3.1. Sediment Characteristics and Mean Concentrations of Heavy Metals
Sediment samples display the pHs between 6.81 and 7.30 with an average value of 7.08, indicating a relative safer environment for heavy metal stabilization. SOM in sediments varied from 3.00 g/kg to 12.91 g/kg with an average value of 7.30 g/kg. Concentrations, Igeo
values and sediment quality guidelines (SQGs) are listed in Table 3
. Average concentrations of Cu, Zn and Cd exceeded their corresponding background values, while average concentrations of Cr and Pb were lower than the background values. In particular, the average measured value of Cd was 2.5 times than its background value. SQG is established to evaluate the toxicity or risk of contaminants to aquatic ecosystems [52
]. When the heavy metal’ concentration is lower than the threshold effect level (TEL), it means that adverse biological toxicity effects rarely occur. When concentration is higher than the probable effect level (PEL), it means that adverse biological toxicity effects frequently occur [52
]. Compared with the corresponding TELs, the enrichment degree of the studied five metals in sediment decreased in the sequence of Cr > Cu > Zn > Pb > Cd. Generally, the concentrations of all heavy metals in 16 sampling sites were lower than Grade II values of the Chinese Environmental Quality Standard for Soils (GB 15618-1995). According to the calculated results of Igeo
), it is obvious that Cd in all sampling sites were under un-contamination to moderate contamination, while sediments was not contaminated by Cr, Cu, Pb and Zn for their Igeo
less than 0. Specifically, Cd in S14 has the highest Igeo
value and Cd in S13 and S14 belonged to moderately contamination. Average Igeo
values of five kinds of heavy metals decreased in the order of Cd > Zn > Cu > Pb > Cr.
3.2. Spatial Distribution of Heavy Metals in Surface Sediments from the Honghu Lake
shows the spatial distribution of Cr, Cu, Pb, Zn and Cd in sediments throughout Honghu Lake. The detailed concentration values of these metals are shown in Figure S2
. To characterize spatial distribution of heavy metals effectively, Honghu Lake was divided into three regions: North Honghu Lake (S1, S2, S7, S8, S15 and S16), South Honghu Lake (S3, S4, S5, S6, S9, S11, S12, S13 and S14), and the outlet of Honghu Lake (S10).
According to Figure 2
a,b, there were similar spatial distributions of relatively higher pollution points for Cr and Cu. The enrichment degrees of Cr and Cu in South Honghu Lake and the east part of North Honghu Lake were higher than the other parts. Concentrations of Cr varied from 59.68 mg/kg (S15) to 101.11 mg/kg (S3). Unlike other heavy metals, only concentrations of Cr (in S1, S3, and S5) exceeded the corresponding PEL, which indicates that adverse biological toxicity effects may frequently occur in these areas. For Cu, the lowest and highest concentrations were found in S2 (17.21 mg/kg) and in S3 (45.37 mg/kg). Based on Figure 2
b, for Cu, 25% of sample concentrations were lower than the TEL (35.7 mg/kg).
For Pb, the highest concentration of 29.20 mg/kg was found in S11 and the lowest concentration of 18.88 mg/kg was found in S2. Based on Figure 2
c, 37.5% of samples of Pb exceeded the background value (30.7 mg/kg), while all of the samples did not exceed the TEL (91.3 mg/kg). The lower part of South Honghu Lake and the west part of North Honghu Lake had relatively higher Pb enrichment degree than the other parts.
Concentrations of Zn varied from 62.64 mg/kg (S2) to 127.12 mg/kg (S15). According to Figure 2
d, 97.75% of samples exceeded the background value (83.6 mg/kg), while only 18.75% of samples exceeded the TEL. Most parts of Honghu Lake were enriched to some t degree except the small portion of south part of North Honghu Lake.
