CHUCK NEWELL: OK, well, today, we're going to wrap up our discussion of matrix diffusion and how it affects monitor natural attenuation by giving you a review of a rapidly changing field and what type of destructive processes occur in these low-permeability zones, such as these clays, these silts, and these limestones. DAVE ADAMSON: Yeah, and this can be really important in terms of the viability of modern natural attenuation to handle the mass that's potentially in those low k zones. And so today, we're going to talk about biotic, biodegradation processes, as well as abiotic, sort of those chemical degradation processes. And there's two key considerations when we discussed this. The first is that we're really primarily talking about chlorinated solvents here. Not allowed of work in this field as focused yet on other contaminants, like petroleum hydrocarbons. And then secondly, we really are talking about an emerging field. There's not a lot of studies have been done to date on this relatively new but important process. CHUCK NEWELL: I'll just echo. It's a really big deal when thinking about this monitor natural attenuation at these chlorinated solvent sites, that even a really slow rate of degradation. And the low-permeability zones can have a big impact on the longevity of these sources and these plumes that are affected by a matrix diffusion. So today, we'll start with the big picture. Is there any attenuation that actually occurs in these clays, and these silts, and these limestones, and things like that? DAVE ADAMSON: Well, I think the answer might be sometimes. At least, we think so. We're starting to get a little bit more evidence based on a lot of lab and even some field studies that show that attenuation might be happening in these low k zones. But again, I want to reemphasize it. There's not a lot of investigation that's been done to date. And again, a slow attenuation this in this case could be really important. CHUCK NEWELL: OK. And then we've got a graph here from Charles Schaefer, a paper that he did. This is for a SERDP study. So some great research coming out of SERDP. The y-axis is this years or the time that's needed for 99% of this mass that's in a rock matrix to go away, if there is a reaction. And the x-axis are these rate coefficients. And it just says that as you get higher and higher rates, as you march farther and farther to the right, then this time to clean up that matrix, which is hard to deal with active remediation, you can't pump any chemicals in there. It just goes down and down. So this is just reinforcing that even a little bit a rate goes a long way to help you out here, right? DAVE ADAMSON: Exactly. CHUCK NEWELL: So let's just go-- we talked a little bit more about what things sort of on a high-level help and what hinder attenuation in these low-permeability zones. So Dave, what might hurt you? DAVE ADAMSON: We've listed several different conditions within here. These are things that favor attenuation. CHUCK NEWELL: Oh, and we're doing favor first. DAVE ADAMSON: Yeah, we'll do favoring. So basically, we've talked about that there's not a lot of advection in these low-permeability units. So in certain sense, that provides a benefit of long retention time so the bugs, basically, that might be in their, degrading these things, have the chance to do it. Secondly, in a lot of these environments there might be reducing conditions. So a lot of the mechanisms that we're interested in terms of biodegradation for attenuation are based on reductive dechlorination for example, or even biogeochemical things that are favorable within reducing conditions. Within these zones, there might be a lot of organic carbon. They might be dominated by silts or organo-clays, where there might actually be something that can serve as a electron donor to drive dechlorination or to form reduce minerals. And then to follow on what that, there might be actually a fairly abundant amount of these reactive mineral species that we talked about during week three present within these low-k units. CHUCK NEWELL: So in terms of the Charles Dickens' A Tale of Two Cities, this is the best of times. So now let's look at sort of the worst of times, what are things that, in this case, hinder this attenuation in these low-permeability zones. Smaller pore throat sizes restricts this migration of microbes and things of that nature, the influx of nutrients, as well. Salinity, that can be a big case. If the high salinity-- it might limit this microbial activity. Limited bioavailability or organic material, if there's not that energy down there for these bacteria to start to work, to jump on the chlorinated solvents, then that reaction may not be very significant. And finally, that this reactivity of these mineral species, some of these iron sulfides, if they're not down there, they're not reactive. Then that's going to be something that may say, I don't have as much attenuation as I might have in other zones. DAVE ADAMSON: The formation of those mineral species has a biological component. It needs microbes to be working in order for those reactive minerals to form. So if the condition of some of those ones on the top that limit the amount of biodegradation activity happened, you can't necessarily even count on reactive minerals to do abiotic reaction. CHUCK NEWELL: So it's something you really want to know. So let's look at some of the ways you can build lines of evidence for evaluating these reactions in low-permeability units of chlorinated solvent sites. What are some of the things you can do? DAVE ADAMSON: Well, the first is you're collecting molecular biological data, basically, to try to confirm if these organisms, the bugs that you are hoping doing the degradation, for example, are present