PEDRO J. ALVAREZ: The past few videos focused on the principles of biodegradation because biodegradation is the process that in many ways serves as the basis for monitored natural attenuation. From a regulatory perspective, if you can show that biodegradation is occurring at an acceptable rate, then this provides strong evidence that monitored natural attenuation might be a feasible remedy. So we have relied heavily on biodegradation as an important and powerful process for natural attenuation. DAVE ADAMSON: That's right. And while I think many practitioners in our field have recognized the importance of biodegradation, there's a lot of new research coming out that highlights other ways that biodegradation can contribute to natural attenuation. These are really data-driven studies that help demonstrate just how much transformation power is associated with some of these under-appreciated processes. In this video, there are two main processes that I'll describe. The first is sustained treatment, and the second is reaction to interface. So let's look at the second one of these first. It turns out that in the subsurface, there are lots of locations that can be considered interfaces. And these interfaces are really where the action is. So the figure at the right is a 2D layout of a plume. And it illustrates a few of the places where different types of interfaces may be relevant as the contaminants move downgradient from the source area. So a few of the more common examples are listed here. And most of these should be relatively familiar as I go through them. So the first is the vadose zone and saturated zone. So this is a transition from a zone in the vadose zone where you might have bioavailability limitations and not very much moisture. And as a consequence, not much biodegradation might occur. And then you transition down into a saturated zone, where biodegradation might be less limited by these factors. But different redox conditions might exist due to less influence from infiltration where you might have a lot of dissolved oxygen. So another interface occurs, then, where groundwater is discharging to surface water. This is the hyporheic zone. And so this is a really dynamic zone where you can get a big influx of nutrients or oxygen based on seasonal stream fluctuations that can really help drive biodegradation. A similar type of interface occurs between surface water and the underlying sediments, which are generally fairly thin zones. But these can harbor significant activity and greatly reduce contaminant flux to the surface water. This interface isn't really considered all that often when we're talking about evaluating MNA because it's not specifically groundwater-related. But it could be very important for reducing risk to receptors. I already mentioned that changes in redox conditions are important in the hyporheic zone. But these redox interfaces can be important at lots of other locations in the plume. In these cases, you may see a rapid change in the contaminant concentration or the types of daughter products because another biodegradation process becomes relevant. And finally, biogeochemical gradients may be created at the interfaces between different geologic zones. Think of the sand in this aquifer in contact with thin lenses of silts and clays that have iron that can help drive both biotic and abiotic processes, or even higher organic content in the silts that may support biological growth. So I've given you a hint of why these interfaces are important. But the basic message is that big changes can occur across short distances, and that these changes can really drive attenuation. The cartoon on the right illustrates this pretty well. This is from one of a series of studies by Jim Spain's research group at Georgia Tech. And they were looking at chlorobenzene and nitrobenzene in the capillary fringe, which basically serves as the transition between the vadose and the saturated zone. They found that a large amount of degraders grew right at this interface, where both oxygen, which serving as the electron acceptor, and the chlorinated benzenes, which were serving as the electron donor in this case, were both limiting. So the influx of both of these were balanced. And you resulted in a pretty narrow reactive zone on the scale of millimeters that prevent any contaminant from migrating into the unsaturated zone. So let me just go over a couple more relevant examples. The first is from a USGS study that was looking at MTBE and other oxygenates in the hyporheic zone. This was by Landmeyer et al., 2010. And the quote here is "MTBE, TBA, and TAME concentrations in groundwater discharge in a five-foot-thick section of the hyporheic zone were attenuated between 34% and 95% in contrast to immeasurable attenuation in the shallow aquifer during containment transport between 0.1 and 1.5 miles." So a very distinct change within a very short distance relative to plume transport was occurring in that hyporheic zone. And then let me follow up with another vadose zone example. So again, this is from Jim Spain's research group at Georgia Tech. And they were looking, again, at the anaerobic/aerobic interface as you transition between the vadose zone and the saturated zone. And at this interface, they found a remarkable capacity to degrade chlorobenzene and the quote here is 2,000 to 4,200 milligrams per meter squared per day for both chlorobenzene and nitrobenzene, and basically eliminated the flux of these contaminants into the vadose zone. Natural attenuation can be an important option to consider as a followup to active remediation operations-- those that stop short of meeting cleanup standards. So it's important to give it a chance before you're embarking on more aggressive types of treatment. I'd like to show you a survey of dozens of sites that have enough data to evaluate natural attenuation as a post-treatment strategy. And we call this process sustained treatment. So what is sustained treatment? Well, the definition for this process is basically enhanced or maintained attenuation capacity within the treatment zone after the end of active treatment-- in other words, after the injections have stopped for, for example, enhanced biodegradation as the remedy. And envisioning what could be responsible for these sort of sustained treatment, there's sort of three different processes that you might imagine happening at an EAB site. And when I say EAB, I mean enhanced anaerobic bioremediation. So these fall in three different categories, the first being sort of processes driven by endogenous decay. So the top part of this panel is showing basically before enhanced bioremediation, where you may have some bacteria and a little bit