Evaluation of the Potential Ecological Risk of Heavy Metal Pollution in Soil and Bioaccumulation Characteristics of Dominant Plants in Siding Pb-Zn Mineland

Abstract-Soils and dominant plants were surveyed in the Siding mineland area. Based on the determination of heavy metal concentration of soil from 8 sections and 14 dominant plant species in different parts, the potential ecological risk was assessed using the Hakanson Index Method. The results indicated that the soil of Siding mineland area was seriously contaminated by Pb and Zn. The risk index values of Pb and Zn were higher than 320, and the degree of ecological hazard was extremely high. While the risk index values of Cu and Cr were lower than 40, and the degree of ecological hazard was low. Statistical analyses showed that mining and smelt activities had a great influence on the concentration and distribution of heavy metals. Pb-Zn and Pb-Cu were likely from a same source, while Cr had a different cumulating character. Furthermore, Pb, Zn, Fe, Mn, Cu and Cr in 14 dominant plant species were at different levels. But the heavy metal concentrations of aboveground part had not reached the level for hyperaccumulators. Among all the dominant plants, Phragmites australis (Cav) Trin.ex Steud., Imperata cylindrical (Linn.) Beauv., Herba taraxaci and Peris vittata L. appeared to have good accumulation of Pb and Zn, and would be very useful in the restoration of Pb and Zn contaminated areas.


INTRODUCTION
There is a dense distribution of non-ferrous metal minelands in south China. The mining activities started long time ago and were intensive. As the mineral resources can't be removed, it brought about a series of social economic and ecological problems, such as long time of occupying, destroying and polluting lands (Luo, et al., 2007). These all changed the aqueous and thermal structures and destroyed the areas and systems for animals and plants. Heavy metal pollution exists universally and is most series (Pen, et al., 2005). It not only limited the development of the mine, but also influenced the agricultural production and inhabitant health around minelands. Siding is the large-scale of state multi-metal mine in China, with the area of 13.64 km 2 . It was finished and put into production in February 1960. Until now, the heartland of the mine had been mining up, left only sporadic mine operating around it.
This work investigated and analyzed the heavy metal concentrations in soils and dominant plant accumulation in Siding minelands. Using the Hakanson Index Method, the potential ecological risk of heavy metal pollution was also evaluated.
The understanding of bioaccumulation characteristics of dominant plants will be useful for heavy metal pollution, scientific management, phytoremediation and ecological restoration.

A. The survey site
The geography coordinate center of Siding Pb-Zn mineland is longitude 109°31′12″E, latitude 25°00′13 ″ N, located at Guangxi Rong-an County, China. The landform there is external ring protuberant with the height more than 400m, and internal billabong with the height about 320m. The soil is subacidity (pH≈6.0). The good condition of aqueous and thermal structures is propitious to plant growing, which provides advantageous climate and geological condition for reclamation and environmental improvement.

B. Sample collection
Soil sampling and vegetation investigation in Siding Pb-Zn mineland were carried out in March 2006. The mineland was divided into 8 areas in terms of mining and vegetation coverage (Fig.1). A area was unquarried; B area was located in sewage outfall of refinery, where there was a small river 10m away. Sewage from the refinery was discharged into the river; C area was located in 50m downstream of the sewage outfall; D area was located in sewage outfall of Laqiong village. Sewage from contaminated area flowed down hillside so that formed a contaminated area of black silt pulpy (about 300m 2 ). There was no plant in the center of the contaminated area; E area was a flat of sewage outfall at the foot of the hill; F area was a tailing dam; G area was a pithead tailing dam; H area have been used for reclamation for 5 years. For each area, Figure 1. Positions of sampling sites 3 samples were collected randomly (3-5 subsamples were merged into one single sample with the sampling depth of 20cm). There were 24 soil samples in all. According to the growing situation, the dominant plants were sampled. All soil and plant samples were sealed with polythene bags and transported into laboratory.

C. Soil and plant analysis
Soil samples were air-dried, ground and passed through a 100 mesh plastic sieve. Plant samples were gently washed with tap water and then rinsed with deionized water. Than they were put into an oven with 105℃ for 30min and 70℃ for 24 hours. Dried plant tissues were ground into fine powder. Soil samples were digested with concentrated HCl + concentrated HNO 3 + HF + HClO 4 (10:5:5:3, v/v) and plant tissues were digested with concentrated HNO 3 . The total metal concentrations (Pb, Zn, Mn, Cu and Cr) in digestates were then determined by a atomic absorption spectrometer (AA-700). The quality control was made by parallel samples and recovery of standard substance (GBW08303) in order to ensure the accuracy and precision.