within low k zone. CHUCK NEWELL: So the number one is bug counts. DAVE ADAMSON: Then you're looking at the daughter product formation. So daughter products are sign of degradation occurring. So you want to see what the distribution of daughter products is between the transmissive zone and the low k zone and use that information to try to help you establish whether degradation is occurring and where it might be occurring. CHUCK NEWELL: So a big slug of cis-DCE in a clay, that might be of a positive line of evidence, right? DAVE ADAMSON: Exactly. And then you're looking at geochemical conditions within the low k zone, again, trying to establish, for example, if there's reducing conditions with in there that would support reductive dechlorination of chlorinated solvents. CHUCK NEWELL: I call it am I swampy, sort of having these anaerobic conditions are going on there. DAVE ADAMSON: And then CSIA data, and we're going to talk more about isotope data in a subsequent series of lectures. But this is something that you can use to tell you whether degradation is actually occurring within those low k zones by seeing whether there's a change in the relative amounts of these various compounds specific isotopes. CHUCK NEWELL: So you can do isotopes conventionally in the plumes and the sands, but you've actually done it in clay's, right? DAVE ADAMSON: Yeah, exactly. And then maybe following that up with a more detailed mineralogical analysis. So taking a look at soil cores, sending them off for analysis such that you can understand whether there's these reactive minerals that are present that might promote an abiotic degradation pathway. CHUCK NEWELL: Iron sulfides, magnetite, and stuff like that. DAVE ADAMSON: Exactly. CHUCK NEWELL: OK. So here I think we've got next some photos of some of the people that are working on this. DAVE ADAMSON: Yeah, again, we showed some of these in the last lecture, but this is Steve Chapman doing some detailed soil coring. And this is an example. We're going to show several slides here from the work that we did at Naval Air Station Jacksonville down in Florida. And so we did the high resolution sampling of two different areas within here, vector 298 soil subsamples from four different locations at building 106. So that was that manufacturing area there. And then we took 166 soil samples from three different locations at building 780. So that was a dry cleaner site. CHUCK NEWELL: OK, ESTCP project, right? DAVE ADAMSON: Yeah, exactly. CHUCK NEWELL: OK. So let's look at some of the results. Here's the case study. The pink is actually where these clays are located. And you see this distribution of these daughter products, these degradation products that provide some clue in what's going on in that low-permeability zone. But it's complicated, because these daughter products could either be produced in low-permeability zone itself or perhaps, produce the transmissive zone and diffused down into this thing. So we're seeing on this graph, it's sort of, the source zone is on the left. And as you march out, you can see more of these daughter products being produced. But in some ways, they seem to be more produced maybe up there in the sands, is that right? DAVE ADAMSON: Yeah, I think that's the predominant pattern here, is that you shift to more degradation products as you're moving down gradient. But those degradation products are mostly present in the more transmissive zones above that clay-- that pink zone that designates the clay. CHUCK NEWELL: Got it, OK. And so with this, is you can then add other tools. Actually, you did at this site, where you actually looked into that dark matter zone and did some other analysis. So let's look at some of those. What else did you do? DAVE ADAMSON: Yeah, and so again, this was a case where we're trying to get an idea of what shifts in isotope ratios. So, what level of fractionation that we might have seen, in the PCE, and the other daughter products that we're seeing in this case. And so we've got a little graph here that's showing the levels of C13 isotope and PCE. What happens to those as you're going in the transmissive zone to the low-k zone. So in that case, we're looking-- going down with depth on the y-axis here. And things moving to the x-axis from left to right are showing increased fractionation, or increasing amounts of degradation in this case. CHUCK NEWELL: And we call it being less negative, right? If you're less negative, which is sort of these spikes off to the right, that means you're seeing bio-degradation-- DAVE ADAMSON: Yeah. CHUCK NEWELL: --in both the top and the bottom sands above and below this low-k unit. This clay, it looks like it's more degraded. Not so much inside the clay. DAVE ADAMSON: The clay, you know, you see fairly similar patterns within there-- which would not be suggested that there's actually degradation going on within these clays-- that the degradation actually happening in the transmissive zone itself, and being more prevalent as you move down gradient. So it fits this conceptual model that, at least at this site, most of the degradation of the parent compound is happening within the transmissive zones, not the low-k unit. CHUCK NEWELL: Cool. OK, well let's look at some of the other lines that I actually did some bug counts, but it was difficult to do that in these clay samples, right? DAVE ADAMSON: It can be a little bit difficult to do that. But in this case, we're showing data then in this table, showing sort of two different locations. So the sands and the clays. And we were able to see the Dehaloccoides in both units, generally higher within the sandier units. But we were able to establish that there is the presence of these key dechloriating organisms, these Dehaloccoides species within those