 Well, thanks everyone for joining us today for another one of HydroTerror's webinar series. We have got a huge number of registrants for this particular topic. And today's topic is landfill caps, the pros and cons of conventional and green alternatives. Joining us today, we have Dr Brent Davie who's come back for more. So many thanks Brent for joining us today. For those of you who don't remember Brent from a previous webinar on landfills, Brent is a principal environmental scientist with Fife. And he has a long history of working in environmental consulting. And what's particularly relevant to today's presentation was he was a leading research scientist on alternative caps for landfills in a program that was called the ACAP program. And some of the slides today that Brent's going to be going through relate back to that research. But a little bit of housekeeping before we get into things. We've covered that one. So we love your questions as you know, and we would like to see them typed in the Q&A section which you'll find on your screen. So if you type into those, I will read those questions out at the end of our presentation and endeavor for Brent and myself to answer those at the end. So please have them coming through. We've got some good early bird questions. We've got about six there. So thanks for sending those through too. So we'll leave a bit of time at the end. Why does Hydrotera do these webinars? Well, we are passionate about sharing knowledge about everything technology and how it can be applied. We believe in the importance of facilitating education, particularly at a time when most people, most organisations just don't seem to be able to find the time to train the industry. So we're trying to do our bit to help there. And we like to take an industry leadership position around improving the way things are done. So that's really why we're sharing this knowledge. And I guess we love to collaborate with people. So having Brent here is an example of that too. So what are we talking about today? So Brent's going to do a general overview of landfills, what happens when a landfill is closed and the post closure covering and ongoing management. He's going to talk, and this is really the core of it, he's going to talk about conventional and alternative capping strategies that can be used. Then I take over and I talk about these transpiration caps in a bit more detail and with more of a focus of how we actually monitor their performance. So without further ado, I'd like to hand over to Brent and thanks very much, Brent. Well, good afternoon, everybody. And I'm delighted to be back again with Hydrotera on this. It's an area I've been interested in for a long time. And I was actually fortunate to be the first research manager for the ACAP program here in Australia in 2006, I spent about 15 months getting it started and building the first two test systems, one of which I'll talk about a little bit today. But I've since spent more than my fair share of time on landfills. So I know a little bit about this job of capping landfills when the landfill is more or less finished. It's actually quite an important part of the whole exercise. So next slide, please. So what do we mean by landfill? Well, they've tended to be depressions in the ground, usually. Very often gullies and that sort of stuff. That's how they start. People chuck rubbish in them and small towns and that sort of thing start building them up. But ultimately, it's interesting that these days in cities, very often the major waste management companies were actually before that quarry companies. So they're the ones with the big holes. And so here in Melbourne, for example, most of our landfills used to be quarries, sand in particular or say the Hanson quarry up north and still is. Removing basalt. I think the one at Werribee's also was a basalt quarry. So you've got these big holes and they're great things to fill. So the fill is virtually any waste that we are trying to dispose of. And the whole idea is that a landfill is at least somewhere specific where we know that we can deal with. The important thing is that the fill never stays the same unless you're talking about clean construction waste, which of course is completely inert. But otherwise, if you've got food wastes and that sort of stuff in it, they decompose and degrade and release various gases that we also want to manage. But for the purpose of this talk also it's important to understand the concept of airspace, which is the space above the landfill next one, please. Okay, and so you wind up with this massive waste. And there are a number of potential outputs from that waste. Perhaps the first one is simply litter stuff that gets blown off the top, washed away by rain. It's one of the first signs that you've got a landfill nearby is very often the wind blown litter. The odor is another one. The inevitably the food, etc. Rotting is an important part of the deal. And the odor is often quite offensive, partly because methane itself has got an odor that a lot of people don't like. But the other thing that the environmental regulators are particularly concerned about is lead shape, which is the liquid that accumulates at the bottom of this massive waste. So, and it potentially causes problems with ground water. Anyone who's ever been near a landfill will also realize that things like seagulls, ibises, you won't see them so often during the daytime, but rats that sort of stuff, they're also a problem. And so in the next slide, I should say that in that massive waste, particularly where you've got biological material. And the MSW in the heading speaks of municipal solid waste, which has all the food scraps, etc. that we households check out. And this is the process that goes on over time, where you can see different gases evolved by the system. And it is a system. It's important even to take into account the settlement that happens, I'll talk about a bit later on, that's the amount of the way the waste actually compacts over time. And gas is evolved by the massive waste. And again, over time, what this rather ratty diagram, which comes from the UK. And that was the best they could produce. And what this diagram doesn't tell you is how long we are looking at. Typically, a putrecible landfill can keep evolving methane for as long as maybe 15 or 20 years. Much beyond that, there's very little methane left, and other things happen. But you need to be aware that a landfill does keep evolving methane in particular for a very long time. So I'll have the next one now. Okay, so what happens when you fill a landfill? And it's just sort of by way of definition, a landfill is filled when it runs out of airspace. Think about it. Most landfills started before the surrounding area develops. And there've been all sorts of pressures on the lateral expanse of these landfills. I can't keep expanding horizontally forever. And also often we have incidents and there's been several in Victoria where residential areas have encroached closer and closer to landfills and then suffered because there's been landfill gas and various other things. So often the only way that a landfill can expand is by increasing its height. So expanding into the airspace. But you do reach a point where you just can't go any higher for a variety of reasons. And then you have to look at what happens next. If I could have the next one please. You know, the good landfills, the ones that are operating to collect municipal waste do try to be good neighbors. And so they try to minimize their outputs. They try to collect leachate, for example, but of course you can't do that if you haven't planned to. A leachate system, a leachate collecting system is not something