of organic carbon, shown here as Foc. And then after bioremediation, after injecting a large amount of substrate and growing up a lot of organisms, you have a much, much higher amount of biomass. And that biomass then over time is subject to slow decay due to endogenous processes. And that can serve as a source of electrons over a long, multi-year time frame to support further degradation. The second process in sustained treatment would be based on the activity of reactive minerals. So in this case, before enhanced bioremediation, you might see some reduced minerals-- iron in the plus 2 form-- that are basically reactive minerals. And then afterwards, you've sort of charged these mineral species up so that you really increase the proportion of those that are in this reduced active form. And then finally, a third process that would fall under sustained treatment is based on electron donor diffusion. So before enhanced bioremediation in this case, you'd add electron donor and be primarily present in that sand. And not very much is present into the clay. But as you add more and more electron donor, you promote a diffusion-based process where you have a high concentration in the sand. It's driving through diffusion that electron donor into the clay such that after bioremediation, you now have a reverse diffusion gradient such that that electron donor that made its way into the clay can then slowly diffuse out into the sand and hopefully sustain additional biodegradation activity over a longer period of time. So one of the things that we wanted to try to look at is what extent does sustained treatment occur at field sites? And I'm going to go over some results from a ESTCP project. This is a federally-funded project to evaluate the performance of in situ treatment technologies at a large number of sites. And this is a 2015 project, so recently completed project. As part of this, we're doing a big data study of regulatory reports, of databases, and things where we could gather information on how these types of technologies performed. And you're doing this through gathering groundwater concentrations within the treatment zone over long periods of time and evaluating performance. As part of this examination of sustained treatment, we looked at that subset of sites, then, that have a long enough period of post-treatment monitoring data to evaluate if sustained treatment is actually happening at these sites. And that was the case at 34 of these sites, where you had three to 12 years of monitoring data after the end of treatment. So let's take a look at some of the results that came out of this study. So these are 34 different sites that I'm going to show you the performance for. And these are data that are showing what's happening in the post-monitoring period based on the order of magnitude reduction in concentration. So there's two bars that are going to represent either site. These are side-by-side bars. And on the x-axis, we're going to show the data individually for these 34 sites. And the left bar for this first site is showing the concentration reduction that was seen from before treatment to the first year after the end of active treatment. And the bar to the right of that one, then, is showing the concentration reduction that occurred from before treatment to at the very last year of the concentration record. So the idea here is that sites that are undergoing sustained treatment are showing evidence of sustained treatment. You'd see a continued decrease in concentration during that longer term period after the end of active treatment. So you'd see these bars continue to go down, or in the better direction, in these cases. So that's an example of what happened here at this first site that's shown up here on this chart. So let's show some of the remainder of the data. We basically saw a large number of these sites that fell into this category of where you saw continued concentration reduction after the end of the treatment period. So these are evidence of sustained treatment. He then saw a smaller subset of sites where basically there was no change in concentration. So these are stable situations where basically there was no change in the concentration from the end of treatment to the end of the concentration record. And then finally, there were a small subset of sites where you actually saw a decreased performance, or a worse performance, such that at the end of the concentration record that the concentration was actually slightly higher than it was at the end of active treatment. So in this case, the two on the left, the blue and the green bars, are indicators to sustained treatment, while the ones on the right are indicators that concentration did rebound and that there was no evidence of sustained treatment. So that was on a site-by-site basis. Let's take a look, then, sort of looking at this again in a different way. And we're going to use Mann-Kendall trend analysis. So Mann-Kendall is a very popular nonparametric statistical test for demonstrating whether there was an actual statistically significant trend with, in this case, concentration data during the post-treatment period. So we're going to look at those same sites. This is 34 sites. And based on Mann-Kendall, we saw that 89% of those 34 sites fell into a decreasing, a probably decreasing, or a stable category. So that's 89% of the sites that did demonstrate the sustained treatment during the extended monitoring record. And a much smaller subset, 11 of these, showed an increasing trend. So that's on a site-by-site basis. We're looking at each of those sites the number of wells that were in there. And we did the similar breakdown on this and we got similar numbers. So we're looking at 83% of the wells falling into that sustained treatment category, while only 17% of the wells indicated an increasing trend where there may have been a concentration rebound. So the key point in this case is that sustained treatment was indicated at the majority of the sites and the wells that were included in this performance study. And that can be very important in terms of managing expectations at these sites and mitigating the chance that rebound, concentration rebound, would occur. So the key points in this lecture, looking at both interfaces and sustained treatment-- interfaces are common to environmental sites and have increasingly been established as locations where significant attenuation is occurring. A sustained treatment is defined as enhanced or maintained attenuation capacity within the treatment zone even after the end of active treatment. And sustained treatment may last for several years at enhanced bioremediation sites and help prevent concentration rebound.
