The Spatial Distribution and Risk Assessment of Nutrient Elements and Heavy Metal Pollution in Sediments: A Case Study of a Typical Urban Lake in the Middle and Lower Reaches of the Yangtze River

Abstract

The ecological environment of urban lakes affected by human activities is deteriorating rapidly. As a source and sink of pollutants in the lake environment, sediments have become the focus of environmental assessments. At present, most of the studies only conduct pollution assessments on surface sediments. In this study, taking the typical urban lakes GanTang Lake and NanMen Lake (G&N Lake) as the background, not only is the planar spatial distribution of their nutrient elements, seven kinds of heavy metals, and As analyzed in detail, but risk assessments are also carried out on the pollution conditions at different depths. The causes of pollution at different depths are analyzed. It is found that in this lake, with the increase in depth, the pollution situation decreases slightly, but the pollution of nutrient elements is severe. There is severe pollution of nutrient elements at a depth of up to 1 m in the whole lake sediment. In the sediments with a depth of up to 1 m, more than 90% of the areas in the whole lake are at or above the moderate pollution level of Hg, and more than 70% of the areas are under slight pollution of Cd, resulting in the ecological risk level of the whole lake being at or above the high-risk level. Urban lake sediment management is inherently complex, driven by multifaceted factors where intensive anthropogenic activities constitute the primary pollution source. This research provides insights to guide restoration strategies and sustainable development policies for lacustrine ecosystems.

1. Introduction

With the acceleration of industrialization and urbanization, lake ecosystems are facing unprecedented challenges [1]. Lakes are transitional zones between water bodies and land. They are crucial environments for many species and significant resources for human society’s water use, leisure, and recreation [2]. In limnological research, the interaction between lakes and streams is defined as a coupled ecosystem. In the entire ecosystem, lakes are regarded as the “receivers” of nutrients and other solutes (including various types of pollutants) and are prone to anthropogenic eutrophication caused by nutrients (e.g., phosphorus and nitrogen) generated by human activities [3]. Due to the interaction among lake depth, stratification, and sediment storage, the conversion rate and retention rate of substances in the lake are also highly correlated with the hydraulic residence time of the lake [4]. Therefore, in general limnological research, lakes are considered as vertical systems, and this is used to emphasize the transport processes between the epilimnion, hypolimnion, and deep sediments. Sediments are one of the important sources of nutrients in lakes, rather than just nutrient sinks, which is more evident in the material transformation and circulation of urban inland lakes, because urban lakes bear the convergence of various substances brought about by more frequent human activities. Meanwhile, heavy metal pollution has gradually become one of the key factors affecting the stability of lake ecological environments [5]. Due to the structure of sediments, they form a natural geosorbent in which pollutants introduced to aquatic environments are stored. These pollutants not only disrupt the balance of lake ecosystems but also pose potential threats to human health that depends on the lakes for survival.

As an important component of lake systems, lake sediments serve as both the main sink for nutrients and heavy metals and as their secondary pollution sources. When external conditions change, accumulated nutrients and heavy metals in the sediment may be re-released into the overlying water, promoting the explosive growth of algae or causing ecological toxicity, thus exacerbating water quality deterioration and forming a vicious cycle [6]. Conducting in-depth research on the distribution of nutrient elements and heavy metal pollution in lake sediments can, to a large extent, reveal the hazards of the current pollution and potential risks, providing important reference bases for the improvement and governance of lake ecosystems [5,7].

To scientifically assess the level of pollution by nutrients and heavy metals in lake sediments, various evaluation methods are widely used. Among them, the Geoaccumulation Index (I_geo) is commonly used to quantify the extent of heavy metal pollution [8], while the Potential Ecological Risk Index can comprehensively consider the toxicity coefficients of heavy metals and their concentrations in sediments to evaluate the potential hazards they may cause to the ecosystem [9]. To assess the pollution status of nutrients in lake sediments, analyzing the total phosphorus and total nitrogen content is the basic approach to directly measure the load of nutrients in sediments. Additionally, the Comprehensive Pollution Index (CPI) [10] and the Organic Pollution Index (OI) considering organic matter pollution conditions are also used to comprehensively assess the pollution levels of nutrients in sediments and their impact on the ecological environment.

In this study, GanTang Lake and NanMen Lake (G&N lake) are a municipal lake ecosystem formed by the connection of two lakes. Given the existence of domestic sewage, agricultural wastewater, and industrial wastewater in the surrounding area, in order to conduct a more comprehensive pollution assessment, the sediments were stratified into three layers for separate pollution evaluations. The Comprehensive Pollution Index and the Organic Pollution Index were used in combination to assess the nutrients in the sediments. Regarding the seven kinds of heavy metals and As pollution in the lake sediments, the commonly adopted Geoaccumulation Index method was utilized to analyze the areas severely contaminated by heavy metals, and the Ecological Risk Index method was applied for verification to ascertain the extent of the seven kinds of heavy metals and As pollution in G&N Lake. Correlation analysis based on the data of each pollutant element was carried out to identify the pollution sources. A sediment database for this lake has been established, enabling spatial distribution analyses and laying the foundation for future remediation and research.

2. Materials and Methods

2.1. Study Area

As illustrated in Figure 1, G&N Lake represents an urban lacustrine system within the middle and lower Yangtze River basin, encompassing a 15.35 km² catchment area. Hydrologically, it integrates runoff from the northern Lushan Mountain Range before discharging into the Yangtze River mainstream. The lake maintains a regulated water level of 15.62 m, with historical flood events reaching 17.42 m (both elevations referenced to the Yellow Sea Datum).

2.2. Bottom Sediment Sampling and Chemical Analysis

Based on the complex surrounding situation of G&N Lake, 115 columnar sediment samples were collected from G&N Lake in October 2022. In order to investigate spatial heterogeneity in 7 kinds of heavy metals, the As distribution, and nutrient source dynamics, the sampling points are distributed according to the grid distribution method. The coordinates of all sample sites were recorded by the Global Position System (GPS).

