Study on Soil Heavy Metal Content and Ecological Risk Assessment of Jiaozhou Bay of China

Sampling and analyzing the soil of coastal public green space in Jiaozhou Bay, Qingdao, a total of 31 plots were surveyed, 242 soil samples were collected, and the types and contents of heavy metals in the soil were analyzed. The ecological risk index method and the ground accumulation index method were used to evaluate Qingdao Jiaozhou Bay. Heavy metal risk in coastal public green space. The results show that the average content of Cr, Cd, Pb, Zn, Hg, Cu and Ni in heavy metals exceeds the natural background value of soil. The content of As, Cr, Pb, Zn and Cu is lower than the value specified in the secondary standard specified in GB15618-1995 Soil Environmental Quality Standard. The content of Hg and Ni is between the values specified in the second to third grades. The content of Cd exceeds the value specified by the three-level standard. The ecological risk index and the low cumulative index evaluation show that the heavy metals with relatively high risk are Cd and Hg.


Introduction
With regards to Qingdao's rapid economic development in recent years, industry has made a particularly outstanding contribution, but also brought serious environmental pollution, especially heavy metal pollution and affect on some cellular and molecular aspects [1][2][3][4][5][6][7][8][9][10] . Jiaozhou Bay, located in the south of Qingdao, is a typical semi closed bay. With the rapid development of the surrounding economy and society, a large amount of industrial waste water and domestic sewage are discharged into Jiaozhou Bay through rivers. Under the influence of ocean current such as economic and also energy, heavy metals in the waste water and sewage are collected to the coastal area of Jiaozhou Bay along with its sediment. Therefore, this paper samples the soil of the coastal public green space of Jiaozhou Bay, and analyzes the categories and content of heavy metals in the soil [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. The potential ecological risk index and index of geoaccumulation are used to evaluate the content of heavy metals in the soil of the coastal public green space in Jiaozhou Bay.

Collection of samples
According to the principles and methods of soil sampling in the Technical Specifications for Soil Environmental Monitoring, and in combination with the principle of distribution of soil points, the soil in the studied area is monitored [30,31], with a total of 8 large soil monitoring points and 31 small soil monitoring points (Figure 1). We collect about 10 cm of soil surface samples, weighing about 1500 g, put into sample bags, and take back to the laboratory for air drying. Specific soil monitoring points information is shown in Table 1.

Analysis and assessment of samples
After air dried, the soil samples are filtered out of stones and other impurities, grinded by mortar, filtered by 100 mesh nylon screen, and stored in brown reagent bottle. IP900 and UV spectrophotometer are used to detect the composition and content of heavy metals in soil after air drying. Each group of soil samples by each method is measured three times in parallel, and the mean value of the three results is taken as the measurement result.

Evaluation methods of soil heavy metals 2.3.1. Potential ecological risk index (RI)
The Potential Ecological Risk Index judges the potential ecological risk of heavy metals according to the type, concentration and toxicity coefficient of heavy metals in the soil [12,[32][33][34][35][36][37][38][39][40][41]. The specific evaluation methods are as follows: (1) The pollution coefficient of single heavy metal is: coefficient of a certain metal, C i is the measured value of heavy metal in soil, and C n i is the evaluation standard of this heavy metal.
(2) (2) The potential ecological risk coefficient of a single metal i is: and T f i is the toxicity coefficient of the heavy metal.
(3) The comprehensive soil ecological risk index at a certain point is: Due to the deposition of heavy metals and the affinity of solids, there is a proportional relationship between toxicity and rarity. The potential biological toxicity of a certain metal element is inversely https://doi.org/10.37358/RC. 20.4.8093 proportional to its richness. Therefore, the toxicity coefficient can be calculated according to the abundance value of heavy metals [42], as shown in Table 2. The Potential Ecological Risk Coefficient describes the pollution degree of a certain pollut.ant, divided into 5 levels from low to high; the Potential Ecological Risk Index (R i ) describes the comprehensive value of the potential ecological risk coefficient of multiple pollutants at a point, which is divided into 4 levels [43][44][45][46], as shown in Table 3.

The Index of geoaccumulation
(1) The Index of Geoaccumulation is widely used in the evaluation of soil heavy metal pollution. The quantitative index of heavy metal pollution degree is determined by the relationship between the content of a certain heavy metal and its geochemical background value. The calculation formula is as follows: I geo = [C s i /(K × C n i )] where C s i is the content of heavy metal element n in the soil; C n i is the geochemical background value of this element in the soil; the constant k is the correction index, that is to say, considering the background value change possibly caused by the rock forming movement, which is usually used to characterize the sedimentary characteristics, rock geology and other influences. In this paper, K is set as 1.5, and the classification standard of the Index of Geoaccumulation and the division of pollution degrees are shown in Table 4. (2)

Data processing
Excel 2007 is used to calculate the heavy metal data, and SAS software is used to analyze the correlation of heavy metals.

Statistics and analysis of heavy metal content in the soil
See Table 5 for the characteristics of heavy metal content in various sample sites of Jiaozhou Bay public green space in Qingdao. The average contents of As, Cr, Cd, Pb, Zn, Hg, Cu, and Ni are 6.71, 127.81, 4.27, 50.06, 188.54, 0.82, 79.64, and 52.44 mg/kg, respectively. The contents of As, Cr, Pb, Zn, and Cu are lower than those specified in the second level standard of GB15618-1995 Soil Environmental Quality Standard. The contents of Hg and Ni are between those specified in the second level and the third level, while the content of Cd is higher than that specified in the third level standard. Except from As, the average contents of Cr, Cd, Pb, Zn, Hg, Cu, and Ni are higher than the natural background values of soil [47][48][49][50][51][52].