Concentrations of Cd varied from 0.30 mg/kg (S2) to 0.53 mg/kg (S14). Based on Figure 2
e, the contents of Cd in all sampling sites exceeded the corresponding background value (0.172 mg/kg), while no samples exceed the TEL (0.596 mg/kg). In addition, the enrichment degree of Cd in lower part of South Honghu Lake, the west part of North Honghu Lake and the outlet of Honghu Lake were relatively higher than the other parts.
3.3. Heavy Metal Chemical Fractions
The proportion of heavy metals chemical fractions of Cr, Cu, Pb, Zn and Cr are displayed in Figure 3
. The proportions of the acid-extractable fraction decreased in the order of Cd > Cu > Zn > Pb > Cr. Figure 3
shows that the bioaccessibility of Cr is at a low level since no more than 2% of Cr occurred in acid-extractable fraction and reducible fraction. Cr mainly existed in residual fraction, ranging from 89.92% to 96.29%, and Cr in oxidizable fraction ranged from 3.46% to 8.52%. Unlike Cr, Cu in sediments was mainly associated to oxidizable fraction and residual fraction, which ranged from 35.19% to 52.30% and from 35.47% to 59.33%, respectively. The proportions of Cu existing in acid-extractable and reducible fractions ranged from 0.34% to 11.34% and from 0% to 3.37%, respectively.
Different from the other metals, Pb mainly existed in oxidizable fraction in all sampling sites except S2, ranging from 53.12% to 80.53%. The oxidizable fraction of heavy metal is produced by activities of aquatic organisms and discharge of organic wastewater, which is relatively stable in sediment. However, heavy metals existing in oxidizable fraction would become higher valence metals with migration in the strong oxidizing conditions. It means that Pb in sediment from Honghu Lake potentially had some ecological risk.
Zn existed mainly in residual fraction, ranging from 64.18% to 78.83%. The amounts of Zn in acid-extractable fraction, reducible fraction, and oxidizable fraction were in the range from 2.00% to 14.31%, from 3.86% to 6.96% and from 13.35% to 21.56%, respectively.
Unlike other metals, Cd had the higher proportion of acid-extractable fraction (19.41–31.47%) while mainly occurred in residual fraction (53.57–72.55%). The acid-extractable fraction is very sensitive to water environment change, which be probably released in acidic or neutral condition and easily transfer into aquatic organisms. It showed that Cd in sediment from sampling sites might have a relatively higher biological toxicity than the other metals.
3.4. Fuzzy Comprehensive Risk Assessment
According to the analysis above, results were obtained by different evaluation indexes such as SQGs, PEI and RAC. The enrichment degree compared with SQGs decreased in the order of Cr > Cu > Zn > Pb > Cd, average ecological risks were decreased in the order of Cd > Cu > Pb > Cr > Zn, and the average bioaccessibilities decreased in the order of Cd > Cu > Zn > Pb > Cr. Results showed that some differences surely existed among these widely used methods, which may confuse decision makers because these methods unilaterally focus on ecological risk based on total content or bioaccessibility based on chemical fraction. Moreover, there are complexity and fuzziness in environmental assessment system, which need to be under quantitative reduce and control. Therefore, a new model with synthetically considering heavy metals’ total content, ecological risk, bioaccessibility and systematic fuzziness is needed. To identify the comprehensive risk of sediment heavy metals efficiently, the fuzzy comprehensive assessment method was established based on PEI, RAC and fuzzy theory. To make the assessment method scientific, rational and high recognition, firstly, potential ecological risk and bioavailability were divided into different levels using fuzzy mathematics, then fuzzy results of PEI and RAC were endowed with weights, and finally comprehensive risk can be calculated as in Equations (5) to (16). Combining Table 1
and Table 2
, according to arithmetic calculation from Equations (1) to (16), an assessment matrix based on average values of Cr, Cu, Pb, Zn and Cd is as follows:
Based on Equation (6), average comprehensive risks of heavy metals in sediments of Honghu Lake were as follows: RiskCr = (1, 0, 0, 0, 0), RiskCu = (0.655, 0.345, 0, 0, 0), RiskPb = (0.882, 0.118, 0, 0, 0), RiskZn = (0.745, 0.255, 0, 0, 0) and RiskCd = (0.032, 0.466, 0.502, 0, 0). According to maximum membership principle, average comprehensive risks of heavy metals decreased in the sequence of Cd (considerable risk) > Cu (moderate risk) > Zn (low risk) > Pb > Cr. It indicated that Cd and Cu were likely to have adverse biological effects on aquatic systems of the Honghu Lake.
