Impact Imposed by Urbanization on Soil Heavy Metal Content of Lake Wetland and Evaluation of Ecological Risks in East Dongting Lake in China

The urbanization is regarded as the major factor related to human activities that may interfere with the natural ecosystem. In this study, we have selected the wetland of East Dongting Lake as the research area. We have collected 180 soil samples (within the range of 0–20 cm, and 20cm-40cm), and we have measured the contents of their physicochemical properties (including salinity, pH value, soil particle composition, soil organic carbon, nitrate nitrogen and rapidly available phosphorus) as well as heavy metal elements (including As, Hg, Cd, Pb, Ni and Cr). We have adopted the methods of multivariate statistical analysis and inverse distance weighted (IDW) interpolation, so as to to reveal the sources and distribution characteristics of heavy metal content in soil in the research area. By adopting the potential ecological risk index (PERI) method proposed by Hakanson, we intend to assess the PERI values of heavy metals. Our research ndings have shown that: (1) 8 sorts of heavy metals have shown positive correlation with each other. As, Hg and Zn have shown a signicantly positive correlation with SOC (P < 0.01); As, Ni, Cr and Zn have shown a signicantly positive correlation with AP (P < 0.01); As and Pb have shown a signicantly positive correlation with Clay (P < 0.01); whereas Hg and Zn have shown a signicantly negative correlation with Silt (P < 0.01); As and Pb have shown a signicantly negative correlation with Sand (P < 0.01). (2) During urbanization, the elements of Cd, Ni, As, Hg and Pb might be enriched due to agricultural and industrial activities, whereas the use of fertilizers and pesticides constitute one of the major factors contributing to the increase of Cd and Pb contents in soil. (3) Inuenced by the varying land patterns and with exception to Cu, the Fe-normalized concentrations have shown signicant variations among different types of land use (P < 0.05). Specically, there is a signicantly higher level of Cd, Zn, Pb and Hg contents in the agricultural land than other types of land use, whereas there is a slightly higher level of heavy metal content in the mudats than that in the grassland. In addition, the content of heavy metals in woodland remains relatively stable, and with exception to As, the content of heavy metals in woodland is the lowest among the ve types of land. (4) The average value of the comprehensive PERI in heavy metals amounts to 555.03, representing a strong degree of ecological risks. Specically, the proportion of sampling points of Cd that have a high level of ecological risks amounts to 36.51%, which is the primary element contributing to heavy metal pollution in the region, especially in the agricultural land.


Overview of the study area
Located on the south side of Jingjiang River in the middle reaches of the Yangtze River, the East Dongting Lake is positioned between 28°59" and 29°38" north latitude, and falls in the range between 112°43" and 113°15" east longitude. The lake area is adjacent to Yueyang City, a renowned historical and cultural city in northern Hunan Province, and covers a total area of 190,000 hm². Moreover, the East Dongting Lake National Nature Reserve constitutes one of the 21 wetland nature reserves of international importance. These sites are designated by the Chinese government for inclusion into the International Convention Wetlands (also known as the Ramsar Convention), and the reserve in East Dongting Lake mainly serves the purpose of protecting the wetland ecology and biological resources in the lake area. Located in the subtropical humid climate zone, this region features ample sunshine and rainfall as well as annual average temperature of 17 C°. In addition, the regional precipitation ranges between 1,200 and 1,300 mm, whereas its frost-free period lasts for 285 days.
Thanks to the distinct ecological environment of the East Dongting Lake, the region is richly endowed with natural resources, whereas the local species feature long history, uniqueness and high rarity. As one of the only two natural river-connected lakes in the middle and lower reaches of the Yangtze River, the Dongting Lake plays a signi cant role in regulating the ood runoff of the Yangtze River and preserving the genes of species or biodiversity.
