease may be due to colder climate conditions and limited vegetation at higher altitudes. A recent study in the Qinghai-Tibet Plateau uncovered distinct distribution patterns of soil mercury in the permafrost region 35. The presence of soil mercury may be attributed to pollution sources like industrial contaminations and the use of mercury-containing pesticides in urban areas. The distribution of soil mercury content is affected by various factors such as land-use type, climate conditions, terrain, vegetation coverage, and human activities. To effectively address soil mercury pollution, further research is needed to investigate the mechanism and interaction of these influencing factors.
3.2 Influencing factors of total mercury distribution in soil of tea garden
A Pearson correlation analysis was employed to explore the connection between Hg and the concentration of other metal elements, as well as pH (Fig. 3). The results demonstrated a significant positive correlation between the Hg and TC, As, Na, and V (p < 0.01), as well as a substantial positive correlation with Zn (p < 0.05). Furthermore, a significant negative correlation was observed between the Hg and K (p < 0.05). Although no significant correlation was found between the Hg in tea garden soil and pH, it is important to note that soil pH has been shown to play a crucial role in regulating various reactions that affect the form, migration, and transformation of Hg in soil. This includes processes such as adsorption-desorption, complexation, oxidation-reduction, and methylation-demethylation 36–38. The lack of correlation in this specific study may be due to the overall strong acidity of tea garden soil and its low degree of variation, which results in minimal impact. Numerous studies have demonstrated the influence of soil pH on these reactions, despite the findings of this particular study. It was surprising to find a strong positive correlation between the levels of mercury (Hg) and total carbon (TC) in the soil. This could be due to the influence of soil organic matter on the accumulation of Hg, as previous studies have shown that organic matter can affect the solubility and mobility of Hg in soil 39. This study highlights the significant impact of soil pH and organic matter on the distribution, speciation, and mobility of Hg in tea garden soil. However, further research is needed to fully understand the mechanisms behind these findings and their potential implications for ecology and human health.
The presence of soil organic matter and the pH level play crucial roles in the behavior of mercury in soil. Research has shown that the adsorption of mercury by soil is influenced by both pH and organic matter 40. Specifically, at lower pH levels, the soil tends to adsorb more mercury, and the presence of organic matter can further enhance this adsorption process. Other studies have also confirmed the impact of organic matter on the adsorption and release of mercury in soil 41. It has been found that dissolved organic matter can facilitate the adsorption of mercury by soil, while also reducing the soil's capacity to release mercury. Additionally, the pH level of the soil can affect the release and uptake of mercury 42. Under different environmental conditions, the pH level can influence the rates of methylation and demethylation of mercury 43. It has been observed that in acidic conditions, the release of mercury decreases as the pH level increases 44. Furthermore, the presence of organic matter in soil can increase the adsorption of mercury, thereby decreasing its release. Overall, these studies highlight the significance of pH and organic matter in regulating the release of mercury in soil.
3.3 Source apportionment of total mercury in tea garden soil
The PMF model was used to analyze the source composition spectra and contribution rates of various heavy metals in the study area, as shown in Table 3. High concentrations of Cu and Zn were found in Factor 1, suggesting a potential association with livestock manure, which often contains these metals as antibacterial agents and additives for animal care. Factor 1 can therefore be identified as a fertilization source. Factor 2 was found to be dominated by Ni, indicating a pollution source related to electroplating alloys, commonly found in wastewater and waste residue from industrial plants. This leads to the designation of Factor 2 as an industrial source. Factor 3 showed a high contribution from As, a metal primarily sourced from coal combustion, designating Factor 3 as a coal combustion source. Factor 4 had a high contribution from Hg, accounting for 72.4% of the total, and is defined as a natural source due to the relatively small proportion of other metals and the concentration of mercury being close to the background value. Figure 4 shows that factor 4 has the most significant impact on Hg, accounting for 72.4% of the total influence. Factors 5, 3, and 2 also play a role in influencing Hg levels. This suggests that the primary source of mercury in tea garden soil is natural, but is also affected by transportation, coal combustion, and industrial production activities. This finding is consistent with previous research indicating relatively low levels of mercury pollution.
