1. Introduction
Heavy metal (HM) contamination in the water and sediments of rivers and lakes has attracted worldwide attention. Compounds such as Mercury (Hg), Arsenic (As), Cadmium (Cd), Chromium (Cr), and Copper (Cu) are examples of contaminating HMs [
1,
2]. Industrial wastewater discharge, household and public sewage, the mining industry, waste incineration, and aquaculture all result in the output of HMs into the environment. Such toxic metals have been detected in various environmental media, including water, sediments, and aquatic organisms [
3,
4]. The contents of typical metals in the surface sediments of the Yalu River Estuary and its adjacent waters were measured as Cr 19.9–90.9 mg∙kg
−1, Ni 8.39–39.7 mg∙kg
−1, Cu 2.9–193.2 mg∙kg
−1, Zn 48.7–183.2 mg∙kg
−1, Cd 0.1–0.5 mg∙kg
−1, Pb 19.3–48.7 mg∙kg
−1, As 2.3–15.9 mg∙kg
−1, and Hg 15.2–302.8 mg∙kg
−1 [
5]. Sediment is an important reservoir of metals in aqueous environments. HMs adhered to sediment not only affect the safety of aquatic organisms and human health but also can be re-released under certain external conditions to become a secondary source of pollution. Therefore, more attention should be paid to the concentrations of metals in sediments and the risks that this poses [
6].
HMs are persistent, highly toxic, and bioaccumulative. They are generally incorporated, precipitated, and enriched in sediments and some aquatic organisms [
7]. Exposure can cause damage to aquatic organisms and the human body [
8]. For example, As is closely related to the occurrence and symptoms of cardiovascular disease, hypertension, liver disease, and tumors [
9]. HM pollution can also cause various histopathological changes in fish. It has been reported that the species mean acute toxicity value (SMAV) of Cu for
Crucian carp was 208.0 μg∙L
−1, and the chronic SMAV was 70.0 μg∙L
−1 [
10]. China and the United States Environmental Protection Agency (USEPA) both list Hg, As, Cd, Cr, Cu, Zn, and Pb as priority pollutants.
As the third-largest freshwater lake in China, Taihu Lake has become a diverse ecoregion with numerous industrial, agricultural, and aquacultural activities. The Taihu Lake basin is one of the most developed areas in China, with a population of 67.55 million. It is also an important drinking water source for native residents. Therefore, it is vital to protect the safety of surface water from pollution, which can influence the health of aquatic ecosystems and humans. The HM pollution and severe eutrophication in Taihu Lake have raised increasing concern from scientists and the local government [
11,
12]. Previous studies have focused on the exposure characteristics and source identification of HM in surface water and sediments of Taihu Lake [
11,
13]. Our previous investigation indicated that the number of fish, zooplankton, macrophyte, and benthic invertebrate species in Taihu Lake has declined significantly since 1980 [
14]. Water pollution, caused by a variety of chemicals generated during anthropogenic activities, is a major threat to aquatic life. Risk assessments are typically used to assess the potential harm of specific pollutants to the ecosystem and human health at concentrations detected in the environment; such assessments play an important role in regulatory decisions. Therefore, it is necessary to investigate the potential ecological and human health risks of HMs in different environmental media to guide the prevention and control of HMs pollution in Taihu Lake. HMs are often introduced into the environment as mixtures. However, the information on the ecological and health risk of combined metal pollution in Taihu Lake is still limited. Additionally, previous studies concentrated on the northern and eastern parts of Taihu Lake, such as Meiliang Bay, Gongshan Bay, Zhushan Bay, East Taihu Bay, and so on. The overall assessment risk of HMs in surface water and sediment of the whole of Taihu Lake is more informative for policymakers to establish pollution control measures.
The main objective of this study was to investigate the distributions of eight typical HMs (Hg, Cr(VI), As, Cd, Cu, Ni, Pb, and Zn) in the surface water and sediments of the whole of Taihu Lake, China, and assess the potential ecological and human health risks of HMs via multiple analytical and assessment approaches. Risk quotients (RQs) were used to assess the ecological risk of HMs in surface water. The geo-accumulation index (Igeo), the pollution load index (PLI), and the potential ecological risk index (RI) were used to assess the degree of contamination and ecological risks of the HMs in sediments of Taihu Lake. Furthermore, the potential health risks associated with drinking water and fish consumption were examined according to the US EPA methods. The in-depth results regarding risk assessments of HMs in the surface water and sediments can be used to improve the environmental management of HMs in Taihu Lake.
