In Situ Remediation of TCE in Silts and Clays in Sweden
This video explains, through interviews with client SGU, and environmental contractor team Envytech and GEO, the in situ remediation of historic groundwater contamination (TCE and DCE) in silt and clay sediments at an active facility in central Sweden. Video length: 7m 26s
At a facility in central Sweden, historic manufacturing activities resulted in the contamination of the soil and groundwater below the site over time. With TCE concentrations measured up to 5,000μg/L and DCE upto 6,000μg/L, it was determined that remediation was necessary in order to protect the wider environment, especially as the site is located close to a water supply area.
Kristen Forsberg (SGU): “My name is Kristin Forsberg and I’m working as project manager at SGU. Our group at SGU are acting on behalf of the government and we are responsible for remediating and investigating contaminated sites that were previously operated by the central government. And we also deal with orphan sites. SGU has several sites that are contaminated with chlorinated solvents. This site was chosen for remediation since it had all the right conditions for serving as a test site: It is relatively small in size, it had relatively moderate levels of contamination. No DNAPLs were found, and the geology consists of clay and silty sediments. We considered a number of different remediation methods: Thermal for example, was thought to be too expensive, compared to the health and environmental risks; In Situ Chemical Oxidation was considered, but as we know, it is a ‘contact sport’, and we found it to be very hard in this kind of clay and silty sediments; Monitored Natural Attenuation would work, we can see we have natural attenuation occuring, but it would take a lot of time, which we didn’t want to wait for.”
After a comprehensive review, SGU opted for an Enhanced Reductive Dechlorination approach, through the injection of controlled-release electron donor substrates.
Kristen Forsberg (SGU): “We have 3 site specific challenges: It is an active manufacturing site, we have a lot of cables and pipes in the ground, and the fine sediments could make it hard to get a good radius of influence. This is why we are carrying this project as a two-step procedure: Step 1 – is the Pilot Trial, Step 2 – The full scale works, which is designed and planned based on the results of the pilot trial.”
Envytech and GEO were the team selected to carry out the remediation works. Envytech and REGENESIS first developed the design of the in situ remediation. Due to the low permeability of the aquifer material, we recommended trialling two of our electron donor technologies in a double pilot study.
Helena Hinrichsen (Envytech): “One of the most important things with carrying out in situ remediation is to carry out a pilot test first, to make sure you verify the data from previous site investigations, as well as to check your selected method will work as designed. If it is, then it’s fine, you can go on and carry out the procedure as planned. But if not, then you still have the time to re-adjust for the full-scale implementation. This makes that you don’t waste time, money and resources on what could be an inefficient procedure. And this is one of the great benefits for doing a trial.”
Jack Shore (REGENESIS): “So on this site we trialled two appropriately soluble electron donors. We trialled 3DMe, a high-volume, low-viscosity product. It typically comes as a concentrate, and we recommend that you mix it 10:1 with water to form an emulsion, which can then be injected into the subsurface. Once in the subsurface, it ferments and releases hydrogen, which can be utilised as an electron donor in the reductive dechlorination process. We also trialled HRC. HRC again is an electron donor, but is of a very high viscosity. And we would typically recommend it for sites where the geology has been reported to be of a very low permeability, such as here, where we have clays.”
The pilot tests were carried out by GEO during summer. And after two months of monitoring, the results confirmed the preferred technology for the full-scale works.
Helena Hinrichsen (Envytech): “As the results from the pilot test show that the ground could take both the volumes of HRC as that of 3DME, we choose 3DMe since it’s a higher volumne product, gets better spreading and has a lot lower viscosity, which makes it easier to mix and inject in these kinds of cold climates. So we are now on site doing the full scale injection with 3DMe.”
The lessons learned from this pilot were used to complete an accurate full-scale design. The full-scale 3DMe design will comprise a series of injections that can be completed within a single week.
John Ulrik Bastrup (GEO): “We started yesterday with doing the full-scale with a spacing of around 4 meters. So, it’s going very well. We are doing it with 3 rigs, direct push, and just before we hit the bedrock, we stop. We are starting from the bottom up. Just before the bedrock we have the Moraine, going into the moraine, and on the top of the moraine is the silt and clay layer will come, and that is a little more tight, so we have to be very ‘polite’ while we are injecting in that space. And we can feel that also on the pressure on the rigs and the pumps, so it’s very good that we log that, so we know exactly how much pressure we can put in, so we can follow what we did in the pilot test.”
Given Sweden’s predominant geology tends to be till or thick marine clays, underlain by crystalline bedrock, there have been concerns that Enhanced Reductive Dechlorination (ERD) is difficult to achieve.
John Ulrik Bastrup (GEO): It’s always interesting to see about clay. We have a lot of clay material in Denmark and also here in Sweden. So can you do this injection work in clay – that is what it is all about – or do you have to have other techniques, other methods to clean up. In fact, it is not always good to do things like this in clay material, but that is why you have to test it before(hand). But in this case, we can see that it is very good. The Pilot test learned us how to do it here. Put the right pressure on the pumps, so it will not come up and make daylighting, but it will into the layers. And we can see, it is different from layer to layer.”
ERD using 3DMe has been successfully applied on thousands of sites across the world, in varying geologies including clays and silts. This Swedish remediation project carried out on behalf of SGU, shows that it is also a worthy remedial option in the Nordic countries and that the presence of clay and silty sediments in the subsurface, in reality is not as challenging or limiting as often perceived. SGU will continue to take groundwater samples to monitor the efficacy of the treatment. Regulatory closure of the site is expected to follow within 3 years post injection.