Simon: So the site we’re gonna talk about today is a petrol filling station that’s situated in London, United Kingdom. It was previously decommissioned about seven years, previously. There’s a workshop on site. This proposed residential development, we knew that potential sources remained on site. We knew that it was situated on a Secondary A Aquifer, which is a moderate risk really in the UK, 350 meters from a surface water body and in the mainly residential setting.
So when we were looking at the site, we had a guide that there was existing sources, potential sources of contamination on the site. These were underground storage tanks, above ground storage tanks, fuel lines, and interceptors . There’s also the form of forecourt area and workshops. We knew from published data that the geology comprised gravels over London Clay. And on the site walkover, we saw that there are residents situated on the north, south and western boundaries of the site.
So we developed a conceptual site model which identified, really, where the main areas of concern were. So, for this site, it was gonna be a residential redevelopment, so a very high-risk classification was given to human health, residential land use. There was also a high risk for the residents off-site if there was a plume migrating off site. There was a high risk to the controlled waters as well. So checkpoint really, is, have I developed a good conceptual site model? Do I understand what is gonna drive the remediation?
Onto the site investigation, so for this site, we initially targeted the potential areas of concern, i.e., the underground storage tanks, the above-ground storage tanks. Then locations were placed for site coverage and to enable a triangulation of groundwater flow and gradient to be determined. We also wanted to make sure that we had boundary monitoring wells adjacent to the residential dwellings that were on the site boundaries, so we wanted to try and establish whether there was a plume migrating to the residential receptors off site.
We then made sure that we put the well screen in the right place. We wanted to intersect the LNAPL plume, and we included speciated lab testing, TPH, and PAH. We wanted to keep some flexibility as well. On sites like this, you always find…you tend to find some something you didn’t expect. So when we left the site, we wanted to make sure that we had properly delineated the plume.
And some of the key information we provided to the solutions provider was we gave them an understanding of the geology. It was 1.5 meters of main ground, about four meters of sands and gravel, overlying London Clay, ground water is at about 2.5 meters depths. We identified an area that had 5 mil of LNAPL, a product on top of water. And we provided some groundwater parameter such as pH, Redox, and DO, which is dissolved oxygen. We also provided an estimated plume dimension for dissolved phase and the unsaturated zone.
On some of the sites, and on this side was one of them, we wanted to consult with the solutions provider, actually, before we went to site to do the investigation to make sure that we provided to them the information that they would need.
Gareth: So, when a site investigation is being completed, one of the key things we’re looking for, for in situ remediation is delineation of that plume. So how far does the plume extend laterally, and how far does it extend vertically? So in this drawing here, it’s basically a drawing taken from a surfer plot, we’ve got a site investigation where there’s been a spill of petroleum hydrocarbons into the groundwater. The groundwater is moving towards the south there. And the site investigation has been completed with the number of wells going into the ground.
And what you can see on the east and west of the core zone, the deeper the blue, the higher the level of contamination, on the east and west of the side of the source on the core zone, there are low concentration. So you found the edge of that plume, if we assume that that isotherm, that contour, is the target we want to remediate to.You can see that east and west we don’t need to go any further.
To the south where the plume is moved with the groundwater, there is little information, so surfer is exaggerating the size of the plume, but it could be true. It could be high concentrations way beyond the site boundary, the same up gradient, not as exaggerated. We do have another well up there and also the groundwater is not moving in that direction, so it’s more likely the contamination is going down gradient.
But what would benefit the site investigation would be extra wells going in, because without them, you’re remediating a site of about that size. And within situ remediation, the costs are essentially directly proportional to the volume in which you’re treating. So it’s worth spending the money, putting in some extra boreholes. You might find that the levels in those boreholes are quite low, and then you found the edge of that plume, so you can remediate a much smaller area.
Looking at the extent of the contamination vertically, the issue can be that the contamination is not homogeneous vertically through the groundwater, more of an issue with chlorinated solvents, but certainly, with petroleum hydrocarbons, it can happen. So if you’re in a heterogeneous formation where you’ve got lots of layers of different material, this is very common in the lowlands of northern Europe. You’ll have productive lenses and then clay lenses where you’ve got a mobile porosity, where a lot of the contamination isn’t going.
So, if you have a long well screen and it’s telling you that you’ve got low dissolve phase concentration, say one milligram per liter of TPH, total petroleum hydrocarbons, that might not be true. What you might have is not much contamination in a lot of that zone, but a permeable zone taking a lot of contamination through which is then being mixed up in the well itself, so vertical delineation is key.
So then when we’ve delineated the size of the plume, we can actually delineate within the plume itself. And why that’s important is because it feeds directly into what remedial approaches and can be brought to bear on the contamination and what is needed. So thinking about this early on while the site investigation is going on ensures that you’re not having to go back and do further work when you’ve already gone to site and done the delineation work.
