Spatial Distribution of Heavy Metals in Sediments of Tamarix chinensis Wetland on the South Bank of Bohai Bay

College of Biological and Environmental Engineering, Binzhou University, Binzhou 256600, China

Abstract In order to understand the spatial distribution of heavy metals in coastal wetlands, Tamarix chinensis wetlands on the south bank of Bohai Bay were selected as the research object. The content of Cr, Mn, Ni, Zn and Cu in soil was determined by sampling method in April 2018. The spatial distribution characteristics of heavy metals and causes were studied. The results showed that except for Cr, Ni and Zn, the content of other heavy metals in this area did not exceed the background value of soil environment in Shandong Province. The study area was slightly polluted by these three heavy metals. In the vertical direction, the content of Cr, Mn, Cu and Ni followed the law of surface layer>middle layer>bottom layer, while the content of Zn was higher in the bottom layer, which needed to be further studied. In the horizontal direction, the content of the five heavy metals was not significantly different according to the distance from the coastline. There was no significant difference in the content of heavy metals between rhizosphere soil and non-rhizosphere soil. There was a significant correlation between the content of heavy metal elements, except Mn, the content of heavy metals was significantly correlated with the content of soil organic matter. The content of some heavy metals was significantly correlated with the content of available phosphorus and ammonium nitrogen.

Key words Bohai Bay, Tamarix chinensis wetland, Sediment, Heavy metal, Spatial distribution

1 Introduction

Coastal wetlands are not only rich in natural resources, but also have important ecological functions, such as protecting rich biodiversity, and purifying environment. It plays an irreplaceable role in maintaining the ecology of coastal areas[1]. Bohai Bay, located in the middle of the Bohai Sea, is a half closed bay. The water in the bay is shallow and the water exchange capacity is poor[2]. The inflow of water from the Yellow River and other rivers has brought it rich nutrients, at the same time, the environmental pollution problem in Bohai Bay has become increasingly prominent[3-4].

Heavy metal is one of the most important pollutants in the offshore environment. Most of the heavy metal pollutants entering the ocean are enriched in sediments. Sediments are considered to be the ultimate accumulation of heavy metals in the marine environment[5-6]. When the environmental conditions change, the balance between the overlying water body and the sediment is broken, and the heavy metal pollutants in the sediment re-enter the water body[7]. Therefore, the spatial distribution characteristics of heavy metals in coastal sediments can reflect the pollution status of the sea area on the one hand[8-10]. The 2008 survey found that the coastal area of Bohai Bay was seriously polluted by heavy metals, in which the over-standard rate of cadmium content in cultured shellfish was up to 50%[11]. This shows that heavy metal pollution has caused serious harm to the regional ecology.

There are related studies on the spatial distribution of heavy metals in coastal wetlands and estuarine wetlands at home and abroad. Liu Jingchunetal. studied the variation of heavy metals Cu, Zn and Cr in the sediments of mangrove wetlands in Zhangjiang Estuary[12]. Through the investigation of the surface sediments on the beach of Yancheng, Jiangsu Province, Zhao Xueqinetal. analyzed the content of Hg, As, Cd, Cr, Cu, Pb, Zn, Ni and other heavy metals in the surface sediments on different beach surfaces in the core area[13]. Tang Luluetal. analyzed the content level and spatial distribution characteristics of heavy metal elements Hg, Cu, Pb, Zn, Cd, Cr and As in surface sediments[14]. Ling Minetal. studied the spatial distribution characteristics of trace elements in wetland soil ofTamarixchinensisfarm in the Yellow River Delta[15]. Luo Xianxiangetal. determined Cu, Pb, Zn, Cd, Hg, As, particle size and total organic carbon in the surface sediments of Laizhou Bay, and pollution assessment method and potential ecological risk assessment method were used for pollution and risk analysis[16]. Foreign studies have shown that the pollutants in wetland sediments will have a direct toxic effect on tidal flat organisms, and ultimately affect human health through the food chain enrichment.

In this project,T.chinensiswetland on the south bank of Bohai Bay was selected as the research area to study the spatial distribution of heavy metals and causes in wetland ecosystem. It can provide a theoretical basis for the effective prevention and control of heavy metal pollution in estuarine wetlands in Bohai Bay area.

