Heavy Metal Pollution and Ecological Risk Assessment in Sediments of Xiling Channel Inland Waterway of Guangdong Province

College of Environmental and Chemical Engineering, Zhaoqing University, Zhaoqing 526061, China

AbstractUsing the sediment monitoring data of five sections of the Xiling Channel inland waterway of the Pearl River Delta, and using Nemerow composite index, the coefficient of variation, and the index of geoaccumulation (Igeo) and the potential ecological risk index, this paper analyzed and assessed the heavy metal pollution of sediments. The results indicate that Cr reached mild pollution; Cu had a large degree of variation, and the changes of Cr and Zn were significant with fluctuation; the enrichment of heavy metals decreased as follows: Cd>Cu>Cr>Zn>Pb>Ni>Hg>As; Cd had the highest degree of enrichment and belonged to moderate pollution; the ecological hazard of heavy metals was Cd>Hg>Cu>Pb>As>Cr>Ni>Zn, and Cd had the highest ecological hazard and was the main controlling factor of potential ecological risk. In conclusion, the sediments in Xiling Channel inland waterway were polluted by heavy metals to some extent, and cadmium was the main pollutant and had the largest potential ecological risk.

Key wordsXiling Channel inland waterway, Sediment, Heavy metal pollution

1 Introduction

Rivers are important surface water resources. The quality of river water bodies and bottom sediments will directly or indirectly affect the environmental quality of the waters discharged to lakes or oceans, accordingly posing a potential threat to local water ecosystems and human health. As the "sink" and "source" of river pollutants, sediment is the carrier, final destination and accumulation tank for the migration and transformation of many pollutants in the environment. Heavy metals in sediments are the result of long-term accumulation, and their concentrations are relatively stable[1].

Heavy metals have a density higher than 4.5 g/cm3and have obvious biological toxicity and are difficult to degrade. Besides, heavy metals cannot be fully absorbed and decomposed in water, and are easily combined with other substances in the water to form more toxic substances, thus people and relevant government departments are caring more about the study on heavy metal pollution and protection and control of sediments[2-3]. Because heavy metals are not easily dissolved after entering the receiving water body, most of them will be subject to certain physical, chemical and biological effects, and then the substances will quickly complete the phase transition process from liquid phase to solid phase, and finally they are accumulated in the sediments[4-5]. The sediment is not only the accumulation place and final boarding place of heavy metals, but also a key part of the water ecological environment. The sediments can absorb various pollutants in the water. However, once the sediment condition is changed due to certain impact, the heavy metals will be released again and become a secondary pollution source, which will pose a potential threat to the water environment system, and then through enrichment and concentration of the organisms at all levels in the food chain, they will eventually enter the human body through biological circulation, causing great harm to human health[6]. Generally, the reference indicator of river water environment quality is the heavy metal content of sediment. Therefore, the research and analysis on sediment heavy metal pollution are of great practical significance and importance[7].

With the rapid development of urbanization, industrialization and agricultural intensification in the Pearl River-Xijiang River Economic Belt Basin, a series of pollution problems have become more serious, and people are caring more about the problem of heavy metal pollution in sediments in the basin. Meanwhile, both domestic and foreign scholars are also very concerned about the research on heavy metal pollution in sediments. In the Twelfth Five-Year Plan period, in line with the principles of developing shipping, developing tidal flat resources and improving the ecological environment, the state and government departments strive to create a low-carbon, green, convenient and efficient high-level waterway in Pearl River Delta[8-9]. In 2014, the scheme forDevelopmentPlanforPearlRiver-XijiangRiverEconomicBeltwas officially approved. This file intends to develop Pearl River-Xijiang River Economic Belt in the pattern of "one axis, two cores, and four groups with extension regions", to create comprehensive traffic passage, and build green and ecological corridor[10-11]. Xiling Channel inland waterway is a key channel for implementingDevelopmentPlanforPearlRiver-XijiangRiverEconomicBelt, and accelerating reconstruction and expansion of Xijiang and Beijiang river channels. At present, due to the fragile ecological environment of Xiling Channel inland waterway, water pollution is deteriorated in some rivers and water ecosystems are degraded. Therefore, it is urgent necessary to actively strengthen the water ecological environment, strive to build an environment-friendly Pearl River, and build green and environmental Xiling Channel inland waterway.