low-k units. So it is evidence that there is the potential for degradation within these low-k units, both in terms of the Dehaloccoides as well as the vinyl chloride reductase. So that's that key bio-marker for showing that the complete degradation pathway all the way to ethene is actually present. CHUCK NEWELL: OK, and if you're interested in the actual bug counts, let's just go into these clays. How many bugs were in there, and those OU3-3 clays right there? DAVE ADAMSON: Yeah. So we're talking about fairly low levels. Two times ten to the fourth cells per gram in this case. So if you're familiar with sort of the ranges that you're associating with those, those would definitely be on the low end, but detectable in these cases. CHUCK NEWELL: OK, so 20,000 per gram sounds like a lot, but it's actually not that much. DAVE ADAMSON: Yeah, and in lot of cases at sites, you're trying to see a couple orders of magnitude that might be associated with really rapid dechlorination. CHUCK NEWELL: Neat. OK, so then you put all this data in one place, right? We're looking at, again, these different graphs. The red is this low-permeability unit, these clays that are out there. We're sort of going from the source zone, which is off to the left, to this downgradient on the right. You combine these three data sets, what are they? DAVE ADAMSON: Yeah, so you've got containment distributions sort of forming this row on the top. And you've got isotope data then in that middle row. And then the bottom row is actually from groundwater data, where you're establishing what the geochemical parameters and daughter product distribution is in that. And these sort of fit into this, helping you build this conceptual model that you might have for this particular site, in relation to whether degradation is occurring within these low-k units. For the most part, at this particular site, it looked like most of the degradation was occurring above those units, or above and below, and not maybe as much within the low-k units themselves. CHUCK NEWELL: OK. So maybe it's not happening really strong here. Well, let's go look at a bunch of cases all around the world. Let's leave this Florida case study. And look, we've got this map of the United States with some of these circles on there. We've got some maps of the world. And these were all of the studies that you could find that talked about attenuation, destruction by degradation, or abiotic degradation in these low-permeability zones, and you put them together in this chapter, the SERDP report. Let's just go to a couple of them. What happened in Japan? DAVE ADAMSON: Well this was one that was really promising. This was actually, as far as we knew, the first study that basically showed that dechlorinating bacteria were present and growing within these sort of low-permeability aquitards in this case. They established that Dehaloccoides was present in higher numbers within those units, as opposed to the aquifer itself. CHUCK NEWELL: OK, so Land of the Rising Sun is the land of the rising Dehaloccoides, is that right? DAVE ADAMSON: Sure. CHUCK NEWELL: OK. How about Florence, South Carolina, another great study. What's there? DAVE ADAMSON: Yeah, and this was a study done by University of Guelph, and they were, again, looking at sort of these bio-markers for dechlorination, and they found that there's definitely elevated concentrations of these things within the clay aquitard itself. They combine that microbial data, then, with isotopic data that backed it up and showed strong fractionation then within these clay units. So strong evidence that degradation was actually occurring in the Low-k zones. CHUCK NEWELL: Big thumbs up there. Even though that's not very far away from your Jacksonville site, which is sort of a thumbs down. So there's a lot of complexity going in here, and we're learning more in terms of all this, so. So we talked about how this degradation can be important. Let's go to the SERDP type site study, and just sort of-- we sort of touched on this in some of the previous lectures. But go to some modeling by Steve Chapman and Beth Parker using their really detailed model called FRACKTRAN, and in a fractured rock, site see what these plumes look like, with and without degradation that's occurring in the fractures in the matrix. And this is without any bio-degradation itself, and then we quickly go to with bio-degradation. DAVE ADAMSON: Yeah, quite a bit different picture. I mean you've got a lot less mass going on in there. The plume hasn't expanded nearly as long, even with a relatively long half life, in this case. CHUCK NEWELL: So I'm going to go back. Here's the no bio-degradation case, looking at how big that that one. Here's this one. We'll just flip back and forth for a while. And you can really see it's got this big impact on here. What half life did they use up there? DAVE ADAMSON: This a 10 year half life in this case. So, pretty reasonable half life. We're not assuming that it's really rapid degradation in these cases. CHUCK NEWELL: Got it. OK, well let's wrap up. I guess our key points, that this whole idea of this attenuation in these low-permeability zones, it's a critical, relatively new MNA process, s dealing at chlorinated solvent sites. DAVE ADAMSON: And then, as most MNA studies, we're dealing-- we're trying to collect various lines of evidence to demonstrate whether this is a relevant process. So in this case, looking for bugs, analyzing isotopes, looking at daughter products. CHUCK NEWELL: But there's only been a few studies completed to date. There's much to learn. DAVE ADAMSON: And based on some of the modeling data that's out there, if you even assume fairly conservative half lives, it could have a potentially huge impact on plume longevity.
