Heavy Metal Pollution and Ecological Assessment around the Jinsha Coal-Fired Power Plant (China)

Heavy metal pollution is a serious problem worldwide. In this study, 41 soil samples and 32 cabbage samples were collected from the area surrounding the Jinsha coal-fired power plant (JCFP Plant) in Guizhou Province, southwest China. Pb, Cd, Hg, As, Cu and Cr concentrations in soil samples and cabbage samples were analysed to study the pollution sources and risks of heavy metals around the power plant. The results indicate that the JCFP Plant contributes to the Pb, Cd, As, Hg, Cu, and Cr pollution in nearby soils, particularly Hg pollution. Cu and Cr in soils from both croplands and forestlands in the study area derive mainly from crustal materials or natural processes. Pb, Cd and As in soils from croplands arise partly through anthropogenic activities, but these elements in soils from forestlands originate mainly from crustal materials or natural processes. Hg pollution in soils from both croplands and forestlands is caused mainly by fly ash from the JCFP Plant. The cabbages grown in the study area were severely contaminated with heavy metals, and more than 90% of the cabbages had Pb concentrations exceeding the permissible level established by the Ministry of Health and the Standardization Administration of the People’s Republic of China. Additionally, 30% of the cabbages had As concentrations exceeding the permissible level. Because forests can protect soils from heavy metal pollution caused by atmospheric deposition, close attention should be given to the Hg pollution in soils and to the concentrations of Pb, As, Hg and Cr in vegetables from the study area.


Introduction
Heavy metals are widely used in the industrial and residential sectors due to their useful properties, such as their strength, malleability, and heat and electrical conductivity [1]. The demand for metals has increased with social development. Consequently, the metal uptake by crops and vegetables grown for human consumption has increased [2,3]. Heavy metals have high densities and are toxic or poisonous at low concentrations. The excess consumption of non-essential trace elements such as arsenic (As) and cadmium (Cd), even at relatively low levels, can cause various diseases, renal dysfunction, endocrine disruption, reproductive dysfunction, and cancers [4][5][6]. Heavy metals may enter the soil through bedrock or from anthropogenic by-products such as solid or liquid waste deposits, agricultural inputs, and industrial and urban emissions [7]. Soil contaminated with metals is a primary source of toxic element exposure to humans. Toxic metals in soils can enter the human body through the consumption of contaminated food crops or water or the inhalation of dust [8,9]. The presence of heavy metals in the soil is an important indicator of environmental pollution [10] and has become a serious issue worldwide.
Increased fossil fuel combustion during the past century is responsible for the progressive change in atmospheric composition [11][12][13]. Coal-fired power plants represent one of the most

Study Area
The JCFP Plant (27 • 28'29" N, 106 • 15'34" E) is located on the edge of Jinsha City in Jinsha County, Guizhou Province, southwestern China ( Figure 1). The JCFP Plant includes eight units with a total installed capacity of 1700 MW. The plant uses coal from coal mines in the adjacent area, such as Jinsha, Qianxi, and Zunyi. The study region is a typical mountainous area, with mountains and hills comprising 92.50% of the total area. The climate of Jinsha County is subtropical humid monsoon. The mean annual temperature (MAT) and mean annual precipitation (MAP) range between 15 and 16 • C and between 800 and 1000 mm, respectively. The main ecosystem types are evergreen broad-leaved forest, coniferous and broad-leaved mixed forest, and montane elfin forest. sword Fern, Imperata cylindrica Linn. Beauv, Lobelia seguinii Levl. Et Vant., Herba artimisiae Sieversianae, and Herba Acroptili Repentis.

Field Sampling
In 2015, soil samples were collected from 41 locations (34 sampling sites in croplands and seven sampling sites in forestlands) around the JCFP Plant ( Figure 1). Vegetable samples (cabbage) were collected from 32 locations where soil samples were also drawn (no cabbage was found at two of the 34 sampling sites in croplands). Soil samples were collected to a depth of 20 cm, and the locations were recorded using a handheld global positioning system (GPS). Soil samples were collected with a stainless steel shovel and immediately packed in self-zip plastic bags. To avoid cross contamination, the shovel was brushed and then flushed with soil from the subsequent sampling site. Each soil sample included mixed soil from four or five plots at each location. Simultaneously, three or four cabbages were sampled and mixed to form the vegetable sample.