that you can put in after your landfill has been in place. Any of you old enough to remember Alice's restaurant, the La La Guthrie, you know, he put his letter with his address on it under the landfill, under the waste you're on. No, it doesn't work like that. You've got to plan to put a liner in and to collect the leachate. But let's assume that's what your landfill has done. That's how it works. But also you put a cover on and the cover as I've drawn here is at least two components to it. And you also need to put some sort of system to relieve the pressure of the gas that is being developed. Very often this is a reticulated pipework system that captures the gas and can use it. So a lot of landfills actually use, generate electricity from the gas they collect and so on. And then you can see in the background of this that's a standard cover on a well managed landfill. Looks like it's kept mown, but it's well grasped and so on. But I guess it's fair to say that a landfill really is only as good as it's kept. Next slide please Rich. So the caps got a number of aims. And it's important to think about all of these things are long term operations. You can't just sort of chuck the waste in and cover it and walk away. This goes on for a very long time. And the idea is that the waste has to be covered for the long term. You've got to prevent nuisance litter. You've got to prevent access to birds and animals. And you've got to particularly prevent pollution to nearby waterways, you know, blowing up litter and all that sort of stuff. You also need to stop rainwater infiltrating into the massive waste. And the reason for this is, as that graph showed earlier, that inevitably putressable waste rots. It breaks down and releases not only gas, but generates this leachate, which is typically quite acidic, very smelly. And by virtue of being acidic, it will transfer lots of metal ions and things and other toxic substances potentially to ground oil. But we also need to manage the landfill gas. And the purpose of camps is to do that to minimize the amount of fugitive emissions. And then, of course, as you'll see in some of my slides later, the visual amenity of these landfills is really not very high. And so the purpose of covering them is to make them look a bit better. Another next slide, please, Richard. It's just generally speaking, and in the most schematic, if you like, way. This is the kind of cover that people put on the waste. If they can afford it, it includes an impermeable GMM brain. That's very expensive material, but it is also very effective because it's completely impermeable. But being largely plastic, it's not something you leave by itself. So it has to have some sort of a final cover on which you can put grass or some other vegetation to make it look good. Underneath, you need various protective layers to protect that GMM brain from the waste. Remember that the waste degrades and compacts and anything sharp that's inside the waste tends inevitably to wind up breaching that impermeable GMM brain they've put on top. And that's one of the main reasons why these things fail, that the waste inside actually breaches the GMM brain. And kind of have the next slide, Richard. So cappings actually, you know, even the conventional capping is fought with difficulty. And I think it's fair to say the industry would agree that conventional caps certainly struggle to meet all their expectations and requirements. For starters, it's not something that anybody can do, just slap some clay over the top of a landfill and hope that it provides an effective cover. They require a great deal of specialist design expertise and crews that are used to working in this sort of environment and handling the clay in such a way that it, you know, the chances of cracking and minimized, and the clay is properly applied across the whole of the area and particularly at the edges. These are some examples of edge defects, you might say, in capping, where you can see the waste is sticking out the sides basically of various landfills. The one at the bottom is a small quarry landfill up in Northern Victoria, where the land is slumped, rather the landfill itself is slumped, and so the cover has been pulled down. And as a result, you can see the foreground by the tree there, they're sort of cracking right along the edge of the perimeter of the landfill area. And so it's also fair to say that conventional caps don't adapt very well to the shrinking and subsiding of landfill context. Next slide please, Richard. And even if they try, as you can see, they're certainly not attractive places. These are just two examples from Melbourne. This is actually almost happening while we speak. The image on the right is not so much a landfill, but a facility that was trying to handle recycled materials and send them off to China and of course got stumped by a Chinese ban on mixed recyclables and that sort of stuff. The material you see here is the material that even they felt they couldn't recycle. And it's pretty horrible. So this is why people hope to capstone. Anyway, given the difficulties with conventional caps, do we have an alternative? And so rather than trying to make, if you like, a dry tomb, a container that is completely impermeable to water and gas, in the mid 90s, the Americans, let's say cotton socks, decided that they might try something else. And this idea was probably promoted initially by plant scientists who understood that plants actually take an enormous amount of water out of the ground with their root systems and the process called transpiration. So there is a net movement. Any plant will tend to withdraw water from the soil and transpire it through the leaves. And if you get this right, then presumably this is a process that you can harness to remove the water. So the idea of transpiration caps or ET caps, evaporation transfer, evapotranspiration caps, phyto caps, all sorts of different names for is that instead of using compacted clay and impervious materials as the cover material matrix, you use something spongy, uncompacted soil or any other material that will will promote the growth of plants or allow the growth of plants and has enough capacity to hold the sort of water that you'd expect to come from rainfall, etc. And the plants can then remove the water by this process called transpiration using their root systems. But it's better than that because inevitably that root zone area develops a population of microorganisms that if there's methane present can break it down. And thereby prevent the escape of methane to the environment, which is one of the other purposes of a landfill cap. And so with time at least these populations develop and the amount of methane that can be effectively destroyed or metabolized is quite substantial. And they can be quite effective at preventing methane from escaping further because you don't need to compact them nearly as much. And because there's potentially a wide variety of media you can use, they should be easier to make. And so that was the basis I guess for the design. And we have the next slide please Richard. So they were a great idea. And even before people started working on them extensively. It was realized that there were a number of issues that you had to be careful about in choosing and building an ET cap. The first thing I guess is simply the slope of the surface. My idea is that you try and run off as much rainfall as possible rather than have it percolate into the landfill contents. You try and have the surface run off run away and you manage that properly. Obviously, the how much water holding capacity of your cap will depend on your climate. So that in turn will determine the thickness of material you