Are the metals causing adverse effects to the environment? In the ecological risk assessment of metals in lake sediment, you can choose several types of methods: chemistry, modeling, evaluating local biota or conducting laboratory toxicity tests. How many methods are enough to say for sure?
In our recent paper, authored by Krista et al., we faced this problem. We studied the Finnish lakes under the influence of metal mining and looked at the situation from several different views. In our earlier study, we started with analyzing metal concentrations – high in sediments, moderate in water. We analyzed the water and sediment chemistry – soft waters, high DOC, high seasonal variation in O2. Based on the chemistry and environmental quality guidelines, there seemed to be increased risk in those four study lakes.
In this current study, we included toxicity tests (L. variegatus, C. riparius, V. fischeri, L. stagnalis) to the picture, together with analysis of macroinvertebrate community structures and metal bioavailability & bioaccumulation studies. Bioavailability was studied with passive sampling (diffusive gradients in thin films) and bioaccumulation by collecting and analyzing body residues in chironomus larvae from the field.
And the results?
Chemistry and benthic organism community structure analyses showed risks in the most-polluted half of the studied lakes. Clear toxicity was seen some of the tests, but we assume low pH to be the reason of that in most of the cases. Metal body residues were not high enough to induce adverse effects and the bioavailability was not connected to observed toxicities.
What did we learn?
Acidic sediments with high sulfide concentrations are tricky, when conducting toxicity studies. There were a bunch of adverse effects observed, but majority of them could be explained by the low pH (simplified: metals + sulfides + oxide -> hydrochloric acid -> drastically decreasing pH)
Different methods may lead to totally different results. Recommendation: Use several test methods to ensure the reliability of your results. Traditional sediment triad approach includes chemistry, toxicity and benthic organism structure. Since metal speciation and bioavailability are important aspects in toxicity, studies of them should be included.
Know your environment. It is easier to evaluate the situation, when you have all the information. One day, we will have enough data to build models for those parts of ecological risk assessment that are still missing. Then, all this knowledge can be transferred to administration and routine environmental monitoring.
Text by Kristiina Väänänen, picture by Jenny Makkonen
Activated carbon is a sorbent with the capability of strongly binding pretty much any organic substance to its surface (a process called adsorption). Since a large share of pollution in aquatic ecosystems concerns such organic substances (for example PCB’s), it would hence be a suitable sorbent to remediate them. Once a pollutant is adsorbed to the activated carbon, it is bound so strong that it is no longer available to organisms. This includes even the case where they eat the activated carbon particles “coated” with the pollutant. The organisms would just pass it through their digestive system, pooping it out unaltered. So, long story short, the idea is to render pollutants harmless to the environment, rather than having to remove them (which additionally leaves the question where to put the removed pollutant).
Unfortunately it has been shown that activated carbon itself can actually be quite harmful to certain animals. Therefore, it is necessary to not focus solely on developing these novel remediation methods to be as effective as possible, but to ensure that they are also safe to apply in the environment. After all, what does it help us if we treat the pollution in a place, but at the same time wreck its ecosystem?
In the paper this post is based on, we mainly examined several different methods of applying activated carbon to polluted sediments (which is where the major share of pollutants in aquatic ecosystems are). You basically have two general options: the more laborious one of mixing the sorbent into the sediment actively, or the more “crude” way of thin layer capping. In the latter method you just cover the polluted sediment with the activated carbon (see picture 1). In the field that would mean all you need to do is to take a shovel and spread the carbon. So, while we did know that thin layer capping would be the easier method to execute, what we aimed to find out in our tests was how it compares in matters of effectiveness and safety.
We simulated the two application methods in the laboratory in test vessels containing sediment from a PCB-polluted site (Lake Kernaalanjärvi, southern Finland). As a test organism we used Lumbriculus variegatus, small worms that burrow through the sediment. The amount of PCB’s that the worms take up from the test sediments told us how well the different treatments work for remediation, while their biological responses (things like their change in body mass) were used as parameters to measure the adverse effects of the sorbent material itself.
The major results published in this paper were both promising and worrying at the same time (picture 2). We found out that both methods are effective in general. Worms living in sediment under a thin layer cap took up ~50% less PCB’s from the sediment than from the untreated, “raw” sediment. When the activated carbon was mixed into the sediment, the uptake of PCB’s was prevented almost completely. So, while thin layer capping is a method that is a lot easier to use (and hence cheaper), it is not quite as effective as mixing the sorbent into the sediment. Nevertheless, one has to also keep in mind, that animals dwelling in the sediment (and the thin layer cap) can mix the activated carbon with the underlying sediment. This process is called bioturbation and it was actually even visible in our laboratory test vessels (picture 1). It’s just a lot slower than mixing sediment and sorbent right away upon application. In addition, mixing via bioturbation of course requires animals to stay on the treated site and not to flee the site in panic when the activated carbon is applied.