2.2 Statistical sources and data processing 2.2.1 Land use data In this study, we have adopted the data extracted from Landsat OLI remote sensing images for the location map of Dongting Lake. First, we have conducted the remote sensing image preprocessing by using the ENVI5.1 software (Kulkarni et al. 2018). Second, we have divided the types of land use by object-oriented and visual interpretation methods (into 6 categories, namely, construction land, grassland, cultivated land, woodland, beach and water body) (Bierschenk et al. 2019). In addition, we have assessed the accuracy of the classi cation map by adopting the error matrix and analyzing the 180 ground control points recorded during eld investigation (Huang et al. 2019). Our research ndings have shown that the classi cation accuracy and Kappa coe cient amount to over 87% and 0.92, respectively, proving the optimal accuracy of classi cation (Ghosh et al., 2019).

Collection of soil samples and test analysis
Based on the principles of sample availability, representativeness and uniformity, we have taken into full account of the varying history of wetlands, patterns of land use, coverage of vegetation and types of soil in the region (Crocker et al. 2021), We have collected 180 soil samples in 2019 and 2020, and we have recorded the eld coordinates via GPS with precision of 3-5 m . In the vicinity of each sampling point, we have taken samples three times for every 50-100 m and fully mixed them to form a complete soil sample (Gu et al. 2019). We have packed all soil samples in polyethylene bags. In the meantime, we have recorded relevant information (i.e., type of landscape, type of soil, coverage of vegetation) in detail, while bringing the samples back to the laboratory for air drying under natural conditions (Chen and Hu 2019). We have collected 180 soil surface (with depth ranging between 0 and 20 cm) samples.
Subsequent to the air drying, we have removed the soil debris and carried out grinding and sifting. In addition, we have further grinded some of the soil in the sample until it can pass through a nylon sieve of 0.149 mm, so as to measure the metal items in soil (Keshta et al., 2020). To reveal the features of soil quality and environmental conditions, we have selected 17 indexes representing the physicochemical properties as well as heavy metals in soil by integrating the methods of both outdoor and indoor measurements (La Colla et al., 2018). Speci cally, we have adopted soil pH meter during eld measurements . To measure and determine the composition of soil particles (clay, silt and sand) by adopting the laser particle size analyzer (Microtrac S3500, made in USA) during laboratory experiments (Huang et al., 2018). We have selected three indicators, namely, soil organic carbon (SOC), nitrate nitrogen (NO3-N) and available phosphorus (AP), fore the determination of soil nutrients. Speci cally, we have adopted the total organic carbon analyzer (HT1300, made in Jena of Germany) to determine the SOC subsequent to the elimination of soil calcium carbonate with hydrochloric acid (Huang et al., 2018). We have determined the NO3-N with ultraviolet spectrophotometry (Chai et al., 2019). In the meantime, we have measured the content of AP through the photoelectric colorimetric method (Lyu and . We have selected 10 indexes in total for the measurements of heavy metals in soil , including As, Hg, Cd, Pb, Cu, Zn, Mn, Fe, Ni and Cr, respectively. Predominantly in uenced by rock and soil weathering , there is abundant content of Fe and Mn in nature, and thus they are oftentimes selected as standardized elements so as to eliminate the impact imposed by natural processes on the spatial differentiation of heavy metals (Manuel Trujillo-Gonzalez et al., 2019). We have sifted the soil sample with a 0.149 mm nylon sieve, and subsequent to 6 hours of high temperature digestion of the mixture of nitric acid, hydro uoric acid and perchloric acid, we have adopted the inductively coupled plasma-atomic emission spectrometry (HITACHIP 4010, made in Japan) for measurements (Manzoor et al. 2018 Descriptive statistics of the physicochemical properties of soil and heavy metal concentrations are shown in Table 2. The results of statistical analysis have shown that the soil texture is mainly composed of silty loam and sandy loam. The pH value of soil ranges between 5.10 and 8.40, representing weak acidity to weak alkalinity. Moreover, the variation of the contents of soil organic carbon, nitrate nitrogen and available phosphorus