2. Materials and Methods
2.1. Study Area and Sample Collection
This study investigated the concentrations of eight HMs in surface waters and sediments in Taihu Lake. The lake is located in the lower reaches of the Yangtze River Basin. During the Mesozoic, large-scale granite invaded the basin of Taihu Lake, and the famous Suzhou granite formed. The elevation of the southwestern mountain is higher than the northeast region. The contents of organic matter and carbonate are relatively low in the sediment of Taihu Lake, which has fewer effects on the contents of metal [
6]. Taihu Lake is the third largest freshwater lake in China, with a surface area of 2400 square kilometers and a drainage area of 36,895 square kilometers. Taihu Lake is a typical shallow lake with a maximum depth of less than 3.0 m and an average depth of 1.9 m. Taihu Lake has abundant rainfall, with an average annual precipitation of 1100 mL. It belongs to the subtropical monsoon climate zone. The Taihu Lake basin is one of the most developed industrial and agricultural areas in China. It is also an important water source, aquaculture area, and aquatic habitat. Along with the rapid economic development and intensive use of water resources, the water quality of Taihu Lake has deteriorated and suffered from eutrophication and cyanobacterial blooms.
A total of 30 fresh surface water and sediment samples were collected from Taihu Lake in October 2019. The sampling sites are shown in
Figure 1. At each sampling location, five independent water/sediment samples were collected and pooled to obtain one composite water/sediment sample. The sediment samples (upper 20–30 cm) were collected using a stainless grab sampler. The water samples were acidified and extracted immediately, and the sediment samples were stored at −20 °C until further analysis. The sediment was air-dried and then ground through a 0.15 mm sieve to remove the visible plant debris and stones and to homogenize the sample. The preparation process avoided contamination and the loss of material.
2.2. Analyses of HM Concentrations in the Surface Water and Sediments
The pretreatment of the water samples and the quantification of HMs were carried out according to the standardized methods from the Ministry of Ecology and Environment (MEE) of China with minor modifications [
15]. The water samples for the detection of HMs were acidified with HNO
3 to a pH < 2 and then stored at 4 °C. The acidified samples were placed in a cool container and immediately delivered to the laboratory. Concentrations of Cd, Cu, Pb, Zn, and Ni in the surface water of Taihu Lake were measured by inductively coupled plasma–mass spectrometry (ICP-MS, Elan 6000, Perkin Elmer, Waltham, MA, USA). As the concentrations in the water samples were analyzed by hydride generation atomic fluorescence spectrometry (HG-AFS) using a Millennium Excalibur system (PSA 10.055. P S Analytical Ltd., Orpington, Kent, UK). Hg concentrations in water samples were detected by vapor generation–atomic fluorescence spectrometry using a Millennium Merlin system (PSA 10.025. P S Analytical Ltd., Orpington, Kent, UK). The Cr(VI) concentrations in the water samples were determined by flow injection analysis (FIA) and the diphenyl carbazide spectrometric method [
16].
In the laboratory, the pretreatment of the sediment samples for Cd, Cu, Pb, Zn, Ni, Hg, and As was carried out according to the standardized methods from the MEE of China with minor modifications [
17]. Briefly, 6 mL of a 3:1 mixture of concentrated HCl and HNO
3 was added to 0.1 g of each sample, followed by digestion in a microwave sample preparation system (Mini WAVE, SCP Science, Quebec, QC, Canada) for 1 h. The concentrations of Cd, Cu, Pb, Zn, and Ni in the digested solutions were analyzed using ICP-MS, and the contents of Hg and As were detected using atomic fluorescence spectrometry (AFS). Cr(VI) concentrations in sediment samples were determined by flame atomic absorption spectrometry after alkaline digestion [
18]. The samples were analyzed within a week. All of the chemical reagents used were of super-pure grade. The experimental glassware was precleaned by soaking in 15% HNO
3 (
w/
w) for at least 24 h, followed by soaking and rinsing with ultrapure water prior to use.
