Okay, well today we're going to go slide into the world of the complex and the sublime sort of with a discussion of a numerical computer models that can be used to evaluate MNA. >> Right, so we've talked about that more complicated models where finite difference or finite element are used to discretize the sub surface,and you apply transport processes, really attenuation processes, to the plume. But how would you put all of this into context? Is there a hierarchy of groundwater models? >> Well if you think that the first tool was that bone that was thrown by that monkey in that movie, 2001: A Space Odyssey, he was probably using that as a weight for the bailer, right? But we've come a long way. So let's talk about the continuum of groundwater tools in this slide right in here. So it sort of starts out that there's different things like the terms on the left. We go from limited, to site data, to site data with simplified assumptions, and then to more complex tools, and models and things like that. >> So in the first category you have things that are hand calculations, so that would be like calculating the groundwater velocity or something like that. >> Exactly. How would you do this though? Would you use the Darcy velocity or the seepage velocity, if you wanted to know how long a plume is getting. >> I would use the seepage velocity. >> Very good, exactly. Then going down examples of site data, it's like different scenario's documents that we develop, where you put these different plumes in different buckets, like taxonomy. >> What about the analytical models then? >> Some examples of those we've talked about, BIOCHLOR, REMChlor, REMFuel. And then you slide into these numerical models, which we'll talk about today. Things like MODFLOW, MT3DMS, Tough, things like that and give you really this detailed site-specific stuff. So let's first just talk about some of the strengths and the weaknesses of these numerical models. So what we're really looking at is that their strengths, that they can really sort of look at this macro-scale heterogeneity, multiple layers, changing hydraulic connectivities. They can simulate recharge and discharge to streams, so you can really connect to the surface in a good way. They can do more complex reactions for sorption and biodegradation. Just overall, they're more realistic, but they do have some limitations right. >> Well, obviously these things take a little more expertise in order to use them, so there's a time investment, there's maybe a money investment that would be associated with these. Then as we'll discuss further, you have to be careful with how you handle matrix diffusion in terms of these models. And then finally, you have to enter your source concentration versus time, right. >> That's right, and those analytic models, you can actually put a source mass in there, and it will use this box model to tell you what those concentrations are versus time. These numerical models typically don't have that. You have to actually enter in this year, it's this concentration. This next year, it's this concentration. You may have to run something like RamCore to get those numbers, so one of these subtle things that doesn't have these built in source models. Here's something from the RT3D, which is a numerical model, they call this reactive model. They have these frequently asked questions, and they talk about when do I use the simple tool versus the complicated tool. In the middle here, they talk about how simple analytical models can be useful for upfront screening assessment, or some sites that lack complexities in flow, geology and reaction. They have simple geology and things like that. Then they say that this RT3D numerical model on the other hand can be used to model the complexities of the flow and the geology and the reaction for natural attenuation, or for some sort of accelerator remediation technology. So, a good sort of way to balance just the different stuff out, okay. So now let's look at what we mean when we talk about discretization, right? So what we're going to do instead of just assuming this one deep flow analytical model, we're going to break up that sub surface into a bunch of different blocks. And so on the left is coming from one of the manuals for mod flow, and it talks about, you can have these different elements in there and each one of the boxes, each one of these cubes is going to have a representation at what the geology is in that space. And then you can define different cells of being boundaries, this one's creating flow, this one's sucking outflow. Then you can sort of get complex side of geology on the right, by sort of changing the thickness of your cells, as you can see in that example, right? >> Yeah, so you maybe have some complexity, but you're trying to at least approximate it with these grids. >> Yeah, so we can really get complex hydrogeology, different layers, the different thicknesses of different units with doing stuff like this on the right. So now let's go look at the reaction terms, and so here's this is sort of what they have in their manual, and you start to get the sense that this is a little more complicated than just putting in this first order decay. You could do that if you want to help represent your attenuation of a dissolved plume that way, but you can also have other ways to look at some of these chemical reactions and things like absorption. Give us some examples. >> Well, you can do linear absorption, Freundlich. You can use Langmuir. You have all sorts of choices in terms of just that one process. >> Right, so you have to consider there's more data that might be required to put that stuff in there, but you have that power to do that with these numerical