fall in the range of 0.05%~2.88%, 0.25 ~ 10.72 mg/kg and 0.72 ~ 26.46 mg/kg. Based on the nutrient classi cation standard adopted in the National Second Soil Census in China (Saranya et al., 2018), the nutrients of soil in the research area remain at a low level. In addition, the contents of Cu, Cr, CD, Ni, Zn, As, Hg and Pb fall in the range of 10.65 mg/kg ~ 39.48mg/kg, 22.19 ~ 76.54mg/kg, 0.8 ~ 1.08mg/kg, 23.09 ~ 54.23mg/kg, 31.34 ~ 114.37mg/kg, 17.91 ~ 148.95mg/kg, 0.15 ~ 0.56mg/kg and 3.98 ~ 35.57mg/kg. By referring to the soil background value and the national second-level standard for the 8 heavy metals in Dongting Lake (Sharma et al. 2019), we have found that the contents of As, Cd and Hg of all soil samples exceed the soil background value, whereas part of the other heavy metal samples exceed the standard. The coe cient of variation of heavy metal elements vary within the range of 0.22 ~ 36.99% (Sibal and Espino, 2018). Speci cally, the concentrations of the elements of Cr, Ni, As and Hg feature a high coe cient of variation (> 17%). By contrast, the concentrations of the elements of Cd, Cu, Pb and Zn feature a low coe cient of variation (< 17%), indicating that they have led to a wide coverage of pollution in the research area (Cardwell et al., 2002).

Correlation of soil data
As shown by Table 3, the relationships among the 8 heavy metals are as follows: There is a signi cantly positive correlation between As and Hg, As and Pb, As and Zn, Hg and Zn. soil and the elements of heavy metals, in addition to the identi cation of pollution sources (Meng et al., 2017). Last but not least, we have adopted the method of Multivariate Analysis of Variance, MANOVA) in this study to explore the distribution of heavy metal concentration among varying modes of land use (Sodango et al., 2018). We have conducted all statistical analyses through the software SPSS18.0, whereas we have applied the softwares of MiniTab17.0 and Origin8.6 to data conversion and statistical mapping respectively (Naylo et al., 2019).

Potential ecological risk assessment of heavy metals in soil
The method of potential ecological risk index (PERI) proposed by Hakanson (Othman et al., 2018) has taken into account the environmental background value, toxicity response coe cient, pollution level and environmental sensitivity of numerous heavy metal elements, and is thus the most commonly used method at home and abroad at present for assessing the ecological risks brought along by heavy metals . The calculation formula is as set out as follows: In the formula, PERI refers to the comprehensive potential ecological risk index (Grybos et al., 2007); E i r refers to the single potential ecological risk index of heavy metal element i in the soil; T i r refers to the toxicity response coe cient of heavy metal element i (Cr = 2, Cd = 30, As = 10, Hg = 40, Cu = Ni = Pb = 5, Zn = 1) (Ciarkowska 2018); C i f refers to the pollution index of heavy metal element i; Ci refers to the measured value of the heavy metal element i, mg/kg; C i n refers to the environmental background value of heavy metal element i (Rastmanesh et al., 2018) (Cu, Cr, Cd, Ni, Zn, As, Hg and Pb are 26.0, 68.0, 0.10, 32.0, 94.0, 14.0, 0.10 and 27.0 mg/kg, respectively). In this study, the classi cation criteria of PERI of heavy metals in soil are shown in Table 1. Speci cally, Eir is divided into 5 grades, PERI is divided into 4 grades.    The soil organic matter, pH values and soil texture can impose a direct or indirect impact on the migration and dissolution of heavy metal elements in sediments (Chen and Hu, 2019). As shown by the analysis of correlation matrix (according to Table 3), the elements of As, Hg and Zn are positively correlated with SOC (P < 0.01); the elements of As, Ni, Cr and Zn are positively correlated with AP (P < 0.01); moreover, the elements of As and Pb are positively correlated with Clay (P < 0.01). On the other hand, the elements of Hg and Zn are negatively correlated with Silt (P < 0.01); whreas the elements o As and Pb have shown a signi cantly negative correlation with Sand (P < 0.01). Relevant studies have shown that the strong absorption capacity of soil organic matter and ne particles (including clay and powder) would facilitate the accumulation of heavy metal elements (Ghosh et al., 2019). However, there has been no signi cant correlation between Mn and Fe with soil organic carbon, indicating that the input of soil organic carbon may be subject to the in uence of external factors (Vongdala et al., 2019).

