Vertical Distribution and Pollution Assessment of Heavy Metals in Mangrove Sediments of Dongzhai Harbor,Hainan

Yefan ZOU, Yong LI, Zhizhong ZHAO, Yinuo JI, Dan WU

School of Geography and Tourism, Hainan Normal University, Haikou 571100, China

Mangroves are plant communities mainly consisting of salt-tolerant evergreen trees and shrubs that grow in the intertidal wetlands in tropics and subtropics,which are regularly submerged by rising tides[1-2]. They are productive, with various ecological functions, and can provide food,nutrients and habitats for microorganisms and animals[1].In addition, mangroves, as one of the most productive, biodiverse and valuable ecosystems in the world[5], can protect the coastal landforms from winds and storm surges,improve the sedimentary environment and purify water and so forth[3-5]. Mangrove sediments contain clay mineral, active metal oxide and organic matters. Due to their high specific surface area, mangrove sediments can easily accumulate varieties of bio-toxicity pollutants. Therefore,they play a double role, with as a carrier and possible source of contami nants[6]. Heavy met als in mangrove sediments may be released bake to water under the condition of change in water environments or external biological effect[7]. More over,the geochemical properties of mangrove sediments were investigated in this study with an attempt to provide references for the treatment of heavy metal pollution, and for the monitoring, management and restoration of mangrove ecosystem.

Materials and Methods

Study area

The mangrove area of Dongzhai harbor (110°32′-110°37′E, 19°51′-20° 01′ N)[8]with tortuous coastline,many bays and gentle lagoon is located in the northeast of Haikou City,northern coast of Hainan Island.It is a shoal-water bay approximate to SNthat formed by continental sink during the Great Qiongzhou Earthquake of 1605. The study area has a large amount of sediments because of the correlation between the north of Dongzhai Harbor and Qiongzhou Strait during flood time[9]. A total of 32 species from 16 families inhabit the shoal-water bay. The mangroves are surrounded by Aricennia marina and Rhizophora stylosa (80%of the coverage rate)[10]. Heavy metals easily introduced to the mangrove ecosystems via natural and anthropogenic processes have major pressure on the whole ecosystem.These heavy metals are originated from disposal of liquid effluents, natural runoff, busy shipping with oil leakage, indiscriminate use of pesticides in agricultural fields,and fertilizer in breeding[11].

The study on the mangrove sediments in Dongzhai Harbor has attracted increasing attention in recent years.Previous papers have reported the horizontal distribution,mobility pattern,combined form and correlation of heavy metals in mangrove sediments,but the vertical distribution and changes in heavy metal concentrations are still unclear.In order to reveal the vertical distribution and variation of heavy metal concentrations in mangrove sediments,the samples of mangrove sediments were collected from 15 sites at different depths of 0-5, 30 and 60 cm to measure the concentrations of eight heavy metals(Cr,Co,Ni,Cu, Zn, As, Cd and Pb). In addition, a geoaccumulation index was used to summarize the contamination status,and analyze the impacts of anthropogenic activities on the mangrove ecosystem.

Sediments collection

Forty-five surface sediment samples were collected from 15 sites at depths of 0-5, 30 and 60 cm in August, 2013. All the samples of surface sediments from each site were placed inside a sterilized plastic bag and sealed. The samples were then dried at room temperature to constant weight; subsequently, they were placed in the digesting block and ground until homogeneous and passed through a 200-mm sieve. All the samples were kept in a sterilized plastic bag and sealed for following analyses. Heavy metal concentrations in sediment samples were determined using inductively-coupled plasma mass spectrometry (ICP-MS), and the data was analyzed by the laboratory of College of Geography and Tourism,Hainan Normal University.

Table 1 The concentrations of heavy metals in mangrove sediments of Dongzhai Harbor mg/kg

Results and Analysis

The distribution of heavy metals in surface sediments of Dongzhai harbor

The pollution status and levels in mangrove areas can be evaluated based on the concentrations of heavy metals in surface sediments. With the assistance of horizontal differentiation in metal concentrations,source of pollution can be traced back and contamination levels will be evaluated to some extent[12]. The range of heavy metal concentrations(Cr,Co,Ni,Cu,Zn,As,Cd, Pb) at all sites was presented in Table 1.The distribution of heavy metals in the mangrove sediments of Hainan Harbor was characterized by comparing with the background values of aquatic sediments in Hainan and soil in northeast Hainan[13].