Based on the preliminary design specifications for the comprehensive remediation project in the lake sector, contaminated sediments within two sub-basins of G&N Lake require dredging interventions due to chronic impacts from flood disasters and overflow pollution. The maximum dredging depths will reach up to 60 cm and 70 cm, respectively, covering a total area of 1.34 km². Additionally, colloid-like semi-suspended deposits at the sediment–water interface (approximately 7–15 cm thick) throughout the entire lake will be removed [11]. In this study, we aim to validate the engineering rationale by aligning sediment sampling depths with the proposed dredging parameters, thereby enabling the assessment of the feasibility of implementing such dredging depths within this lake’s ecological restoration scheme.

Sediment cores were retrieved using a stainless steel cylindrical corer (inner diameter: 10 cm), with each core vertically stratified into three layers, namely surface (0–20 cm), middle (20–40 cm), and bottom (40–100 cm), using a pre-cleaned stainless steel splitter.

A certain amount of sediment was collected at each site, wrapped in aluminum foil, sealed in argon-purged aluminum foil bags, transferred away from light to the laboratory, and stored at 4 °C in dark refrigeration.

Each ground sediment sample was lyophilized and sieved to obtain the <63 μm fraction, then split into three replicates. And then TN, TP, and OM were measured by the national standard methods (HJ 717-2014, HJ 632-2011, and HJ 615-2011, respectively). Finely powdered sediment subsamples (of approximately 1 g each) were dissolved in 15 mL of concentrated hydrochloric acid and 5 mL of concentrated nitric acid (3:1 ratio aqua regia). The mixture was digested at 120 °C for 1–2 h. Upon cooling, the solution was diluted to 30 mL with deionized water and filtered using filter paper. For the material prepared in this way, the concentration of trace metals (Cd, Cr, Cu, Ni, Pb, and Zn) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (HJ 491-2019).

2.3. Assessment of Sediment Contamination

2.3.1. Geoaccumulation Index Method

Under natural conditions, lacustrine sediments primarily originate from watershed soil erosion. Sediments and soils within the same region typically share a common parent rock material and have undergone similar weathering processes, indicating a high degree of correlation between them. Due to the specificity of sedimentary processes and hydrological regimes across different areas, utilizing background elemental values of soils defined at the same watershed scale can provide a relatively accurate indication of the degree of element enrichment observed within the sediments [12,13].The Geoaccumulation Index (I_geo) proposed by Ekissi [14] was implemented to evaluate metallic contamination levels in sedimentary deposits. This classification system comprises seven distinct categories, where the highest classification tier corresponds to a 100-fold enrichment compared to baseline geochemical concentrations, as detailed in

Table 1. The background values of 7 kinds of heavy metals and As in the soil of Jiangxi Province [13].

Province	Background Value (mg·kg−1)
	Cu	Zn	Pb	Cr	Cd	Ni	As	Hg
Jiangxi Province	27.30	69.40	32.30	45.90	0.11	18.90	14.90	0.084

.

In the conventional GI model, the GI of the m-th heavy metal (I_m) is

$${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}={\mathrm{l}\mathrm{o}\mathrm{g}}_2\left[{\displaystyle \frac{{\mathrm{C}}_{\mathrm{n}}}{\mathrm{k}{\mathrm{B}}_{\mathrm{n}}}}\right]$$

(1)

where C_n—concentration of the analyzed metal in sediment; B_n—geochemical background for the analyzed metal; and k is a constant of change, taking into account the environmental background value changes caused by sediment differences, with k = 1.5. The method is used to calculate the detected concentrations at the sites and the corresponding background values. The calculated values are compared in

Table 2. Classification criteria for heavy metal pollution levels in sediments using the Geoaccumulation Index method.

Classification	Igeo	Pollution Status
0	≤0	Uncontained
1	(0, 1]	Uncontaminated to moderately contaminated
2	(1, 2]	Moderately contaminated
3	(2, 3]	Moderately to heavily contaminated
4	(3, 4]	Heavily contaminated
5	(4, 5]	Heavily to extremely contaminated
6	>5	Extremely contaminated

to determine the pollution grade of the corresponding element at the corresponding site.

2.3.2. The Multi-HM Potential Ecological Risk Index

The Potential Ecological Risk Index (PERI) method represents a comprehensive approach for ecological risk assessments rooted in aquatic sedimentology principles [15]. This methodology systematically integrates the ecotoxicological impacts of heavy metals with their environmental behavior through the application of standardized and quantifiable indices, enabling comparative risk evaluations across different contaminant species. The computational framework of this assessment tool is expressed as follows:

$$\mathrm{R}\mathrm{I}={\sum }_{\mathrm{i}=1}^{\mathrm{n}} {\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{T}}_{\mathrm{r}}^{\mathrm{i}}\times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{f}}^{\mathrm{i}}}{{\mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}}}$$

(2)

The Potential Ecological Risk Index (RI) constitutes a quantitative assessment framework where Eⁱ_r denotes the potential ecological risk factor of an individual metal. Herein, Tⁱ_r represents the metal-specific toxicological weighting coefficient, with established values for common contaminants: Ni (5), Cu (5), Pb (5), Zn (1), Hg (40), As (10), Cr (5), and Cd (30). The parameter Cⁱ_f corresponds to the measured concentration (mg·kg⁻¹) of target metal ii in sediment samples, while Cⁱ_n indicates the geochemical baseline concentration (mg·kg⁻¹) derived from regional background surveys. Since the G&N Lake ecosystem is located in a typical urban area within the middle and lower reaches of the Yangtze River, the arithmetic mean of heavy metals contents in Layer A soil from Jiangxi Province was selected as the background value. According to the calculated values of Eⁱ_r and RI, compare with

Table 3. Classification criteria for potential ecological risk levels of heavy metals in sediments.

Eir	Potential Ecological Risk Level	RI	Potential Ecological Risk Level
≤40	Low level	≤150	Low risk
(40, 80]	Medium level	(150, 300]	Medium risk
(80, 160]	Moderate level	[300, 600]	Heavy risk
(160, 320]	Considerable level	>600	Serious risk
>320	High level

to determine the pollution risk grade of the corresponding site.

