The traditional flux chamber for measuring the leaching of contaminants does not allow supply of oxygen into the chamber. The traditional method therefore deteriorates the conditions for the benthic organisms where the measurement is done, which leads to inaccurate measurement of the flux from the seabed. One of the reasons for this is that the biological life on the seabed are contributing to a remoulding and mixing of the seabed sediments. This is caused by benthic organisms which affect and mix the sediment as they move and migrate. This process is named bioturbation.

- Contaminated seabed sediments can be found in lakes, fjords or in ocean areas which have been widely exposed to emissions of pollutants. When we do measurements with this method, we are first and foremost concerned with the quantification of organic pollutants that are leaching out into the water from the seabed, explains Espen Eek. He is the Technical Lead for contaminated sediments at NGI.

In several port and harbour areas, polluted seabed has been capped with the clean materials, as a remedial measure. This has worked well and resulted in improved water quality.

It is always important to assess the condition on the seabed after remediation has been carried out in an area.

In order to measure flux from the seabed sediment samples can becollected for analysis in the laboratory or flux chambers can be used to measure flux in situ. In situ measurement has the advantage of measuring the flux without disturbing the sediment by sampling or handling in the laboratory.

The traditional in-situ method for measuring the amount of contaminants that are released into the water, a closed chamber, a so-called flux chamber, is placed on the seabed. The purpose is to measure the amount of toxins released from the seabed and is captured in a sorbent material inside the chamber over a certain period of time.

- This method has been shown to give a good measure of the diffusive flux from sediments. However it has been recognized that the oxygen in the water inside the flux chamber can be consumed, thus deteriorating the conditions for the benthic organisms inside the chamber and reducing the effect bioturbation enhanced flux of environmental toxins. When I discussed this with my colleague, Gerard Cornelissen a couple of years ago, we concluded that we could do the measurements of leaching by pumping purified water into the flux chamber in a continuous process. This would keep the concentration of oxygen inside the chamber at ambient level, Espen Eek explains.

Fluxchamber 600

NGI therefore built a prototype of such a chamber, based on the traditional flux chamber design, but with additional features to allow flow of water through the chamber. In collaboration with PhD student Lukas Mustajärvi, supervised by researcher Anna Sobek and other colleagues at Stockholm University, the prototype was tested on polluted seabed in the Baltic Sea. Lukas Mustajärvi defended his doctoral thesis, where this work was included, in Stockholm, 12th January 2018.

- We see a difference between the two sampling methods, which correspond with our hypotheses that bioturbation increases the sediment to water flux of organic contaminats, says Espen Eek. He was one of the authors contributing to the scientific paper in the Environmental Pollution magazine, "In situ benthic flow-through Chambers to determine sediment-to-water fluxes of hydrophobic organic contaminants legacy".

Maaleutstyr miljoegifter sjoebunn 700

Preparing the flux chamber on deck prior to installation on sea bottom.

He thinks the new method will be useful in connection with the cleanup on the seabed, as it provides more accurate measurements of the condition before and after the mitigation measures have been implemented. The new method has until now, only been used for research purposes. However we believe, this can also be a useful tool in field surveys to map the risk associated with contaminated sediments, or to measure the effects of the remediation in contaminated sediments.

- First and foremost, this is an important step in the effort to understand the leaching and exposure of environmental toxins on the seabed. We have gained more knowledge and insight, which will be useful in future work on this topic, Espen Eek emphasizes.

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FACTS

  • NGI has contributed to the environmental cleanup in several contaminated harbour areas, including in Trondheim, Oslo, London and at Herøya.
     
  • Reliable measurement methods before and after capping of polluted seabed sediments are important in order to determine the effect of the mitigation measures.
     
  • A flux chamber is a type of enclosed box that is placed on the seabed with contaminated sediments. The chamber has a sorbent material that captures the organic contaminants which have leached out into the water from the seabed, so that these can be measured and quantified.
     
  • Bioturbatjon is a common term for biological processes that contribute to mixing and moving soil or sediments. In seabed sediments benthic organisms are the main contributors  to bioturbasjon. Charles Darwin was the first who described these processes.

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