Analytical Methods
The soil samples were air dried at room temperature in the laboratory and then homogenized and passed through a 2 mm sieve (preparation for determining the soil properties) after drying to a constant weight. Finally, the soil samples were ground in an agate mortar and passed through a 0.14 mm sieve for heavy metal analysis. The cabbage samples were dried in an air-blowing thermostatic oven after washing with deionized water prepared using a water purification system (Nex Power 2000 from Human Corporation, Seoul, Korea). Simultaneously, approximately 10 g of each plant sample was weighed and dried to a constant weight at 45 °C for water content analysis. Soil samples used for determining heavy metal content were digested according to United States Environmental Protection Agency (USEPA) procedures [22]. Pb, Cd, Cu and Cr concentrations were determined using inductively coupled plasma atomic emission spectroscopy (5300 V, Perkin Elmer Corporation, Waltham, MA, USA). Hg and As concentrations were determined using atomic fluorescence spectrometry (AFS-933, Jitian Corporation, Shanghai, China). The plant samples for metal content determination were digested with 4 mL of concentrated nitric acid (HNO3) and 1 mL of hydrogen peroxide (H2O2) using a microwave digestion system. The total content of the studied elements in plant samples was determined using an atomic absorption spectrometer (ZEEnit 700P, Jena Corporation, Jena, Germany). Analytical blanks were processed for all determinations. The analytical procedures used to determine heavy metals in soil samples and cabbage samples were

Field Sampling
In 2015, soil samples were collected from 41 locations (34 sampling sites in croplands and 7 sampling sites in forestlands) around the JCFP Plant ( Figure 1). Vegetable samples (cabbage) were collected from 32 locations where soil samples were also drawn (no cabbage was found at two of the 34 sampling sites in croplands). Soil samples were collected to a depth of 20 cm, and the locations were recorded using a handheld global positioning system (GPS). Soil samples were collected with a stainless steel shovel and immediately packed in self-zip plastic bags. To avoid cross contamination, the shovel was brushed and then flushed with soil from the subsequent sampling site. Each soil sample included mixed soil from four or five plots at each location. Simultaneously, three or four cabbages were sampled and mixed to form the vegetable sample.

Analytical Methods
The soil samples were air dried at room temperature in the laboratory and then homogenized and passed through a 2 mm sieve (preparation for determining the soil properties) after drying to a constant weight. Finally, the soil samples were ground in an agate mortar and passed through a 0.14 mm sieve for heavy metal analysis. The cabbage samples were dried in an air-blowing thermostatic oven after washing with deionized water prepared using a water purification system (Nex Power 2000 from Human Corporation, Seoul, Korea). Simultaneously, approximately 10 g of each plant sample was weighed and dried to a constant weight at 45 • C for water content analysis. Soil samples used for determining heavy metal content were digested according to United States Environmental Protection Agency (USEPA) procedures [22]. Pb, Cd, Cu and Cr concentrations were determined using inductively coupled plasma atomic emission spectroscopy (5300 V, Perkin Elmer Corporation, Waltham, MA, USA). Hg and As concentrations were determined using atomic fluorescence spectrometry (AFS-933, Jitian Corporation, Shanghai, China). The plant samples for metal content determination were digested with 4 mL of concentrated nitric acid (HNO 3 ) and 1 mL of hydrogen peroxide (H 2 O 2 ) using a microwave digestion system. The total content of the studied elements in plant samples was determined using an atomic absorption spectrometer (ZEEnit 700P, Jena Corporation, Jena, Germany). Analytical blanks were processed for all determinations. The analytical procedures used to determine heavy metals in soil samples and cabbage samples were assessed for quality control using certified reference materials GBW-07403 and GBW10020. The uncertainty of the analytical procedure was within 10%.

Quantification of the Soil Pollution Level
To assess the contamination level and determine the anthropogenic effect on heavy metals in the soils of the study region, the enrichment factor (EF), contamination factor (CF) and geo-accumulation index (I geo ) values were calculated.