require. And then you've got a whole range of plant issues that to do with the extraction and transpiration of the water. You need plants that can transpire a lot. You need plants that can establish themselves rapidly. And if they won't, how do you manage the risk of erosion until they get big enough to keep the surface stuff moving around? So, you know, what are the best species? Are they necessarily native or imported or whatever? And indeed, do we simply plant a set of plants and walk away? Or do we think about a difference sequence of plants over time? And, you know, for starters, it's probably true that you wouldn't want to be running ET caps. We'd have to spend a bit of time thinking about them and getting the work in places that are very humid with lots of moisture in the air, etc. So that there's not so much requirement for plants to transpire water from the root zone. How does the plant cover evolve over time? And, you know, particularly will the medium you're planning to use for your cover actually support plants? These are really quite critical issues. And, of course, methane actually is not so much toxic, but it smothers root zones. By excluding oxygen, it can cause plants to struggle because they can't get oxygen to their roots. Next slide, please, Richard. Anyway, in 2006, the AACAP program started and it was inspired in many ways by a trial that was run by the Willertland Field. It started in about 2004, I think, where a couple of trial plots of these alternative covers were established. And the guys at Willertland, actually, all they were using for cover material was what's called quarry scalpings, which is just the dirt scraped off the rock before they then get the vassals out. And they set up these two trial plots, which, as you can see, the vegetation is quite established and looks quite attractive over time. The landfill I talked about in the north has a section where they did put one of these transpiration covers on years ago, and they use wallaby grass as the material to, and that is actually quite effective transpiring up there. Next slide, please, Richard. Okay, this is the first of the test cells that the AACAP program established. Down at the Cedarland Field at Glinterst, it was a range of controls and test cells, comparing side by side, photo caps and conventional covers. And in this particular case, the cells were actually dug into the waste, sorry, dug into the cover that was already in place so that the very control for the conventional cap was simply all the surrounding area. But you can see there is a succession of plants over time. And, you know, 15 years later, there's quite an established plant community on the surface of these things, but it does come and go a bit as the 2017 photo shows it's a bit stressed at times. And so there are some issues that we can talk about. Unfortunately, as far as I can establish the facilities that AACAP put in place are not currently being monitored and worked on, but that may not be the case. I'm just trying to establish what's going on because as this indicates, there's a lot of long-term work that is being done. Next slide, please, Richard. So, nevertheless, what the AACAP program has been able to show is that ET caps do work. They reduce drainage just as well as conventional caps do and in many ways their behaviour is more predictable. They are at least as good as ordinary caps have oxidising methane and reducing fugitive methane emissions. More important perhaps is that they're sustainable. They look after themselves much more than conventional caps and they're self-healing. They resist the cracking that happens in a conventional cap because they tend to conform much better with the surface of the waste map. And you can choose the plant species according to the local area and climate and that's another important advantage. You don't have to scour around to find clay of a specific characteristic in your environment. You can use, you know, just local plant species, but you do need to test it. You don't need to use clay or geomembrane. You don't even need soil. The land, the other covers at Lindos, for example, used a mixture of sand and compost and I'll come back to what you can and can't do with the media shortly. And really the only maintenance is looking after the plants themselves. And if you have some sort of fissure or the plant dye water, it's relatively easy to fix. If, on the other hand, you've got a conventional cover and you start getting cracks in the clay, they are not actually amenable to just being patched up or whatever. It doesn't work that way. And they once cracked a landfill cover doesn't really uncrack in the conventional clay format. That's one of their problems that they don't repair themselves for a while. The important thing perhaps about ET caps to consider is that it's entirely possible to adjust them and design them so that they will allow some water to get into the waste. And that can actually support faster decomposition. Now, I mentioned that landfills are long term exercises, but nevertheless, they probably don't operate fully for more than sort of 2025 years. So, in a sense, anything that gets the waste decomposed to a stabilized state is very useful. And, you know, before the landfill closes, so there is an option to, in a sense, turn the whole practice of landfill management around so that the legacy impacts are nowhere near as great. Next slide please, Richard. If there are some issues with conventional covers and alternative covers are so fantastic, why isn't everybody using them? This is a critical and important question. They're not simply ET covers are not simply sort of set and forget type systems. They do need care and attention. And I think it's that partially that lack of understanding and and sort of whispering around the industry that says, no, don't go there. They don't work. That's not true. But they do need care and attention and understanding of where the issues are. You can't have an evapotranspiration camp that hasn't got plants on the evapotranspiration. So you've got to worry about the plants. You've got to understand the ways in which they transpire as species that your climate and seasons, that sort of stuff. And in particular, the characteristics of the soil, the amount of water it will hold. The harder it holds that water, that's this function called soil suction and soil suction as Richard will explain a bit later on. But basically what that means is that the higher the soil suction, the harder the plant has to work to actually extract water from the ground with its roots. Its structure and things like total organic carbon, they are all critical issues in getting the right soil. You have to take care about the medium and you can't just chuck any old dirt on the top of your waste so that you just can't sit it and forget it. And it's for these sorts of reasons that the regulators still aren't very comfortable with ET caps. I think it's fair to say that they're becoming more so, partly because there are now well established guidelines for designing and using ET caps. And they've actually been approved for landfill covers across Australia. I think that may not be quite the case in West Australia or it will happen. The reason in West Australia is that the capping with sand is a bit of an issue. But I think that's being sorted out. Anyway, one of the ways in which the EPAs of this world can be satisfied is by the data you can produce in experimental systems like the ACAP project we're trying to do. And as you can see, there's a number of variables that we are looking at to or that we need to look at to be able to assess how effectively a cover is working. Any cover doesn't matter what kind of design, but