So in the source area where the spill occurred, so we have an underground storage tank, we’re gonna have extensive adsorbed contamination. We’re gonna have free product, so fuel floating on the surface of the groundwater. That’s light non-aqueous phase liquid, and then we’re gonna have high levels of dissolved phase contamination.
Now, that will then move downgradient if the groundwater is moving and then the slide is moving to the right, and what you’ve got then is a free product plume. So this is defined by the LNAPL, the light non-aqueous phase liquid, maybe just a thin layer. We’re often told that, yeah, we’ve got free product, but there’s no soil contamination. It tends to be with petroleum hydrocarbon sites that, if you’ve got free product, you’re gonna have a smear zone. It may be very small. What I mean by smear zone is the soils at the top of the groundwater have absorbed contamination on them, so there is a potential for a secondary source.
Downgradient from this at the distal end of the plume, at the end of the plume, you’re gonna have the dissolved phase hydrocarbons. And that basically will peter out that will go to nothing. So when you’re looking at this plume, you want to look from the DQRA what the remedial targets are, and that will define the distal end of your plume where we need to remediate. So anything further downgradient from that, essentially is attenuating naturally.
So that allows us to delineate the plume laterally. Now, if we look at this animation from the University of Birmingham, what we got is fuel being spilt through the vadose zone, so if you look at the column on the left, the yellow section at the top is unsaturated. So we’ve got some oil being spilt from the surface, and it goes down to the groundwater which looks green on that column on the left, and basically spreads out on the surface of the groundwater itself.
Now, if we look at the second column, the water, you’ll notice in the animation that the groundwater level actually gets pushed down. This is important because as the free product sits on top of the groundwater, it weighs something. It might be lighter than water, but it weighs something, so it’s pushing that groundwater level down. So you’ll find that your smears zone, your adsorbed contamination, actually exists below the groundwater level due to the weight of the fuel itself.
If you look at the third column, you’ll see that as the LNAPL comes down, it starts to spread out in the capillary zone above the groundwater, so just above the groundwater where capillary action is bringing up some of the water into the vadose zone. As the oil moves down, it finds that these pore throats are blocked by water, and so it starts to spread out. So when you have a spill of oil, it’s not just sitting at the groundwater level, it’s sitting above the groundwater level, and it’s actually depressing that groundwater so that you get a much greater spread of the contamination on the soils at that point.
And of course, if you have seasonal or tidal fluctuations, you gonna get a move of the contamination up and down across the soils. Dissolved phase contamination under this, you can assume three meters of dissolved phase contamination from the surface conservatively. If you are able to delineate better, it may be that what you can do is have a targeted dose with an increased treatment towards the top of the water column because the petroleum hydrocarbons float. That’s where the worst of the contamination is gonna to be.
So why do we want to zone the plume? The reason is because it directly informs what approach we’re going to take. So if you look at this graph, we’ve got treatment efficiency on the y-axis, so the higher you are on the y-axis, the better the efficiency of the remedial approach. The contaminant concentration is on the x-axis, so that’s increasing to the right. So on the right for petroleum hydrocarbons, we’ve got a lot of free products. On the left, we’ve got very low dissolved phase concentrations.
So starting on the right, if you’ve got free product on the site, in the core zone, in that LNAPL plume, if you have good access to that free product, if there’s a lot of free products sitting on the surface or more than a skin, more than a rainbow sheen, if you’ve got an amount that can be pumped off, you should pumped that off. It is very efficient way of getting rid of the contaminant mass.
But what you’ll find is, if you have low dissolved phase target, and again, this is information that you need from the site investigation, from the consultant early on, you’re not going to be able to get to those targets just using physical remediation, as I’m sure many of you have encountered out there when you’re doing a pump and treat system. You can be stuck on site for a very long time, trying to get to those low concentrations.
So at the other end of the scale, biological degradation can deal with dissolved phase contamination, get down to very low concentrations. However, it can’t deal with free product. So depending where you are in the plume, you may not be able to start with biological degradation. If you’ve got free product, 99.9% of that fuel is not biologically available. It’s only the part that’s dissolving into the groundwater where the aerobic microbes live that can be degraded by the bacteria.
And in the middle, you’ve got chemical oxidation. It’s essentially good at everything, it can break down free product, but if you can remove that free product, it’s a cheaper option. It can break down low dissolved phase concentration, but it’s a contact sport. So the less contamination you have in the groundwater, the less contact you get, the less efficiency of treatment you get. So chemical oxidation is good where you have removed as much free product as you can, but you still have high residual contamination, so a skin free product or high dissolved phase concentrations.