2.3.3. Comprehensive Pollution Index Method

The Comprehensive Pollution Index (FF) methodology was employed to assess the contamination status of total nitrogen (TN) and total phosphorus (TP) in surface sediments. This approach involves the sequential calculation of individual pollution factors based on single-element evaluation, followed by the comprehensive classification of pollution levels through integrated index analysis [10]. The specific calculation formula is as follows:

$${\mathrm{S}}_{\mathrm{n}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}}{{\mathrm{C}}_{\mathrm{n}}^{\mathrm{s}}}}$$

(3)

$$\mathrm{F}\mathrm{F}=\sqrt{{\displaystyle \frac{{\mathrm{F}}^2+{\mathrm{F}}_{\mathrm{m}\mathrm{a}\mathrm{x}}^2}{2}}}$$

(4)

where FF is the Comprehensive Pollution Index; S_n is the pollution index of total nitrogen (TN) or total phosphorus (TP), where n is TN or TP; Cⁱ_n is the measured concentration of TN or TP; and C^s_n is the standard content of TN or TP. TN and TP were 1000 and 320 mg·kg⁻¹, respectively. F is the average value of S_TN and the S_TP of the TN or TP pollution evaluation index; F_max is the maximum value of S_TN and the S_TP of the TN or TP pollution evaluation index. According to FF, the comprehensive pollution degree of TN and TP in sediments is divided into 4 levels. According to the calculated values of S_TN, S_TP and FF, compare with

Table 4. Standard for the classification of TN and TP pollution levels in sediments by the Comprehensive Pollution Index method.

Classification	STN	STP	FF	Comprehensive Pollution Level
1	≤1.0	≤0.5	≤1.0	Non-contaminated
2	(1.0, 1.5]	(0.5, 1.0]	(1.0, 1.5]	Mild contaminated
3	(1.5, 2.0]	(1.0, 1.5]	(1.5, 2.0]	Moderately contaminated
4	>2.0	>1.5	>2.0	Heavily contaminated

to determine the pollution grade of the site.

2.3.4. Organic Pollution Index Method

The Comprehensive Pollution Index (CPI) method integrates total nitrogen (TN) and total phosphorus (TP) concentrations into a composite indicator to characterize the contamination status of surface sediments. While this integrated approach provides enhanced representativeness compared to single-element pollution indices, it systematically neglects organic constituents. To address this limitation, the Organic Pollution Index (OI) was incorporated to holistically evaluate sedimentary organic contamination through multi-parameter analysis [16]. The formula is as follows.

$$\mathrm{O}\mathrm{I}=0.95{\mathrm{C}}_{\mathrm{T}\mathrm{N}}+{\displaystyle \frac{{\mathrm{C}}_{\mathrm{O}\mathrm{M}}}{1.724}}$$

(5)

where OI is the Organic Pollution Index; TN is the measured value of total nitrogen content; and OM is the measured value of organic matter content. According to the OI, organic pollutants in sediments are classified into 4 levels, as shown in

Table 5. Classification criteria for organic pollutants in sediments using the Organic Pollution Index method.

Classification	OI	Pollution Level
1	≤0.05	Non-contaminated
2	(0.05, 0.2]	Mild contaminated
3	(0.2, 0.5]	Moderately contaminated
4	>0.5	Heavily contaminated

.

2.4. Quality Analysis/Quality Control

To ensure the accuracy of sampling points, the grid point layout method is carried out on the map before sampling. During the sampling process, GPS coordinate positioning is performed on the actual sampling points, and these coordinates are used for the recording, mapping, and calculation of actual samples, thus ensuring the accuracy of the coordinates of sampling points.

All sample tests are carried out in laboratories with China Metrology Accreditation qualification and are operated by the staff therein. For every 10 out of the 115 sampling points, one point is randomly selected for sampling twice as parallel samples. One standard sample is measured among every twenty samples tested, and one quartz sand sample is measured as a blank sample among every ten samples to ensure the accuracy of the samples. Each sample is subjected to three parallel determinations, and the results are expressed as the average of the three parallel tests. The Shimadzu AA-7000 (Shimadzu Corporation, Kyoto, Japan) was used for the detection of Pb, Cu, Zn, Ni, Cd, and Cr contents. The Haiguang AFS-9750 (Haiguang Instrument Corporation, Beijing, China) was used for the detection of Hg and As contents.

3. Results

3.1. Spatial Distribution of Nutrients in Sediments

The planar spatial distribution patterns of key nutrient elements (TN, TP, and OM) across tripartite vertical sediment strata are systematically presented in Figure 2.

The values obtained from the detection of nutrient elements in the lake are shown in

Table 6. Descriptive statistics of each nutrient element content.

Element	Mean	Max	Min	Standard Deviation	Coefficient of Variation
TP (mg/kg)	1455.39	11,720.00	317.00	911.99	0.63
OM (g/kg)	49.64	95.50	14.73	13.84	0.28
TN (mg/kg)	2483.53	4320.00	699.33	649.89	0.26

. To facilitate comparative spatial analysis, the lacustrine systems were partitioned into distinct zones:

Gantang Lake was divided into the northern, central, and southern sectors, and Nanmen Lake was divided into the western, central, and eastern sectors, based on hydrodynamic gradients and basin morphometric characteristics.

The distribution of organic matter is shown in Figure 2a. The average organic matter content in the three layers of sediment decreases with the increase in depth. The average organic matter contents of the three layers are 59.24 g·kg⁻¹, 48.35 g·kg⁻¹, and 41.48 g·kg⁻¹, respectively, and the spatial positions of the main polluted areas are basically the same.

In the surface sediment, the organic matter content is the highest in the area near the north of Gantang Lake. The closer the lake is to the south, the lower the OM content. In Nanmen Lake, the OM content is the highest at the western and eastern corners of the lake, and the content is relatively high in the middle part of Nanmen Lake near the south bank.

In the medium-layer sediment, compared with the surface sediment, the OM content in Gantang Lake decreases significantly. The OM content is relatively high, reaching about 70 g·kg⁻¹ near the northern shore area of the lake. In Nanmen Lake, the OM content is higher in the western and middle parts than in other parts, and the OM content at a few points in the middle is higher than that in the surface sediment.

In the bottom-layer sediment, the OM content in the north of Gantang Lake is significantly higher than that at other points, reaching about 50 g·kg⁻¹, and the content in other areas is about 40 g·kg⁻¹ or lower. In Nanmen Lake, the content in most areas is lower than 40 g·kg⁻¹. There are some points with a relatively high OM content in the middle of the lake, and their positions correspond to the points with a high OM content in the middle-layer and surface sediments. The distribution of the above interpolation maps indicates that there are obvious organic carbon pollution sources in the north of Gantang Lake and the middle of Nanmen Lake, and the pollutants have infiltrated into the sediment with a depth of up to 1 m, or the continuous pollution deposition in these areas has lasted for a longer time.