Quality control for the sediment samples was achieved by the use of certified reference materials (GBW07312) produced by the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences. Analytical reagent blanks were prepared with each batch of digested samples and then analyzed in the same way for background correction. Each sample was measured in triplicate to assess the accuracy and precision. The analytical results for the reference materials were within 10% variability, and the relative standard deviations (RSDs) for the triplicate samples were less than 10%. The detection limits were 0.01 mg∙kg−1 for Cd, 0.05 mg∙kg−1 for Cu, 0.1 mg∙kg−1 for Pb, 0.2 mg∙kg−1 for Zn, 0.4 mg∙kg−1 for Ni, 0.001 mg∙kg−1 for Hg and As, and 0.25 mg∙kg−1 for Cr(VI). The recovery rates of Cd, Cr(VI), Cu, Pb, Zn, Ni, Hg and As were 96.8–105.3%, 91.1–105.2%, 94.7–106.8%, 95.1–106.3%, 93.2–105.4%, 94.6–107.6%, 97.5–103.1%, and 97.5–103.8%, respectively.
2.3. Ecological Risk Assessment of HMs in Surface Water
The ecological risks of HMs in the fresh surface water were assessed by the method of risk quotients (RQs). The RQs were calculated as the ratio of the measured concentrations of individual metals to the aquatic life criteria [
19]. The acute and chronic aquatic life criteria of HMs for Chinese native freshwater organisms were retrieved from peer-reviewed literature and official reports. The acute and chronic RQs were calculated in this study. The RQs were categorized as follows: (1) RQ ≥ 1.00, high ecological risk; (2) 0.10 ≤ RQ < 1.00, medium ecological risk; and (3) RQ < 0.1, low ecological risk.
2.4. Ecological Risk Assessment of HMs in Sediments
2.4.1. Geo-Accumulation Index (Igeo) Method
The I
geo method proposed by Müller is commonly used to assess the degree of contamination by HMs in sediments [
20]. The index quantitatively expresses the degree of pollution by individual metals in particular locations as the ratio of the measured concentration to the geochemical background concentration. The I
geo values were calculated using Equation (1):
where C
n is the measured concentration of an individual metal in a sediment sample (mg∙kg
−1); 1.5 is a correction coefficient considering that the diagenesis might cause background value fluctuation, and B
n is the background value of the individual metal. In this study, the values of B
n for the eight metals were adopted according to the previous study [
21]. The I
geo values were categorized as follows: (1) I
geo < 0, pollution level 0, indicating no pollution; (2) 0 ≤ I
geo < 1, pollution level 1, indicating no to moderate pollution; (3) 1 ≤ I
geo < 2, pollution level 2, indicating moderate pollution; (4) 2 ≤ I
geo < 3, pollution level 3, indicating moderate to heavy pollution; (5) 3 ≤ I
geo < 4, pollution level 4, indicating heavy pollution; (6) 4 ≤ I
geo < 5, pollution level 5, indicating heavy to extremely heavy pollution; and (7) I
geo ≥ 5, pollution level 6, indicating extremely heavy pollution [
22].
2.4.2. Pollution Load Index (PLI) Method
The PLI method is commonly used for evaluating the combined effects of the contaminants [
23,
24], and this index was used to measure the eight HMs in the present work. The smaller the index value, the lower the pollution degree.
Here, C
n is the measured concentration of an individual metal in a sediment sample (mg∙kg
−1); B
n is the background value of the individual metal; PI is the pollution load index of an individual metal; and PLI is the pollution load index of eight HMs at a given sample site. The PI and PLI are divided into four categories, as shown in
Table 1.
2.4.3. Potential Ecological Risk Index (RI) Method
The RI method was also applied to calculate the potential ecological risk of a single metal and the sum of the eight HMs in the sediments of Taihu lake [
25].
Here, EI is the potential risk of an individual metal; T
i is the toxicity effect coefficient with values of Hg = 40, Cr(VI) = 2, As = 10, Cd = 30, Cu = 5, Ni = 5, Pb = 5, and Zn = 1; RI is the ecological risk of eight HMs in sediments. The categories of the EI and RI values are shown in
Table 2.