Enrichment Factors
The extent of heavy metal contamination in the soils of the study area was assessed based on the determined concentrations and the baseline values of heavy metal concentrations in Guizhou Province. The enrichment factor of each element was obtained with Equation (1), which was modified froḿ Cujić et al. [18]: where EF M is the enrichment factor of element M, [M] is the concentration of the element M, and [Fe] is the concentration of iron. The subscripts " Stu " and " Gui " indicate the concentrations of Fe in the studied area and Guizhou Province, respectively. Here, Fe was used as the reference element for geochemical normalization because it is associated with fine solid surfaces, its geochemistry resembles that of many heavy metals, and its natural concentrations tend to be uniform [18,23]. According to previous studies, five contamination categories can be created based on the enrichment factor, as specified in Table 1 [24,25].

Contamination Factors
The CFs were obtained by dividing the determined value by the baseline value (Equation (2)): where CF is the contamination factor, C Stu is the heavy metal concentration in the soil sample, and C Gui is the baseline in Guizhou Province. Based on their intensities, the contamination levels were classified on a scale from 1 to 6, with the highest classification (6) indicating that the metal concentration is 100 times greater than the level expected in the Earth's crust [18].

Geo-Accumulation Index
I geo was used to evaluate the degree of heavy metal pollution in the soils from the study area. This index was calculated with Equation (3): where I geo is the geo-accumulation index, 1.5 is the background matrix correction factor introduced to account for possible differences in the background values due to lithospheric effects, C stu is the heavy metal concentration in the soil sample, and C Gui is the baseline concentration in Guizhou Province. Seven classes were established based on I geo : I geo < 0, class 1 (uncontaminated to moderately contaminated); 0 < I geo < 1, class 2 (moderately contaminated): 1 < I geo < 2, class 3 (moderately to heavily contaminated); 2 < I geo < 3, class 4 (heavily contaminated); 3 < I geo < 4, class 5 (heavily to extremely contaminated); 4 < I geo < 5, class 6 (extremely contaminated); 5 < I geo, with class 7 being an open class that comprises all values of the index higher than class 6. The heavy metal concentrations in class 7 may be 100 times greater than the geochemical background value [18,26].

Quantification of the Potential Risk to Humans
To evaluate the potential risks of heavy metals to humans in the study region, the accumulation factors (AFs) of the studied heavy metals in cabbage were determined. AFs were calculated using Equation (4): where C cabbage is the heavy metal content in cabbage (dry weight), and C Soil is the heavy metal content in the soil. Here, AF < 1 suggests that cabbage the specific element does not accumulate in cabbage; 1 < AF < 2 reflects low accumulation; and AF > 2 reflects high accumulation [27].

Statistical Methods
Data processing and statistical analysis were performed using

Heavy Metal Content and Properties of the Soils from the Study Area
The descriptive statistics of the analysed heavy metals and the soil properties of soil samples from 41 locations in croplands around the JCFP Plant are summarized in Table 2.  27.98 and 31.66·mg kg −1 , respectively. Moreover, the mean pH in soil samples from croplands was slightly lower than that in soil samples from forest areas. Notably, all of the mean concentrations of heavy metals in soil samples from croplands around the JCFP Plant are higher than those in soil samples from forests. In addition, the mean values of nutrient elements, such as total nitrogen (TN), total phosphorus (TP), organic matter (OM), and total organic carbon (TOC), in soil samples from croplands were much lower than those in soil samples from forests.According to the National Soil Resource Survey of China, the baseline levels of Pb, Cd, Hg, As, Cu and Cr in Guizhou Province are 35.20, 0.66, 0.11, 20.00, 32.00 and 95.50 mg·kg −1 , respectively [28]. Generally, soils from croplands in the study area were polluted by Pb, Hg, As and Cu. The mean levels of the studied heavy metals, except for Hg, in the soils from forestlands were lower than the baseline levels of heavy metals in Guizhou Province. However, this conclusion is based only on mean values. For example, the Pb content in soils from 10 sampling sites located in croplands was lower than 35.20 mg·kg −1 , and the minimum value at these sites was 4.73 mg·kg −1 . Conversely, the Pb content in soils from two sampling sites located in forest areas was higher than 35.20 mg·kg −1 , and the maximum concentration reached 39.01 mg·kg −1 .