the fundamental principles are that you're looking or trying to quantitate, transpiration, evaporation and the two of them go together. So that they have a net effect on how much water is removed. You need, of course, to know the precipitation and runoff. So how much rainfall, how much just drains down the slope, all that sort of stuff. But there is also what's called lateral flow within the cover. So that needs to be measured and then how much percolates into the waste and how much leachate you produce. So these are all variables that if you're going to be promoting a new design or whatever or any design to the EPAs, you need to address these issues when you're setting out the design. Next please Richard. Yes, one way is to use what are called lice emitters. And lice emitters are basically big tubs that are designed and constructed in some such a way that there is a big area of your cap within them. So that edge effects and that sort of stuff are minimized and you can actually work on the cover material at scale. This diagram just summarizes the various components of where you have a conventional cover compared with a photo cover. And the idea being that you can then instrument them as Richard will talk about, you need to know things like temperature, soil moisture, soil suction, conductivity, a whole lot of other parameters that all need to be looked at. And to also understand that these things are not totally idiot proof. And there are problems with things like the meters that you use to measure water flow. If you don't understand that they need to be maintained and that sort of thing. Next slide please Richard. So this is just an example of the construction of the lice emitter pans that happened with linter's for example. They certainly not your average bench top devices. The minimum dimensions of the lice emitter that you can see in the middle or the pan, the middle image at the bottom there. That is 20 meters by 10. So it's quite a substantial size and quite an amount of work to build. And some of the other photos there just show some of the building was going on. The two images on the right are just showing in this case the institution of soil moisture meters or soil moisture sensors. And you've got to remember that what you are doing is trying to instrument a potentially unstable medium for the long term. So a lot of work has to go into how you connect it up to the various leads that go to other things, how you protect those. And so the sort of longevity of how you place these sensors etc is quite important. With that I think I'm done Richard. Knowing that you have got some more to say about particularly this stage of things. So I'll hang around and let you take over. Thank you. Thanks Brent. That was excellent. So I'm going to talk about monitoring of these structures that Brent has been suggesting as a better way to go for capping in various situations. Just looking a little bit at the current regulations that people in Victoria would be familiar with the best practice environmental measures for landfills. We have a couple of key numbers which apply to all caps and transpiration caps don't get any special treatment as such in terms of their overarching performance. So what is the performance, the magic number? 75% of the seepage rate through the basal liner at the bottom of the landfill. Okay, which is 10 litres per hectare today per day is the target. So you can't have more coming in than is going out is really the emphasis there. So that's ultimately what we're trying to measure is how much water would be coming through these caps. And we're measuring it against that compliance criteria. If you want to see more information about such things just you can have a look online at the Beppham. It's a Victorian EPA publication. So that's a compliance number, but to actually measure is this cap working as designed. There's a lot more things to that. And that's really these measurements are, in my opinion, more important than your compliance number, particularly during the early phase. Most of these sites say sort of allow for about five years to establish vegetation and that sort of thing. So you're wanting to know is the cap sort of starting to work as it's designed as the plants establish themselves. What changes are we seeing? And they're very interesting caps because they're quite dynamic, you know, and from a monitoring perspective, they're a lot more interesting than measuring a conventional cap because there's not much to measure at all. But a little bit on the basics of what we're trying to measure here. The transpiration cap is like a sponge. Okay, so think of soil moisture as water retained in a sponge. The bottom right hand picture there shows some in that the sort of black, the black patterned particles, their soil particles, then between the particles you have some moisture, which is in blue, and you also have some air voids. Now the way that water moves through a soil depends on how much water there is. So water loves soil. It gets very attracted to it and it effectively binds to the surfaces of the soil until there's too much water. And we call that when the channels between those soil particles build up enough, we call that free draining. And that's when water actually drains by gravity down through the soil. But in these transpiration caps, we try to avoid that scenario because we don't want a whole lot of moisture draining all the way through it. So we want to keep the moisture content less than that. So we don't want free draining or the bit that does free drain we want to catch before it comes all the way through the cap. So very important to understand that really what we're manipulating is soil moisture in the unsaturated zone. Okay, that's what trees typically drink. And we get rain comes onto the top of the cap and it infiltrates through forget lots of rain. It becomes free draining until it gets down and more of that moisture gets absorbed and then we reach a zone which is still unsaturated. And so that holds onto that moisture long enough for the trees to suck it back out. Now keep in mind it's not really trees, it's grasses, a lot of grasses. So we're looking at deep rooted grasses. So how does the moisture move around? Well, it really moves around by suction, right? A lot of people measure water content, right? So that's how much water's in there that really moisture doesn't move around by just how much is in there. It actually moves around by suction or another term for its water potential. So how do we change the water potential? The less water there is, the more suction there is. And so that water effectively moves towards a lower area of potential. So you see the top little drawing I've got there? It sort of shows what's generating this suction. So leaves of grasses and things create suction. So we've got negative 100 MPa up there. So very high level of suction occurring there at the surface of the leaf. And then as we come further down, we ultimately end up back in the soil and we've got a relatively lower suction. So the plant is able to suck that moisture out of the soil. When it rains into the soil, the suction becomes less and less as it gets more saturated because the moisture is not being held to those particles. So what we want to do is have a bit of a balance. We want the plant sucking enough moisture that there's still a relatively high level of suction in the soil to hold on to that moisture. Otherwise it would all just come whooshing through and out the bottom. What's the risk with these lovely caps? Well, if we don't get our vegetation established, and the water, when it does rain and it reaches that point of those water channels being filled, the water will flow through. And