Taking Xiling Channel inland waterway as the research object, we analyzed the pollution of heavy metals (As, Hg, Pb, Cu, Zn, Cr, Ni, and Cd) in sediments, and carried out the survey and assessment of river sediments, in order to provide reliable and scientific basis for ecological protection, prevention and control of heavy metal pollution and sediment treatment risk assessment of Xiling Channel inland waterway.

2 Experimental materials and methods

2.1 Overview of the study areaXiling Channel inland waterway flows through Guangdong Province, mainly including Guangzhou, Foshan, and Zhongshan cities. These three cities have average annual temperature of 22℃, they have long summer and short winter, warm climate, plentiful rainfall and rich heat, belong to subtropical monsoon climate, and both the wind and rain are adjusted by marine meteorology. The water bodies near the study area mainly include five waterways: Donghai, Ronggui, Hongqili, Xiahengli and Zhenxiang. Donghai waterway originates from the western boundary of Nanhua and extends to the eastern part of Yinggezui. It is 21 km long and is mainly located in the southwest of Shunde County. It is named Donghai because it is located in the east of the mainstream of Xijiang River. Ronggui waterway originates from Yinggezui of Zhongshan City, flows through Rongqi, and reaches the Banshawei in the east, and it is about 19 km long, and the flow area is 319 km2. Hongqili waterway flows through the southwestern side of Nansha District of Guangzhou City, adjacent to Shunde City and Zhongshan City. It originates from Banshawei in Shunde County. After passing through Ronggui waterway, it flows to the southeast and flows through Wanqingsha Town in Nansha District of Guangzhou, and flows to the ocean through Hongqimen. Xiahengli waterway runs from Yishatou to Jiaomenkou, about 14 km long. It belongs to the Class I inland waterway and meets the Jiaomen waterway. It is adjacent to the Pearl River Estuary and is surrounded by water on three sides. It is peninsula-shaped. Zhenxiang waterway starts from Nanshakou to Dajiaozui, about 7 km long.

2.2 Sample collection and processingAccording to the actual situation of Xiling Channel inland waterway, we collected the sediment sample in river sections. Along the direction of water flow, and set the monitoring sections for each waterway. Combining the distribution characteristics of regional water bodies and characteristics of surrounding environment, we arranged five monitoring sections (Table 1).

With the aid of a piston column bottom sampler, we collected 3 pieces of the 0-20 cm mud samples of the sediment. After mixing evenly, we sealed the samples in a clean polyethylene bag and marked, and transported back to the laboratory for storage under refrigeration (-20℃). Before the analysis, the collected sediment samples were air-dried in a cool place to remove plant roots, stones and other impurities. After grinding through an agate mortar, we stored them at a low temperature through a 100 mesh nylon sieve.

Table 1 Sampling sections of sediments in Xiling Channel

Sample No.River nameLocation of sampling sectionGeographical coordinateW1Donghai WaterwayQijiao Bridge section(22°45′18.77″ N, 113°06′49.75″ E)W2Ronggui WaterwayWangqi Bridge section(22°46′58.73″ N, 113°17′11.63″ E)W3Hongqili WaterwaySection at intersection of Ronggui Waterway tributary and mainstream(22°48′05.98″ N, 113°20′56.11″ E)W4Xiahengli WaterwayXiahengli Bridge section(22°48′16.65″ N, 113°32′39.99″ E)W5Zhenxiang WaterwaySection at Zhenxiang Waterway dredging point(22°44′07.73″ N, 113°36′01.66″ E)

2.3 Sample determinationWe used a full-automatic graphite digester (ST-60) to digest the sediment samples with hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid system[12-13]. Then, we measured the contents of heavy metals Cd, Cr, Cu, Ni, Pb, As, Hg and Zn in the sediment using ICAP 6200 inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermos Scientific, USA). The basic chemical properties of the soil were determined by soil agrochemical analysis[14]: the water and soil were mixed at a ratio of 2.5∶1 to measure the soil pH.

2.4 Assessment standards and methods

2.4.1Assessment standards. Since there is still no quality standard for river sediments, according to the characteristics and behavior of the water environment, it is determined that the environmental quality assessment standards for river sediments in this study will adopt Category 2 standard inMarineSedimentQuality(GB18668-2002) and the relevant requirements of the Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995), with reference to the background value of soil heavy metals in Guangdong Province[15], as shown in Table 2.

2.4.2Evaluation methods. (i) Nemerow composite index method.Nemerow composite index is a weighted multi-factor environmental quality index based on single factor index assessment. It is based on extreme or maximum values. This method has been widely applied to calculate the comprehensive pollution index. The calculation formula for this method is as follows:

Pi=Ci/Si(i=1, 2, 3,...,k;p=1, 2, 3,...,m)

(1)

wherePidenotes the single pollution index of pollutanti,Ciis the measured value of pollutant, andSiis the soil environment standard value of heavy metal.