Spatial Distributions of Heavy Metals in Soils from the Study Area
The spatial distributions of the studied heavy metals in the soils from the study area were obtained by empirical Bayesian kriging interpolation and are shown in Figure 2. The mean concentrations of Pb, Cd, Hg, Cu, As and Cr in soil samples from croplands around the JCFP Plant were 46.02, 0.62, 0.70, 26.40, 35.51 and 52.62 mg·kg −1 , respectively. Additionally, the mean concentrations of Pb, Cd, Hg, Cu, As and Cr in soil samples from forest areas around the JCFP Plant were 33.66, 0.58, 0.30, 11.99, 27.98 and 31.66·mg kg −1 , respectively. Moreover, the mean pH in soil samples from croplands was slightly lower than that in soil samples from forest areas. Notably, all of the mean concentrations of heavy metals in soil samples from croplands around the JCFP Plant are higher than those in soil samples from forests. In addition, the mean values of nutrient elements, such as total nitrogen (TN), total phosphorus (TP), organic matter (OM), and total organic carbon (TOC), in soil samples from croplands were much lower than those in soil samples from forests.According to the National Soil Resource Survey of China, the baseline levels of Pb, Cd, Hg, As, Cu and Cr in Guizhou Province are 35.20, 0.66, 0.11, 20.00, 32.00 and 95.50 mg·kg −1 , respectively [28]. Generally, soils from croplands in the study area were polluted by Pb, Hg, As and Cu. The mean levels of the studied heavy metals, except for Hg, in the soils from forestlands were lower than the baseline levels of heavy metals in Guizhou Province. However, this conclusion is based only on mean values. For example, the Pb content in soils from 10 sampling sites located in croplands was lower than 35.20 mg·kg −1 , and the minimum value at these sites was 4.73 mg·kg −1 . Conversely, the Pb content in soils from two sampling sites located in forest areas was higher than 35.20 mg·kg −1 , and the maximum concentration reached 39.01 mg·kg −1 .

Spatial Distributions of Heavy Metals in Soils from the Study Area
The spatial distributions of the studied heavy metals in the soils from the study area were obtained by empirical Bayesian kriging interpolation and are shown in Figure 2.  The spatial characteristic maps illustrate high variations in the heavy metal content of the soils, except for Pb. However, the other heavy metals exhibit certain similar spatial characteristics. The heavy metal concentrations in soils collected from the southeastern, eastern and northeastern parts of the studied area are generally lower than those in soils collected from the southwestern, western and northwestern parts. Additionally, no heavy industry exists in the study region. The wind in the study area blows mainly from the northeast and southeast directions. The northeast wind is often re-directed by the mountains in the southern part of the study area, becoming a southeast wind. Consequently, large amounts of fly ash created by the JCFP Plant are deposited in the northwestern region of the study area. We believe that the JCFP Plant contributes little to Pb pollution in soils in the study area. The Pb pollution in soils from the study area is mainly associated with agricultural activities, such as fertilization and pesticide application. However, the JCFP Plant is an important source of other heavy metals (Cd, Hg, As, Cu and Cr) in the soil. The spatial characteristic maps illustrate high variations in the heavy metal content of the soils, except for Pb. However, the other heavy metals exhibit certain similar spatial characteristics. The heavy metal concentrations in soils collected from the southeastern, eastern and northeastern parts of the studied area are generally lower than those in soils collected from the southwestern, western and northwestern parts. Additionally, no heavy industry exists in the study region. The wind in the study area blows mainly from the northeast and southeast directions. The northeast wind is often re-directed by the mountains in the southern part of the study area, becoming a southeast wind. Consequently, large amounts of fly ash created by the JCFP Plant are deposited in the northwestern region of the study area. We believe that the JCFP Plant contributes little to Pb pollution in soils in the study area. The Pb pollution in soils from the study area is mainly associated with agricultural activities, such as fertilization and pesticide application. However, the JCFP Plant is an important source of other heavy metals (Cd, Hg, As, Cu and Cr) in the soil.