D. Evaluation method and standard of heavy metal pollution
The potential ecological risk index was used for evaluation of heavy metal contamination in soil, which was suggested by Hakanson (1980) basing of heavy metal's characteristic and environmental action . This method associated with heavy metal concentration, ecological effect, environmental effect and toxicology to evaluate single element or multiple elements quantitatively.
This method was also used to evaluate heavy metal pollution of sedimentation (Jia, et al., 1997;Ma, et al., 2003;Xiang, et al., 2006). The calculation formulas are as follows:  (Liu, et al., 1999). In order to evaluate the heavy metal pollution more objectively, the present work chose the background value of local soil as the reference value. The standard parameter of toxic response made by Hakanson is: Zn(1) < Cr(2) < Cu(5) = Ni(5) = Pb(5) < As(10) < Cd(30) < Hg(40) (Lars Hakanson, 1980). As lacking of the parameter of toxic response of Mn, this study had not evaluated of its potential ecological risk. The grading standard of coefficients and indices of heavy metals' ecological risks was showed in table 1.

A. Heavy metals in soil
The concentrations of Pb, Zn, Mn, Cu and Cr in the soils were 5934.2, 23285.3, 447.0, 78.8 and 51.7 mg/kg ( Table 2). The order of metal concentration was Zn>Pb>Mn>Cu>Cr. The concentrations of Pb, Zn, Mn, Cu in the 8 sampling areas were much higher than soil background values in both Guangxi and China, with the concentrations of Pb, Zn higher than the Grade III of Environmental Quality Standard for soils. The concentration of Pb was 18212.0mg/kg, which was 36 times to the value of Grade III of Environmental Quality Standard for soils. The highest concentration of Zn with 75300.9mg/kg of tailing dam was 150 times to the value of Grade III of Environmental Quality Standard for soil. The highest concentrations of Cu and Cr (146.0mg/kg and 101.0mg/kg) were from the sewage outfall of refinery. They were both lower than Grade II of Environmental Quality Standard for soils. As regard Mn, the highest concentration (846.0mg/kg) appeared in the flat of the sewage outfall at the foot of the hill， which was 5-folds of soil background values in Guangxi and 1.7-folds of soil background values in China. The variation coefficients of Pb, Zn were high, which indicated the distribution of Pb, Zn in different sampling points was greatly different and the mining activity significantly influence the concentration and distribution of heavy metals.

B. Correlations of heavy metals
In order to realize the correlations of heavy metals, the Spearman Ranking Correlation Coefficient and Kendall ζ Correlation Coefficient were calculated ( Table 3 ) . The correlation between heavy metals could estimate whether they came from the same source. In natural world, heavy metal pollution is complex with several metals compound. If they significantly correlated, the possibility of homeology is grate. The source may come from natural (source of geochemistry), or compound pollution resulted from human activities (Yang, et al., 2007). The two correlation coefficients both showed that there was a significant correlation between Pb and Zn, or Pb and Cu (table 3). Therefore, there was great possibility of their homeology, and in accordance with reality. In addition, the Spearman Ranking Correlation Coefficient showed that there was a very significantly correlation between Cu and Zn, while the Kendall ζ Correlation Coefficient also displayed significant correlation. The results showed that the two calculating methods were consistent. The correlation between Cr and other heavy metals was low, indicating special accumulation of Cr.
RI of 8 sampling areas were all above 300 (table 4), which indicated that potential ecological risk in each area was strong. The order of the ecological risk level was: B > F > G > D > C > E > H > A. It meaned: sewage outfall of refinery>tailing dam>pithead tailing dam>sewage outfall of Laqiong village>50 meters downstream of the sewage outfall>reclaimed area>unquarried area. The result showed that waste water and residue from smelting were main pollution sources in Siding mineland. The potential ecological risk of surrounding area of sewage outfall even exceeded that of tailing dam. Therefore, management must be strict and audit to cleaning production of processing heavy metals ought to be strengthened.

D. Community analyze of dominant plants
In Siding mineland, 14 species of higher plants that belonged to 11 families, including 3 gramineae and 2 euphorbiaceae, had been investigated. The main artificial cultivated plants were fruit trees, vegetables and cash crops. Dominant species were almost herbaceous plant and mainly belonged to gramineae, which due to their abundant species, fast growing, widely distribution and resistant of arid and dry.

E. Concentrations of heavy metals in dominant plants
In order to study the absorption and accumulation to Pb, Zn, Mn, Cu and Cr by dominant plants quantitatively and to find accumulators with heavy metal resistance, the concentrations of heavy metals in dominant plants were analyzed. The accumulated ability of plants is depended on biological features of the plants, soil types, nutrient elements and heavy metal availability. Concentrations of Pb, Zn in all plants were in the normal level. But they unreached to the critical concentrations (1000mg/kg and 10000mg/kg) (Baker, et al., 1983) of the hyperaccumulator. Concentrations of Pb, Zn in root of Pteris vittata L. were 467.93mg/kg and 20425.31mg/kg. Meanwhile, concentrations in its stem and leaf were 536.62mg/kg and 2537.56mg/kg. It proved that Pteris vittata L. had a good toxic resistance of Pb, Zn, that was accordance with the study by An et al (2007). Ageratum conyzoides L. had a concentration of 703.14mg/kg of Pb in the stem and leaf, which closed to 1000mg/kg. Further research could be carried out to study its potential accumulation.