Table 3
Source contribution for different elements by positive matrix factorization
Element | Source component spectrum(mg/kg) | | Source contribution rate(%) |
Factor 1 | Factor 2 | Factor 3 | Factor 4 | Factor 5 | Factor 6 | | Factor 1 | Factor 2 | Factor 3 | Factor 4 | Factor 5 | Factor 6 |
Hg | 0.000 | 0.008 | 0.011 | 0.081 | 0.012 | 0.000 | | 0.2 | 6.7 | 9.9 | 72.4 | 10.7 | 0.0 |
Cu | 10.02 | 0.00 | 0.39 | 1.87 | 0.00 | 0.00 | | 81.6 | 0.0 | 3.2 | 15.2 | 0.0 | 0.0 |
As | 0.18 | 0.72 | 8.18 | 0.00 | 0.00 | 0.93 | | 1.8 | 7.2 | 81.7 | 0.0 | 0.0 | 9.3 |
Cd | 0.006 | 0.000 | 0.002 | 0.006 | 0.000 | 0.029 | | 14.5 | 0.0 | 4.4 | 13.6 | 0.0 | 67.5 |
Ni | 0.07 | 10.65 | 0.57 | 0.49 | 0.00 | 1.15 | | 0.6 | 82.4 | 4.4 | 3.8 | 0.0 | 8.9 |
Zn | 7.31 | 1.90 | 1.92 | 7.76 | 16.52 | 32.31 | | 10.8 | 2.8 | 2.8 | 11.5 | 24.4 | 47.7 |
Pb | 0.00 | 2.12 | 0.00 | 0.00 | 48.22 | 8.40 | | 0.0 | 3.6 | 0.0 | 0.0 | 82.1 | 14.3 |
The primary sources of soil mercury content are natural processes such as rock weathering, volcanic activity, and soil formation, as well as anthropogenic activities like fossil fuel combustion, mining, smelting, pesticide and fertilizer usage, and industrial wastewater discharge. In urban and suburban areas, soil mercury pollution is mainly caused by industrial wastewater discharge and vehicular exhaust contaminations. Soil mercury pollution in Guilin and its surrounding areas is mainly found near industrial zones and traffic routes45. Similarly, in Fushun City, heavy metal pollution in urban and rural vegetable gardens is primarily caused by industrial wastewater discharge, waste gas contaminations, and the use of fertilizers and pesticides 46. Additionally, studies have shown that soil mercury levels are influenced by factors such as soil type, land use patterns, and soil pH. For example, in the southwestern region, agricultural soil mercury levels are mainly affected by the soil parent material and land use patterns, with paddy soil having significantly higher mercury content compared to other soil types 47. To address soil mercury pollution, various measures can be taken, including controlling industrial pollution sources, reducing fossil fuel combustion, using pesticides and fertilizers more responsibly, and improving wastewater treatment processes.
3.4 Health risk assessment of total mercury in soil of tea garden
The amount of mercury present in the soils of tea gardens has a strong correlation with the potential risk of mercury exposure from consuming tea. Our calculations, as shown in Table 4, indicate that the daily intake of Hg from tea consumption in the studied region ranges from 0.011 to 0.132 micrograms per kilogram of body weight per day (ug/kg bw/day), with an average intake of 0.050 ug/kg bw/day. When assessing the risk of Hg exposure using the Hazard Quotient (HQ) metric, we found that the average HQ value is 8.75% ± 3.54%, indicating that the overall exposure level is significantly below the threshold for concern. However, it is important to note that the highest recorded HQ value among all sampling sites was 23.10%, observed at a location in Jindou Town, Yongchun County, where the soil Hg content measured 296.45 ug/kg. The study's assessment of mercury exposure risk from drinking tea is based solely on mercury levels in tea garden soils, which has its limitations. Factors such as differences in mercury accumulation between young and old tea leaves, variations in tea processing methods, and the impact of different brewing techniques can all affect the actual risk of mercury exposure. While the current risk levels are considered safe, it's important to consider the potential risk from tea grown in areas with high mercury pollution. Future research should aim to include these factors in risk assessments to gain a more comprehensive understanding of mercury exposure from drinking tea.
Table 4
The statistical evaluation of daily potential mercury intake and hazard quotient in the tea-consuming population
| Min | Max | Mean | Std. |
PDI(ug/kg bw/day) | 0.011 | 0.132 | 0.050 | 0.020 |
HQHg(%) | 1.95 | 23.10 | 8.75 | 3.54 |
Limited research has been conducted on mercury pollution in tea garden soils, but recent studies have assessed the health risks associated with this type of pollution. One study investigated the potential harm of prolonged consumption of tea contaminated with mercury on the nervous system, kidneys, and other organs 48. Another study conducted a spatial health risk assessment of mercury pollution in soil from a contaminated site in China, revealing significant health risks to residents, especially children and pregnant women 26. Additionally, a health risk assessment of mercury pollution in agricultural soil in Shiquan County, Shaanxi Province, found that the pollution mainly originates from industrial contaminations and mining activities, posing a certain health risk to local residents 49. These assessments highlight the need for effective measures to reduce exposure and protect human health from soil mercury pollution, and suggest that different assessment methods and measures should be adopted based on different pollution situations and environmental conditions.