Evaluation of soil heavy metal pollution 3.2.1. Evaluation of ecological risk index
The single ecological risk factor value and potential ecological risk index value of each heavy metal are calculated with the soil natural background value as the reference standard, and the results are shown in Table 6. Most of the RI values of the samples of the coastal public green space in Jiaozhou Bay of Qingdao are less than 150, indicating that the potential ecological risk index is low. The potential ecological risk coefficients of samples 3, 6, 9, 11, 12, 13, 14, 15, 17, 19, 20, 21, 22, 24, 25, 26 and 29 are extremely high. The sequence of ecological risk of heavy metals in each sample site is Y20＞Y15＞Y22 ＞Y14＞Y9＞Y17＞Y13＞Y19＞Y21＞Y26＞Y6＞Y24＞Y12＞Y29＞Y25＞Y11＞Y3＞Y31＞Y30 ＞Y8＞Y28＞Y27＞Y2＞Y1＞Y18＞Y10＞Y16＞Y5＞Y4＞Y23＞Y7.It can be seen that there is still a great risk in the soil of the coastal public green space in Jiaozhou Bay. Among the eight heavy metals analyzed, Zn and Cr have high contents, but their toxicity coefficients are very low, 1 and 2 respectively, so the ecological risk is the lowest; Cd and Hg have low contents, but their toxicity coefficients are high, 30 and 40 respectively, so their ecological risk is relatively high compared with other heavy metals. The order of the average single ecological risk factors of eight heavy metals is Cd＞Hg＞Cu＞Pb＞As＞Cr＞Ni＞Zn.

Index of Geoaccumulation Evaluation
Among the eight heavy metal elements analyzed, the average index of geoaccumulation of As, Cr, Cu and Ni are all less than 0, indicating that the coastal public green space of Jiaozhou Bay is almost free from the pollution of these four heavy metals; while the average index of geoaccumulation of Cd, Pb, Zn and Hg is more than 0, 2.10 for Cd specifically, while 0-1 for other 3 heavy metals, indicating that the coastal public green space of Jiaozhou Bay is subject to the medium pollution of Cd and minor pollution of other 3 heavy metals. The order of the average index of geoaccumulation of all metal elements is Cd＞ Hg＞Pb＞Zn＞Cu＞Ni＞Cr＞As.

Analysis of Sources of Soil Heavy Metals
Most of the coastal public green space in Jiaozhou Bay is located at the estuary of the rivers, and some are located around the factory. The main sources of heavy metals in the soil are the heavy metals carried by the surrounding factories and the factories in the upstream of the rivers. In this paper, the content of heavy metals in the soil of the coastal public green space of Jiaozhou Bay was statistically analyzed, and it was found that the content of each index accorded with the normal distribution. On this basis, the correlation of heavy metals was analyzed, and the corresponding correlation coefficient was calculated. The results are shown in Table 8.
When the confidence level (both sides) is 0.01, the correlation is significant. It can be seen that the correlation among Zn, Cu, Pb is relatively large, and that between As, Cd, Hg and other metals is relatively low, with poor correlation. According to the above ecological risk index and index of geoaccumulation, the pollution degree of coastal public green space in Jiaozhou Bay is relatively low, but the pollution degree in some areas is relatively high; the heavy metals with relatively high risk are Cd and Hg, and the correlation between Cd and Hg and other heavy metals is poor, which can be considered that Cd and Hg are affected by human activities; while the other heavy metals are not affected by human activities, still in a clean state [53][54][55][56][57][58][59][60][61][62].

Conclusions
1) The average contents of As, Cr,Cd, Pb,Zn, Hg,Cu, Ni are 6.71, 127.81, 4.27, 50.06, 188.54, 0.82, 79.64 and 52.44mg/kg respectively. The contents of As, Cr, Pb, Zn and Cu are lower than those specified in the second level standard of GB15618-1995 Soil Environmental Quality Standard. The contents of Hg and Ni are between those specified in the second level and the third level, while the content of Cd is higher than that specified in the third level standard. https://doi.org/10.37358/RC. 20.4.8093 2) The single ecological risk factor value and potential ecological risk index value of each heavy metal are calculated with the soil natural background value as the reference standard. Most of the RI values of the samples of the coastal public green space in Jiaozhou Bay of Qingdao are less than 150, indicating that the potential ecological risk index is low. The potential ecological risk coefficients of samples 3, 6,9,11,12,13,14,15,17,19,20,21,22,24,25,26, and 29 are extremely high.The order of the average single ecological risk factors of eight heavy metals is Cd＞Hg＞Cu＞Pb＞As＞Cr＞Ni＞Zn. Among the eight heavy metal elements analyzed, the average index of geoaccumulation of As, Cr, Cu and Ni are all less than 0, indicating that the coastal public green space of Jiaozhou Bay is almost free from the pollution of these four heavy metals; while the average index of geoaccumulation of Cd, Pb, Zn, and Hg is more than 0, while 0-1 for other 3 heavy metals, indicating that the coastal public green space of Jiaozhou Bay is subject to the medium pollution of Cd and minor pollution of other 3 heavy metals. The order of the average index of geoaccumulation of all metal elements is Cd＞Hg＞Pb＞Zn＞Cu＞Ni＞Cr ＞As.
3) Correlation analysis shows that Cd and Hg are affected by human activities, while other heavy metals are not affected by human activities, still in a clean state.