Factor analysis of soil data
In terms of the heavy metal elements distributed in soil, we could draw on reference elements to eliminate the in uence of natural processes, so as to facilitate the quantitative description of human factors . Fe is primarily originated from the weathering of rocks and soils and is less affected by human factors in general (Cardwell et al., 2002). According to Table 2 and Table 3, the concentration of Fe features a low coe cient of variation and has shown a signi cantly positive correlation with other metals in soil, which indicates that Fe can be used as a reference element for standardized data and can correct the effect imposed by the size of soil particles on the distribution of metals (Feng et al., 2004). Therefore, we have analyzed the raw data and the Fenormalized data (i.e., the ratios of Cu, Cr, Cd, Ni, Zn, As, Hg and Pb to Fe) to further determine the relationship between soil physicochemical properties and heavy metals, in addition to the sources of heavy metal elements (see Fig .2 and Table 4) (Jiang and Guo, 2019) Variance. As shown by the rotation factor load matrix, Factor 1 (F1) accounts for 18.03% of the total variance, which includes As, Fe, Mn, Zn and soil organic carbon. This result also conforms to our research ndings during the analysis of correlation matrix. Factor 2 (F2) mainly includes nitrate ammonia, Pb and soil particles, and the factor load all exceeds 0.50. Ni and Cr constitute the primary control factors of Factor 3 (F3), whereas the factor load amounts to 0.6, accounting for 10.69% of the total variance. The closely related indexes in Factor 4 (F4) include heavy metals Cd and Hg, whereas their factor load amount to 0.57 and 0.55, respectively. For the Fe-normalized data, we have also extracted four factors, accounting for 65.34% of the total variance. F1 only includes the elements of Pb, Ni, Hg and Cd, whereas their loads all exceed 0.70; F2 includes the Fe-normalized AP and particle composition of soil, with their loads all exceeding 0.70. This indicates that the effect imposed by soil particle size on heavy metals becomes smaller after Fe standardization. F3 extracted from the Fe-normalized data contains the elements of Mn and Zn, whose loads amount to 0.7 and 0.8, respectively. F4 of the Fe-normalized data contains the pH value, soil organic carbon and Cr, whose loads amount to 0.70, 0.79 and 0.67, respectively. In general, soil particle components can impose a signi cant impact on the physicochemical properties of soil and the distribution of heavy metals. The average contents of Cd, Ni, As, Hg and Pb in the research area all exceed the soil background value of Hunan Province. Therefore, based on the results of factor analysis, we may infer that the contents of Cd, Ni, As, Hg and Pb are mainly subject to the in uence of human factors.The surrounding area of the East Dongting Lake is the major area of agricultural production in northern Hunan Province. Consequently, a large amount of agricultural wastewater originated from non-point pollution sources ows into the Liaohe River every year. Hence, human activities may be one of the primary reasons for the enrichment of the aforementioned ve elements of heavy metals.The relationship between the contents of Cd, Pb and AP indicate that the use of chemical fertilizers and pesticides in the research area may be one of the major reasons behind the increase of Cd and Pb contents in soil. In certain developed countries in Europe and America, the cadmium content in phosphate fertilizer is subject to legislative restrictions. Furthermore the content of cadmium permitted for use is continuously reduced in the standard, whereas the application amount is kept under stringent control (Khalifa and Gad, 2018). However, in the agricultural area surrounding the East Dongting Lake, the application of phosphorus fertilizer has not been subject to stringent control, and consequently the area is plagued with the abuse of phosphorus fertilizer. Based on relevant studies, the largest source of phosphorus was net input of food/feed in 1985-1994. However the application of phosphorus fertilizer took its place to become the largest input source in 1995-2015 (Wijesiri et al., 2018), followed by food/feed net phosphorus input, whereas the smallest source of input was non-food phosphorus.