what's the problem of that? Well, when you have lots of leachate, you have lots of disposal costs. And some sites where they haven't got this right, they end up spending a fortune in disposal of leachate. So that's the risk. And so that's why you've got to be so careful with establishing your vegetation. So how much can we trust a plant, I suppose, is what it's all about? Well, I think we've lived off plants for many years, and we know that they need to be looked after, and that's how they grow well. I was listening to Brent, he was saying, you don't want to grow these things, you know, where there's lots of moisture and it's tropical, but got to remember that's also where plants grow well, right? So we have this dilemma with transpiration caps. Best spot for a transpiration cap if you weren't thinking about the vegetation would be a desert, right? But worst spot for a transpiration cap could also be a desert. Why? Because it's hard to get things established to grow. So you have this sort of dilemma of finding the right balance. And people do modelling on vegetation and that sort of thing. And on the thicknesses of these caps to work out, the optimum places to do this and the optimum thickness of the soil cap itself. So there used to be a couple of packages. One was called Vados W, which was an unsaturated flow model, and another one called Soil Cover, which was the original one. What did I want to show you on this? A few interesting things. So there's this concept of effective root depth, okay? What does that mean? It means, well, a plant doesn't suck water from everywhere. It only sucks it from where you've got good established root systems, okay? And it only sucks it from where its suction is sufficient to pull a moisture from the soil. So if you look at this picture on the left, this is just one out of straight off the internet. But it was interesting to note that when you look at the distribution of roots, there's about 70% of the moisture that's coming from really in that top 50% of the root depth. So sometimes people talk about what's the maximum root depth of something, but we've also got to talk about and think about, well, and how much of that moisture is coming across that thickness of soil. So if you plant grass and it grows and it's got, you know, five centimetre depth of roots, well, once the water gets past that, it's won the battle of getting to the waste, right? So we have to have a battle to hold on to that. So we want deep-rooted grasses that suck a fair amount of water, but are also reasonably easy to maintain. Hope you're following me. On the right-hand side here is a sort of schematic of a journey of water through a photo cap. It's not a photo cap. The picture, it's just any old thing where we're monitoring soil moisture, but the terms are important. So if you have heavy rain, you have surface runoff. And when I say heavy rain, what do I mean? I mean, it's exceeding the capacity for the water to move through the soil. So that's when we get runoff to the side. So Brent talked about needing to have, you know, reasonably steep surfaces to allow some of that runoff to occur. That is important. But runoff only occurs if the intensity is exceeding the capacity for that moisture to move down through your soil profile. So from my experience, a lot of the time, moisture does go straight down into these caps, right? You don't always see a lot of surface runoff coming off them. So it is about then making sure you can get it back out, right? So if you look at those little blue arrows coming down the journey and getting to the grass roots in this particular diagram, it's sort of saying that the top section, we've had enough rain that it's free draining water. And that's why it's got the thing saying gravitational water. Then we hit the zone, which they've called soil water holding capacity. This is that unsaturated zone where the moisture is holding onto the soil and it's not all draining out the bottom. That's the zone where the vegetation does its work and it pulls the moisture back out of these caps and we hold onto the moisture rather than having too much deep percolation. Now what's deep percolation? That's water that's got beyond your root zone. So if you don't have a very mature grass, then deep percolation can be pretty shallow and it's still gone past, right? So it's very important to get the vegetation established. So I'm just going to talk a little bit about how we measure those things because really measuring that movement of water is measuring the performance of the caps. Check the time. I'm going to skim over a couple of these just to some schematics for a bit short for time. You do need to measure weather, right? You need to know how much rainfall has been coming on. You also want to be able to calculate a vapour transpiration. So you need to know things like solar radiation and that sort of thing too. In the soil moisture side of things, we need to measure two things. We need to measure the volumetric water content, and we measure that as a percentage typically, right? So how much water is in the soil as a percentage of the total volume of soil is what that's about. We also need to measure the soil water potential. And remember, I was talking about it's this potential and the variance in the potential which drives how water moves around in the soil. So as I said, water doesn't always move down. It moves to that point of highest suction. So we can measure that directly using various sensors that we put in those lysimeters that Brent was mentioning earlier. So what is this talk about lysimeters? So as Brent mentioned, they're a bit like a giant bucket. But what we're trying to do is construct exactly the same sort of cap within that bucket as is across the whole site. It's important to understand that it's not a small exercise. Building a lysimeter is a big exercise, right? We then measure the water that comes out the bottom of that, that's your deep percolation. That's that flux number that the EPA is saying is the critical number, the amount of seepage we're allowed to have. So a very important number that comes out the bottom of these. But it's not really that important versus understanding the processes that are working in the cap itself. Because at the end of the day, what are you going to do? You know you've got too much coming through. What's causing too much to come through is that the fact that the plants aren't performing. And so we need to have senses to tell us how well our plants are sucking out that moisture. So it's sort of one measure is a compliance measure. The others are telling us what's wrong with our machine, right? They are constructed in the field and they're big things to build. And I've got a couple of pictures for you. So this is one that we installed recently. Thank you for letting us use these photos. So you can see there's a weather station, not a lot of vegetation, because this is when it's being built, right? Obviously they're going to plant a lot of vegetation across this. But a nice weather station here. And then each of these Lysimida groups have a series of soil moisture sensors and suction sensors effectively down profile to measure the variance in soil moisture. Next photo. So just a couple of things that we really do need to measure on these. So we need to have the climate parameters. We're getting that from that weather station. We need to understand the vegetation conditions. And this is a walkover assessment, you know, using a botanist, for example, because you want to get estimates of the plant density, plant growth and its health. You know, we all know it can be difficult to grow plants. So that's where the effort needs to go to make these work. We