So looking at a site, you would choose different techniques across the site, and you’d also use these different techniques over time as the remediation project continues. So, if we’ve got a petrol filling station here on the left, which looks a lot like a factory, but they’ve got an underground storage tank that has been leaking into the groundwater. So red is bad in this drawing here, the contamination is moving down through the groundwater from top left to bottom right.
What you could do in the source zone is, take out the tank, take out the worst of the soils, and then move on to chemical oxidation and biological degradation. So you’re dealing with that free product. You’re dealing with the adsorbed phase, then dealing with the high levels of contamination. And then if you need to get to low concentrations, you can go on to biological degradation.
Just downgradient, you may not have enough free products that it’s worthwhile using an extraction system, so you can move straight onto a grid of chemical oxidation, followed by biological degradation to degrade the residual dissolve phase. Downgradient of this in the plume, you might have a series of barriers where you’ve done direct push, so that third arrow shows a direct push rig, driving rods into the ground, injecting a small amount of product into the ground.
For a petroleum hydrocarbon, we’re gonna be talking about oxygen release compound, which I’ll come on to. You’re injecting about a half of 1% of the pore volume, so you’re not blocking the permeability of the formation, so the groundwater continues to flow through where you’ve injected the product. But what it does is it creates a highly aerobic zone through which the contamination is broken down as it moves into that zone.
So you can inject it through direct push or at the end of the site, that fourth arrow. We’ve got some wells. You can put it into wells in the form of socks, etc. That creates a zone at the edge of the site that prevents any of the contamination moving off site to potential receptors beyond the site boundary.
Simon: So now, some other key sites information and that was needed to be provided, the saturated aqueous, the thickness. This is not just needed for the solutions providers. It’s needed by the consultant when they’re doing their detailed quantitative risk assessment. All these parameters are required by both parties, so hydraulic conductivity, groundwater, gradient, and flow direction, FOC which is fraction of organic carbon, and the speciated laboratory testing.
It’s also important to look at the distances to any receptors. So in green, you can see the actual parameters that were derived from the petroleum site. So we could both use this information, and solution provider could use it for their design. We could use this within our DQRA to come up with realistic targets.
And little checkpoints at the bottom, one of the key information that you need for your DQRA is the more information site-specific information you can obtain from sites, then you generally take out the conservatism from the DQRA, and you get more realistic remedial criteria.
Gareth: Okay. So other key site investigation information from the solution provider’s point of view were interested in what the effective porosity on the site is. This will allow us to calculate the amount of impacted water that there is and work out the contaminant loading, essentially that allows us to do stoichiometric calculations, on say, for instance if we’re doing biological degradation, how much oxygen is required to break down the contamination through biological degradation, and therefore we know how much oxygen is in our product, and therefore how much product we need to apply. So that’s the sort of information we’re looking for there.
Permeability of the site has a number of effects, but just touching on a couple. If you’ve got high permeability, it allows you to use advective systems, so basically something that can pump or high-volume treatments, chemical oxidation. As I said, it’s a contact sport, you need a high volume in there. So you need a high permeability on the site to be able to do this. And on this London site, we did have that.
Some sites that have low permeability, and you know, there may be clay, silt, etc., in which case it might be difficult to get chemical oxidant in, so you look to be using an Oxygen Release Compound. It’s a low volume product. You inject it into the ground and then it relies on diffusion to move that dissolved oxygen through the groundwater. It’s basically the movement of a chemical through water, not the movement of water containing a chemical. It’s diffusion of the active ingredient through the subsurface.
Permeability, potentially, can limit options. So, you may have very high concentrations that you want to use chemical oxidation, but you’ve got a low permeability, so you’re gonna struggle to get an effective dose into the ground, so you may need to switch to a different technology. You may need to switch to enhanced biological degradation, earlier than you wanted, so potentially losing efficiency. But it’s a trade-off due to the permeability.
The other side of things, if you remember a couple of slides back, I showed you a barrier application. If you’ve got very permeability, if you’ve got very high flow, you’ve only got a certain amount of time in which to treat that contamination as it moves through. So that information is key for us to design a barrier that’s essentially thick enough, that’s deep enough to deal with that contamination as it moves into the treatment zone.
And if you’ve got a heterogeneous site where you’ve got high permeability and low permeability, you’re gonna need to think about a number of approaches, so that can deal with both. And I’ll show you that on this next slide. So this is a test that was done by Doner and Sale at Colorado State University, that the light material is a sand high permeability, the other material, are clay lenses.
The idea here is to show you that a contamination gets into these permeable zones but then diffuses into the immobile porosity, into the low permeable zones. So you can then treat these permeable zones very readily with a pump and treat system, but then you’re left with a problem, and I’ll show you that now.