Indices of Heavy Metal Pollution in Soils from the Study Area
The EF ranges of Pb, Cd, Hg, As, Cu and Cr in soils from croplands in the study area were 0.02-1.98, 0.02-8.14, 0.77-16.97, 0.08-2.42, 0.05-1.43 and 0.11-1.06, respectively ( Table 3). The EF ranges of Pb, Cd, Hg, As, Cu and Cr in soils from forest areas were 0.30-0.76, 0.18-0.84, 0.36-1.98, 0.02-0.89, 0.08-0.82, and 0.01-0.23, respectively. Based on the mean EFs, the enrichment of heavy metals in soils from croplands in the study area can be classified as no enrichment to minimal enrichment, except for Hg, which exhibited moderate enrichment. The enrichment of heavy metals in soils from forest areas can be classified as no enrichment to minimal enrichment. Notably, all the mean EF values of heavy metals in soils from croplands are higher than those of heavy metals in soils from forestlands. This result suggests that forests are likely effective shields, protecting soils from pollution caused by atmospheric deposition. Previous studies have suggested that metals are derived from mainly crustal materials or natural processes if their EF values are between 0.05 and 1.50 and likely from anthropogenic activities if the EF values are higher than 1.5 [18,29]. All the EFs of Cu and Cr in soils from croplands in the study area were within the range of 0.05 and 1.5. Thus, Cu and Cr derived from mainly crustal materials or natural processes in the study area. The EFs of Pb, Cd, Hg and As in soils collected from some the sampling sites in croplands were higher than 1.50. Therefore, anthropogenic activities are non-ignorable sources of heavy metal pollution in the study area with respect to these elements, particularly Hg, as 73.52% of the EFs of Hg exceeded 1.50. The EFs of Pb, Cd, As, Cu and Cr in soils from forestlands were within the range of 0.05 to 1.5, but 57.14% of the EFs of Hg in soils from forestlands were greater than 1.50. In summary, the concentrations of the studied heavy metals in soils from croplands were generally higher than those from forestlands. The EFs values indicated that heavy metal pollution in soils from croplands was more extensive than that in soils from forestlands. Pb, Cd, As, Cu and Cr in soils from forestlands derived from mainly crustal materials or natural processes, whereas the Hg in soils from forestlands likely originated from atmospheric deposition. The range of I geo values for Pb, Cd, Hg, As, Cu and Cr in soils from croplands in the study area were −4.76 to 1.03, −5.04 to 2.44, 0.40 to 3.91, −2.89 to 1.80, −3.17 to 0.85 and −2.09 to −0.50, respectively. The range of I geo values for Pb, Cd, Hg, As, Cu and Cr in soils from forestlands were −1.12 to −0.39, −1.46 to 0.06, −0.24 to 1.66, −4.67 to −0.06, and −6.19 to −0.89, respectively. Based on the mean values of I geo for Pb, Cd, As, Cu and Cr, the soils from both cropland and forestland areas could be categorized as class 1 (Table 3), meaning these soils are uncontaminated or moderately contaminated. However, the mean I geo value of Hg in soils from cropland was 1.81; thus, Hg exhibited moderate to heavy contamination in these areas (class 3). The mean I geo value of Hg in soils from forestlands was 0.79, suggesting that Hg pollution in soils from forestlands was moderate. Notably, all of the I geo values of Hg in soils from cropland were greater than 0, and five I geo values for Hg in soils from forestlands were greater than 0. Thus, Hg pollution is likely a significant problem in the study area. Some I geo values for Pb, Cd, As and Cu in soils from croplands were greater than 0. Specifically, all of the I geo values for Cr in soils from both croplands and forestlands were lower than 0, and all of the I geo values for Pb and Cu in soils from forestlands were lower than 0. In conclusion, Hg is the top-priority element among the studied elements in soils around the JCFP Plant.
In the present study, the CF values indicate that the soils are contaminated with certain metals, whereas the EF and I geo data suggest either no or moderate contamination of most metals. As shown in Equations (1)-(3), the concentrations of Fe in the studied area and Guizhou Province were employed to eliminate the effect of regional geochemical process, and the background matrix correction factor (1.5) was introduced to account for possible differences in the background values due to lithospheric effects in calculating the I geo data. However, the CF values were calculated by dividing the determined value by the baseline value. We believe that these values have their own advantages. The results from the EF and I geo data could be used for global comparisons. However, the results from CF values are more likely of local interest.

