DAVE ADAMSON: I'd like to close out this week's discussion of the principles of biodegradation by looking at a particular way to apply these principles, specifically using a mass balance approach. I think most folks in our field are familiar with the mass balance concept, particularly if they're chemical engineers that somehow found their way into the environmental field. These same mass balance principles can be used when you're dealing with contamination in the subsurface. There's lots of different ways to apply them, a few of which we'll talk about in this video. You just have to recognize that the amount of degradation that occurs is a function of a lot of different factors, including thermodynamics and kinetics of various reactions, as well as the presence, availability of various reactants, all of which could be a function of site hydrogeology. So first, let's take a look at biodegradation reactions from the electron acceptor perspective. Consider this maybe a one slide refresher on material that Pedro covered earlier during this week. And remember, when we're talking about electron acceptors, some may be directly involved in containment and attenuation, and others may be competing. So I'm going to put up a chart here that basically shows the types of electron acceptors, the reactions that they undergo, the byproducts, as well as the redox potentials that they work under, as well as the order of the reactions. So again, we're dealing with oxygen, nitrate, ferric iron reduction, sulfate reduction, and then methanogenesis, the formation of methane using carbon dioxide as the electron acceptor. So again, these are reactions where you're trying to look at what competition you might have from these particular electron acceptors based on the geochemical environment that you're present in. In these cases, when you're worried about that or worried about tracking these in a mass balance approach, there's certain things which you're measuring the depletion of. So in this case, the circled ones are oxygen, nitrate, and sulfate. And there's other reactions in which case it's easier to measure the byproduct. So for example, iron reduction, you may measure the amount of dissolved iron that's being formed. Or in methanogenesis, you may actually be measuring the amount of methane that's being formed. So this leads us to the term biodegradation capacity. This is basically looking at stoichiometry, and then using the knowledge we gain from this, in terms of electron accepting processes, to determine the amount of contamination that an aquifer can assimilate or degrade. We refer to this, then, as biodegradation capacity. And we express it in terms of milligrams per liter or on a flux basis, milligrams per year. So this is an important concept for natural attenuation reactions, where the presence of electron acceptors are basically promoting the reaction that we're interested in. In this case, an example would be the oxidation of petroleum hydrocarbons. So let's look at an example with benzene. So the benzene stoichiometry under aerobic conditions is shown here. C6H6 benzene is degraded. One mole of that is degraded by 7.5 moles of oxygen. In this case, we're looking at mineralization all the way to CO2, in which case 6 moles of CO2 are formed. So the basic stoichiometry of this is shown here. And we can calculate this on a mass basis of saying that we need about 3.08 grams of oxygen consumed per gram of benzene being mineralized. In this case, we're making the assumption that we're ignoring biomass formation. In reality, we might not need this much oxygen. But for the sake of this example, we're showing it as the full mineralization reaction here. And this is for benzene. And you can consider that benzene is often present with other BTEX compounds. So if you are to represent a sort of average BTEX based on these stoichiometries, a good ratio to use would be something like 3.14 grams of oxygen consumed per gram of BTEX mineralized. So this is a pretty important ratio that we can use when we're looking at our plume dynamics. So in this case, if you measure dissolved oxygen concentration up gradient of a BTEX contaminated zone, you can calculate the potential aerobic biodegradation capacity for BTEX. So that's shown in this little cartoon, where you've got 7 milligrams per liter of DO upgradient. And that would basically mean that within that contaminated zone, you'd have the capacity to degrade 2.2 grams of BTEX. We refer to this sort of as utilization factors. So a BTEX utilization factor is that 3.14 grams of oxygen per gram of BTEX being degraded. And we'll show a chart on the next slide of utilization factors for other electron accepting conditions. But we can use this for our benefit of also then saying how much of that biodegradation capacity is being expressed? Or how much degradation is actually occurring? So this can account for, then, changes across a contaminated zone, as shown in this little cartoon, where we show upgradient within the contaminated zone and downgradient. We have a change of dissolved oxygen from seven milligrams per liter to two milligrams per liter, meaning that within that contaminated zone, we have an expressed biodegradation capacity for BTEX of 1.6 milligrams per liter. So let's further explore this concept using some data collected at Kessler Air Force Base. So we're showing concentrations of various electron acceptors basically background, and then downgrading of the source. So this is dissolved oxygen, nitrate, iron, sulfate, and methane. And you see the relative concentrations at each of those locations. We can then use utilization factors. So these, again, are stoichiometrically derived ratios of the amount of this electron acceptor being used relative to, for example, BTEX in this case. So again, that 3.14 for dissolved oxygen is that same