So we should very soon see some fluorescein coming in. This is the contaminant that’s being spilled, and it’s moving through the groundwater, contaminating all the mobile porosity. And you see, it’s starting to diffuse into the edges of the clay there and immobile porosity. We switch on a pump and treat system and very early on we have great success. We remove a lot of that contamination. We remove a lot of that contamination. I’m gonna try that again.
So you’ll see, it remove a lot of that contamination, but then watch out for the trails coming from the clay, and the fact that that clay is glowing, you’ve got contamination that’s diffused into that immobile porosity and then it’s leaking back out. This might stop you get into your low dissolved phase targets, so particularly important in your plume zones.
Okay. So, geochemistry, don’t just look at the contaminants of concern. We need to look at other lines of evidence. So very easy things to do on the side is look at dissolved oxygen while you’re there. It’ll give you a very quick snapshot of what the plume is doing. So in aerobic conditions, for biological degradation to be going on aerobically, you’re looking for a greater than one milligram per liter, essentially. Aerobes tend to cease at about 0.5 milligram per liter. I’ve written anaerobes there, but I meant aerobes.
Redox is another thing to test on the site. Obviously, a pinch of salt with the results from your redox meter, they do tend to drift. They do need to be calibrated a lot. Generally, on a site, you’ll be seeing anything from -400 which was deeply anaerobic, to +800 millivolts which is highly aerobic. It’ll give you a snapshot of current plume conditions, which I’ll come on to in the next slide.
For aerobic degradation, you wanna create conditions of about 150 millivolts or above. You tend not to see this on a site where there has been a spill, that that petroleum hydrocarbon will be actually using up a lot of the naturally available oxygen. You’re often gonna be seeing much lower than that within the plume itself.
Iron and manganese, soluble forms of iron and manganese, there are a good other way of looking at the…whether it’s oxidizing conditions or reducing conditions. As you get more oxidizing conditions, essentially, the iron and the manganese is converted into solid forms, and it settles out. So the soluble forms will decrease as you get a higher redox, as you get a higher dissolved oxygen. So you can look at these in the center of the plume. That gives you more evidence as to what’s going on in the plume itself.
The other thing to look at is the BOD and COD, biochemical oxygen demand, chemical oxygen demand. What that does is it allows us to spot things that we might not have already seen that will act as an oxygen demand. If we’re gonna put oxygen in to biologically degrade this contamination, there might be something going on. There might be a lot of reduced metals on the site, or the oxygen sinks. There might be a peat layer that hasn’t been picked up, giving off a lot of humic acid. There maybe a broken sewer. It has happened before on the site. These things are going to use up your product that you’re trying to use to remediate the contamination, and you have to take account of these things.
So going back and having a look at a plume, this time we’ve got a dissolved phase plume. So we’ve got the source area where we had the spill, and then we’ve got a dissolved contamination moving to the right across the slide. When it first was spilt and we first got the dissolved contamination moving through the groundwater, it was aerobic groundwater. So the microbes used the oxygen, reacted it with the contamination in a respiration reaction that releases energy which they adsorb and that allows them to grow and multiply. So that’s why they do it.
Now, as they did that, that was degrading the contamination, but very quickly, the oxygen and natural oxygen is used up. And the only way it can get back into the center of that plume is by moving in from the sides or upgradient. So what you find is you get an aerobic corona on the outside of your plume, where biological degradation may be occurring, aerobic biological degradation may be occurring.
But in the center, you often get deeper and deeper anaerobic conditions. It’s pretty common when you’re looking at petroleum hydrocarbon sites to find that there’s actually sulfate reduction going on, and so you’re down about -150, -200 millivolts, something like that in the center. So, as you’re going across your site, doing your initial site investigation, look at the redox, look at the dissolved oxygen. You can start to get a quick idea of, actually, where is this plume before your results come back from the laboratory, just based on what’s going on in the ground, what are the microbes telling us by the way they affect geochemistry.
The other thing is if you dig a hole, and that’s my best artwork there, for a hole on the site, if you’re doing trial pitting, you’ve spilt fuel. It’s a clear liquid. You’ve spilt it into sand, and then when you dig your hole, it’s all black. We’ve all seen this. Now, what that is, is it’s showing you that it’s deeply anaerobic conditions. You’ve got iron sulfides being laid down, which are black, and they’re sticking to the soils and that’s what you’re seeing.
So immediately, before your analysis comes back, it’s telling you that it’s very anaerobic in there. There’s very little energy given from these reactions to the microbes to break down the contamination. So it could well be the natural breakdown of your plume is very slow. And if it’s very slow, potentially, it’s going off site.
