Baltic Breakfast: Dredging polluted sediments comes with risks – better alternatives exist

Many coastal areas in Sweden are contaminated, often due to past industrial activities such as sawmills, pulp and paper mills, steelworks and ports. A common method for remediating these areas is dredging, after which the removed sediments are disposed of on land or in deeper water. This has been the case in the heavily polluted harbour of Oskarshamn, where 700,000 cubic metres were dredged at a cost of SEK 630 million.
However, such remediation can also cause new problems: disturbing the sediments may resuspend pollutants and spread contaminants to other areas.

“Contaminants that were safely buried can come up instead and expose fish and other animals,” says Professor Jonas Gunnarssson, who has spent decades researching contaminated sediments and potential solutions, most recently through the CAPTIVE project, which has included studies of the yet untreated outer harbour of Oskarshamn.

Other methods for remediation

Other methods for treating contaminated sediments include monitored natural recovery (basically leaving the sediments as they are and following what happens over time) and so-called capping, where a layer of material, such as sand, is placed on top of the sediment to isolate contaminants.

In conventional capping, the layer can be up to a metre thick, but it is also possible to perform capping using a thinner layer of material with a sorbent, often activated carbon.

“The animals will bioturbate this material and mix it into the sediment, and the carbon will then sorb the contaminants,” Jonas Gunnarsson explains.

Activated carbon or activated biochar is also used for detoxification purposes in cases of poisoning, as well as for water treatment. The reason this material is so effective is that it contains a large number of micropores, providing a very high surface area: one gram of activated carbon has a total surface area of about 1,200 square metres (30 × 30 metres).

Testing activated carbon in Norway

Together with colleagues from different organisations (such as NIVA and NGI – the Norwegian Geotechnical Institute), Jonas Gunnarsson began testing activated carbon for sediment remediation in the Norwegian Grenland fjord some years ago.

“We took tonnes of activated carbon and put them inside a big boat,” he says. “And we mixed activated carbon with clay that came from a clean site and added salt, and then we pumped this down to 30-metre depth and 90-metre depth.”

The idea is that pollutants that do not dissolve easily in water but instead bind to sediment particles will preferentially sorb to the micropores of the activated carbon, and the method was shown to work.

“We have seen in many studies now that we can decrease between 60 and 90 percent of the uptake of contaminants in fish and in sediment biota,” says Jonas Gunnarsson.

Effects on benthic communities

However, the thin-layer capping treatment also turned out to have unexpected and unwanted effects on benthic animals, and Jonas Gunnarsson and the other researchers in the CAPTIVE project, run by Stockholm University in cooperation with KTH Royal Institute of Technology, have since tested multiple types of activated carbon and materials to mix with it, in order to find a solution that is both effective and without negative effects.

The experiments have shown that powdered activated carbon (15–150 µm particle size, so-called PAC) works best in terms of sorption efficiency, but it tends to resuspend in the field and can also cause negative effects on animals.

“When it’s taken up in the animals the activated carbon does not only take away the contaminants, but it also binds all the essential nutrients, the food,” Jonas Gunnarsson explains.

Granular activated carbon (150 µm–5 mm, so-called GAC) shown to be less effective, but does not harm the animals. The question then arose as to whether it might be possible to use a material with a particle size somewhere in between.

“Most of the benthic fauna feed on particles that is just under 100 µm, so perhaps we could go over that, but under 300 µm…”

Mesocosm experiments at Askö Laboratory

To investigate this further, the researchers collected large sediment samples from Oskarshamn and transported the box cores to the Askö Laboratory, where several experiments were conducted.

To summarise years of experiments, Jonas Gunnarsson concludes that granular activated carbon in the size range 100–250 µm was shown to reduce the bioavailability of many pollutants by up to 95 percent, without negative effects.

Even more promising, the material was shown to work well when mixed with polonite – a calcium silicate mined and produced in Poland, often used in wastewater treatment to trap phosphate and metals.

“Polonite sorbs several metals and reduce their bioavailability, and also works as a good carrier material for activated carbon, but it should not be overdosed as that can cause negative ecological effects by increasing alkalinity,” Jonas Gunnarsson says.

When experimenting with dosage, the researchers found that a dose corresponding to an increase of the sediment’s own organic carbon content by 1.5 percent is sufficient. In Oskarshamn, this corresponded to 600 g GAC per m², which could be mixed with the same weight of polonite. However, the optimal dose and sorbent type must be evaluated on a case-by-case basis.

Microbial responses to pollution

A research study led by Jonas Gunnarsson’s colleague Divya Pal (who were unable to attend the Baltic Breakfast seminar in person) has taken a closer look at how microbes in the sediment respond and adapt to pollution.

An experiment was conducted in which the researchers took 40 sediment cores from Oskarshamn, where the bacteria had been exposed to copper for half a century, and 40 cores from a clean area outside Askö, to the laboratory. Then new RNA and DNA analysis methods were used to examine how the microorganisms responded to different copper contamination levels during different oxygen conditions.

The results show that the microbes from Oskarshamn was two times more tolerant to copper than the ones from Askö. They tolerance was also higher during hypoxic (low oxygen) conditions, when bioavailability of copper is lower, than under oxic conditions.
This indicates that there are more factors to consider when taking management decisions about polluted sediments than the concentration of the contaminants, Jonas Gunnarsson emphasises.

“We have to understand what the pollution history, what the oxygen conditions are and how long time the bacteria and animals have been exposed because that will affect how tolerant they are,” he says.

Disturbing sediments as you would with dredging will change the environmental conditions and increase oxygenation, and make many contaminants more bioavailable.

“We have to look at the bioavailability rather than the concentrations, and bioavailability will decrease with time. In old contaminant sites as Oskarshamn only a smaller part of the contaminants were actually bioavailable.”

The results of the studies performed in the CAPTIVE-project put a question mark to the whole 630 million remediation treatment done in Oskarshamn.

“When toxicity is low it’s better to not dredge, but leave it as it is or maybe do this thin layer capping”, concludes Jonas Gunnarsson.

Microbial responses to pollution

A research study led by Jonas Gunnarsson’s colleague Divya Pal (who was unable to attend the Baltic Breakfast seminar in person) has taken a closer look at how microbes in the sediment respond and adapt to pollution.

In one experiment, the researchers collected 40 sediment cores from Oskarshamn, where bacteria had been exposed to copper for half a century, and 40 cores from a clean area outside Askö. New RNA and DNA analysis methods were then used to examine how the microorganisms responded to different levels of copper contamination under varying oxygen conditions.

The results show that microbes from Oskarshamn were twice as tolerant to copper as those from Askö. Their tolerance was also higher during hypoxic (low-oxygen) conditions, when the bioavailability of copper is lower, than under oxic conditions.
This indicates that more factors need to be considered when making management decisions about polluted sediments than just contaminant concentrations, Jonas Gunnarsson emphasises.

“We have to understand the pollution history, what the oxygen conditions are and how long time the bacteria and animals have been exposed, because all that will affect how tolerant they are,” he says.

Disturbing sediments, as is done during dredging, changes environmental conditions, increases oxygenation and can make many contaminants more bioavailable.

“We have to look at the bioavailability rather than the concentrations, and bioavailability will decrease with time. In old contaminant sites as Oskarshamn only a smaller part of the contaminants were actually bioavailable.”

The results of the studies conducted within the CAPTIVE project raise questions about the SEK 630 million remediation carried out in the inner harbour of Oskarshamn.

“When toxicity is low it’s better to not dredge, but leave it as it is or maybe do this thin layer capping,” concludes Jonas Gunnarsson.

Text: Lisa Bergqvist

Watch a recording of the seminar