Summary and conclusions, Miljøstyrelsen

Study of leaching from harbour sediments under different redox conditions

Summary and conclusions

The purpose of the project is the collection of data for total content of a number of chosen parameters in harbour sediments from three types of harbours plus data on the distribution between the adsorbed and the dissolved phase under reduced and oxidised conditions. Data have also been collected with the aim of evaluating the development in redox conditions over time for sediments disposed of in an environment where oxygen is available (above the water line). The data have been used to model the leaching from two types of disposal facilities:

Disposal under water in a coastal disposal site, where sediments are assumed to remain in reduced form, and
Disposal in a landfill (in accordance with the Landfill Directive), where oxidation of the sediments can occur.

Finally, an evaluation is made of the suitability of batch-tests (normally used to evaluate leaching from inorganic waste) for the evaluation of leaching from disposed sediments.

The collected data and modelling is used as a basis for the assessment of the exposure related to landfilling either in the facilities similar to the existing land deposits exclusively for pumpable dredged materials (in Danish "spulefelter") or in Directive complying landfills. The assessments could be used as a paradigm for this type of future exposure assessments.

It should be noted that the project does not address the legal issues related to the possibilities of landfilling of sediments in the existing facilities ("spulefelter") or in landfills complying with the Landfill Directive. For instance, the Landfill Directive strictly prohibits the landfilling of liquid waste.

As part of the project, data has been collected from three types of harbours: fishery, industry and leisure boating, and for the chosen parameters: PAH (polyaromatic hydrocarbons), and TBT (and the degradation products hereof), and the metals Cd, Pb, Cu, Zn, Ni, Hg, and As.

The samples have been collected in KAYAK cylinders so as best to ensure the collection of the fine particles with the aim of picturing the degree of contamination as precisely as possible. This method also ensures that the samples can be kept anaerobic. The sampling sites have been chosen in collaboration with the three harbours, at places were substantial sedimentation can be expected to occur.

The pore water has been pressed from the samples by vacuum-filtration and both sediment and pore water have been analysed under anaerobic conditions. Water has been re-added in an amount similar to what has been removed, and the sediment has been thoroughly oxidised. This caused redox-levels to rise from negative values to 150 – 200 mV. The filtration process was then repeated under aerobic conditions. Finally, batch-tests were carried out in accordance with the procedure described in the Danish implementation of the Landfill Directive.

The analytical results did not show an unambiguous relation between redox condition and leachability for the investigated metals and organic compounds (evaluated on the basis of calculated K_d-values, the ratio between adsorbed and dissolved phase of the compound). But the observed leachability varied substantially between the three harbours. As expected, the leachability varied between the metals where As, Ni, Cd, and Zn are the least sorbing (mentioned in the order of increasing adsorption), while the leachability of Pb, Cu and Hg depends on the redox conditions.

The adsorption of Pb increases with increasing oxidation rate, which is also true for Ni and As too a lesser degree. For As, the sorption decreases with increasing oxidation for the sediment from Gilleleje. For Cu sorption decreases with oxidation of the sediment while leachability of the other compounds is more a function of the overall composition of the sediment and varies significantly for the three harbours. There is no significant pattern in the leachability of PAH and TBT in relation to redox conditions, which is also not to be expected.

As mentioned, there is substantial variation of leachability between the three harbours. The calculated K_d-values (and thus the sorption) for Copenhagen Harbour increased substantially for all metals (except Cu) with the transition from strictly anaerobic to batch-test and aerobic conditions. This pattern is only similar for Pb and Ni for the two other harbours and to a lesser degree. There is no discernible pattern for the organic compounds related to the specific harbour. This is probably partly due to the relatively large variation between the single samples both with respect to total content and to pore water concentration.

The reason for the increase in sorption with increasing oxidation could be the change in composition of the organic matter. The oxidation presumably removes some of the more easily dissolved organic matter to which some of the metals are bound, leaving less dissolvable organic matter that will lead to an increase in the overall binding of the metals. Another reason could be the reduction in chloride due to addition of fresh water. Chloride forms dissolvable complexes with several metals and the presence of more or less chloride thus can influence the overall concentration of the metals in solution.