So if you look at the benzene degradation rate, if we look at the blue line which is the anaerobic half-life, if you’re in natural conditions where it’s an aerobic, the half-life is 24 months, to why benzene tends to linger on these sites. And, obviously, it drives the risk. So that benzene could well be getting off the edge of the site, getting to your receptor. If you look at the pink line, the aerobic half-life goes to 10 days, so that’s from 24 months to 10 days. So in anaerobic conditions, benzene breaks down very readily. So immediately, that benzene stops being a risk. It’s not gonna make it to the edge of the site, and it’s degraded.
Simon: Okay. The plume delineation that we identified on the petrol filling station. You can see in yellow, there was some unsaturated source material. That was encountered when we took out the underground storage tanks. There was an area in green which was identified as having the LNAPL plume. There’s about five millimeters. Dissolved phase plume was over 50% the site. And if you can see, the red dotted line is the calculated remedial criteria for the aromatic c8 to c10. So this was our target concentration. It was the main driver on this site.
Another sites, you can have fun with your contouring package, and we’ve got sort of a lot more datasets. You can start to then identify quite quickly where the real areas of concern are that this is the information that the solution provide they can then target. You know, you can never clean up the site 100%. But if you get rid of…you do an 80% reduction in the source mass or something like that, you wanna target the main areas of concern. So you don’t wanna target those lower areas that are not gonna give you the best result.
So on the remedial strategy document, which has to be prepared before any remedial actions are undertaken. So on this site, we prepared one. We identified who was responsible for what. So this is between the consultant and the solutions provider, who was responsible for what. We had a wide scatter approach to looking at potential remedial options, you know, what is actually the problem on this site, the site that you’re analyzing. So on this site, the issue was a groundwater issue.
So we had…really, the approach was for groundwater remediation. However, we knew that there would be some soil, so you just know that there’s gonna be some unsaturated soil materials, so we wanted to allow for that. You wanted to allow, as well, for discovering something unexpected. When you are remediating a site, this often is encountered. And so, it really depends on the consultant and the solution provider, acting quickly.
We had to define, as well, the remedial criteria, the frequency of testing, what the criteria were, and the post remedial monitoring program. It’s also quite useful to provide, if you’ve had discussions with the solution provider, provide the information data sheet that you may have had back from them. So we did that on this site. We included that within the remedial strategy, and then this document went off to the regulators for their approval, which was achieved, and then the remedial works commenced on the site.
The other thing you want to think about is, are there any environmental controls? So in this site, we had residential properties. We were concerned about whether there was gonna be noise or odor or dust issues, so appropriate mitigation measures had to be in place which included some monitoring.
It’s also worthwhile discussing the site with the solutions provider to see if there’s some joined up thinking that you can do, so you try and think of some cost savings. You know, consultants don’t always have all the answers, so why not have a chat with your solutions provider and see if they can come up with a solution.
So in this site, this was the DQRA groundwater remedial criteria. So we provided the guide as to the reduction that was required. Generally, it’s harder to achieve the remedial criteria if they are lower. So you can see at the bottom, there’s some of the OCs that we encountered. Now, these are quite tricky to clean up. Generally, the benzene will go fairly quick, and the aromatic will go quick. But we were concerned on this site about the persistent low concentrations. So we wanted to make sure that the solution’s provider understood that and would target those within the remedial actions that undertaken.
It is also worthwhile, just checking with the solution’s provider, are these achievable? Can you achieve these criteria? What’s gonna be the implication on time and cost? Is it realistic time and cost? So just get a real-world opinion of these criteria, see if they are achievable.
Gareth: Okay. So, the remediation was completed by Rake Remediation. I’ve split it up into a couple of phases just to explain the approaches that were taken. So we can see here that we have the tank area and the interceptor area that was the worst of the contamination. What happened here is the tanks were removed and disposed of. There was impacted soil that was dug out and remediated. And then there was free product within…I’ll show you an excavator. This is the excavation.
There was free product within the groundwater at the base of the excavation, so that was pumped out. And then oxygen release compound was placed into the base of the excavations in order to deal with the residual contamination. So, it does this through enhanced natural attenuation, ORC advanced. Oxygen release compound advanced formula is specifically designed for in situ remediation. It comes, generally, in the form of a powder, but I’ll show you the pellets in a couple of slides.
It’s a calcium oxyhydroxide. And what we’ve done with it is intercalated a phosphate group. Now, if you took peroxygen, a calcium peroxygen, and added it to water, what you would see is oxygen being released and bubbling out of that water. Now, you might think, great, that’s what we want. We want the oxygen, but it’s bubbling out of the water, and you want to keep that oxygen dissolved in the water.
