The bottom sediment of water bodies is an archive of all human activity. It is a reservoir of nutrients but also of harmful substances – and it can also turn into a source of emissions.
- Text Marianne Mustonen
- Photos Niko Jouhkimainen and Jarkko Akkanen
“Most lakes tend to be remediated due to eutrophication. Water bodies have also been remediated due to the presence of harmful substances, but clearly less frequently,” says Research Director Jarkko Akkanen from the Department of Environmental and Biological Sciences.
A lake’s environmental history can be studied through its sediment. For example, the caesium fallout from the Chernobyl nuclear accident shows whether a particular sediment layer was formed at or after the time of the accident.
Sediment acts as a reservoir for harmful substances, but it can also become a source of emissions, even after the original source of contamination no longer exists. Harmful substances stored in the sediment may have an adverse effect on the water body and its biota.
“Substances released from the sediment can be harmful to benthic organisms. These substances may also accumulate in the food chain and end up on our own plates. The consumption of fish from such areas may need to be restricted,” Akkanen notes.
“In Finland, environmental matters are generally well taken care of. If an area is known to have had industrial activity, it is possible to assess where emissions from that activity may have spread. But the question is: do we know all our emission sources? As the climate changes, water currents may shift and sediments may begin to move.”
Threshold values for harmful substances, similar to those available for contaminated soil, are still lacking for sediment both in Finland and at EU level. Contaminated areas are remediated as needed, for example when former industrial sites are converted for residential use.
However, contaminated sediment cannot simply be excavated and deposited on land. If harmful substance thresholds are exceeded, the sediment can no longer be returned to the water body. When depositing sediment on land, risks must be minimised: the harmful substances found in the sediment are effectively encapsulated within a mound.
Akkanen’s research group has studied sediment remediation in several locations, including in Lake Kernaalanjärvi in southern Finland. Historical PCB emissions from the Tervakoski paper mill have been monitored since the 1980s, and fish consumption restrictions have been imposed for the lake.
“PCB concentrations in fish appear to be slowly decreasing. The latest fish consumption restrictions are from 2010,” Akkanen says.
“This may be due to clean sediment accumulating on top of the contaminated layer, naturally encapsulating the harmful substances. These substances are persistent, so the feasibility of water body remediation must be assessed carefully. For instance, the dioxins in the sediment of the River Kymijoki were decided to be left alone and not dredged, so no remediation has been carried out, either.”
Significant PCB concentrations were also measured in Viinikanlahti in central Tampere. The area is now being remediated for residential use.
“Viinikanlahti is an old industrial site, so sediment will likely need to be removed. A wastewater treatment plant has also been located there,” Akkanen explains.
Central Joensuu, on the other hand, is where the former Penttilä sawmill once operated. A major remediation project was carried out on the soil and the sediment of the log pond, and some of the contaminated material was encapsulated on site, in a landscaped mound.
The most recent study by Akkanen’s research group examined the condition of water bodies located near the Paukkajanvaara uranium mine in Eno, eastern Finland.
The area of the mine that operated in the 1950s and 1960s has been remediated, but uranium concentrations in the downstream ponds remain among the highest in the world.
Jarkko Akkanen
Research Director
The benthic organisms and fish contain high levels of uranium, yet anglers are still occasionally seen there.
“The ponds have not been remediated in any way in this respect, but I do not know how widely this is known. Perhaps the anglers don’t eat the fish,” Akkanen hopes.
“We are more interested in the toxicity of uranium and its passage through the food chain than in the radiation it emits. Our latest doctoral dissertation will provide additional information specifically on the uranium found in Paukkajanvaara.”
Materials that bind harmful substances can be used in sediment remediation
Dredging contaminated sediment is a drastic measure for which alternatives would be welcome. Akkanen’s research group has examined possibilities for on-site remediation.
“Contaminated sediment can be covered with clean soil. A geotextile can be placed over the sediment, followed by clean soil, although this is still a rather drastic and labour-intensive measure,” he explains.
In essence, a geotextile is a filter fabric that does not in itself remediate anything. It keeps soil or sediment in place but allows water to pass through.
“Materials that bind harmful substances can also be added to the sediment to prevent the substances from spreading. Activated charcoal would work well, but it may be harmful to benthic organisms. We have studied activated charcoal in pellet form, which may work better. Geotextiles, activated charcoal and heavier soil materials could also be added in layers on top of the sediment.”
It can be difficult to find someone cover the costs of sediment remediation. The original source or the company causing the contamination may no longer exist, leaving the taxpayer to pay the bill.
“This is why it is good to know at least the locations of the most heavily contaminated areas before planning any construction. Some of this knowledge is partly based on educated guesses, but there may be no data available on individual sites.”
“When it comes to organic substances, remediation works well, but efforts to control metals such as uranium have been less successful. The chemistry of metals in water bodies is far more complex, as metals are sensitive to changes in factors such as dissolved oxygen levels or water acidity,” Akkanen notes.