As could be seen from Table 1,the concentrations of heavy metals in sediments followed the order of Cr ＞Zn ＞Ni ＞Cu ＞Pb ＞Co ＞As ＞Cd.The concentrations of Cr, Co and Cu were above the background values of aquatic sediments in Hainan and lower than the background values of soil innortheast Qiongzhou[13].Pb concentration was lower than both background values.

As shown in Table 1, the vertical variation coefficients of heavy metals in surface sediments in the study area ranged from 0.20 to 0.92. Almost every heavy metal expressed minor variation coefficient lower than 0.40 and lower dispersion degree,except for Cd which had a variation coefficient of 0.92 and a higher dispersion degree.The high metal concentrations found in this area could probably be a result of the anthropogenic activities, such as high population density,increasing domestic sewage, busiest shipping lane with oil leakage and spill, prosperous tourism, heavy sea traffic and developed breeding industry (such as shrimp,duck).

The concentrations of heavy metals like Cr, Co, Ni, Cu and Pb showed a similar spatial variation at all the sites,follow the sequence of GT-1-01 ＞QT-2, GN-2 ＞NWS-2. At the site QT-1, the concentrations of Cr and Zn were higher than that of other metals.According to investigation,heavy metals at the site QT-1 were usually not exchangeable with outside ocean because the site QT-1 in low terrain was restricted by some factors, such as a large amount of death plants,apparent odor in sediments which contained fine mucky soil and high content of clay with strong absorption. GN-2 with sparse and dying plants was less polluted by heavy metals than QT-1, because the heavy metals,especially Pb and Zn could be discharged through the drainage ditches nearby GN-2.

Table 2 The concentrations of heavy metals in mangrove sediments mg/kg

The vertical distribution in surface sediments of Dongzhai Harbor

As shown in Fig.1,the mean concentrations of heavy metals in mangrove sediments of Dongzhai Harbor ranged in an decreasing order of Cr(84.40-86.87 mg/kg)＞Zn(61.04-79.82 mg/kg)＞Ni (43.22-44.78 mg/kg)＞Cu(20.62-24.59 mg/kg)＞Pb(21.25-23.70 mg/kg) ＞Co (17.45-17.63 mg/kg)＞As(8.38-9.11 mg/kg)＞Cd (1.70-2.51 mg/kg).

The enrichment regularity of heavy metals was proved. Generally,the concentrations of Cu, Zn, As and Pb were significantly negatively correlated with depth (i.e. the deeper the sediment, the lower the concentration of heavy metals). And all the sites exhibited the strongest enrichment of Zn.The mean concentration of Co had minimal difference at different depths.Based on the fact that Cr mean concentration in upper layer was slightly higher than that in the other two layers,indicating that upper layer could be easily affected by leaching. This is the reason why more heavy metals are enriched in relative lower layers. However,the mean concentrations of Ni and Cd in middle layer were slightly higher than that in upper and lower layers.The concentration and distribution of heavy metals were affected by the source of sediment, absorption ability of sediment and human activities.Fine particle such as mucky soil had high absorption ability, and thus Cu, Zn, As and Pb were enriched in upper layer.

A crucial step for evaluating the impact of sediment pollution and the level of contamination affecting a given area is to establish a reference background (Fig.2). The background value of aquatic sediments in Hainan[13]was considered, comparing with the mean concentrations of heavy metals. In addition,Ni background value of soil in northeast Hainan was selected here because Ni background value of aquatic sediments in Hainan was not found[13].

As shown in Fig.2, the logarithms of the ratios of mean concentrations of heavy metals to the background values of aquatic sediments in Hainan were greater than zero(except Pb and Cd at 60 cm deep), indicating that the mean concentrations of the eight heavy metals in mangrove sediments at different depths (except Pb and Cd at 60 cm deep) were higher than the background values. Meanwhile, Cd concentration was as 25 times as high as the background value. In fact, Cd and As with higher concentration maystem from anthropogenic sources which include dense shrimp pond,duck breeding, indiscriminate use of pesticide and fertilizer in agricultural fields,oil leakage and plumbing.

The concentration variation of heavy metals in mangrove sediments of Dongzhai Harbor

Changes in the concentration of heavy metals over time have significant influence on mangroves,even the whole mangroves wetland ecosystem.The mean concentrations of heavy metals sampled in May of 2013,May-Sep. of 2009, and May-Aug. of 2011 were compared,as shown in Table 2.