The distribution of total nitrogen (TN) is shown in Figure 2b. The average total nitrogen content in the three layers of sediment decreases with the increase in depth. The average total nitrogen contents of the three layers of sediment are 2946.88 mg·kg⁻¹, 2409.48 mg·kg⁻¹, and 2103.74 mg·kg⁻¹, respectively. The spatially polluted areas of the entire lake are basically the same, and the main polluted areas are in the north and middle of the entire lake.

In the surface sediment, in Gantang Lake, the TN content at a few points in the southern area is relatively low, less than 2000 mg·kg⁻¹, and the total nitrogen content in other parts is around 2550 mg·kg⁻¹. However, the total nitrogen content at most points in the north of Gantang Lake is as high as over 3200 mg·kg⁻¹. In Nanmen Lake, the content at a few points in the western and eastern corners is relatively high, reaching around 3900 mg·kg⁻¹. The total nitrogen content at most points in the middle of the lake is as high as 3000 mg·kg⁻¹, and at a few points, it is around 1800 mg·kg⁻¹.

In the medium-layer sediment, in Gantang Lake, the total nitrogen content at a large number of points in the northern area reaches 3000 mg·kg⁻¹, and the content at points in the middle and southern areas is around 2000 mg·kg⁻¹. In Nanmen Lake, there are a few points in the western and middle areas where the total nitrogen content reaches over 3000 mg·kg⁻¹, and the total nitrogen content at other points is around 2500 mg·kg⁻¹. In the eastern area, the total nitrogen content at half of the points is below 2000 mg·kg⁻¹, and at the other half of the points, it is around 2500 mg·kg⁻¹.

In the bottom-layer sediment, in Gantang Lake, only the points in the northern area reach 3000 mg·kg⁻¹. In the middle area, the total nitrogen content at a small number of points is above 2500 mg·kg⁻¹, and the content at most points in the middle area is the same as that at points in the southern area, around 1800 mg·kg⁻¹. In Nanmen Lake, the total nitrogen content at most points in the western and middle areas is close to 2500 mg·kg⁻¹, and in the eastern area, the total nitrogen content in most parts is below 2000 mg·kg⁻¹.

The distribution of total phosphorus is presented in Figure 2c. The overall content in the lake sediments is relatively high. The total phosphorus content in the three layers of sediments of the G&N Lake ecosystem is basically consistent, ranging from over 500 mg·kg⁻¹ to less than 4000 mg·kg⁻¹. However, the total phosphorus content at a few points in the medium-layer sediment is exceptionally high. These points are located in the middle of Gantang Lake and the middle of Nanmen Lake, where the total phosphorus content is as high as over 7500 mg·kg⁻¹.

3.2. Spatial Distribution of Heavy Metals in Sediments

The planar spatial distribution characteristics of seven kinds of heavy metals and As across tripartite sediment strata in urban lake ecosystems are systematically presented in Figure 3.

As

Table 7. Descriptive statistics of each element content.

Element	Mean (mg/kg)	Max (mg/kg)	Min (mg/kg)	Standard Deviation	Coefficient of Variation	Mean Value of Sediments in Chinese Lakes [17]	Background Value
Hg	1.60	24.60	0.14	2.74	1.70	0.05	0.08
As	10.41	24.10	3.52	3.92	0.38	12.10	14.90
Pb	86.53	208.00	11.00	38.05	0.44	31.00	32.30
Cu	64.12	195.00	16.00	24.10	0.38	31.70	27.30
Zn	149.74	319.00	46.00	50.49	0.34	88.00	69.40
Ni	54.33	119.00	18.00	20.40	0.38	36.80	18.90
Cr	108.36	238.00	46.00	25.38	0.23	85.00	45.90
Cd	0.42	2.58	0.05	0.37	0.88	0.19	0.11

, the concentrations of various elements in the three layers of sediment in GN lake are compared with the average values of metal elements in the sediment of Chinese lakes and the background values of soil in Jiangxi Province, as shown in the table. In this lake, since the average concentration of As is lower than the background value and the average value of the sediment, the concentrations of other metals are relatively high.

Figure 3a delineates the vertical stratification of arsenic concentrations, revealing a progressive decline in mean values from surface (11.70 mg·kg⁻¹) to intermediate (10.44 mg·kg⁻¹) and deep layers (9.16 mg·kg⁻¹), with spatially persistent contamination footprints across sedimentary horizons.

In surface sediments, Gantang Lake exhibits localized anthropogenic hotspots in northern sectors (max 13 mg·kg⁻¹), and the content in other parts is relatively consistent, around 10 mg·kg⁻¹. In Nanmen Lake, the contents in the western and central regions reach 24 mg·kg⁻¹, which is higher than the background value, while the content in the eastern region is relatively low, around 9 mg·kg⁻¹.

In the medium-layer sediment, in Gantang Lake, only a few points in the northern and central parts have an arsenic content reaching 15 mg·kg⁻¹, which is higher than the background value, and the content in other regions is relatively low, less than 10 mg·kg⁻¹. In Nanmen Lake, the western and central parts of the lake have a relatively high arsenic content, reaching over 13 mg·kg⁻¹. The arsenic content in the eastern region is relatively low, around 11 mg·kg⁻¹.

In the bottom-layer sediment, in Gantang Lake, the arsenic content at a few points in the north reaches 13 mg·kg⁻¹, and the content at other points is consistently around 7 mg·kg⁻¹. In Nanmen Lake, the arsenic content at a few points in the western, central, and eastern parts reaches around 13 mg·kg⁻¹, and the content in other parts is all below 10 mg·kg⁻¹.

In G&N Lake, the arsenic content at most points is close to the background value, and only a few points in the western region of Nanmen Lake are affected by point-source pollution.

The content distribution of cadmium in the three layers of lake sediments is presented in Figure 3b. The average contents of the three layers of sediments from top to bottom are 0.60 mg·kg⁻¹, 0.48 mg·kg⁻¹, and 0.32 mg·kg⁻¹, respectively, all of which are more than three times higher than the background value. The distribution of cadmium in the entire lake is relatively even. The cadmium content in both the surface-layer and bottom-layer sediments is below 1 mg·kg⁻¹. In the medium-layer sediment, only a few points in the southeast area of the eastern lake have a cadmium content reaching 4 mg·kg⁻¹, while the cadmium content at other points is below 1 mg·kg⁻¹. The sediments in the entire lake with a depth of up to 1 m are all severely polluted by cadmium.