F. Accumulation and transfer characteristics by different plants
The concept of hyperaccumulator was proposed by Brooks in 1977 (Brooks, et al., 1977). Plants ideal for phytoremediation should posses multiple traits. They must have short growing period, resistance of plant diseases and insect pests, large biomass of aboveground part, and accumulate more than two heavy metals simultaneously.
To known the accumulation ability of plants further, Biological Accumulation Coefficient (BAC) and Biological Transfer Coefficient (BTC) were calculated (Table 7). BAC represents accumulation of one heavy metal (Salt, et al., 1995). As easy to collect, the aboveground part of plant is important in actual soil heavy metal remediation. It is that, larger the BAC, stronger the ability of accumulating heavy metals. BAC of aboveground part of plant larger than 1 is a prominent feature that to distinct the hyperaccumulator from other plants. BTC can reflect the plant ability of transferring heavy metals from root to their aboveground part. Heavy metal concentration in root is usually much higher than that in stem and leaf. But in hyperaccumulator, situation can be opposite. Generally, there should be more ionic transfer or channel protein to promote heavy metals loading to xylem . As presented in table 7, BAC of Pteris vittata L. for Cr was 2.27. However, Pteris vittata L. was not a hyperaccumulator for Cr due to the Cr concentration (141.  When BTC was concerned, it seemed that Pteris vittata L. and Herba Taraxaci had a higher Pb transfer rate. Rosa laevigata Michx. and Phragmites australis (Cav.) Trin. Ex Steud. had a higher Cr transfer rate. And Rosa laevigata Michx. also had good Mn transfer ability. Herba Taraxaci showed good relocation abilities of Pb, Zn, Cu, Cr, Mn with BTCs larger than 1.
In all investigated plants, no hyperaccumulators had been find. However, some dominant plants in mineland such as Pteris vittata L., Imperata cylindrical (Linn.) Beauv., Ageratum conyzoides L., Herba Taraxaci) displayed good resistance to heavy metals. These species can be uesed as the pioneers for ecological restoration of mineland in early period, which can not only promote the vegetation coverage rate quickly and conserve water and soil, but also accumulate some of heavy metals.

IV. CONCLUSIONS
The heavy metal concentrations were very high in mineland siol. Most of them were exceeded the soil critical concentration for agricultural and forestry production and plant normal growth. There were significant differences among heavy metal concentrations. Heavy metal pollution in Pb mineland soil was serious, with the major pollution factors of Pb and Zn. Significant correlations between Pb-Zn, Pb-Cu and Cu-Zn illustrated their possibility of homology. This was accordance with the conclusion of Baoshan Pb-Zn mineland studied by Yue (2004). There was significant positive correlation between Pb-Cu, and very significant positive correlation among Cu, Pb, Zn (Yue, 2004). The correlation between Cr and other heavy metals was not obvious. This was a proof of the special accumulation of Cr.
Evaluation of the potential ecological risk of heavy metal pollution showed that pollution of Pb, Zn was serious in Siding mineland. The trend of pollution situation followed the order: sewage outfall of refinery>tailing dam>pithead tailing dam>sewage outfall of Laqiong village>50 meters downstream of the sewage outfall>reclaimed area>unquarried area. Pollution in sewage outfall of refinery was the most seriously because the sewage discharged into rivers nearby. Therefore, the government department should control more strictly in order to make sewage discharge meet the standard. The ecological risk of reclaimed area was light. In other words, after reclaimation, the heavy metal pollution situation in mineland was a bit abated.
Investigation of dominant plants in mineland indicated that different plants had significant differences of the same heavy metal absorption. And different parts of one plant also had differences to heavy metal absorption, with root>stem>leaf usually. Although concentrations of Pb, Zn were high in mineland, plant accumulation was not ideal, with BACs lower than 0.5 on the whole. This was in accordance with the study of Pb-Zn mineland in Zhejiang by Bi De (Bi, et al., 2006). That was to say, plants in Pb-Zn mineland surely had some resistance to Pb. Even though, accumulation of all investigated plants did not meet the critical concentrations of hyperaccumulator, they were all dominant plants in local with large biomass and heavy metal resistance. Phragmites australis (Cav.) Trin. Ex Steud., Imperata cylindrica (Linn. ) Beauv., Herba Taraxaci and Pteris vittata L. had obvious Pb and Zn accumulation, and can be used as pioneer plants for ecological restoration in Pb-Zn mineland.