2 Materials and methods

2.1 Soil sample selectionThe sampling time is April 2018. In the direction of the coastline vertical toT.chinensiswetland, one section was set up every other 1 km, and a total of 3 sections were set up. One sampling point was set for every 0.5 km in the direction from sea to inland on each section. According to the growth status of plants, rhizosphere soil and non-rhizosphere soil were collected. The soil layers were divided as follows: surface layer (0-30 cm), middle layer (30-60 cm), bottom layer (60-100 cm). At least three soil samples were collected from each sampling site, mixed and sampled by method of quartering, put into a sealed bag and brought back to the laboratory for analysis. The sampling point layout is shown in Fig.1.

Fig.1 Sampling point in the study area

2.2 Sample treatment methodsThe collected sediment samples were dried naturally indoors. The stones and plant residues were picked out and ground through 100-mesh screen. About 0.3-0.4 g samples were weighed by electronic analytical balance (one ten-thousandth). After HCL-HNO3-HClO4digestion, the solution was prepared for the determination of heavy metals.

2.3 Methods for determination of samples in the study area

2.3.1Determination of heavy metals in samples. The content of Cu, Zn, Cr, Ni and Mn in samples was determined by atomic absorption spectrophotometer produced by Shimadzu Company of Japan (method for determination of heavy metals). The formula for calculating the metal in the sample is as follows:

C=c×v/m

whereCis the concentration of heavy metal elements in the sample (mg/kg, dry weight);cis the concentration of the digested solution obtained from the standard curve (mg/L);vis the volume of solution after being put to a constant volume (L);mis the weight of soil samples participating in digestion (kg).

2.3.2Determination of other indexes in samples. We referred to the Agrochemical Analysis of Soil edited by Bao Shidan for the determination of other physical and chemical indexes of soil.

2.3.3Statistical analysis methods. First of all, Excel was used for preliminary arrangement, and then SPSS software was used to do statistical description and analysis. Graphic drawing was completed by using the drawing function of OriginPro7.5.

3 Results and discussions

3.1 Statistical description of heavy metals in surface sedimentsAccording to the background value study of soil environment in the old Yellow River region of Shandong Province, the background values of Cr, Mn, Ni, Zn and Cu in the surface soil of Shandong Province were 30.97, 18.71, 42.31 and 13.46 mg/kg, respectively.

From Table 1, it can be seen that the average content of Cr in the surface soil of the study area was 33.64 mg/kg, which was 1.08 times of the environmental background value, and the maximum value was 46.95 mg/kg, 1.52 times of the background value. The average content of Ni was 17.68 mg/kg, which was 0.94 times of the environmental background value, lower than the environmental background value, and the maximum value was 23.45 mg/kg, which was 1.25 times of the background value. The average content of Mn was 260.46 mg/kg, which was 0.61 times of the environmental background value. The average content of Cu was 7.53 mg/kg, which was 0.56 times of the environmental background value, lower than the environmental background value. The maximum value of Mn was 306.23 mg/kg, which was 0.72 times of the background value. The maximum value of Cu was 11.92 mg/kg, which was 0.86 times of the environmental background value, lower than the soil environmental background value. This showed that the content of Mn and Cu in this study area was on the low side. The average content of Zn was 29.98 mg/kg, which was 0.71 times of the environmental background value, and the maximum value was 86.89 mg/kg, and 2.05 times of the background value. The surface Zn skewness was greater than 0, indicating that there were more sample points with value less than the mean value in the surface soil. The coefficient of variation of the content of the five heavy metals was all in the range of 10%-28%, which was consistent.