Agricultural production and population growth constitute the major sources of phosphorus input derived from human activities, and their negative ecological effects cannot be neglected (Ciarkowska, 2018). In terms of the element Pb, the application of pesticides may lead to increased concentration of the heavy metal Pb in surface soil. However, as the human activity becomes increasingly intense, the irrigation and drainage of the water used for agricultural irrigation can easily lead to the migration of the element Pb (Ye et al. 2019). Therefore, the Pb content of surface soil is rather low, which conforms to data extracted from the site that indicate a lower content of Pb than background value.

Effects of urbanization on heavy metals in soil
Over the past few decades, there has been a large-scale renovation and urban development in the vicinity of the East Dongting Lake, leading to constantly increasing population in the region. In particular, the recent development of economic circles, new farms and industrial parks around the East Dongting Lake has led to a large amount of domestic sewage and waste. Along with the industrial and agricultural sewage, such pollution enters into the East Dongting Lake from the ground surface, imposing a severe impact on the ecological quality. Given that these heavy metals will persist in wetlands, they will seriously undermine the stability of wetland ecosystem (Ghosh et al., 2019). During urbanization, heavy metal concentrations in soil will increase over time in general, but not all the elements of heavy metals in the research area reveal the same trend (Huang et al., 2019). In different regions and with exception to Cu, the Fenormalized heavy metal concentrations have shown signi cant variation among different types of land use, indicating that the progress made in urbanization can impose a signi cant impact on the distribution of heavy metal elements. As shown in Fig. 3, in the agricultural land, there is a signi cantly higher level of Cd, Zn, Pb and Hg contents in the agricultural land than other types of land use (P < 0.05) (Mama et al. 2020), and relevant studies have shown that these four kinds of heavy metals are related to the application of phosphate fertilizer (Imchen et al., 2018). In the grassland, there is a rather lower level of agricultural and industrial pollution, contributing to a relatively low overall content of heavy metals in this region (Zhang et al., 2019b). In addition, the effects of natural leaching facilitate the migration of heavy metals from the surface to the deep soil, among which Pb and Zn are the elements that have experienced the most migrations ). In the mud ats, due to the evaporation of water, a large area of open space has been formed. Part of the space is used for waterfowl breeding, resulting in contamination to some extent, whereas the reed and miscanthus sacchari orus have massively grown on other parts of the mud ats. Through the enrichment of soil nutrients, they have effectively improved the soil quality by absorbing a certain concentration of heavy metal elements in soil. Nevertheless, its content of heavy metals is slightly higher than grassland . In the woodland, the lamellar structure of plant community can weaken the effects of leaching and soil erosion, leading to enhanced physicochemical properties of soil. Moreover, the content of heavy metals accumulated in the woodland is relatively stable, which has the lowest level of heavy metal contents among the ve types of land with exception to As (Gao et al., 2018). Along with the advancement of urbanization, and with exception to the heavy metal Cu that does not change signi cantly, the contents of heavy metals in other types of land all vary to a certain extent (Konstantinova et al., 2019).