want to measure the water flux as a surface runoff, if we can, very difficult to do, by the way. We want to measure a water flux, the leakage through the cap. We use a Lysimida to do that. We want to understand the soil moisture variations to understand how well our vegetated layer is performing. And someone needs to keep inspecting the cap to make sure that it's not falling apart because of what Brent was saying. The waste that's underneath is consolidating, and that's affecting the cap's structure. I'm going to skip over this one running out of time. So to measure the evapotranspiration rates, there's various ways you can do that. Thanks for Gordon for putting a picture of an irrigation device in there, but really it's just to show you what transpiration is and that sort of things. But we use Lysimidas. There's a technology called edi-correlation that can be used to cost the fortune. It's really got research applications. Or you can use various coefficients to estimate evapotranspiration based on various vegetation types. And that's really like a percentage of your evaporation readings. This just shows you some of the sensors. So this is a sensor that you can derive volumetric moisture content from. The picture on the right shows how they're deployed into a pit that's being constructed in the Lysimida to deploy the sensors. This is a suction sensor or tensiometer, as they're known. These are deployed into that same trench that you saw reasonably close to those soil moisture sensors. So in the one area down the profile, you have both moisture content and suction readings. Now, the seepage that comes out the bottom of these Lysimidas. You'll remember on Brent's picture, we had a drainage line coming out. The flows are very small. That's a good thing, right? We want no flows coming through those things. But typically the flows are very small. And so you need a sensitive way of measuring those flows. And this picture on the left here has these tipping bucket gate mechanisms. So the water comes out those pipes where you've got those blue arrows there. And as those buckets fill up, they tip and that gives you a flow rate. Now, remember the flow rates, it's not so much needing to know a daily flow rate, right? It's about understanding the flow rate over a much longer period. So often the performance of these caps would be evaluated over how did it perform for the whole year. So these things need to be robust. But they do need to be able to handle low flows. From a maintenance perspective, we've been to sites to support these sorts of things. And quite often you'll find these tipping bucket rain gauges have got a lot of sediment in them, particularly in the early phase of the monitoring. And that's because you have loose materials which come down those drainage lines and end up in those. So it's really important to maintain these, otherwise you can get to the end of your five-year trial and not have enough good data to really decide if it's working or not. So that's the end of my presentation. Now, we're going to shift to just a little summary of what we've learned. So I think conventional landfill caps contain waste, but not necessarily prevent... What's that say Brent? I can't actually see it on my hand. Not necessarily prevent water infiltration, leading to more leachate formation or the escape of methane. Evapotranspiration caps can contain waste and limit leachate production, but they need care and construction and maintenance if they're going to perform as intended. All right. So over Q&A, so as I mentioned at the beginning, we had a few good questions that came through the early bird questions. Number one, how do regulators typically respond to alternative cover systems? Are they happy with this? Well Brent, I think you sort of covered that a bit, but do you want to talk to that a little bit more about where the current status is? I think probably the applicants who've tried to get ET covers approved would have more accurate and reliable advice than myself. But my understanding is that the regulators still remain to be completely convinced, and that's where the lysimeter data that you've been talking about that I talked about is so crucial. And I think people haven't appreciated that that's as important as it really is and the role it plays. There are guidelines, and unfortunately, unless you are a member of the Waste Management and Resource Recovery Association, WMRA, it's a very expensive exercise to get them, but that's where you'll find them. I haven't exactly seen what the final version is, but those guidelines are accepted by the EPAs across the country and form the basis of how these things are judged. But I think until we start getting more experience with them, and remember that in Australia, for example, the first of these only went in 2004, and most of the experimental ones have not gone in actually overwaste. Most of these have been set up in trial cells that haven't been actually capping waste. And until that happens, and of course, you're in a catch-22 there, most regulators won't let you take an area that's been conventionally covered and cut a hole in it and stick a fighter cap in the hole. That's simply not allowed at the moment. So there are some issues still. I believe they're being addressed. Partly because one of the pressing issues for a lot of municipal associates, councils, et cetera, is what do you do with dead landfills? The landfills that were there in the 60s and 70s and have been abandoned, closed off, had football labels put over the top, but that's all. And to cap them conventionally would be more a defence budget to actually do that sort of thing. So ET covers were seen and probably still are as a way of dealing with legacy landfills. Thanks, Brent. We're going to restrict you to shorter answers to get through the quantum that are coming in. But that was good. Yes, no. I would say that on a couple of Lysimita projects on landfills that we're working on, the mechanism that seems to have been adopted is that the auditor will require a prolonged Lysimita trial, which is built over the waste, right? And providing that all functions well, well, they're happy with that, with the cap. So really it's, I guess, the philosophy is, yes, we're okay with you building it, but we're going to keep an eye on it. If it doesn't work out in the end bill, that's the problem for the landfill operator. So where you see these Lysimitas going in there, the regulator has sort of let them at least proceed with the concept. And right now we're reviewing the performance of one of those that's been out there for probably seven years now. So next question, are there any specific areas of the regulation of landfills that you think need to be improved and or removed? I'm not involved enough to know, I have to admit. You'd need to speak with landfill operators. We should have heard from you earlier, Brent, was that you'd like the guidance on the transpiration caps to be a bit more accessible than hitting away in the WMRRs side of things. So perhaps there's a need for the regulator to adopt it as one of their own controlled docks and have it up on the EPA website perhaps. Yeah, I'd agree with that. Absolutely, Richard. Absolutely. Next one's slightly off topic. Sustainable landfills, PFAS soils. Do you have a view of, do they need to be treated any differently? These sites with PFAS? Yes, they do simply because PFAS is rather different from most contaminants in the sense that it's highly water soluble and moves with water. But whether or not ET caps, for example, are suitable for capping PFAS repositories, I simply would not know, it would need to be something that was researched and backed up by a whole heap of data, like