In Part 1 of this Blog post we took a look at the on-site sediment remediation with activated carbon. Now we will gain a small insight to the first field trial of the method in Finland, which was started by our group in August 2015.
The test site lies in Lake Kernaalanjärvi, which was contaminated with PCBs between 1956 and 1984. There was a steady, unnoticed discharge of the chemicals from a paper mill upstream one of the lake’s feeding rivers (Tervajoki). Since no one noticed that leak for so long, quite an amount was released to the river and ended up in the lake eventually. The fact that this is still a problem nowadays, even though the leak was shut down over 30 years ago, gives you a hint on the persistency of PCBs in the environment.
Since the lab trials had not only shown the high efficiency of activated carbon, but also potential risks of the sorbent particles themselves, we applied it only to a small plot of 300 m2 within the lake. This way we don’t mess up a whole lake, if the side effects are bad, but we also don’t waste too much money, if the clean-up potential is not as good as seen in the lab. The plot lies in the south end of the lake – the most contaminated area. This is right where the contaminated feeding river enters into the lake (see satellite image). With it come the PCBs, usually attached to suspended particles that settle as sediment once the water flow speed gets low enough.
For the remediation works, we ordered about 1000 kg of pressed pellets consisting of activated carbon and clay (Sedimite™). The latter increases the density of the pellets, making them sink faster to the bottom of the lake. This fast-sinking property makes handling and applying the activated carbon really easy. You can basically just shovel them out of a boat onto the water surface and they sink straight down onto the sediment. With pure activated carbon – usually a powder – that would be unthinkable. In part 1 of this post you could see a picture of the mess we can easily create already in the lab when we handle activated carbon powder. Add a bit of wind or rain (which we rarely see in the lab) to that and you might not have the greatest work day of your life. Even if the powder finally reaches the water, most of it would just get suspended in the water column and settle after days at wherever the water flow brought it.
For the application of activated carbon we had to first of all change our “lab rat” attitude to field-trial-mode: In the lab, we usually work with precisely measured doses, carefully applied in controlled environments. In the field, we had to take more of a “rough estimate” approach. We started by measuring a 10 x 30 m field on the lake, marking it with buoys and ropes. For better orientation and to achieve a more even layer of activated carbon, we diverted the plot into 5 x 5 m intersections, which were handled one at a time. After we had applied (read: shoveled) all of the pellets onto the test site, we took some sediment core samples of the freshly covered site. Luckily we could see that the pellets had actually worked as intended and we achieved a quite good layer of activated carbon on top of the sediment.
Now – about one year later – we checked in to see how the field looks like. We took core samples on the same spots again and unsurprisingly, the field looks a lot different. Wind and waves have affected the plot heavily: a lot the sorbent has been swept away. In addition, a thick layer of new sediment has covered what was left on site.
How this is affecting the remediation potential and the adverse effects of activated carbon, we plan to find out in the near future. We have scheduled a lot of monitoring works, such as surveys on the condition of the local sediment fauna and changes in the PCB uptake by the organisms living on our plot.
Text by Sebastian Abel, photos by Sebastian Abel and Jarkko Akkanen