The content distribution of metallic nickel in the three layers of lake sediments is shown in Figure 3c. The average contents of the three layers of sediments from top to bottom are 55.19 mg·kg⁻¹, 55.48 mg·kg⁻¹, and 53.38 mg·kg⁻¹, respectively, all of which are more than three times higher than the background value.

In the surface sediment, in Gantang Lake, there is a straight line running from the northeast to the southwest in the eastern part. This line consists of points with a relatively high nickel content, reaching over 80 mg·kg⁻¹. There is a parallel line to its northwest, and the nickel content at these points reaches 60 mg·kg⁻¹. In Nanmen Lake, the content at most points is relatively balanced, around 50 mg·kg⁻¹, and only in the northeast corner is the nickel content relatively high, reaching over 100 mg·kg⁻¹.

In the medium-layer sediment, in Gantang Lake, it shows the same trend as the surface-layer sediment. The pollution zone runs from the southwest to the northeast, with the content reaching over 80 mg·kg⁻¹. There is also a parallel pollution zone to its northwest, but the range is relatively smaller. In Nanmen Lake, a few points in the western and central parts have a nickel content reaching over 65 mg·kg⁻¹, while the point with an extremely high nickel content in the northeast corner in the surface layer does not appear in this layer.

In the bottom-layer sediment, similar to the surface and medium-layer sediments, in Gantang Lake, there are non-continuous lines composed of points with a high nickel content, reaching over 80 mg·kg⁻¹. Parallel to it are non-continuous scattered points with a high nickel content, reaching over 60 mg·kg⁻¹. In Nanmen Lake, except for a few points in the western and central parts where the nickel content reaches 60 mg·kg⁻¹, the content in other parts is around 45 mg·kg⁻¹. The sediments in the entire lake with a depth of up to 1 m are all severely polluted by Ni.

The content distribution of metallic copper in the three layers of lake sediments is presented in Figure 3d. The average contents of the three layers of sediments from top to bottom are 85.60 mg·kg⁻¹, 55.30 mg·kg⁻¹, and 50.79 mg·kg⁻¹, respectively, all of which are more than one time higher than the background value.

In the surface sediment, in Gantang Lake, the copper content in the north is higher than that in the south. The content in the north is approximately 100 mg·kg⁻¹, while that in the south is around 70 mg·kg⁻¹. In Nanmen Lake, the content in the west is higher than that in the east. The highest content is in the northwest corner, reaching over 150 mg·kg⁻¹, while the content in the southeast corner is below 70 mg·kg⁻¹.

In the medium-layer and bottom-layer sediments, the copper content is relatively evenly distributed. Most points have a copper content below 80 mg·kg⁻¹, and a few points have a copper content of around 100 mg·kg⁻¹. These points are consistent with those with a high content in the surface sediment.

The content distribution of chromium in the three-layer lake sediments is shown in Figure 3e. The average contents of the three layers of sediments from top to bottom are 112.52 mg·kg⁻¹, 105.83 mg·kg⁻¹, and 103.97 mg·kg⁻¹, respectively, all of which are more than twice the background value. The chromium content is the highest in the middle of the entire lake, and the content in other regions is relatively balanced. The specific analysis by layer and region is as follows.

In the surface sediment, the content in Gantang Lake is relatively balanced. Only a few irregular points in the central region have a content reaching 130 mg·kg⁻¹, and the content in other regions is below 110 mg·kg⁻¹, still far higher than the background value of chromium. In Nanmen Lake, the content in the western and central regions is relatively high, with most areas reaching 130 mg·kg⁻¹, and the highest content exceeding 220 mg·kg⁻¹. The content in the northern and southern parts of the lake is slightly lower, around 100 mg·kg⁻¹, but still far higher than the background value.

In the medium-layer sediment, the content in Gantang Lake is balanced, with most points having a content of around 100 mg·kg⁻¹. In Nanmen Lake, the chromium content in the western region is above 130 mg·kg⁻¹, and the content in the central and eastern regions is relatively balanced, with a few points reaching 130 mg·kg⁻¹.

In the bottom-layer sediment, the chromium content in Gantang Lake is balanced, with most points having a content of around 100 mg·kg⁻¹. In Nanmen Lake, a few points in the western and central regions, as well as a few points in the center of the eastern region, have a content reaching 130 mg·kg⁻¹ or above, and the content in other parts is around 100 mg·kg⁻¹. The sediments in the entire lake with a depth of up to 1 m are all severely polluted by chromium.

The content distribution of zinc in the three layers of lake sediments is shown in Figure 3f. The average contents of the three layers of sediments from top to bottom are 187.67 mg·kg⁻¹, 137.66 mg·kg⁻¹, and 125.96 mg·kg⁻¹, respectively, all of which are more than one time higher than the background value. In the entire lake, a large number of points in the area from the south to the east of the upper-layer sediment have a relatively high zinc content. The zinc content in the middle and the northwest corner of the medium-layer sediment is relatively high, but lower than that in the upper layer. In the bottom-layer sediment, the content at a few points in the northwest corner, the north, and the middle is relatively high. The points with a high zinc content are basically the same in the three layers of sediments, and the zinc content at most points decreases with the increase in depth. Regarding the detailed regional distribution:

In the surface sediment, in Gantang Lake, the zinc content in the northern region is relatively high, reaching over 180 mg·kg⁻¹, while the content in the southern region is relatively low, around 130 mg·kg⁻¹. In Nanmen Lake, the points in a large area from the southwest to the northeast and in the north reach over 200 mg·kg⁻¹. The content in the area slightly south of the middle is slightly lower, around 180 mg·kg⁻¹, and the content at a few points in the south and the southeast corner is the lowest, around 100 mg·kg⁻¹.

In the medium-layer sediment, in Gantang Lake, the zinc content at a few points in the northern region and the middle is relatively high, reaching 180 mg·kg⁻¹, and the content in other regions is below 150 mg·kg⁻¹. In Nanmen Lake, the content in the middle region and the southeast corner is higher than 180 mg·kg⁻¹, and the content in other regions is around 130 mg·kg⁻¹.