Heavy Metal Content of Cabbages
The mean values of Pb, Cd, Hg, As, Cu and Cr in cabbages (fresh weight) from the study area were 0.38, 0.07, 0.01, 0.06, 0.51 and 0.43 mg·kg −1 , respectively. The descriptive statistics of heavy metal concentrations (mg·kg −1 ) in cabbages from the study area are shown in Table 4. To assess the risk of heavy metals in vegetables from the study area, the heavy metal content of the cabbage samples was compared with the "maximum levels of contaminants in foods" (fresh samples), namely, Pb (0.10 mg·kg −1 ); Cd (0.20 mg·kg −1 ); Hg (0.01 mg·kg −1 ); As (0.05 mg·kg −1 ) and Cr (0.50 mg·kg −1 ). These permissible values were established by the Ministry of Health and the Standardization Administration of the People's Republic of China [30]. Approximately 97.06% of cabbages from the study area had Pb concentrations exceeding the maximum level; 8.82% of cabbages Cd concentrations exceeding the maximum level; 18.75% of cabbages had Hg concentrations exceeding the maximum level; 34.38% of cabbages had As concentrations exceeding the maximum level; and 18.75% of cabbages had Cr concentrations exceeding the maximum level. No information is available regarding the limit value of Cu. Therefore, heavy metal pollution of vegetables is a serious problem in the study area, and considerable attention should be given to these elements, particularly Pb.

Accumulation Factor
Vegetables can absorb and accumulate heavy metals at concentrations sufficiently high to cause clinical problems in both animals and humans [31]. Moreover, the same vegetable can accumulate Pb, Cd, Hg, As, Cu and Cr differently. The results of this study indicate that cabbage accumulates different elements at different rates ( Table 5). The AF of Pb ranged from 0.02 to 0.77, with a mean valve of 0.18; that of Cd ranged from 0.02 to 8.39, with a mean value of 1.85; that of Hg ranged from 0.01 to 0.54, with a mean value of 0.15; that of As ranged from 0.00 to 0.27, with a mean value of 0.03; that of Cu ranged from 0.05 to 1.21, with a mean value of 1.85; and that of Cr ranged from 0.02 to 0.56, with a mean value of 0.11. Based on these mean AF values, the accumulation ability of cabbage exhibited the following order for the studied elements: Cd > Cu > Pb > Hg > Cr > As. Only Cd slowly accumulated in cabbage.

Conclusions
The sources and levels of heavy metal pollution in the study area were investigated by conducting a statistical analysis of heavy metals in 41 soil samples (34 from croplands and 7 from forestlands) and 32 cabbage samples collected around the JCFP Plant.
The results of the EF analysis indicate that Cu and Cr in soils from both croplands and forestlands in the study area derived from mainly crustal materials or natural processes. Pb, Cd and As in soils from croplands originated partly from anthropogenic activities, but these elements in soils from forestlands derived from mainly crustal materials or natural processes. Hg in soils from both croplands and forestlands resulted from mainly anthropogenic activities (fly ash generated by the JCFP Plant). The geo-accumulation index analysis indicates that no Cr exists in soils from the study area. However, Hg concentrations in soils in both croplands and forestlands are a severe problem. Moreover, Pb, Cd, As and Cu were observed in soils collected at some sampling sites in cropland areas, and Cd and As were observed in some soil samples collected from forestland areas.
In summary, the JCFP Plant is a point pollution source of Cd, Hg, As, Cu and Cr in soils around the power plant. Hg concentrations in soils around the JCFP Plant is a particularly serious problem. Pb deposition in soils from the study area is caused mainly by agricultural activities. Forests are effective shields, protecting soils from pollution caused by atmospheric deposition. Additionally, Cd was observed to accumulate in cabbage. Based on the "maximum levels of contaminants in foods", the heavy metal content of cabbage around the JCFP Plant poses serious risks. Efforts should focus on ensuring the safety of vegetables for consumption by local residents.