The mean concentrations of most heavy metals increased over time. In May of 2013,the mean concentrations of heavy metals were higher than that in 2009 and 2011, except that Cu, Zn and Pb concentrations in May, 2013 were lower than in May-Aug, 2011.Due to environmental deterioration and ecological destruction, the mangrove areas which had decreased sharply were subjected to serious ecological threat. In order to protect mangrove ecosystem, more stringent measures should be taken to forbid the breeding industry,even though the local government had taken strict measures to control the shrimp breeding in Hainan mangrove natural reserve.

Table 3 Muller’s classification for geoaccumulation index(Igeo)

Geoaccumulation index(Igeo)

Geoaccumulation index (Igeo), a quantitative index, was originally defined by Muller[16]in order to evaluate the contamination values by determining and defining metal contamination in sediments. The parameter is shown as follows:

Where Igeois the geoaccumulation index. Cmis the measured concentration of the examined element m in the sediment sample and Bmis geochemical background value of the element m, mg/kg. The factor 1.5 is introduced to minimize the effect of possible variations in the background which may be attributed to lithologic variations in the sediments[17]. The seven different classes of geoaccumulation index along with associated sediments pollution status are given in Table 3[18].

Igeovalues used to calculate the eight heavy metals in the 15 sites at depths of 0,30 and 60 cm can be seen in Fig.3.

The calculated indices of geoaccumulation (Igeo) of the studied metals at all the sites are presented in Fig.3.The Igeoshowed minor differences,follow the order Cd ＞As ＞Co ＞Cr ＞Zn ＞Ni ＞Pb. Only Cd showed higher Igeo,the other metals had similar Igeo ranging from -1.00 to 2.00. The Igeo of Cd revealed that the site DGK-2 was moderately polluted (Igeo= 1.25),and other sites were strongly polluted by Cd(4.95 ＞Igeo ＞3.00).The.Igeo of As at the site GN-2 was 3.21, indicating that GN-2 was strongly polluted;the Igeo of As at other sites ranged from 0.48 to 1.56, suggesting that all the sites were unpolluted to moderately polluted or moderately polluted.However,the Igeo revealed that all the sites were unpolluted to moderately polluted with Co and Cd (1.00 ＞Igeo＞0). The Igeoof Cu and Zn varied from 0 to 1.00 at sites QT-1-01,QT-1-02,QT-2, GN-1 and GN-2, indicating that the five sites were unpolluted to moderately polluted by Cu and Zn,while their Igeovalues at the other 10 sites were below zero,indicating that the 10 sites were unpolluted by them. There was no Ni or Pb pollution at all the sites(0 ＞Igeo),except that QT-1-01 was unpolluted to moderately polluted by Ni(1.00 ＞Igeo＞0). In summary, the concentrations of the eight heavy metals at QT-1-01,QT-1-02,QT-2,GN-1,GN-2 and GN-3 were higher than at other sites.Cd and As whose concentrations were closely related to anthropogenic activities were the major source of heavy metal pollution and they were considered as the most toxic and potential toxic metals, respectively[19]. The six sites described above had few chance to exchange with outside because of their low terrain, but strong absorption because of the mucky soil.In addition,most of the six sites located near sewage ditches or densely populated areas were greatly affected by agricultural, breeding and local development activities.

Conclusions

In this study, the mean concentrations of heavy metals in mangrove sediments at different depths were higher than the background values of aquatic sediments in Hainan, decreasing in the order of Cr ＞Zn ＞Ni ＞Cu ＞Pb ＞Co ＞As ＞Cd. The concentrations of Cu,Zn,As and Pb had positive correlation with depth.The enrichment of Cr in upper layer was slightly lower than that in other layers. The result indicated that upper layer was easily affected by leaching, causing high enrichment of Cr in middle and lowerlayers. On the contrary, the concentrations of Ni and Cd in middle layer were slightly higher than in upper and lower layers.

The concentrations of heavy metals showed an increasing trend over time,following the sequence of 2013 ＞2011 ＞2009. According to geoaccumulation indices, Cd and As are the major causes of severe heavy metal contamination, besides Cd is more harmful to environment than As. The Igeovalues of the eight heavy metals at sites QT-1-01, QT-1-02, QT-2, GN-1,GN-2 and GN-3 are mostly higher than at other sites,because the six sites are more affected by human activities.

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