In the bottom-layer sediment, in Gantang Lake, the zinc content at a few points in the northern and middle regions reaches 180 mg·kg⁻¹, and the content in other regions is below 150 mg·kg⁻¹. In Nanmen Lake, except for a small area adjacent to the north of Gantang Lake and a few points in the middle with a content of around 180 mg·kg⁻¹, the content in other regions is below 135 mg·kg⁻¹. The sediments in the entire lake with a depth of up to 1 m are all severely polluted by zinc.

The content distribution of lead in the three layers of lake sediments is shown in Figure 3g. For metallic lead, the average contents of the three layers of sediments from top to bottom are 96.94 mg·kg⁻¹, 82.05 mg·kg⁻¹, and 84.23 mg·kg⁻¹, respectively, which are more than twice the background value. In the entire lake, the area with the highest lead content is in the northern region of Gantang Lake. In the three layers of sediments, the lead content in Gantang Lake is higher than that in Nanmen Lake, and the overall pollution trend is consistent.

In the upper-layer sediment, in Gantang Lake, the lead content in the northern part is relatively high, reaching over 140 mg·kg⁻¹. The lead content in the central area of the western part of the lake is around 120 mg·kg⁻¹, and in the southern region, it is below 120 mg·kg⁻¹, showing a gradually decreasing trend from north to south. In Nanmen Lake, the lead content in the central-western and central regions is around 70 mg·kg⁻¹, and the lead content at points in the eastern region is all below 50 mg·kg⁻¹.

The spatial distribution of lead content in the medium-layer sediment is basically the same as that in the surface sediment, but the overall content has decreased.

The spatial distribution of lead content in the bottom-layer sediment is basically the same as that in the surface-layer sediment. The overall content has slightly increased compared with the medium-layer sediment. The number of points with a lead content reaching 165 mg·kg⁻¹ in the north of Gantang Lake and the number of points with a lead content reaching 120 mg·kg⁻¹ in the south are higher than the number of points reaching the corresponding contents in the medium-layer sediment. Also, the number of points with a lead content reaching around 70 mg·kg⁻¹ in the western and central regions of Nanmen Lake is higher than the number of points with the corresponding content in the medium-layer sediment.

The content distribution of mercury in the three-layer sediments of the lake is shown in Figure 3h. In the surface sediments, the mercury content at each point varies little, all below 3 mg·kg⁻¹, but exceeds the background value.

In the medium-layer sediments, there are a small number of points in the northern area of Gantang Lake where the mercury content is more than 14 mg·kg⁻¹, more than ten times higher than the background value. The content in other parts is relatively balanced, around 0.1 mg·kg⁻¹.

In the bottom-layer sediments, the distribution is similar to that of the medium-layer sediments. In the northern part of Gantang Lake, there are some points where the mercury content is greater than 14 mg·kg⁻¹, and there are also a small number of points with an even higher content of more than 20 mg·kg⁻¹. The content in other parts is around 0.1 mg·kg⁻¹.

In summary, across the three sediment layers, the arsenic content remained comparable to the background value. In contrast, cadmium, chromium, nickel, copper, zinc, lead, and mercury exhibited concentrations substantially exceeding their respective background levels. These elevated elements have induced significant ecological contamination, exerting detrimental impacts on the G&N Lake ecosystem. Notably, mercury demonstrated the most severe pollution, with concentrations at several sampling sites surpassing the regulatory threshold by over an order of magnitude (>10-fold).

3.3. Evaluation Sediments by the Organic Pollution Index Method

When the three layers sediments are calculated by the Organic Pollution Index method, it is found that the OI values of the three-layer sediments are all much higher than 0.5, and the evaluation grades are all severe pollution. The specific calculated values are shown in Table S1.

3.4. Evaluation of Sediments by the Comprehensive Pollution Index Method

The S_TN, S_TP, and the comprehensive pollution index FF are calculated through the Comprehensive Pollution Index method. The detailed calculation results of S_TN and S_TP are shown in Table S2.

The number of points reaching different evaluation levels in the three-layer sediments and the corresponding FF of each layer of sediment are shown in

Table 8. The quantitative statistics of evaluation grades for polluted sites by the Comprehensive Pollution Index Method.

Different Layer Sediment	The Number of Each Pollution Level About STN				The Number of Each Pollution Level About STP				FF
	Non-Contaminated	Mild Contaminated	Moderately Contaminated	Heavily Contaminated	Non-Contaminated	Mild Contaminated	Moderately Contaminated	Heavily Contaminated
Surface	0	1	1	113	0	0	0	115	6.52
Medium	1	3	22	89	0	0	0	115	10.18
Bottom	4	15	35	61	0	2	3	110	9.41

.

In the surface sediments, the S_TN values of only two points are lower than two, and most areas are evaluated as having severe TN pollution. The S_TP values are all greater than 1.5, and the FF values are greater than 2.0. Thus, it can be seen that the TN, TP, and overall pollution in the surface sediments are all severely polluted.

As shown in Table 8, with the increase in depth, the number of points with severe pollution in the medium layer decreases and further decreases in the bottom layer. However, since more than half of the points are severely polluted and the FF values also indicate severe pollution, in the Comprehensive Pollution Index evaluation method, the pollution levels of TN and TP in G&N lake are classified as severe pollution.

3.5. Evaluation of Sediments by the Geoaccumulation Index Method

The Geoaccumulation Index evaluation method was used to evaluate the seven kinds of heavy metals and As contents at 115 points in the three-layer sediments. According to the calculated I_geo values, an ArcGIS interpolation map was drawn, as shown in Figure 4 and Figure S1.

In the evaluation of the Geoaccumulation Index of eight elements, the pollution of As to the G&N Lake area is the lowest. Almost all sampling points are uncontained, and only three points in the surface layer are uncontaminated to moderately contaminated.

For the five elements of Pb, Cu, Zn, Ni, and Cr, the highest level of pollution is moderately severe, with a small number of points being moderately to heavily contaminated and most points being moderately contaminated or uncontaminated to moderately contaminated. Other images can be found in the Supporting Information (Figure S1).

The number of points reaching each evaluation level is shown in Figure 5.

Cd has the most serious pollution in the surface layer. A small number of points are heavily contaminated, while most points are moderately to heavily contaminated, uncontaminated to moderately contaminated, or moderately contaminated, and a few points are not polluted. The number of points reaching a higher pollution level gradually decreases with the increase in depth, while the number of uncontained points increases with the increase in depth.

