One of the things that we touched briefly on during the first few lectures on the abiotic degradation is this concept that's how do you demonstrate that it's a relevant process at your site? Yeah, this is a really important concern. I think a lot of practitioners have some or maybe even a lot of familiarity with how you show that biological reductive dechlorination is happening. But they really might not have much experience at diagnosing whether abiotic degradation is an important contributor at their site. >> Yeah, good point. And in my experience, there's lots of similarities in the type of data you collect. But there's also some unique parameters for this abiotic stuff that need to be considered. Just as well as some challenges that are inherent to demonstrating, sometimes it's tough to do, mm-hm. >> Yeah, we'll touch on some of those. I think a good way to start this discussion is to maybe take a look at some conceptual models of how you might see a chlorinated solvent plume. So the first one here is sort of this PCE-dominated source zone. And you see these byproducts of reductive dechlorination as you maybe move in higher concentrations away from that source area. All the way down to ethene, the terminal product of that reductive dechlorination. So in this case, you're seeing these reductive dechlorination products The concentrations decrease with distance and time. You've got strongly reducing conditions with measurable organic carbon. And then you maybe got some microbial data that establishes that you got these biomarkers for dechlorination activity. So this type of site, you'd think would be more attenuation due to biodegradation. >> Key point there, if you see a lot of these cDCE, it's likely being produced by a biodegradation reaction. And so it's a key marker for seeing biodegs out there. >> Yeah, so that's one of type of sight that you might be familiar with but consider this other type. Again, you've got PCE as the source and it might extend a little bit downgrading into that source. And maybe you see a little bit of TCE as well, but you don't see these footprints of these reductive dechlorination products. So no degradation products detected in this case. Concentrations again are decreasing with distance in time. You may have strongly reducing conditions because of organic carbon that's there. But microbial data in this case indicate that there's no biomarkers for dechlorination. For this type of scenario, you got pretty good evidence that attenuation is not being caused by biodegradation. That it's more likely that it's abiotic degradation that's the most relevant pathway. >> Okay, but, Dave, let's talk about cysts, dichlorethane cysts DCE. This can be degraded by these abiotic processes? >> It definitely is a potential product, but in most cases, considered a thoroughly minor, part of a minor pathway in this case. So in these type of sites, you'd be much, much less likely to see cysts as you would at a biological reductive dechlorination-dominated type thing. >> So those are the two extremes that we're seeing in these two pictures. But sometimes you got a site that's in the middle and you're not sure how much is abiotic, how much is biotic. >> Yeah, and so in this case, you definitely want to consider collecting more data. When we collect data for natural attenuation, again, we're talking about establishing these sort of lines of evidence. And those are shown here of sort of standard protocol for most MNA-type studies. And you're dealing with concentration trends and plume footprints that might support that primary line of evidence. And you may be looking at degradation products and geochemical signatures associated with the second line of evidence. So these are all sort of standard things that you're measuring as part of any MNA process. >> And the way I think about these is sort of a high level way is these lines of evidence. Line of evidence one is this confirmation of construction of mass. So I call it I shrink, therefore I am. In terms of Voltaire and that type of stuff. Line of evidence two, I think about it more, am I swampy? Are there these indications of very anaerobic conditions that are say, creating methane? So just high level is the way I think about these things. >> Okay, that works for you, that's great. When we're dealing with abiotic pathways, we sort of have to then think of these secondary and tertiary lines of evidence. And we have to maybe consider collecting data that's related to the mineral or soil components. Doing microbial tests that might have a different sort of focus than you're doing for biological natural attenuation. And then against maybe setting up microcosms in order to establish rates. >> Key thing about the microcosms, if you're looking for the abiotic brackets, they would be sterile microcosms, right? >> Yeah, so again, we touched on some of the reaction pathways. But, here's a great example of looking at TCE and we see it go through this sort of this more unique pathway where you're forming chloroacetylene and then acetylene. And so this is different than what you would see with biological reductive dechlorination. So if you see these sorts of things it would be an indication that abiotic degradation is happening. So that uniqueness in certain ways would help you distinguish between what's going on. >> But it's just easier said than done, trying to find that acetylene for certain reasons that you'll explain, right? >> Yeah, why is it easier said than done? Well, basically you're dealing with biodegradable compounds. Something like acetylene might not stick around for very long after you collect it. Again it's also highly volatile. So not only potentially being degraded, but potentially being lost when you're trying to collect it. And then you're talking about fairly low levels that are being formed. So you're dealing with method detection limits and things it might be hard to quantify. >> And what are the sort of project we're actually going to spends some time trying to find this settling using some new methods, right? >> Yeah, but the big deal here is then is you might want to try to go looking for settling but false negatives are an issue. The compound might be gone by the time the sample gets from the field and ordered the lab. So you wouldn't be able to actually distinguish that it's happening. In terms of geochemical