Unlike other metals, the comprehensive risk of Cd belonged to the considerable risk level, which is because Cd had a moderate ecologic risk and a high level of bioaccessibility. Since the membership values of moderate risk level and considerable risk level were very close and the results is greatly affect by subjective factors, further monitoring and expert discussion about the risk level of Cd is necessary when decision making. Although the bioaccessibility of Cu was close to the moderate level, its comprehensive risk was at low risk level because of its low ecological risk. The membership degree of low risk level (0.507) and moderate risk level (0.493) was also close. Under principle of maximum risk protection, Cu was determined as moderate risk. If the absolute difference between memberships of two risk levels is less than 10%, the final risk level can be determined as the higher level for warning. Average comprehensive risk of Cr in sediments belonged to the low risk level because of its very low ecological risk and quite low biological risk. For Cr, Zn and Pb, they were also at the low comprehensive risk level due to their low ecological risk and low biological risk.
Spatial distributions of the calculated comprehensive risk levels and their probabilities for heavy metals in each sediment sample from Honghu Lake are shown in Figure 4
and Figure 5
, which were drawn according to the results listed in Table S1
. Compared with other metals, Cd in sediments was at the highest risk level varying from moderate risk to considerable risk. The areas under considerable risk were about 43.75% of Honghu Lake, with 57.14% samples belonging to considerable risk with about 50% membership degree (Figure 4
e and Figure 5
e). The central part of South Honghu Lake (S4, S5, S6, S9 and S12), east part of North Honghu Lake (S1) and outlet of outlet of Honghu Lake (S10) were the priority control areas. Moreover, sites including S2, S3, S4, S5, S6, S7, S8, S9, S11, S12, S13, S14, S15 and S15 need to be paid more attention when decision making because the probabilities of their considerable risk level was 40–50%. Spatially, the comprehensive risks of Cd decreased in the order of S5 > S10 > S1 > S12 > S4 > S9 > S6 > S15 > S13 > S16 > S11 > S3 > S8 > S7 > S14 > S2. Moreover, risk levels of Cd were not completely determined in S3, S6, S9, S11, S13, S16 and S12 because their calculated membership values of moderate risk level and considerable risk level were quite close.
According to Figure 4
b, for Cu, there were 81.25% and 18.75% of areas under low risk and moderate risk, respectively. The areas around S1, S12 and S14 of South Honghu Lake were of concern. Specifically, the membership degrees in S1, S3, S9, S11 and S16 were around 50% (Figure 5
b). It is necessary to pay more attention to these sites during corresponding decision making. The comprehensive risks of Cu were in the descending order of S12 > S14 > S1 > S11 > S16 > S9 > S3 > S4 > S10 > S2 > S8 > S7 > S6 > S15 > S5 > S13.
Comparing with Cd and Cu, the comprehensive risks of Cr, Pb and Zn were at lower level. Comprehensive risks of Cr and Zn in sediments from 16 sampling sites were all at low risk level. Calculated risk values of Cr in samples except S9 were extremely close, and S9 was slightly higher than other sampling sites. Comprehensive risks of Pb decreased in the order of S2 > S5 > S3 > S4 > S6 > S7 > S10 > S8 > S13 > S9 > S12 > S16 > S14 > S15 > S1. Comprehensive risks of Zn in sediment at all sampling sites except S1 (belonging to moderate risk level) were at low risk level. Calculated comprehensive risks of Zn decreased in the order of S1 > S12 > S9 > S10 > S13 > S14 > S16 > S15 > S11 > S8 > S6 > S2 > S3 > S4 > S5 > S7.