The concentrations of 8 sorts of Fe-normalized heavy metals and their signi cant differences are shown in Fig. 3. Overall, the element of As has shown the highest median value, followed by Zn, Cr, Ni, Cu, Pb, Cd and Hg. However, although the concentrations of all sorts of Fe-normalized heavy metals in agricultural land are much higher than in other land-use patterns, the analysis of variance indicates that the concentration of Cu and Ni normalized against Fe has no signi cant variation among the 8 sorts of heavy metals in varying types of land use (P > 0.05). The Fe-normalized As concentration amounts to 99.52 mg/kg in the agricultural land, signi cantly higher than the concentration of 70.99 mg/kg in grassland. Similarly, the Fe-normalized Hg concentration amounts to 0.56 mg/kg in agricultural land, signi cantly higher than 0.38 mg/kg in woodland and and 0.37 mg/kg in grassland. The Fe-normalized Cd concentration in agricultural land amounts to 1.02 mg/kg, signi cantly higher than that of woodland and grassland, which amounts to 0.77 mg/kg, and also higher than the concentration of 0.81 mg/kg in the construction land. The Fe-normalized Pb concentration amounts to 26.48 mg/kg in agricultural land, signi cantly higher than that in woodland and grassland (19.52 mg/kg) as well as construction land (19.76 mg/kg). In general, except for the high concentration of Fe-normalized heavy metals in agricultural land, and the relatively low concentration in grassland and woodland, the concentration of heavy metals is lower in natural area than in areas subject to human activities.
3.5 Potential ecological risk assessment of the heavy metals in soil Judging from the potential ecological risk coe cient of single heavy metal Eir (Table 5), Cd features the highest risk coe cient with the mean value of 292, whereas the sampling points that have their E.56ir ≧ 320 account for 36.51% of the total, indicating strong ecological risks. As for other heavy metals, including Cu, Zn, As, Hg, Pb, Ni and Cr, the average value of their potential ecological risk coe cients amount to 4.66, 0.80, 63.37, 182.10, 3.69, 6.40 and 1.45, respectively, indicating a low level of ecological risks. Judging from the research results, the coe cients of variation of Zn, As, Cr and Pb are relatively large, each accounting for 32.71%, 36.89%, 41.91% and 32.18%., The results indicate that in the East Dongting Lake, the elements of Zn, As, Cr and Pb have large variations of ecological risks, which are related to the distribution of pollution sources in the research area. According to Table 5, the average value of the comprehensive potential ecological risk index (PERI) for heavy metals amounts to 555.03, representing strong ecological risks. The maximum PERI value amounts to 637.74, whereas the minimum value amounts to 360.88. In addition, the coe cient of variation is 9.17%, indicating that PERI of heavy metals is subject to little variation, and the elements of Hg and Cd constitute the major contributing factors.
Judging from the spatial distribution of potential ecological risks of heavy metals (Fig .4), there is an evident pattern of spatial distribution for the heavy metals of soil in the research area. shown high ecological risks in the agricultural land in the west of Dongting Lake wetland. The elements of As, Ni, Hg, Pb and Zn have shown extremely high ecological risks in the central part of Dongting Lake wetland, especially in the areas dedicated to aquaculture and rice planting. Over the past few decades, a considerable number of lakes and wetlands have been reclaimed for agricultural cultivation and shery breeding. While improving the soil fertility, the long term use of fertilizers and pesticides can lead to increased content of heavy metals in soil. As a result, the extensive application of chemical fertilizers and pesticides will not only lead to the contamination of surface soil by heavy metals, but also exacerbate the potential food safety risks derived from heavy metals. Therefore, judging from the perspective of food safety, we shall pay extra attention to the bioaccumulation of Cd in aquatic products, rice and other sorts of food crops. In the northwest and southeast of East Dongting Lake, the agricultural land and mud ats have shown high comprehensive PERI values.

Discussion
Based on the aforementioned analysis of the distribution characteristics of heavy metals, the identi cation of pollution sources and the assessment of heavy metal pollution, we may conclude that the heavy metals in the wetland soil of Dongting Lake have already shown strong ecological risks. Therefore, it is urgent to restore the wetland soil in this area. Subsequent to contamination, the soil can impose threats to human health through the food chain. To avoid their harm to human health, it is necessary to eliminate the heavy metals from the soil.