models. Now let's go this document that talks about how to apply this RT3D model for chlorinated solvent sites. And just to give you a sense of here's this outline, if you have these different problems that you want to solve. I have Chloroethenes only, I do this stuff on the right. If I've got a mixture of things, I do this stuff on the left. But it talks about which dials you turn, which processes you want to use, but you sort of get the sense we're getting more complexity here. So there's more power, but it does take time to sort of to figure this stuff out. Let's go through a list of attenuation reactions that are available in this RT3D numerical model here. They list out of a bunch of different ways of doing this. Dave, what's your favorite here? >> I'm a bio guy. I like the double Minot model. >> Okay, but they have instantaneous reaction, which was used in the bioscreen. But it combines this power of really being able to better simulate the complex geology. You can get sort of a wide variety of these reaction terms to sort of simulate all this stuff. Let's look at an example. >> Okay. >> Okay, so let's look at a numerical model, this is a great paper by Rasa, Doug McKay and colleagues. And it's basically looking at a vertical slice in the subsurface. There's this picture here with a lot of different cells, and the reason I'm using a lot of three meter modeling zones, so it's not that much, ten feet, but they have 60 vertical layers in there, because they are modeling matrix diffusion of oxygen through a silt layer getting into the this MTBE plume. And so the deal is that the MTBE plume is on the left, and this what this picture look like in this numerical model from 2004 to 2006, that MTBE plume looks like it's going away. And why is that? >> Well, it's degrading, and it's degrading in part of TBA, and that's shown on those right panels, so you see the purplish color sort of increase in as you go through time. >> The daughter product, right? So this model will handle some of that reaction, but then it also handles this very sort of important piece that you could not do in analytical model, has that layer that the oxygen diffusing through. Now, we talk about this in some of the previous lectures. This matrix diffusion piece, because that's a very low probability in that the options going through in their model, you have to be careful with that, and here is this paper from two of the folks from the University of Guelphs, Steve Chapman and Beth Parker, Tom Sale from Colorado State, Liam Donor. But they try to model this green tank experiment that we showed in several lectures, right, about matrix diffusion, and let's look on the bottom left, how many of these nodes or these cells do they need to model this in ModFlow MT3D? >> Well they got up to 10,000 nodes. >> So I need a lot. Very thin layers to this, and what was their key conclusion right now with numerical models when you're looking at matrix diffusion? >> Or in this case the quotes require a much higher resolution than commonly practiced to simulate this process in matrix diffusion. >> So just a warning out there if you're really thinking about this matrix diffusion process, you may need to be careful about how you do this to make sure you capture this correctly. There's still a lot of work being done sort of in this area, but one thing you could do is that if you don't want to apply these models, there are these things called these type sites. And this is in this state of the science review of management of contaminants and low permeability zones, Dave a little bit about that project, who's the authors on that? >> It's primarily Tom Sale, but Beth Parker, yourself and- >> And you. >> Well yeah, I contributed to part of this, yeah. >> And so, this one said there's a modeling chapter where they set up one of those really, really powerful models, HydroGeoSphere, and then with model, what they call different type sites. And so, if you don't have the time to model something, you could say, well I think my site's most like this two-layer sand or clay, or my model's got a fractured network, It's fractured rock. And let's look at that example here. And you can sort of see their domain, and that's showing up on the left. What are all the lines up there in that top panel? >> Those are individual fractures within that domain, right? >> Yeah. So you might say in Texas terms, that's a mess of fractures. >> That's a mess of fractures. Exactly. >> So they sort of are, they put those in there and they say this is a highly fractured rock, and then they've got different sizes of aperture size and the frequency and the hydraulic conductivity and the concentration. And then they let it rip, and then they can say well you can see these graphics in here just by observing if your site sort of looks like this on the top left, maybe not exactly, but you can get this idea of the style of the site. And this shows that over 20 years 50 years and a hundred years, this is what this plume would look like, and then in this case that got some degradation in here, so that's why its starts to fade out after a year 100. So some powerful staff in terms of these type sites in that if your really looking at matrix diffusion chlorinated solvents, those types of thing. So lets rap up and just say that the numerical models can model things that analytical models cannot. >> And then you pointed out several of the limitations, and they're powerful, but you don't often have a modeled source term, and you may need those really fine grids in order to capture matrix diffusion. >> And the type site can be sort of a middle ground of looking at this stuff, you see what a site that sort of represents your site might look like.