So by intercalating this phosphate group, we call it controlled release technology, what we do is we slow the release of oxygen to about 9 to 12 months. What that does is it raises the dissolved oxygen in the groundwater. It raises the redox to stimulate the growth of indigenous aerobes. They then break down that contamination. We support that biological degradation over a period of 9 to 12 months, from a single application.
So as I say, it comes as a powder. You mix it with water, generally, to create a mechanical suspension, a slurry, and then it’s injected into the ground. It targets dissolved phase contamination. So in terms of petroleum hydrocarbons, you’re looking at treating something 10 milligram per liter or below as a rule of thumb. Above that, you’re starting to see a sheen, you starting to see globules of free product, and you’re starting to think, “Do I need to do some chemical oxidation before I move on to ORC advanced?” But this will treat the dissolved phase and take you down to the target concentrations.
So it can be used alone if you’re just in the plume. Here we are in the core, so what we’ve done is we’ve had some primary treatment in terms of physical removal of the secondary source of the contaminated soils and of the LNAPL, and then we followed on with ORC. Here’s an example from a site in a chalk bedrock. You’re starting with fairly high concentrations in a number of wells. The injection on site does not take very long. It’s a low volume product. It can be applied very simply to the site, so there’s not much disruption on site. But then, it provides 12 months of treatment from a single application.
So all that needs to be done after that is the consultant comes back in, the consultant then just do the monitoring. The consultant, hopefully, feeds us back the results of the monitoring, so that we can comment on how well it’s doing, whether we’re happy, whether there’s anything been missed, etc. So you can see the results here.
These are the pellets that I mentioned, and these were used on the London site. It’s really just a compressed version of the powder that we’ve got to reduce the amount of dust produced if you’re placing into an excavation. Here’s a video of it being placed into an excavation. So basically, you can hand cast it in or you can use an excavator bucket. You can see, there’s a little bit of dust but not the end of the world. If you put the bucket down a little bit lower, there would be less dust, and basically you spread the dose across the excavation base to target the residual contamination.
It’s not going to deal with all the contamination downgradient. Oxygen is a valuable resource in the subsurface. It will penetrate the base and the walls of the excavation for about a meter or so, at which point it will be used up. So if you have got contamination further down gradient, you’re gonna need to do something more, which brings me onto the second phase.
So once this work was completed, wells were drilled downgradient because we had high levels of contamination in the groundwater beyond the extent of the excavation. So rather than digging up the whole site, what could be done is wells were drilled down, injection wells were drilled down into this plume, and regenOx applied. It’s a chemical oxidant, which I’ll come on to in the next slide. And repeat applications were completed of this to degrade the residual LNAPL and the high levels of contamination in the groundwater. So multiple applications were done.
And then ORC in the form of a slurry was then applied into the subsurface to degrade the residual contamination beyond that. So at this phase, we’ve got chemical treatment and biological treatment, and further up gradient we’ve had that physical treatment followed by the biological treatment.
Okay. So in situ chemical oxidation, it’s a redox reaction between the chemical oxidant and the organic contaminant. You want them to make contact. You inject an oxygen into the ground, itself or highly charged ions that it produces, contact the organic contaminant and smash it up. They break it down. It’s chemically burning the contamination in the subsurface. It’s good for rapid mass production. It becomes less good for very low concentrations because of the lack of contact.
So you target residual LNAPL and high concentrations of dissolved phase. RegenOx is one of our chemical oxidants. We have persulfOx as well. It’s been around for about eight years. The idea behind the product was to make chemical oxidation safe to use. And what it does is it gives you Fenton’s reagent reaction, a chemical oxidation reaction that takes place over two to four weeks rather than instantaneously in order to give you the same amount of chemical oxidation but none of the increase in temperature or the increase in pressure associated with chemical oxidation.
It also avoids targeting solid organics or services, so it’s able to take chemical oxidation into places that it hasn’t been able to be used before. It’s been used on petrol forecourts, petrol station forecourts, where people are actually actively filling the cars. It’s used in domestic oil spills in people’s houses. It’s just taken a powerful technique and made it very straightforward to use. It allows follow-on enhanced natural attenuation using ORC advanced as well.
So it comes in two parts. The part A is sodium bicarbonate, and it carries a hydrogen peroxide on a ligand structure. So what that means is when you inject it into the groundwater, it slowly releases hydrogen peroxide over about two to four weeks. It’s 80% spend after two weeks but It will last to about four weeks. The Part B is an iron silicate catalyst. It comes in the form of a gel. Now, it wants to be a solid. It’s nucleated. It’s trying to crystallize. We’ve kept it at a high pH so that it can’t grow.