(2)

Table 2 Assessment standard for environmental quality of sediments

(mg/kg)

Table 3 Grade of soil pollution degree of Nemerow composite index

GradeNemerow composite indexPollution gradeIPcomprehensive≤0.7Clean (safe)II0.7 3.0Highly polluted

(ii) Coefficient of variation method. The coefficient of variation is a measure of the level of change in each measurement, also known as the standard deviation[8]. The calculation formula is as follows:

(3)

(4)

whereCVis the coefficient of variation of heavy metals,Snis the standard deviation of heavy metal elements in each cross section;Lnis the mean value of the contents of each heavy metal element.

(iii) Index of geoaccumulation (Igeo) method. The index of geoaccumulation (Igeo), proposed by Muller, Professor from Institute of Sediment Research, Heidelberg University, Germany), is used for quantitative research and analysis of heavy metal pollution of sediments in the aquatic environment. This index can be calculated by the following formula:

Igeo=log2Cn/(K·Bn)

(5)

whereCndenotes the measured heavy metal concentration (mg/kg),Bnis the background value of soil heavy metal,Kis a constant (the value of background fluctuation caused by various factors), usually take 1.5[16].

Selecting a different background value will result in a large difference in the calculation results, so choosing a goodBnvalue is very important for the calculation of the Igeo value. According to the previous studies, in the study of the status of heavy metal pollution in sediments, the geochemical background values of the relevant parameters are usually calculated using the sediment or soil heavy metal background values in the area. Since this study area is located in Guangdong Province, we used the background value of soil heavy metals in Guangdong Province as the background value for this pollution assessment. Igeo and pollution level of heavy metals are listed in Table 4.

(iv) Potential ecological risk index method. The potential ecological risk index method, proposed by Hakanson, is based on the characteristics and environmental behavior of heavy metal elements in the sediment, and it has been widely applied in research and assessment of heavy metal pollution of sediments. Through summarizing and combining the various aspects of ecology, environmental science and biotoxicology, this method can quantify the potential risk of heavy metals[17-19]. The calculation formula is as follows:

(6)

Table 4 Relationship between the index of geoaccumulation and pollution level

IndexIgeoPollution indexGrade 0Igeo≤0CleanGrade I0< Igeo≤1Mildly pollutedGrade II1< Igeo≤2Slightly moderately pollutedGrade III2< Igeo≤3Moderately pollutedGrade IV3< Igeo≤4Slightly highly pollutedGrade V4< Igeo≤5Highly pollutedGrade VIIgeo< 5Seriously polluted

Table 5 Heavy metal background value and ecological toxic reaction coefficient

Heavy metal∥mg/kgAsHgPbCuCrZnNiCdCin8.90.08361750.55127.70.04Tir10.040.00552.015.030.00

Table 6 Relationship between potential ecological risk coefficient, potential risk index and risk degree

Risk degreeMildModerateHighVery highExtremely highPotential ecological risk coefficientEir<4040≤Eir<8080≤Eir<160160≤Eir<320Eir≥ 320Potential ecological risk indexRI<150150≤RI<300300≤RI<600RI≥600-

3 Results and analysis

3.1 Results and analysis of sediment status monitoringSediment status monitoring and statistical results of Xiling Channel inland waterway are listed in Table 7.

Table 7 Sediment status monitoring and statistical results of Xiling Channel inland waterway

ItemW1W2W3W4W5MeanMarine Sediment Quality(GB18668-2002)Category 2 standardGrade II standard of EnvironmentalQuality Standards for Soils(GB15618-1995)BackgroundvaluepH6.766.986.656.877.086.87-6.5-7.5-As∥mg/kg6.97.46.16.65.76.5465258.9Hg∥mg/kg0.0610.0920.0520.0570.0490.0620.50.500.08Pb∥mg/kg23.811162.625.685.661.7213030036Cu∥mg/kg159844226849.410010017Cr∥mg/kg54289766196115.215020050.5Zn∥mg/kg69.419284.551.5126104.6835025051Ni∥mg/kg134025162624505027.7Cd∥mg/kg0.180.220.150.200.170.181.50.300.04Total∥mg/kg182738298183408362---

According to Table 7, all monitoring indicators of four monitoring sections (W1Donghai Waterway, W3Hongqili Waterway, W4Xiahengli Waterway, and W5Zhenxiang Waterway) met the Category 2 standard inMarineSedimentQuality(GB18668-2002) and the Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995); Cr indicator of monitoring section W2(Ronggui Waterway) exceeded Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995) by 44.5%, and other monitoring indicators met the limit value of the standard.