Table 1 Statistical description of heavy metals in surface sediments of Laizhou Bay

HeavymetalMinimumvaluemg/kgMaximumvaluemg/kgMeanvaluemg/kgStandarddeviationmg/kgCoefficientofvariation%SkewnessKurtosisCr17.2242.2033.645.8317.34-0.730.96Ni13.1023.4517.682.1512.150.401.21Mn215.78306.23260.4624.359.350.21-0.66Zn24.5886.8929.9811.3137.735.0626.26Cu2.8211.927.532.2529.92-0.18-0.42

3.2 Distribution characteristics of heavy metal elements in the vertical directionThe average content of Cr, Mn, Ni, Zn and Cu in the surface layer (0-30 cm) of soil in the study area was 33.64, 260.46, 17.68, 29.98, 7.53 mg/g, respectively. The average content of Cr, Mn, Ni, Zn and Cu in the middle layer (30-60 cm) was 32.51, 258.87, 17.03, 26.08, 6.45 mg/kg, respectively. The average content of Cr, Mn, Ni, Zn and Cu in the bottom layer (60-100 cm) was 27.94, 253.65, 16.87, 27.07, 5.60 mg/kg, respectively. The content in the surface layer was slightly higher than that in the middle layer, and the content in the middle layer was slightly higher than that in the bottom layer. Fig.2a showed that with the increase of depth, the average content of Cr, Mn, Ni and Cu decreased gradually, showing the change trend of 0-30 cm>30-60 cm>60-100 cm. From Fig.2b, 2c and 2d, it can be seen that the average content of Cr, Ni and Cu in the three sections showed a downward trend, but there was little difference in the content of heavy metals in the three layers. One of the possible causes is atmospheric dry and wet deposition. For example, Spokes L.etal. found that 20% to 70% of the heavy metals such as Cr and Zn in the sediments of the southern bay of the North Sea came from atmospheric wet deposition. The variation trend of Mn content in the three sections is as follows: it showed a downward trend in Section A; it decreased at first and then rose in Section B, but there was little difference in the content in each layer; the content in Section C decreased at first and then increased, but the content in the bottom layer was higher than that in the surface layer, possibly because the content of trace elements in the surface layer of this area is mainly controlled by primary geochemical conditions[15].

Fig.2 Vertical distribution of heavy metals in all sample points and soil sections

The Zn content of the three sections showed the variation characteristics of 0-30 cm>60-100 cm>30-60 cm. If the section changes of heavy metals Cr, Mn, Ni, Zn and Cu are caused by the above two factors, then the maximum value in the non-surface layer may be due to abnormal human activities or the input of atmospheric pollutants during this period.

3.3 Distribution characteristics of heavy metal elements in horizontal directionAll the sample points are divided into three areas according to the distance from the sea, namely, the offshore area, the middle area and the open sea area. The average content of soil Cr in the study area was middle area (35.97 mg/kg)>offshore area (31.53 mg/kg)>open sea area (29.99 mg/kg). The minimum values of Cr were 19.57 mg/kg in offshore area, 18.68 mg/kg in middle area and 17.22 mg/kg in open sea area, respectively. The maximum values were 48.54 mg/kg in the middle area>40.38 mg/kg in the open sea area>40.37 mg/kg in the offshore area, respectively. It can be inferred that the Cr part of the sea water is enriched in the coastal area with the rising tide and ebb tide, and migrates to the open sea area. In the open sea area, due to the pollution of life or industry, the Cr in the soil was on the high side, and at the same time, it migrated to the lower middle area, so that the content of Cr in the middle area was higher. The average content of Mn was offshore area (270.59 mg/kg)>middle area (267.88 mg/kg)>open sea area (235.64 mg/kg). The maximum and minimum contents also showed the law of offshore area>middle area>open sea area. From the analysis of Table 1, it can be seen that all the values of Mn in this experimental sample point were lower than the environmental background value, indicating that this area was not polluted by Mn. The law shown in Fig.3 may indicate the trend of Mn migration from coastal areas to inland in this area, indicating that the source of Mn in this region may be seawater inflow. The average content of Ni was offshore area (18.19 mg/kg)>open sea area (17.49 mg/kg)>middle area (16.93 mg/kg). However, both the minimum value and the maximum value showed that the offshore area>the middle area>the open sea area, while the skewness of the offshore area and the middle area was greater than 0, and the skewness of the open sea area was less than 0. That is to say, most of the values of the sample points in the offshore area and the middle area were less than the average value, and most of the values in the open sea area were larger than the average value.

