Bioavailability of Heavy Metals Using Simultaneously Extracted Metal/Acid Volatile Sulfide in the Sediments of Lake Burragorang, NSW, Australia

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The different results reported and discussed are the average of minimum of two determinations. Blank determinations were carried out wherever necessary and the corrections were made if required. During the analysis for different parameters blanks, duplicates, spikes and standards were processed on 5% basis. The percentage recovery of spiked samples in metal determinations ranged from 94 to 104%, which indicate that the results are accurate and unbiased. Relative percent difference of duplicate measurements was less than 10%, which is a satisfactory precision. The relationships among different variables were determined by calculating Pearson’s coefficient of correlation. All statistical analyses were carried out using statistical software SPSS version 17.0 for Windows.

3. Results and Discussion

3.1. Spatial and Vertical Distribution of Physico-Chemical Parameters

The physical-chemical parameters of sediment cores are shown in Table 1. Sediment pH value ranged from 6.0 to 7.8 represented slightly basic to neutral to slightly acidic nature. Highest pH viz. basic nature was observed in core DWA 18. Organic matter in sediments mainly derives from phytoplankton, zooplankton and other organic materials. Organic matter decreases with depth at sites DWA27 and DWA2 (near dam wall). Interestingly down towards the Cox river in north (DWA18) organic percentage became relatively constant while going down. At DWA35 similar trend was observed for organic percentage, however, two peaks found at 10 and 25 cm layers. The organic matter values are not very high (10% to 12%) except at DWA2 at a depth of 35 cm (21.4%). The values between 12 and 14 are indicators of the allochthonous origin of organic matter ^[13]. The percentage of carbonate contents were more or less constant with a slight decrease at the bottom at all sites except DWA35. In general, low carbonate content in sediments (<10%) is attributed to terrigenous materials transported from catchment and near catchment area by winds, floods and waves ^[14].

3.2. Spatial and Vertical Distribution of Acid Volatile Sulfide

The distribution of AVS with depth in sediment cores are presented in Table 2 and Figure 2. Sediment samples at all sites and depth contained AVS concentrations above the lower limit (0.01 µmol/g) ^[9]. The highest sulfide levels were obtained from site DWA2 (range from 0.59 to 0.12 µmol/g), while lowest levels were obtained from site DWA35 (range from 0.25 to 0.09 µmol/g). AVS concentration at all four sites was low at the surface (5 cm) compare to intermediate depths. Two peaks of AVS were found at 10 and 20 cm layers in cores from DWA2 (near dam wall) and at 10 and 25 cm in core DWA27 and DWA35. In core DWA18, two peaks at 15 cm and 25 cm were identified. From the above distribution patterns, it appears that the AVS contents generally decrease along the profile with peaks at various depths. These profiles of AVS indicated non-steady deposition processes with instabilities during deposition ^[15]. These results are consistent with the findings of other studies indicated that surficial concentrations of AVS were smaller than those in deeper sediments [8,16-21].

The factors affecting the AVS contents in sediment are organic matter, the rate of sulfate reduction and redox status of sediments ^[22]. There were no correlation exit between AVS and organic matter at all locations (Table 3). So, organic matter may not be major controlling factor for AVS content. Relatively high Eh at surficial sediment ^[23] and downward diffusion of O₂ to several centimeters of the sediment–water interface cause the depletion of AVS by oxidation ^[24]. Therefore, the AVS content was very low in the top 5–10 cm of the sediment cores. Below this depth the peaks of AVS content in the profiles may be due to lower Eh, increased sulfate bacterial activity ^[25] and sudden change in depositional conditions ^[26]. At greater depth, the activities of sulfate reducing bacteria would decrease and AVS become reduced to FeS₂, therefore, AVS concentration decreased gradually with the sediment depth ^[8].