The average content of As and Hg did not exceed the background value of soil heavy metals, and other monitoring indicators exceeded the standard by different degrees. The average total content of eight heavy metals in the sediment of Xiling Channel was 362 mg/kg , of which the pollution was most serious in Ronggui Waterway (W2), and the average total content of all heavy metals was 738 mg/kg; the condition of Donghai Waterway (W1) and Xiahengli Waterway (W4) was slightly better, and the average total content of heavy metals was 182 and 183 mg/kg, respectively.

3.1.1Analysis of the content of heavy metals in sediments. The average content of heavy metals in the sediments of Xiling Channel inland waterway at five monitoring points is shown in Table 7. According to Table 7, Cr was the largest in the average content of heavy metals in the sediments at these five monitoring points, up to 115.2 mg/kg. The order of the average content of each heavy metal element was Cr>Zn>Pb>Cu>Ni>As>Cd>Hg.

3.1.2Analysis of the spatial characteristics of heavy metals in sediments. We analyzed the spatial characteristics of heavy metals in sediments of Xiling Channel inland waterway and the results are shown in Fig. 1. According to Fig. 1, except for As, other heavy metal factors exceeded the soil background value by different degrees, but basically did not exceed the Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995), and the maximum value of all heavy metals appeared in the W2section.

Fig. 1 Spatial distribution characteristics of heavy metal content in sediments

The spatial distribution characteristics of each heavy metal: As and Hg had a relatively uniform spatial distribution and the distribution rules were basically the same, the order of heavy metal content at each monitoring point is W2>W1>W4>W3>W5; the spatial distribution of Pb, Cu, Cr and Ni was quite different, and the distribution rules were almost the same, the order of heavy metal content is W2>W5>W3>W4>W1; the spatial distribution of Zn was not uniform, the order of heavy metal content in each monitoring section was W2>W5>W3>W1>W4; the spatial distribution of Cd was relatively uniform, and the order of heavy metal content was W2>W4>W1>W5>W3.

3.1.3Correlation analysis of the heavy metals in sediments. From Table 8, it can be seen that, except Cd and each heavy metal, As and Pb, Cu, Cr, Zn and Ni, Hg and Pb, Cu, Zn and Ni, Pb and Cr showed a weak positive correlation, and there was certain positive correlation between the remaining heavy metals. At the 0.05 level, there was positive correlation between Hg and As and Cr, between Zn and Pb, between Cu and Cr, between Cu and Cr, between Ni and Cr and Zn; at the 0.01 level, there was a high positive correlation between Cu and Pb and Ni, between Pb and Ni. In other words, Cu and Pb, Cu and Ni, Pb and Ni may be homologous pollution. Heavy metal elements in sediments of Xiling Channel inland waterway come from different sources, the comprehensive composite pollution characteristics are significant.

Table 8 Correlation coefficient (Pearson correlation) of heavy metals in sediments of Xiling Channel inland waterway

CorrelationcoefficientAsHgPbCuCrZnNiCdAs1Hg0.879∗1Pb0.0610.5261Cu0.1600.6100.990∗∗1Cr0.6010.909∗0.8270.883∗1Zn0.3360.7350.948∗0.969∗0.933∗1Ni0.2560.6780.964∗∗0.975∗∗0.916∗0.944∗1Cd0.7950.8170.2400.3710.6800.4660.3771

3.2 Assessment of sediment pollution statusIn this study, we used Nemerow composite index, the coefficient of variation, and the index of geoaccumulation (Igeo) and the potential ecological risk index to make accurate and effective assessment of heavy metal pollution of sediments at Xiling Channel inland waterway.

3.2.1Sediment pollution assessment: Nemerow composite index method. According to Table 9, the average value of the heavy metals in the sediments of Xiling Channel inland waterway was less than 0.7, except for As, Hg, Pb, and the other five heavy metal factors had a maximum pollution index higher than 0.7. In sediments of Xiling Channel inland waterway, Nemerow composite index of Cr was 1.10, reaching Grade III (mildly polluted); Nemerow composite index of Cu was 0.78, the pollution grade was low, belonging to Grade II (practically clean); Nemerow composite index of other heavy metals was lower than 0.7, belonging to Grade I (clean, not polluted). Although the pollution level of most heavy metals ius in the scope of clean-practically clean, early measures are needed to prevent heavy metals in the sediments from accumulating to the level of mild pollution.