Why we’re doing that is we want to inject it as a liquid, so you add a little bit of water to it. You inject it into the subsurface. It spreads through the subsurface. As it does, the pH drops due to the buffering of the soil, and the crystals grow within it. And what you get is emplacement in the subsurface of these tiny particles of iron silicate and that’s the catalyst. What happens is the contamination absorbed to this, the oxygen meets the contamination on the surface. There’s a catalyzed chemical oxidation, reaction, the Fenton’s reagent reaction, that breaks down that contamination.
Millions of these reactions occur over about two to four weeks, giving you a powerful chemical oxidation without that increase in temperature and pressure. So the sort of results you see in the groundwater. You have a high level of contamination. You inject the ones, last about two to four weeks, breaks down the contamination. Oh, looks good, and then you get a rebound. And what that’s due is the desorption of the contamination in the smear zone as well.
So you reapply and you get a rebound again. It should be that it’s less contamination because you’re removing the contamination from the soil, so there’s less contamination in the system as a whole. And you repeat the process, normally two, maybe three times. It was twice on the London site. And then when you’re getting your rebound, you know that you’ve removed that smear zone. You can then move on to enhance biological degradation, knowing that you’re not fighting against desorption. So that’s the chemical and biological integration.
On this side, it was injected through wells. Just a quick note on injection wells, we’re all used to putting in wells where you can pump things out. There might be pumping wells for pump and treat systems. There might be monitoring wells for taking samples. If you’re gonna put in a well where you’re gonna inject, that well will be under pressure, and so it’s gonna want to rise out of the ground.
So that top part of your completion, you need to put in a sand cement mix without any bentonite in there so that you get a good hold in the ground. Once the wells are in place, you then put a packer down into the well. That is me, looking quite grumpy on the site. I’m used to being in the office with coffee in my hand these days. But I’ve got what’s called a doughnut Packer with a stinger assembly in my hand. So the stinger is just a pipe. You put that into the well, you inflate the packer, that’s the black part. And that holds it firmly in the well itself, and then you can inject the product into the well without damaging the top of the well by connecting. Okay.
Simon: So, how do you deal with unexpected contamination? Well, on this site during the demolition works, when they took up the slab, pieces of asbestos containing materials, ACMs, were identified which the most cost-effective solution was to hand pick it. So a team of category B trained people were quickly mobilized to site and handpicked the visible ACM. During those works, you can see the photo on the left, there’s a need for some asbestos monitoring pump setup, and obviously appropriate PPE were worn.
So the key here is that needed rapid mobilization. Unexpected contamination, if a consultant says, “I’ll be with you in another two weeks,” basically no work can carry on for two weeks. It slows down the program. So we had to respond quite quickly and gather thinking hats on together with the solutions provider and come up with a was the most cost effective and rapid response.
So on to the verification process, so the consultant verified that the remedial works complied with the agreed remedial criteria. Now, what generally happens, and that happens on this site, was the solutions provider said, “I think we’re getting close to the remedial criteria.” So the consultant then came out and checked that this was the case. It’s an independent check.
We provided some geochemistry parameters, not just the COCs. So again, this is pH redox and DO. These are useful to the solutions provider, give extra information. And this information was fed back as soon as possible, so things could be adjusted on site, pumps turned on, turned off. They could then establish when they needed to do their dosing of various products as well.
So, anything that they needed adjusting, we could then be proactive. Just the check points at the bottom, advocating independent verification. Just if you have the same person verifying results or the remedial works that they’ve done, it can cause some questions to be asked by the stakeholder on the independence.
So the key results from the remedial works on this site, about 400 cube of saw [SP] was treated on site via by remediation of the CL:AIRE Code of Practice. About 500 cube of site one material was hand-picked, with some of it being reused as bank fill within the tank excavation and the surface was removed from site, but at a low-rate cost, had it not been hand-picked.
Dissolved phase concentrations were reduced by between 80% and 90%, and all the remedial criteria were achieved. So this is a time versus concentration for aromatic 8 to 12. And you can see the various activities. The consultant was taking groundwater samples during the remedial work, so we could monitor. You can see the bounce back, the green line. When the pumps are turned off, it bounces back and then some products were added, and you can see the reducing concentrations.
So we had three months of consecutive compliant monitoring towards to end. This is the results for VOCs. If you remember, I said that these are often difficult because the criteria for these are inevitably quite low, and so they can be difficult to achieve. They were achieved on the site which was good news. And then the product thickness went fairly quickly. That was when the…mostly reduced during the active pump and treat.
So a sign off the current status of this site is the remedial verification and post remediation. Groundwater reports have both been accepted by the regulatory authorities. The developer is building and will be called back once the surface cover is brought on, and we can validate that, and then the final conditions can be discharged.