ratio that we calculated on an earlier slide. So we can use that as a multiplier in these cases to understand how much BTEX degradation capacity there was. So it's fairly simple, straightforward math in these cases. But we get, for dissolved oxygen, for example, a biodegradation capacity of 0.5 and so on. As we look at the other electron accepting processes, we actually see that sulfate and methanogenesis are much more important in terms of the amount of BTEX that's being degraded. And the total amount of BTEX that would be degraded in this case is basically the sum of all electron accepting processes. So 16.7 milligrams per liter as BTEX. So those are petroleum hydrocarbons. Chlorinated solvents we have to handle a little bit differently. And there's a lot of different ways that we can use mass balances in our favor. Again, these are electron acceptors typically. So our approach here is basically using biodegradation capacity expressed in terms of the concentration of daughters that are generated in the source zone. What do we mean by daughters? Well, PCE is an example here. We're measuring the amount of TCE, DCE, vinyl chloride, and ethene that might be formed as a result of that expressed biodegradation capacity, the difference in the upgradient and the downgradient. But there's a couple other things we need to then consider. We need to also think about the changes in concentration between the upgradient and the source for electron donors-- so for example, hydrogen being the primary electron donor for these processes-- as well as the change in concentration between upgradient and source for competing electron acceptors, those things that might not be as efficient for driving reductive dechlorination. And the way that we typically handle these things, then, this is looking at the amount of hydrogen equivalents. So we're looking at the electron donor in a mass or concentration basis. So think if you have one milligram per liter of organic carbon, a general substrate, that that's going to generate somewhere on the order of 0.1 to 0.4 milligrams per liter of hydrogen, depending on what you're dealing with, via fermentation process. So that hydrogen can do a couple of different things. And the thing we want it to do is dechlorinate our contaminant. So in this case, one milligram per liter of hydrogen can dechlorinate 21 milligrams per liter of PCE to ethene. So that's a good thing. That's what we're trying to get that electron donor to do. But that's not the only thing that's happening. We've also got all those other electron acceptors that might be present to rob that hydrogen. So for each milligram per liter of hydrogen that we have available, it's going to be consumed by 0.125 milligrams per liter of oxygen that might be present. And other electron acceptors that might be present would represent additional things. So in this case, this is not the reaction that we want to see happening in terms of the available hydrogen that we have to do the reaction that we do want to have, reductive dechlorination. So we've put some sort of useful ratios here that you can use as a reference material in terms of various electron donors and the amount of hydrogen that they produce, the various electron acceptors and the amount of hydrogen that they might consume, and then looking at the various contaminants, like PCE and TCE, and the individual reductive dechlorination steps, the amount of hydrogen equivalence that would be required in these cases to drive those reactions. So you can think of these as reference materials and sort of use them, along with stoichiometry, in order to perform mass balance type calculations. So we'll look at an example here. We're going to look at this in terms of electron acceptor consumed on a hydrogen equivalent basis. So you can imagine that if you had dissolved oxygen, nitrate, and sulfate present within this contaminated site, and you're calculating the amount of electron acceptor consumed and using the ratios that were shown on the previous page, you can convert all of these mass per year basis to a hydrogen equivalent basis. And again, calculating things on a mass per year basis is a very easy calculation if you know the amount of groundwater flow that you're dealing with. Converting things from a concentration to a mass basis is not difficult. So in this case, if these were the competing electron acceptors, you're talking about a hydrogen equivalent of about 10.8 kilograms per year. Now we'll look at the reaction we do want to promote, reductive dechlorination. And we'll look at the daughter products formed on the same kilogram per year basis, and then converting those to hydrogen equivalents. So in this case, if you've got the formation from PCE of these levels of TCE, DCE, vinyl chloride, and ethene, and convert those to hydrogen equivalents, you see that you've got a 3.6 kilogram per year hydrogen equivalent, which is lower than the hydrogen equivalent of the competing electron acceptors. But still, a relatively significant pool of those electrons are going to reductive dechlorination. And if you further consider the amount of organic carbon it would take to drive this reaction, we're talking on an order of 110 kilograms per year to basically sustain this current level of activity. So the key points in this case are listed here. But basically, think of mass balances as a way to keep track of contaminant degradation, both in terms of electron donors and electron acceptors. Calculations for hydrocarbons rely primarily on the biodegradation capacity concept-- how much electron acceptor is needed. And mass balance calculations for chlorinated solvents, which tend to be the electron acceptors in these cases, often rely on a conversion to hydrogen equivalents, which allow you to calculate how much electron donor is needed to drive the reaction.