And that’s exactly where our more worrying results come in: the adverse effects of the sorbent itself. With both application methods it became quite apparent that the worms did not really like our miniature-scale remediation works. They lost their appetite almost completely, stopped feeding and hence lost a lot of weight. While that may sound like a desirable achievement to some humans, for our worms that could be a serious issue.
A possible explanation for this sudden loss in appetite was found on electron microscope images that we took (picture 3). It looked like the activated carbon had quite some detrimental effect to the worms’ gut walls. Their microvilli, which are responsible for nutrient absorption from the gut content, were damaged severely in most worms exposed to sediment treated with the sorbent – no matter with what application method. The exact mechanism on how activated carbon causes this kind of damage remain obscure; one suggestion for example is mechanical abrasion (the carbon particles are quite sharp), but also the strong sorption capacity of the material might be involved.
One interesting thing we saw was that thin layer capping with activated carbon can have quite a devastating effect on Lumbriculus variegatus. This is not too surprising, since the organisms are exposed to a high dose of pure activated carbon at the sediment-water interface. However, when we mixed the activated carbon with clay before applying (thus creating a thin layer cap that resembles natural sediment that is enriched with the sorbent), the adverse effects were a lot less severe. This doesn’t mean there were no more adverse effects, but rather that they were at a comparable level to our other tested application method of mixing the activated carbon into the sediment.
From the results seen in this study we were able to draw some conclusions and implications for future field applications. To sum up, both methods are effective. What the thin layer capping method lacks in immediate effectiveness, it makes up for with its easier application and lower costs. When it comes to the adverse effects, we showed that neither one of the methods has a significant advantage over the other – if certain precautions, like avoiding to apply pure activated carbon, are made. So when deciding on a method, the important factors are mostly the available budget and equipment. Thin layer capping is a better option for sediment remediation in cases where special equipment required for other methods cannot be brought in easily (remote areas) or simply in cases where funds are limited. However, before deciding whether or not to utilize activated carbon in general (and big scale), we will have to make sure that its own adverse effects to the environment are not worse than the pollution effects!
Lastly – if you check our blog post on the first field trial of activated carbon based sediment remediation in Finland, you will probably spot some of these implications already “in action”!
It was time for a field trip, once again. In my project, I have been sampling lake waters, sediments and benthic organisms for several times. I’ll go to the field either during late winter (April) or in autumn (October). Surprisingly enough, it is easier to work in winter, when you have a solid ground – meaning half a meter of ice. In winter, you just saw a hole and start working. It is much easier to get to the lake with a snowmobile than with a large boat trailer.
For a researcher working mostly in office or lab, it is always fun to go outside. In lab, it often takes months and months to get any results. In field, it’s easier to feel you have accomplished something. It is also a good reminder that our lab conditions are far away from ”real life” in nature. Each time in field, we face surprises: the weather is impossible, benthic organisms have disappeared, fisher’s nets are exactly in the planned sampling point or the equipment break in the middle of nothing. A perfect opportunity to develop your problem-solving skills!
The lakes are mostly located 200-300 km from our university, meaning that you have to prepare everything carefully. If you leave something behind, too bad! This time we got everything we needed. Our goal was to collect chironomids (larvae stage of a non-biting midge) from lake bottoms. We are happy to have a technician with creative mind: He has built us a pump to collect the bottom sediment. The sediment is taken to a boat (120 l at the time) and sieved in buckets on board. This is repeated as long as we have enough chironomids – most often meaning 1200-1500 l of sediment going through our hands. The work is hard and muddy, the daylight hours are short.
Happily enough, the weather was great. No rain, no ice cover. In picture below, you see the nice surprise we had one autumn: We arrived to the lakes and they were frozen. It is not an easy task to break even a thin ice layer for several hundred meters.
First three lakes were rather easy. We had a larger boat and there were lots of chironomids to be collected. For the last two lakes, the situation was getting trickier: the lakes were small and shallow, so we needed to change to a smaller boat. Firstly, the roads to the lakes were almost non-existent. And secondly, it was almost impossible to get the boat to our final lake. Yup, the picture below is from a lake. We wore wading boots, because we sunk to our knees in the mud. And since the water was really low, we had to push the boat for more than hundred meters. It is also much more difficult to work in such a small boat.
Thank you Kari, Jenny and Nina for your hard work! Without you, I would still be standing next to our first lake, probably crying.
Text by Kristiina Väänänen, photos by Kristiina Väänänen, Jenny Makkonen and Jarkko Akkanen.
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