So some lessons learned. We learned the value of a really good conceptual site model at the start so we could target the areas of concern we had, and then provide that information back to the solutions provider. It was also good to get a reality check on the remedial criteria, were they achievable, were they appropriate. And we had constant communication between ourselves and the solutions provider, so information was flowing backwards and forwards. This meant that the client in the end had a successful delivery on time, on budget.
Gareth: So in conclusion, I just wanna say that taking a holistic approach, again by holistic, I mean considering the site investigation with the remediation, with the verification, with the sign-off, all together and considering that from the start and keeping that in mind, it will save time and money for the client. Early consideration of a potential remedial approach, how that is gonna be verified, how that’s gonna be completed on the site, even as you just start going to site to break ground for the first time can be done through communication with a solutions provider. It potentially avoids missing information and having to go back to the site, potential delays from that. And crucially, it increases the certainty of the remedial approach, and therefore the success of the eventual remediation.
So thank you very much for your time. I believe we’ve somehow managed to finish on time. So there is time for a couple of questions. If you haven’t asked any questions yet and you want to, please type them in now, and we’ll try our best to go through. We probably got time for a couple. If we don’t answer your questions, then we will certainly get back to you on email afterwards.
So, please if you have to go, just let us know what your question is, and we’ll get back to you. So questions, we’ve got one here says, “What do you recommend when a plume is going under a building that cannot be removed?” Do you wanna start this?
Simon: Yeah. I mean, this is quite commonly encountered, say on developed properties or industrial sites that are active. They are in use. So it leads really… The only option is to consider in situ treatment. So you have to then think about how you’re gonna do in situ treatments. It’s going to be a series of injection wells, recovery wells. It’s going to need careful monitoring and design. And it’s something that will need both consultant and solutions provider to work together. Any comments, Gareth?
Gareth: Sure. Just on a practical note, when you’re looking at in situ remediation, you have to consider the radius of influence of the treatment that you’re using. So if you have got a contamination under a site, say you put…or say you’re upgradient of the building itself where you can inject, that oxygen is not going to go all the way under the building and deal with everything that’s there.
You’re gonna have, depending on what the formation is, maybe four meters, maybe five meters, if you’re lucky, of treatment zone around that. So if that still remains an issue, you can look at higher volume treatments using RegenOx. You’re gonna have to start looking whether you can get in the building and do injection. You can look at trying to flush material underneath the building. You can go on to looking at directional drilling, but that very rarely, actually goes ahead. Potentially, you’re looking at downgradient barriers to then deal with that residual contamination as it moves underneath the building, but every site is different.
Simon: Should I read this one out? “Will a solutions provider charge for advice?”
Gareth: No, generally. I’m probably gonna regret that. No, for a certain it’s in our interest that remediation goes well, and it’s in our interests that as much information can be gleaned from the site as is possible. You know, whether you’re talking to us or another solutions provider or remediation contractor, you know, that information is absolutely key. So we need that information in order to design. So usually, it’s just part and parcel of the service that we provide.
Simon: Okay. Here’s another question. So, I feel a bit like Paul and John from the Beatles here. What other remedial approaches considered for this site? So, yes, they were. The options appraisal looked at all the potential remedial approaches, saw vapor extraction, other things like that. But it was felt that the most cost-effective and sure way of achieving the remedial criteria, because it was mostly a dissolved phase groundwater problem. So we had to target and reduce those dissolved phase concentrations in the groundwater, so it was felt that the approach that was taken was the right one.
The’re coming in thick and fast. What happens when you’re not able to achieve the remedial targets?
Gareth: Obviously, that is not ideal. But in terms of the planning of the remediation, you have to take these things into account. So on a site like this, what would be the issue, would be either we didn’t reach the dissolved phase targets in the groundwater. So, you need to plan for a contingency action, whatever that may be.
In the case of this site, it would most likely be going back and doing further ORC advanced injections. Now, if the site’s being developed, it’s not necessarily the end of the world. You can get in with a very small direct push rig and get into soft verges, etc., and do injection work there. That’ll just continue the treatment until you then hit that target. Usually a contingency, if one goes ahead, it’ss a fraction of the size of the initial treatment. So yeah. I’ll read that.
Okay. Was there a requirement for the OC, hydrocarbon impermeable gas membranes following the remediation?
Simon: And this comes back to your risk assessment, so in your human health modeling. On this site, we allowed for the indoor air inhalation pathway. So, we class that as active. So the remedial criteria assumed that there was no membrane in place. So we did sufficient reduction to justify not having a hydrocarbon resistant barrier. However, a barrier was put in but it wasn’t hydrocarbon resistant. It was just a slightly enhanced barrier on this. It was 2000 gauge low permeability membrane, and that’s been accepted by the regulators as well.
Gareth: Okay. I think that’s about all we got time for. So I’ll just say, thank you.