Cleaning up the large number of groundwater contamination sites is a significant and complex environmental challenge. The environmental industry is continuously looking for remediation methods that are both effective and cost-efficient. Over the past 10 years there have been amazing, important developments in our understanding of key attenuation processes and technologies for evaluating natural attenuation processes, and a changing institutional perspective on when and where Monitored Natural Attenuation (MNA) may be applied. Despite these advances, restoring groundwater contaminated by anthropogenic sources to allow for unrestricted use continues to be a challenge. Because of a complex mix of physical, chemical, and biological constraints associated with active in-situ cleanup technologies, there has been a long standing focus on understanding natural processes that attenuate groundwater contaminant plumes. We will build upon basic environmental science and environmental engineering principles to discover how to best implement MNA as a viable treatment for groundwater contamination plumes. Additionally we will delve into the history behind and make predictions about the new directions for this technology. Any professional working in the environmental remediation industry will benefit from this in-depth study of MNA. We will use lectures, readings, and computational exercises to enhance our understanding and implementation of MNA. At the completion of this course, students will have updated understanding and practical tools that can be applied to all possible MNA sites.
Reactions at Interfaces and Sustained Treatment: Emerging Concepts
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来自 Rice University 的课程
Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
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从本节课中
Biodegradation Principles
In this series of lectures, we will learn when biodegradation can or cannot occur and what conditions are conducive to faster degradation.
与讲师见面
Pedro AlvarezGeorge R. Brown Professor of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
Charles NewellVice President
GSI Environmental Inc.
David AdamsonSenior Environmental Engineer
GSI Environmental Inc.
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.