The pollution of Hg is the most serious. In the surface sediment, the evaluation level of most points reaches moderately to heavily contaminated, and a small number of points reach an extremely contaminated level. The number of points with a pollution level above severe has little difference among the three-layer sediments. With the increase in depth, the number of points evaluated as moderately to heavily contaminated gradually decreases, and the number of points evaluated as uncontaminated to moderately contaminated gradually increases.

3.6. Evaluate the Pollution of Sediments by the Multi-HM Potential Ecological Risk Index

The potential ecological risk assessment method was used to evaluate the potential risk levels reached by the eight elements’ contents in the three-layer sediments at 115 sampling points. The number of sampling points reaching each level is shown in Figure 6. The E_rⁱ values and RI values calculated by the PERI are shown in Table S3.

As can be seen from the table, elements such as As, Pb, Cu, Zn, Ni, and Cr are all at a low level in the three-layer sediments. However, for the Hg element, most of the sampling points in the three-layer sediments are at a medium level or above. In the surface layer, the number of sampling points reaching the high-risk level accounts for more than 65%.

For the Cd element, most of the sites in the three-layer sediments are at a medium-risk level or above. In the surface layer, 40% of the sites reach a considerably serious risk level. With the increase in depth, the number of sites evaluated as a low level and a medium level gradually increases, while the number of sites reaching a high level decreases. However, 73% of the sites still remain at a medium level or above.

The number of potential ecological risk assessment grades achieved at each site in the three layers of sediments is shown in Figure 7. Almost all sites are at the heavy risk grade or above. Therefore, through a comprehensive assessment of the potential ecological risks of eight elements, as shown in

Table 9. E_rⁱ mean value and contribution rate of metals.

Metals	Mean Eri Value of Surface Sediment	Contribution Rate	Mean Er Value of Medium Sediment	Contribution Rate	Mean Er Value of Bottom Sediment	Contribution Rate
Hg	653.98	74.32%	659.77	79.69%	978.71	87.48%
As	7.90	0.90%	7.02	0.85%	6.043	0.54%
Pb	14.80	1.68%	12.60	1.52%	12.78	1.14%
Cu	15.57	1.77%	10.32	1.25%	9.33	0.83%
Zn	2.66	0.30%	2.00	0.24%	1.81	0.16%
Ni	14.52	1.65%	14.58	1.76%	14.01	1.25%
Cr	12.38	1.41%	11.68	1.41%	11.36	1.02%
Cd	158.09	17.97%	109.95	13.28%	84.77	7.58%
RI	879.91		827.92		1118.80

, it is found that most of the sites in the three-layer sediments are at the heavy-risk and serious-risk levels, with only a small number of sites at the low- or medium-risk levels. As the depth increases, the risk gradually decreases, but most of the sites still remain at the severe-risk level or above.

According to the average value of various metals’ E_rⁱ in the sediment, the contribution rate of each metal to the RI value can be calculated, as shown in Table 9.

In the Potential Ecological Risk Index assessment of three-layer sediments, the excessively high E_rⁱ value of Hg is the main cause of the high RI value. Moreover, as the sediment depth increases, the E_rⁱ value of Hg also increases, along with the contribution rate, indicating that the ecological risk of Hg is higher in deeper sediments.

4. Discussion

4.1. Correlation Analysis of Pollutant Sources

For the current situation of serious exceedance of various nutrients and metal elements and high ecological risks in G&N Lake, the R language 4.4.2 was used to conduct correlation analysis on the contents of each element, and the possible pollution sources were summarized.

The overall distribution test diagrams of each element are shown in the Figure S2. The correlations between elements in the three-layer sediments, as shown in Figure 8, revealed significant positive correlations between TN and OM; moderate positive correlations between TP and Zn; moderate positive correlations among As, Zn, and Cd; and moderate positive correlations between Cu and Zn. The low positive correlations between TP and As, Cu, and Cd could be attributed to similar industrial and agricultural activities or atmospheric deposition in the surrounding areas [18]. The moderate positive correlation between TP and Zn is likely due to the combined use of phosphorus fertilizers and zinc-containing pesticides. The moderate positive correlations among As, Zn, and Cd are likely caused by industrial wastewater discharge or the mixed use of arsenic-containing pesticides, cadmium–phosphate fertilizers, and zinc-based fertilizers. The moderate positive correlation between Cu and Zn is attributed to industrial wastewater discharge or the co-application of copper pesticides and zinc fertilizers [19,20,21,22].

The element correlations in the three-layer sediments are largely consistent, indicating that the nutrient and heavy metal pollution in the sediments share similar pollution sources or migration pathways. Notably, Hg and Cd, the two most polluted elements identified in the previous analysis, showed no correlation in the three-layer sediments.

This phenomenon can be explained by two factors:

(1)

Differences in pollution source industries: Hg pollution primarily originates from coal combustion and mercury mining, whereas Cd pollution is closely linked to metallurgical processes (e.g., zinc or lead smelting) and agricultural activities [19,23].

(2)

Differences in adsorption mechanisms: In the lake environment, humus exhibits a significantly higher adsorption capacity for Hg than Cd, while Cd is predominantly adsorbed by iron–manganese oxides [24,25].

4.2. The Current Pollution Situation of the Urban Lake

The drainage zones in the central urban area before the implementation of storm-sanitary sewer separation retrofits (pre-2021) are shown in the Figure 9 below. Due to crude local drainage management practices, wastewater from areas with separated sewer systems inevitably entered the wastewater treatment plant via combined sewer pipelines. For an extended historical period, phosphate fertilizer plants and other chemical enterprises were located around this lake [11].

Based on the survey results of nutrient elements, seven kinds of heavy metals, and As, it is visibly evident that the northeastern sector of the aquatic system is significantly less affected than other areas.

According to the concentrations and evaluations of nutrient elements, 7 kinds of heavy metals, and As in the sediments of G&N Lake, the comparison of the pollution status with other lakes is shown in the following two tables.

The nutrient element contents in different regions are shown in

Table 10. Comparison of the current contents of nutrient element pollution in other regions.