3.3. Spatial and Vertical Distribution of Simultaneously Extracted Metal

In all the sites among HCl-extractable metals (SEM) the Cd concentrations were lower and Zn were highest. Cu, Ni and Pb were intermediate. Vertical distribution of SEM is shown in Table 2 and Figure 3. No significant variation found in Cd concentration with depth in all the four sites. Concentrations of Cu, Ni and Pb at all stations are more or less same at all depths. Zn concentration generally decreased with depth except few peaks in cores DWA2, DWA27 and DWA35.

Table 1. Characteristics of Lake Burragorang sediments

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Table 2. Concentrations of AVS and SEM in sediments of Lake Burragorang

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Table 3. Correlation coefficient between AVS and organic matter

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Table 4. Percentage of SEM-metal and total metal

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SEM-metal concentrations and their percentages to SEM value are presented in Table 4. The concentrations of five acid extracted-metals differed considerably, zinc accounted for approximately 47– 69% of the total amount of the acid extracted-metals, whereas the contribution of cadmium to SEM was only 0.17- 0.63%.

Table 5. Guidance on determining metal toxicity to benthic organisms in freshwater sediments (values in mg/kg) [32]

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The SEM/AVS ratio presented in Table 2 showed that these simultaneously extracted metals at all stations were higher than AVS and their ratio was found greater than 1, which indicates that available AVS is not a major metal binding component for Lake Burragorang sediments and possibly contained metals potentially bioavailable to benthic organisms. However, the levels of AVS concentrations measured in the Lake Burragorang sediments are low compared to values reported in the literature for fresh water sediments ^[27]. AVS concentration depends on season and depth. Many researchers observed variation in AVS levels with season ^{[27, 28, 29, 30]}. Major decrease occurred in the winter because in colder temperatures, FeS formation rates were lower and a smaller supply of FeS-rich particles was brought up from below by bioturbation ^[28].

For the present study, sediments for SEM and AVS were collected for analysis in winter season and hence the low level of AVS was found. Low AVS concentrations indicate that most metals are bound by sediment constituents other than AVS ^[9]. A study conducted by Lawrence Berkeley National Laboratory (LBNL) also had sites with SEM-AVS values greater than one due to relatively low AVS values and not necessarily high concentrations of metals. No toxicity to benthic organisms was observed from these LBNL sites ^[29]. Other constituents in the sediment, such as iron and manganese oxides and organic matter, may have decreased the bioavailability of heavy metals ^[4]. The unbound metals toxicity to benthic organisms can be explained by analyzing individual SEM concentrations according to their threshold effects level (TEL) and upper effects threshold (UET) levels (Table 5). All the locations had individual SEM concentrations lower than their TEL. Even though these investigated metals were bioavailable in the sediment their individual metal concentrations are not expected to be toxic to benthic organisms.

4. Conclusion

The results showed that these SEMs at all stations were higher than AVS and their ratio was found greater than 1, which indicates that available AVS is not sufficient to bind with the extracted metals for Lake Burragorang sediments and possibly contained metals potentially bioavailable to benthic organisms, however, AVS values in this study are low compared to values reported in the literature for fresh water sediments so SEM/AVS ratio could be high owing to relatively low AVS values and not due to high concentrations of metals. Further, the seasonal cycle of SEM and AVS can alter the potential toxicity criteria of sediments. The present study was done during winter period. As reported earlier significant decrease occurred in the winter period because microbial activity reduced in colder temperature. In other words sediment that is non-toxic during the period of high AVS (i.e in summer) can become potentially toxic during periods of low AVS (i.e in winter) ^[31]. In the current study even though these investigated metals were bioavailable in the sediment their individual metal concentrations are not expected to be toxic to benthic organisms as all locations had SEM concentrations lower than their threshold effects level (TEL). However, slight increase in SEM above TEL will be detrimental for aquatic system as available AVS in sediment of Lake Burragorang is not sufficient to bind with the extracted metals. Further, continuous and constant monitoring of Lake Burragorang sediment for SEM and AVS is recommended to protect the aquatic system from the toxic effect of free metals.

Figure 2. Vertical distribution of AVS

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Figure 3. Vertical distribution of SEM

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