lice embedded, et cetera. So next question is best industry practice with examples for disposing of tailings and waste rock on same landfill. I think it means the same as landfill, if any. So certainly what I've seen is waste rock dumps, for example, will adopt very similar methodologies as these transpiration caps. And in fact, that's really where they started. So yeah, similar approaches are adopted. That's for sure. Do you have anything you wanted to add? The main issue with disposing of tailings is that you do need to manage the amount of acidity that gets through and save the amount of percolation. And so operating your ET cover to maximize the amount of transpiration and minimize the amount of percolation of acidic drainage into it is probably a useful thing as well. I've certainly seen with the construction of caps over waste rock dumps and also some tailings is that they try to put in a capillary break, which is a coarse aggregate layer that you then build your soil cover layer on top of. And it's sort of a little bit counterintuitive, but what that does is it stops capillary rise of your sort of particularly saline solutions that can be coming out of your tailings from impacting the vegetation you're trying to grow over the top, for example. All right, next question. This is also a little bit counterintuitive. Drilling bores into the cap. Do you have a view on... Have you done it? Have you drilled holes into caps? No, but again, the beauty of ET caps is that they tend to be self-healing and you should be able to... It's perhaps not the drilling of bores that's the problem, it's what happens afterwards. You've got to be able to seal, if you're putting, say, groundwater monitoring or sampling well through a cap, you've got to be able to seal the outside so that whatever you're doing only goes up and down the end, inside the tube you put in. So doing that in a conventional cover is always a bit tricky and doing it in an ET cover, at least the cover should conform to the outside of the ball. So you need to bear that in mind. All right, so they're the early bird questions out of the way. We're about eight minutes over, so we'll keep charging on. Are you okay another 10 minutes, Brent? Yeah. All right, we'll charge on into these questions. Matt Taylor, what would you say is the general useful life of a traditional cap before they tend to start failing? Would you expect to get at least a decade or so out of one? I think a decade is at least reasonable, but say I've seen a couple now where 15 to 20 years is starting to see quite significant cracking and leaking of gas from them, which is why we know that some landfills keep generating methane for that length of time. But also, I am not that familiar with the current industry averages and these sorts of things. Not the right person to talk to, unfortunately. Next question. With photo caps, how deep is the porous soil from surface level to the waste mass? What would the minimum need to be in comparison to conventional, final caps? Okay, that's relatively straightforward and not in the sense that it depends entirely on the seasonal rainfall and the porosity and holding capacity of your soil. For example, the linter's photo cap had about a metre and a half of capping material, whereas the conventional cover in complete thickness was only about half a metre. So, in a sense, you need to remember that photo caps can actually cost you a bit of airspace. You need to allow for that. But it's not just a particular formula that says you need this many metres of photo cap cover. You need to do the calculations of just how much water you're going to have to do and that affects the thickness, of course. I might just add to that. Earlier, I was talking about the modelling that can be done. So, there are models you use to do simulations. So, there was one called VATOS W which you put in like a daily rainfall data set for a particular region and do your simulation and it calculates your sort of evapotranspiration rates sort of thing and you keep increasing the thickness of your soil until you are modelling and achieving your desired seepage rate at the bottom of it. So, typically to get the thickness right you need to do some modelling. Okay, next question from Julian Marshall. Could ET be added to existing clay capping to have trees and shrubs naturally self... How do you get trees and shrubs naturally self-seeding which had to be removed as directed by EPA, fear of damaging clay caps? I think this is all about how do you control the vegetation and what's the management... The answer there is two parts. One is that the reason the EPA should keep trees and shrubs off conventional landfills is exactly right. They will breach the clay cover and damage it and there's another question further down what happens when trees die all that sort of stuff. This on a conventional cover is bad news. If you... There would be no reason why you can't add an ET cover on top of a conventional clay cover provided you make sure there is that capillary break and that there is enough height of the ET cap to allow your plants to grow without breaching the underlying clay. But I'm sure that's entirely doable with the issue of what happened when the trees die. In a conventional cap that causes breaks, cracks that sort of stuff but in an ET cap the very nature of the medium normally means that it simply collapses around the dead roots etc. It doesn't cause a problem. Next question, I think it's an important one is can we please provide that reference so it is the WMRR reference isn't it? Yeah, I'd have to look it up. I don't have it on hand but I can send it around. It's not actually that easy to find. It's looking pretty hard for that at the moment. So let's come back to Dinesh on that one. Dana, wind all the issues with deep rooted plants if they die fall over and potential to expose waste. It needs to look at the rooting depths. We sort of answered that one before. Certainly if a big tree falls over it does compromise the cap. There's no doubt about that. It would also be fair to say that if you've started an ET cap and you wind up with big trees on it by the time they get to dying and falling over they've ended the waste and have been hit something toxic. It might be argued that the waste underneath is fairly stable and this is less of a problem and I get back to the fact that ET covers are certainly much more self healing than conventional covers. You do have to allow the appropriate depth and some of the work that was done in the early ET cap program did show that you tried not to get really deep rooting plants in your cover and that if you had the right mix of plants it actually reduced the opportunities for some of the more deep rooting species to establish themselves as droppings. Next question. What's the pros and cons of using a drain gauge as opposed to a pan lysimeter? That's yours. I think so. A pan lysimeter I'm assuming you're estimating evapotranspiration from your evaporation pan data and applying a coefficient to it. One's assumed based on crop coefficients if I've got this right whereas the other one is a true direct measurement of the performance of what's been built. In the end the regulator pushes towards these lysimeter structures and literally draining out the bottom of them because it's simulating exactly what's been put in place including the vegetation. Where you're using things like coefficients and apologies if I've misunderstood your question they're based on a certain maturity of that vegetation in itself for achieving that evapotranspiration so that they are an approximation. So that's the downside. Next question. Why are these not recommended to be used in fresh waste young fill