Location	TP (g/kg)	OM (g/kg)	TN (g/kg)
This Research	1.46	49.64	2.48
Yangtze River Basin [26]	0.75	2.21	0.98
Southeastern River Basin [26]	0.66	3.04	1.49
Southwestern River Basin [26]	0.31	2.50	1.41
Northwestern River Basin [26]	0.78	1.50	0.93
Yellow River Basin [26]	0.85	1.47	0.73

. As shown in the table, by comparing this study with the average nutrient element contents in the sediments of various regions in China, it can be seen that the organic matter content far exceeds the average value of each region, being about more than 15 times that of other regions. The TP and TN contents in G&N Lake are at least 1.5 times more than those in the sediments of other regions. The Yangtze River Basin area is the background area of G&N Lake, being approximately twice that of G&N Lake. From this, it can be seen that the nutrient elements of this urban lake are seriously affected by human activities. Flood disasters and surrounding pollution emissions have led to serious accumulation of lake nutrient elements. Engineering measures such as ecological dredging, pollution monitoring, or restrictions on the inflow into the lake are needed to protect this lake.

The comparison table of 7 kinds of heavy metals and As contents in different regions is shown in

Table 11. Comparison of the current contents of 7 kinds of heavy metals and As in other regions.

Location	Hg (mg/kg)	As (mg/kg)	Pb (mg/kg)	Cu (mg/kg)	Zn (mg/kg)	Ni (mg/kg)	Cr (mg/kg)	Cd (mg/kg)
This research	1.60	10.41	86.53	64.12	149.74	54.33	108.36	0.42
Wuliangsuhai Lake, China [3]	0.17	10.13	29.17	18.01	38.69	25.9	32.76	0.54
Taihu lake, China [27]	0.11	9.82	33.55	34.14	105.55	36.23	68.09	0.14
Wadi El-Rayan Lake, Egypt [28]	0.02	3.08	24.50	29.69	116.59	83.13	122.34	1.50
Tangxun Lake, China [29]	0.3	11.22	41.7	47.4	147	40.9	108	0.77
Volvi Lake, N. Greece [30]	–	–	38.3	23	49.6	34.2	105	0.7
Hope Lake, USA	–	–	17.3	22.5	129	–	–	0.39
Gorges River, AUS [31]	–	–	67	30	157	13	39	–
City park lake, Shanghai, China [32]	–	–	32.40	41.30	156.30	–	–	0.24
Mean value of sediments in Chinese lakes [17]	0.053	12.1	31	31.7	88	36.8	85	0.194

.

Compared with the seven kinds of heavy metals and As contents in lake sediments from different regions or countries, it is found that except for the Cd content, the contents of the other seven kinds of heavy metals and As in the sediments of G&N Lake are at a relatively high level in the table, indicating that this lake is more severely affected by human activities. Both G&N Lake and Tangxun Lake are typical urban lakes. The similar contents of As, Pb, Zn, and Cr in the sediments suggest that the sources of the three heavy metals and As pollutions in the urban lakes in the two typical cities are similar. Besides the influence of background values, the seven kinds of heavy metals and As are more likely to accumulate in the sediments of urban lakes, thus causing serious ecological risks. In addition, the Hg content in the sediments of G&N Lake far exceeds that of other lakes. Even Tangxun Lake, which is also a typical urban lake, does not have such a high Hg content, indicating that G&N Lake is the most severely polluted by Hg. In the future, when it comes to the ecological protection and restoration of the lake, attention should be paid to the agricultural and industrial emissions of Hg pollution around the lake.

In recent years, numerous cities in the middle and lower reaches of the Yangtze River have successively initiated comprehensive water environment remediation projects. The city where G&N Lake is located is currently constructing such an initiative. This transformation entails reforming the long-established drainage system, which will fundamentally alter urban lake management practices. Urban lake management will no longer face persistent threats from extensive combined sewer networks. Consequently, this will significantly diminish the sources of elements such as carbon (C), nitrogen (N), and phosphorus (P) entering lake sediments, aligning management approaches closer to near-natural conditions. We contend that implementing source separation of wastewater streams and establishing long-term maintenance mechanisms for urban drainage networks—thereby mitigating overflow pollution caused by pipe failures and flooding—represents key priorities for urban lake management [11].

5. Conclusions

This study comprehensively evaluates the pollution of nutrient elements, seven kinds of heavy metals, and As in the sediments of G&N Lake through a method of stratified sampling and assessment. Based on the detection data of each element and its geographic location, detailed interpolation maps of element concentrations were created to obtain detailed spatial distribution information for each element. The pollution status of nutrients was analyzed using the Organic Pollution Index and Comprehensive Pollution Index methods, while the pollution status of seven kinds of heavy metals and As was analyzed using the Geoaccumulation Index and Ecological Risk Index methods. Finally, correlation analysis of each pollutant element was conducted using the R language. The conclusions are as follows:

• Nutrient Elements: The concentrations of organic matter, total nitrogen, and total phosphorus in the three layers of sediment are almost entirely at severe pollution levels.
• Seven kinds of heavy metals and As: The content of the seven kinds of heavy metals and As at all monitored depths across all sampling sites is higher than the background values of the seven kinds of heavy metals and As in Jiangxi Province’s soil. Among these, the risk assessment for Hg pollution is the highest, with the most severe Hg pollution located in the northwest corner of the lake. More than 90% of the 115 monitoring points across the lake have reached at least moderate pollution levels regarding Hg. Over 70% of the points show at least slight pollution concerning Cd, with the eastern half of the lake experiencing the most severe Cd pollution. The ecological risk across the entire lake is considered to be at a significant risk level or higher.
• The correlations among elements in the three layers of sediment are largely consistent. Elements showing moderate correlation or higher can be attributed to common industrial and agricultural discharges in the surrounding areas.

Based on the study results, comprehensive sediment restoration and remedial management will be carried out across G&N Lake, aiming to improve the sediment properties and mitigate their negative impacts on the aquatic ecosystem. For the TN, TP, OM, Hg, and Cd pollution that reaches up to 1 m deep, during the subsequent ecological reconstruction process, long-term monitoring of water quality and sediment pollution status needs to be conducted on the basis of controlling pollution sources. It is also necessary to establish a long-term management system. For severely polluted areas, covering techniques (such as using clean sand or other materials to cover the sediment) or planting hyperaccumulator plants should be considered to increase the stability of the sediment and reduce the resuspension of pollutants.