2.5. Human Health Risk Assessment for Metal in Surface Water
2.5.1. Non-Carcinogenic Health Risk from Drinking Water and Fish Consumption
Drinking water and fish consumption are the most important exposure routes for metal ingestion from surface water for residents living near Taihu Lake. The non-carcinogenic health risks for a single metal (Hg, Cd, Cu, and Zn) associated with drinking water or fish consumption were assessed using hazard quotients (HQs) in the present study. The non-carcinogenic health risk is defined as the ratio of the lifetime average daily dose (ADD) to the reference dose (RfD) [
26]. An HQ of greater than 1.0 indicates possible harm to human health.
where ADD is the average daily dose from a single route (mg∙kg
−1∙d
−1); RfD is the reference dose of a single metal (mg∙kg
−1∙d
−1); HQ
W is the hazard quotients from drinking water; HQ
F is the hazard quotients from fish consumption; THQ is the total hazard quotient for an individual metal resulting from drinking water and fish consumption.
In this study, the total non-carcinogenic THQ (TTHQ) was calculated as the sum of the individual metal THQ values according to the method of the U.S. EPA [
26,
27]:
The lifetime average daily exposure dose (ADD) of HMs through the consumption of drinking water or fish consumption was calculated according to the methods of the U.S. EPA [
25,
26].
where ADD
W is the average daily exposure dose from drinking water (mg∙kg
−1∙d
−1); ADD
F is the average daily exposure dose from fish consumption (mg∙kg
−1∙d
−1); C
W is a single metal concentration in the water (mg∙L
−1); DI is the drinking water intake (mL∙day
−1); EF is the exposure frequency (365 days∙year
−1); ED is exposure duration (70 years); BW is the body weight for Chinese residents (kg); AT is the averaging exposure time for non-carcinogens (365 days∙year
−1 × number of exposure years); FI is the fish consumption of native residents (g∙day
−1); C
F is the concentration of a single metal in fish (mg∙L
−1); BCF is the bioconcentration factor (L∙kg
−1). The exposure parameters, including DI, BW, and FI, for the residents living around Taihu Lake, were adopted. A higher ADD value indicates that the health risks posed by HMs are higher.
2.5.2. Carcinogenic Health Risk from Drinking Water and Fish Consumption
The carcinogenic risk is defined as the incremental probability that an individual will suffer cancer due to exposure to certain contaminants during one’s whole lifetime. The carcinogenic health risks ® for single metals (Cr(VI), As, Ni, and Pb) associated with drinking water or fish consumption were calculated as follows [
26,
27,
28].
where SF is the oral slope factor, ([mg∙kg
−1∙day
−1]
−1); R
W is the carcinogenic health risks from drinking water; R
F is the carcinogenic health risks from fish consumption; TR is the total carcinogenic risk for an individual metal. A cancer risk value R between 10
−6 and 10
−4 means an acceptable risk level. Otherwise, there could be a carcinogenic risk to human health.
The total carcinogenic risk (TR) was calculated as the sum of individual metal R values according to the method of the U.S. EPA [
27]:
2.6. Data Analyses
The descriptive statistics (including the mean, median, standard deviation, and coefficient of variation (CV)) were calculated to describe the HM concentrations in the sediments and water using Origin 10.5 software. The spatial distributions of the HM concentrations and the values of Igeo and RI were visualized with ArcGIS 10.3 software.
4. Conclusions
The results of the present study indicated that certain HMs posed ecological and health risks in the surface water and sediments of Taihu Lake. The acute risks of HMs to aquatic organisms were acceptable, while the chronic risk quotients of Cu, Ni, and Pb (>1.0) were found in the surface water of Taihu Lake. The risk assessment results revealed a moderate degree of HM risk in the sediments of Taihu Lake. Moreover, the ecological risk of HMs in the northern area of Taihu Lake was found to be higher than in the southwestern and eastern regions. Cd and Hg were identified as the primary contaminants in the sediments of Taihu Lake, with higher values of Igeo and PLI. The non-carcinogenic risk levels for Hg, Cd, Cu, and Zn in the residents living around Taihu Lake were acceptable. However, the ingestion of Cr(VI), As, and Ni through drinking water and fish consumption poses certain health risks and thus require greater attention. As such, it is necessary for the local government to develop clean technologies to reduce the emission of HMs into the aquatic environment and to strengthen the monitoring and management of HMs in the water, sediments, and edible organisms. Northern Taihu Lake has become a “hot spot” of pollution, which should be the focus of concern for policymakers. The control measures for sewage discharge from urban development in the “hot spots” must be more concerned.