The Dongting Lake Wetland is located at the end of several major rivers in Hunan Province, and constitutes a necessary place to enter into the Yangtze River.
The discharge of agricultural, industrial and domestic sewage in the surrounding areas has led to signi cant enrichment of heavy metals in sediments, and has imposed severe negative effects on the ecological environment of the wetland. Besides the interference of human factors, the composition of ne soil particles and anaerobic conditions of wetlands also constitute the primary environmental factors contributing to the adsorption of heavy metals in sediments.
The East Dongting Lake has signi cant implications for the safe production of food and the regional ecological security in central and southern parts of China. The particular background of sediments and the unreasonable interference with human activities will elevate the accumulation of heavy metals in wetland soil to toxic levels. This will cause huge risks to human environment and food health, and ultimately compromising the sustainable utilization of regional natural resources. At present, increasing efforts of returning farmland to lake and ecological protection are made in the entire Dongting Lake area. To cope with the problem, we must rst trace back to its original source and identify the key issues.
Therefore, during future studies, we intend to add more detailed collection and analysis of the samples of heavy metals in soil in uenced by industrial and agricultural production activities. Moreover, we will adopt more scienti c methods of source analysis to identify the features of spatial variation for heavy metals in soil at small scales, so as to reveal the interference factors in a more accurate manner, and provide useful reference for the ecological environment protection in the region.

Conclusion
Pb are lower than the soil background values, whereas the concentrations of Cd, As, Ni and Hg all exceed the soil background values in Hunan Province. Except for Cd, Hg and As contents that are higher than the national standard, the rest of the heavy metal contents are lower than the national standard. The composition of soil particles is the primary physical factor of control affecting the distribution of heavy metal concentrations. The elements of Cd, Cu, Ni and Zn may derived from a common source of human activities, especially the impact imposed by industrial activities. Continuous application of agricultural fertilizers may increase the contents of Cd and Pb in the soil.
(2) The progress made in urbanization can well re ect the evolution of heavy metal concentration. The effects of early soil erosion and natural leaching can impose a certain impact on the distribution of heavy metals in soil. As the human interference becomes ever more intense, the continuous application of agricultural fertilizers, industrial activities and urban expansion have signi cantly increased the accumulation of heavy metal contents in soil.
(3) In uenced by the varying modes of land use, the Fe-normalized heavy metal concentrations are relatively low in woodland, except for the high concentrations of Fe-normalized heavy metals in agricultural land.
(4) The element Cd exists as the most severe form of heavy metal pollution in the East Dongting Lake area, which has brought along extremely high ecological risks. In addition, the distribution tendency is consistent with the spatial distribution of comprehensive potential ecological risks, especially in the areas dedicated to aquaculture and agriculture. Measures including but not limited to the expansion of ecological land, development of ecological agriculture (such as rice crab breeding), strengthening of the management of Dongting Lake Nature Reserve and regular dredging of ditches, can effectively enable us to reduce the possibility of heavy metals entering into wetlands, thus alleviating the threats imposed by heavy metals to the soil and water environment in the research area. Last but not least, the excessive enrichment of Cd may lead to health issues. Therefore, from the perspective of food safety, we ought to extra attention to the bioconcentration of the Cd element in aquatic products and rice.
Declarations 1) Ethics approval and consent to participate: Not applicable.
2) Consent for publication: All authors read and approved the nal manuscript.
3) Availability of data and materials: The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request. 7) Acknowledgements: The authors are grateful to the Forestry Bureau of Hunan Province (FBHP) for providing the case study data. The authors would like to thank the reviewers for helping to substantially improve the quality of the article. The authors would also like to thank the Editage consultant team for reviewing the English language of the manuscript and for their helpful suggestions Maps on the Geographical Location of Research Area and the Distribution of Sampling Points. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Spatial Distribution of Single Potential Ecological Risks and Comprehensive Potential Ecological Risks of Heavy Metals. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.