areas? I suspect that it mainly if you like regulatory inertia and that we haven't enough examples of where this has been adopted. But remember also that most landfill operations the waste the cell is open for some considerable length of time a year or two possibly longer depending on the size of the cell and how much waste has been deposited each day. And the caps are really not put in place until the whole cell is completely filled. So it's not quite as simple as it looks in terms of saying look if you've got a fresh landfill put a fire cover on it. The question of whether you can do in a sense a temporary fire cover is interesting because for example one of the things that the ACAP program actually showed is that the grass cover on a conventional cap actually transpires away a lot of the water. That's quite an important phenomenon. And therefore rather than seed the temporary areas of your new landfill with just grass perhaps we could look at other species of plants that have higher transpiration potential and look better and that sort of thing to make these things work. But I'm probably not the right person to answer that completely. I'm going to do a pretty good job. Okay, so next question from Aidan Hall. It probably depends a lot on landfill contents but what are the considerations around the time it takes for landfill cells to subside and the vegetation types used in a transpiration cap? Could a landfill cell subside after trees and vegetation are established? And what effects would this have on the vegetation effectiveness of the cap? Okay, I believe the beauty of the photo cap system the ET caps is that as that waste collapses the cap keeps up with it and you why because the cap doesn't depend on the geometry of how each of the plants are related to each other physically in space it would self heal and adapt very effectively and probably become more effective with age. The main difficulty may be that if the cap essentially becomes a dish and the runoff tends to pool in the middle then you may have a problem because most of them depend on the fact that the runoff runs away from the cap rather than in the middle. That may be the only difficulty that you'd have in this circumstance but you can repair that by simply adding more growing material until it has the right profile. All right, well I think we've got time for the last five questions but no more. So Dinesh is trying to stitch you up with a reference that says that recommends photo capping being the best option for legacy landfills. I think the word best there might be a bit loaded. So I think what we might do Dinesh is we'll add a few recommended references to the slides that we're putting up onto our website and we'll include the link to that other guidance document as well on that. So you've got those readily available. Dinesh, I guess the other answer is there may not be regulatory guidance but there's almost certainly likely to be economic guidance that says you've got this legacy landfill and you want me to spend $5 million per capita on it. That sort of decision is we are reaching a point where we can quite clearly show that there are considerable cost benefits in using ATK. Okay, so Hamish Mackenzie, famous name in landfills Mackenzie, is there any potential for utilizing wasted construction materials e.g. crushed bricks and glass in the evapotranspiration cap soil medium? Limited and it depends very much on the state in which that material is placed. We for example in the ACAP program tried using crushed waste construction materials, brick concrete etc. as the drainage layer and it was much too irregular and with lots of sharp corners etc. to be useful say in a life similar course problems. You could possibly dilute your growing medium with this material but there are two considerations you must bear in mind. One is that you've got to have plants growing. It has to be something plants can grow in and grow well and it has to look good. The next question is from your final cover to have all sorts of bits and pieces sticking out of it as would probably happen if you use this in the final cover. Remember that aesthetics is a criterion that various people look for in the performance of ACAP. Next question from a world famous horticulturalist and my brother Alex and soil micro flora slash fungal associations and the effectiveness on methane control. Good question. Alex the short answer to that is not enough. I do believe that there were possibly two PhD students in the ACAP program whose work started well after I left who looked into these sorts of things but the ongoing micro flora work and indeed the methane control aspects I think are right for further research. Next question. Anonymous attendee. Any use of shallow dishes in sandy soils to retard infiltration into cold water for plant establishment? I'm not quite sure what this question means. I think what it's inferring is if you have a site with sandy soil and you get heavy rainfall and get some infiltration into it it would be beneficial to have some kind of dishes layered through it to trap that infiltrating water that would then be as the soil dried out re-dispersed back into the soil for plant growth. Probably yes but you'd have to say that it would take some careful design and a full blown research type system to demonstrate that it could work but I agree that something that slows down and it may not be dishes, it could be remember you construct the fighter cap in layers, inevitably you can't sort of chuck a litre and a half of stuff in a cap all at once it's all laid down in layers, it would be possible but it would have to be checked and shown to work in a trial system. One way to achieve a similar thing to that is through that sort of capillary break mechanism I mentioned earlier if you have a layer which has got very wide soil pores and then you put finer grain material above it you can wet up that soil that sits above it more than you normally would be able to before it starts to dry and flow and that's simply because there's a much lower suction in the capillary layer remember I mentioned suction and that water moves from areas of high suction from low to high so that's one way to achieve that. Last question, now Dinesh has exceeded his two question what is the downside of sealing cap cracks with compost as opposed to bentonite? I assume you're talking about conventional covers with cracks in them because generally speaking if an ET cover is constructed properly it keeps conforming so it doesn't crack and that's one of the whole reasons that they're chosen but if you're trying to repair cracks in a conventional cover repairing them with compost may work provided you've got sort of plants growing in the material etc but you can't in any way guarantee that and trying to repair them with bentonite, yes you'll repair the crack but the forces that cause the cracking in the first place will apply next door and so it's a hiding to nowhere it's I believe repairing conventional cover cracks with bentonite or any other clay material that sort of stuff is really a hiding to nowhere because it'll just keep happening it may help reduce the amount of fugitive emissions etc but it won't solve the problem and it may actually be that the longer term solution is actually going to apply an ET cover on top as one of our other attendees suggested All right Brent, well that has brought us to a close thank you very much for presenting and thank you very much for so many people for having and asking all those great questions really appreciate that so great to see you again Brent and thanks for sharing your knowledge today Thanks Richard, thanks for the opportunity and good luck with all of this Thanks mate, see you