In summary, it can also be explained that the Ni in this area had the trend of migrating from coastal areas to inland. The average content of Zn was the middle area (29.65 mg/kg)>the open sea area (29.39 mg/kg)>the offshore area (28.35 mg/kg). However, there was little difference in the content of the three, and the difference between the minimum values was also very small, and the value of only one sample point in the middle area was much larger than that of the other sample points. This may be due to some special factors. Therefore, on the whole, there was no significant difference in the distribution of Zn in the horizontal direction. The average content of Cu in open sea area (7.36 mg/kg)>middle area (6.70 mg/kg)>offshore area (6.42 mg/kg). The difference between the maximum values was small, and the difference between the minimum values was also within a reasonable range. From Table 1, it can be seen that the content of Cu in this area was much lower than the background value of soil environment in Shandong Province, so it can be inferred that this area was not polluted by Cu, and there was no significant difference in the distribution of Cu in the horizontal direction.

3.4 Distribution characteristics of heavy metal elements in roots and non-roots of plantsIt can be seen from Fig.3 that there was little difference in heavy metal content between rhizosphere soil and non-rhizosphere soil. The content of Cr in rhizosphere soil (35.83 mg/kg)>the content of Cr in non-rhizosphere soil (33.42 mg/kg); the content of Zn in rhizosphere soil (33.16 mg/kg)>the content of Zn in non-rhizosphere soil (32.75 mg/kg); the content of Mn in rhizosphere soil (254.33 mg/kg)

Table 2 Statistical description of heavy metal contents in different regions in the horizontal direction

RegionHeavymetalMinimumvaluemg/kgMaximumvaluemg/kgMeanvaluemg/kgStandarddeviationmg/kgCoefficientofvariation%SkewnessKurtosisOffshoreareaCr19.5740.3731.534.960.16-0.37-0.51Mn212.28320.13270.5925.590.09-0.27-0.68Ni14.1524.0518.191.940.110.620.82Zn23.6439.3628.353.040.111.171.81Cu2.1910.646.421.770.28-0.42-0.04MiddleareaCr18.6848.5435.976.710.19-0.390.01Mn201.92311.24255.8721.290.08-0.050.72Ni13.1023.8516.932.320.140.550.50Zn23.2486.8929.6510.230.354.6225.32Cu4.0611.066.701.650.250.41-0.43OpenseaareaCr17.2240.3829.996.250.21-0.44-0.99Mn195.33267.88235.6415.420.07-0.600.61Ni6.8322.3317.493.270.19-1.022.50Zn23.3839.6329.395.340.180.72-0.92Cu2.8211.927.362.290.31-0.11-0.61

Fig.3 The average content of topsoil heavy metals between rhizosphere soil and non-rhizosphere soil

4 Conclusion

In this paper,T.chinensiswetlands on the south bank of Bohai Bay were selected as the research object, and the effects of vertical distribution, horizontal distribution of Cr, Mn, Ni, Zn and Cu and plants on the distribution of heavy metals were analyzed. And the effects of other physical and chemical properties of soil on the spatial distribution of heavy metals were also studied. The following conclusions are drawn:

(i) The content of Cr, Ni and Zn in the surface sediments ofT.chinensiswetland on the south bank of Bohai Bay exceeded the soil environmental background value of Shandong Province, but the excess value was very small. This showed that the study area was less polluted by these three kinds of heavy metals. (ii) In the vertical direction, the distribution of Cr, Mn and Cu follows the law of surface layer>middle layer>bottom layer. The accumulation of heavy metals in the surface layer of soil may be related to human activities in the vicinity of the study area. This may also be due to the selective absorption of heavy metals by plants, resulting in the migration of heavy metals to the surface of the soil. (iii) In the horizontal direction, the content of five kinds of heavy metals was not very different according to the distance from the coastline. It can be seen that there was low correlation between the accumulation of heavy metal elements in sediments and land-ocean interaction. (iv) The results of correlation analysis showed that there was a good correlation in the content between the five heavy metals. Except for Mn, there was a good correlation between the content of other heavy metals and the content of organic matter in soil. There was a good correlation between the content of some metals and available phosphorus content, soil salt content or ammonium nitrogen content. There was no correlation between the content of five heavy metals and pH or the content of nitrate nitrogen in soil.