conditions, so that second line of evidence. And this is a list of sort of what you might be looking for and which one of these be associated with a supporting biodegradation. So then generally looking at negative ORP, negative oxidation reduction potential. Elevated levels of iron, sulfate present but decreasing with distance. Elevated sulfide, methane, and organic carbon. So this is all supporting all those individual players in this biogeochemical process. >> And just a quick sort of shout out about dissolved oxygen. There's this CSDCP project. Carmel Labronze working on it, John Wilson, Todd Wiedemeier. Saying hey, be careful with that dissolved oxygen nourishment. Where it's easy to sort of get some false positives in that thing in terms of aerating it. And so there's discussion in their work about how to really apply dissolved oxygen and get those measurements in the right way. >> Yeah, and that's why you'd want to collect geochemical data on a variety of different parameters in order to sort of establish where you're sitting. >> Exactly, mm-hm. >> Okay, so those are geochemical conditions. Moving into sort of mineral analysis, there's a lot of different options that are potentially available. A lot of these are sort of on the research-oriented side, but we've listed a few of them here. Magnetic susceptibility as a surrogate for magnetite concentration. You can mineral speciation, mineral surface area. You've got these methods that are designed to measure acid volatile sulfides or chromium extractable sulfides. Or then just using microscopy. One we'll highlight here, and we'll actually talk about again in the next lecture is looking at magnetic susceptibility. So there's these fairly standard and straightforward meters that are out there in order to measure the amount of susceptibility in a particular soil sample. And then relating that to a particular concentration of those minerals. >> Two quick questions, is it a soil sample or water sample you stick in that instrument? >> Generally dealing with soil in these cases. >> Because you're looking for the minerals, right? >> Yeah. Okay and second is that a field instrument or a lab instrument? >> A lot of people use these as lab instruments, but there are field options available for doing these sort of things. So these are pretty useful relationships in order to establish some ideas of attenuation rates, which we'll talk about in the future. But the bottom line here is that there are a lot of different options available. Some might be a little bit more complicated than others, but there is ways to sort of get this information. >> What's the key reference in the field? Well if you want more information on this, and we've touched on this before, this EPA 2009 document. It goes through a lot of the methods that are associated with collecting these sorts of mineralogical data and so very good resource in terms of getting more information. So moving into microbial data, again microbes can be important components to these reactions. We're looking at it a little bit differently. In this case, this is an example of a commercial lab microbial insights that can perform these sorts of analyses. But what they're looking for in these sorts of, are biomarkers for iron reducers, the sulfate reducers, and things that are sort of involved in that biological component. Doing the reduction to get the reactive minerals actually to form. >> So these aren't the bugs that are degrading the contaminants. These are the bugs that are making the minerals? >> Right, right. Microcosm data, you can then also collect in sort of on your own lab scale. Again, setting up these little mini reactors and trying to determine what sort of things are in there, what sorts of rates that you get with that. And I, myself, have set up a lot of microcosms, and I like them, so let me talk about some of the pros in this case. Again, we're talking about fairly controlled conditions. I can do what I want to, in terms of setting these sort of reactions up. I can probably more easily measure the degradation products because I can maybe take more samples to find out a settling that might be in there. And then establishing rates for me is pretty straightforward. >> Well, Dave, that all sounds great, but let's do a point, counterpoint. Some of the downsides on this is maybe they're not that representative of what's actually happening in the field. So you have to be careful in extrapolating this lab stuff to the field. And with some of these microcosm it can be difficult to get that sterile stuff. So it's hard to separate abiotic versus the biotic. And it's often these black box experiments can be tough. And there's limited diagnostic information on how much is out there in the abundance of minerals and stuff like that. So point and counter point in terms of these microcosts. >> Sure, so other purchases are out there. There is compound specific isotope analysis and we're going to have a little section later so I don't want to deal with this too much. But there is this idea that you can use compound specific isotope with signatures within this compounds. To actually distinguish between whether a compound is undergoing a biological degradation pathway or an abiotic degradation pathway. Examples as shown in this plot, where they show different reaction pathways. It can be shown as something degrades, as you see fractionation, as you see change in these carbon and chlorine isotope signatures. That you may be able to use that to say in this case whether chemical oxidation was happening or abiotic zero-valent iron, or different hydrolysis, or dehalogenation reaction was happening. So we'll revisit this later on. >> Nate, okay. >> So just to sum up a few of our key points here, basically, primary line of evidence for abiotic degradation is the identifying unique products that might be happening from those reactions. Things like acetylene. >> But the key thing is certainly is this key marker for many of these reactions, but it's not conservative, hard to measure, hard to find out there in the field. >> There's other approaches it can use to help support these analysis. Things like the microbial analysis or the mineral analysis that can serve as secondary lines of evidence. And this lab-based microcosms can provide rate information that you'll be little careful on scaling up to the few.