Table 9 Pollution degree of heavy metals in sediments at Xiling Channel inland waterway

ElementPimax(Pi)max(Pi)2PcomprehensivePollution degreeAs0.260.300.090.28Grade I (clean)Hg0.120.180.030.16Grade I (clean)Pb0.210.370.140.30Grade I (clean)Cu0.490.980.960.78Grade II (practically clean)Cr0.581.452.091.10Grade III (mildly polluted)Zn0.420.770.590.62Grade I (clean)Ni0.480.800.640.66Grade I (clean)Cd0.610.730.540.68Grade I (clean)

3.2.2Coefficient of variation method. From Table 10, it can be seen that the variation degree of heavy metal elements in the sediment at Xiling Channel inland waterway was: Cu>Pb>Zn>Cr>Ni>Hg>Cd>As, showing highly discrete characteristics of the data, indicating that human activities may exert certain impact on the channel. The distribution rules of heavy metals in sediments along Xiling Channel inland waterway are shown in Fig. 2.

Table 10 Coefficient of variation for heavy metals in sediments at Xiling Channel inland waterway

Sampling pointAsHgPbCuCrZnNiCdMean∥mg/kg6.540.06261.7249.40115.20104.6824.000.180Standard deviation0.600.01533.8930.5746.8050.129.440.024Coefficient of variation0.090.2400.550.620.410.480.390.130

The results show that Cr and Zn changed greatly along the channel, and the change trend was similar; the changes of Pb and Cu were relatively slow, and the change rules were general; the change rules of Ni were not obvious; As, Cd and Hg along the channel were basically unchanged. In the monitoring points W1to W3, the distribution of heavy metal content fluctuated greatly and there was obvious peak value. In the monitoring points W3to W5, the distribution of content of each heavy metal factor was relatively flat, and there were no obvious peaks and valleys. Such distribution was mainly related to the actual sewage discharge and hydrological conditions of the river channel.

Fig.2 Distribution pattern of heavy metals in sediments at Xiling Channel inland waterway

3.2.3Sediment pollution assessment: Index of geoaccumulation (Igeo) method. From Table 11, it can be known that the channel sediment has been polluted by heavy metals to a certain extent. Among them, only two monitoring points were not polluted by Pb, Cu, Cr and Zn in the entire channel, and other monitoring points show different degrees of pollution. The Igeo range of Pb was between -1.18 and 1.04, 40% of the sampling point reached Grade I, and 10% of the sampling point reached Grade II and have been slightly polluted; the Igeo of Cu was between -0.21 and 1.94, 60% of the sampling points were at Grade I and above, reaching a mild pollution level; the Igeo of Cr and Zn were between -0.49 and 1.93 and between -0.58 and 1.33, respectively, 40% of the sampling points reached Grade I, and 20% reached Grade II, showing mild pollution; the Igeo of Cd was between 1.32 and 1.87, all sampling points reached Grade II, and the pollution level has reached slightly moderate pollution.

Table 11 Assessment results of Igeo and pollution degree of heavy metals in sediments at Xiling Channel inland waterway

Sampling pointIgeoAsHgPbCuCrZnNiCdW1-0.95-0.98-1.18-0.77-0.49-0.14-1.681.58W2-0.85-0.381.041.941.931.33-0.051.87W3-1.13-1.210.210.790.0050.14-0.731.32W4-1.02-1.07-1.08-0.21-0.31-0.58-1.381.74W5-1.23-1.290.661.420.340.72-0.681.50Mean-1.03-0.950.190.950.600.45-0.791.58Pollution gradeGrade 0Grade 0Grade IGrade IGrade IGrade IGrade 0Grade IIPollution degreeCleanCleanMildly pollutedMildly pollutedMildly pollutedMildly pollutedCleanSlightly moderatelypolluted

The enrichment level of each heavy metal in the sediment was Cd>Cu>Cr>Zn>Pb>Ni>Hg>As. The sediments were basically not polluted by As, Hg and Ni, belonging to clean grade; the pollution level of Pb, Cu, Cr, and Zn was Grade I, reached mild pollution; Cd pollution was serious and reached the level of slightly moderate pollution. Therefore, the prevention and control of Cd pollution in sediments at Xiling Channel inland waterway should not be slackened. Besides, the pollution of other heavy metals should not be neglected, to prevent the continuous deterioration of the pollution.