Sorption to the sediment under anaerobic conditions seems to be a function of the oxygen-consumption-capacity of the sediment resulting in stronger binding the larger the oxygen-consumption-capacity. Following oxygenation of the sediments, the differences in metal-binding are significantly smaller. This means that by oxygenation of sediments with a large oxygen-consumption-capacity, there is a relatively higher increase in the metal release than by oxygenation of sediments with a lower oxygen-consumption-capacity. It should be noted that the above interpretations should be taken with caution since the underlying amount of data is relatively small.

In general, it can be stated the binding of contaminants to the sediment is a complex process, and that the assessment of the leachability of the compounds from the sediment after disposal will have to be based on specific evaluation.

Based on the experimental values the specific leaching from each harbour has been calculated and compared with the relevant quality criteria. For disposal under the water line the calculations show that primarily PAH and TBT exceed the quality criteria. For Copenhagen Harbour the criteria were also exceeded for the metals (except Cu and Hg). It should be noted that due to experimental problems the calculations are based on only one calculated K_d-value for the anaerobic sediment, whereas the calculations for the other harbours are based on two values.

It should be noted that the leached amount of compound is reduced substantially when the filling of the disposal site is completed due to the large reduction in water flow after completion, while the concentration stays the same.

The leached amounts (and concentrations) can for the organic compounds in principle be reduced due to retardation in the surrounding dams and the subsequent possibility of degradation. A calculation based on a limited amount of available degradation data shows that a fairly large reduction can be expected. This is due to the very slow relative velocity of PAH and TBT in the water leaching through the dams and thus the very long time it will take the compounds to pass the dams. Based on these very preliminary calculations, PAH-concentrations in the water surrounding the disposal site seem to comply with the quality criteria with a reasonable margin while the TBT concentrations are just below the criteria. For the metals, the retardation in the dams will result in a very large delay in the break-through of the contamination.

The disposal facilities that are described above are in principle similar to the existing "spulefelter" apart from the fact that the disposal is carried out without the addition of water. As a supplement calculations are also carried out with addition of water and with the reuse of the injected water. Due to the increased amount of water, the maximum concentration is reduced during filling of the site while the total leached amount after filling is the same independent of the method of filling.

Finally, a calculation has been made of the importance of particulate matter for the overall contaminant load. The calculations have been carried out for the outlet of excess water and for traditional dumping of dredged material from harbours on the seabed. These calculations show that particulate matter in any case will increase the contaminant load to a very large degree.

Overall, the model calculations show that disposal of sediments under the water line and without liner in principle can be carried out in a number of cases without exceeding the Danish quality criteria for surface waters. In other cases this will not be possible, and collection of the leachate will be necessary. This will partly be dependent on the overall contaminant content, partly on the content and composition of the organic matter in the sediment. Finally, it will depend on how the disposal facility is constructed (e.g. the composition of the dam material) and the method of filling (e.g. the method of reduction of outlet of particulate matter).

For the disposal in a landfill in accordance with the Directive, calculations were carried out to estimate the development of the oxygen profile in the landfill. The calculations were carried out for Copenhagen Harbour being the harbour with the lowest oxygenation capacity. Based hereon, it was estimated that oxygen saturation only would reach 2 m into the landfill over a period of 100 years. This is in accordance with observations from other studies showing very little oxygen transport in fine-grained material. It should be noted that the calculations are highly dependent on water content (similar to what is observed in the field), since a high water content will inhibit the diffusion of oxygen into the sediment and thus the development of the oxygen profile. It will thus take very long time before any considerable part of the sediment in a landfill is oxidised. Leaching from a land based disposal site will therefore be very similar to leaching from a site where sediments are filled below the water line. In a land based landfill, leaching will also be reduced after filling is completed, but here an active removal of water will take place, increasing the overall flux. However, in land based landfills, the leached compounds can be collected and treated before outlet to the recipient.

The experiments carried out show that batch tests do not seem to give results that are similar to either anaerobic or aerobic pore water concentrations, and that no discernible pattern in the results can be observed. It should be noted that none of the methods used can be said to be well established or standardised for use with sediments to an extent that allows for an evaluation of the influence of the method on the results.