3.2.4Potential ecological risk assessment. According to Table 12, the comprehensive ecological risk index of heavy metals in all five monitoring sections exceeded 150 and reached the moderate ecological risk. The ecological risk of various heavy metals was Cd>Hg>Cu>Pb>As>Cr>Ni>Zn. As, Pb, Cu, Cr, Zn and Ni contributed less to the potential ecological risk index, and the ecological pollution was mild, the potential ecological risk index of Hg in W2section was moderate ecological risk, but Cd had a contribution rate of 57.7%-74.2% to potential ecological risk index, and the potential ecological risk index was 135, the ecological risk was the highest, so it was the main controlling factor for potential ecological risk.

Table 12 Potential ecological risk index for heavy metals in sediments at Xiling Channel inland waterway

Sampling pointEir<40AsHgPbCuCrZnNiCdRIW17.7530.503.314.412.141.362.35135.00186.82W28.3146.0015.4228.8211.453.767.22165.00285.99W36.8526.008.6912.943.011.664.51112.50176.17W47.4228.503.566.472.421.012.89150.00202.26W56.4024.5011.8920.003.802.474.69127.50201.26Mean7.3531.008.5714.534.562.054.33135.00207.40

According to Table 13, the overall heavy metal pollution in sediments at Xiling Channel inland waterway belonged to moderate ecological risk, and should attract attention and should be strengthened for environmental protection.

Table 13 Potential ecological risk degree for heavy metals in sediments at Xiling Channel inland waterway

Sampling pointEir risk degreeAsHgPbCuCrZnNiCdRIW1MildMildMildMildMildMildMildHighModerateW2MildModerateMildMildMildMildMildVery highModerateW3MildMildMildMildMildMildMildHighModerateW4MildMildMildMildMildMildMildHighModerateW5MildMildMildMildMildMildMildHighModerateMeanMildMildMildMildMildMildMildHighModerate

4 Conclusions

Based on the survey of heavy metal pollution in sediments of Xiling Channel inland waterway, we analyzed the content, spatial characteristics and correlation of heavy metals. The results indicate that heavy metal elements in sediments of Xiling Channel inland waterway come from different sources, the comprehensive composite pollution characteristics are significant. Specifically, the Cr indicator of W2monitoring section (Ronggui Waterway) exceeded Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995); and monitoring indicators of other sections met the Category 2 standard inMarineSedimentQuality(GB18668-2002) and the Grade II standard ofEnvironmentalQualityStandardsforSoils(GB15618-1995).

Using the sediment monitoring data of five sections of the Xiling Channel inland waterway of the Pearl River Delta, and using Nemerow composite index, the coefficient of variation, and the index of geoaccumulation (Igeo) and the potential ecological risk index, we assessed the heavy metal pollution of sediments. According to the survey results, Nemero composite index of Cr was 1.10, belonging to Grade III, reached mild pollution; the coefficient of variation of Cu was 0.62, and the degree of variation was large, which may be related to coastal pollution and actual hydrological conditions; the Igeo of Cd was in the range of 1.32-1.87, so Cd had the highest degree of enrichment and reached slightly moderate pollution; Cd contribution rate to potential ecological risk index was 57.7%-74.2%, potential ecological risk coefficient was up to 135, the ecological risk was the highest, so Cd was the main controlling factor of potential ecological risk[19-21]. In summary, the main pollutant in the sediment of Xiling Channel inland waterway is cadmium pollution. The cadmium pollution mainly comes from the diffusion of cadmium in the industrial waste gas along the wind, and it accumulates in the soil around the plant through natural sedimentation; industrial wastewater containing cadmium is used to irrigate farmland, so that the soil is contaminated by cadmium. Most of the cadmium-containing wastewater comes from the mining industry, smelting refining and electroplating industry along the river. A large amount of cadmium-containing wastewater is discharged into the river and leads to cadmium pollution. Once the environment is polluted by cadmium, cadmium can be enriched in living organisms and cause chronic poisoning through the food chain[20,22].

Therefore, in order to prevent and control the pollution of sediments of Xiling Channel inland waterway, related departments should carry out environmental protection work, strictly implement cadmium standard, raise people’s environmental protection awareness, advocate environmental protection, and related enterprises should strictly obey related provisions of environmental protection policies and regulations, produce environment-friendly products and the wastewater should reach the discharge standard.