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.
Applicability of MNA for Different Contaminant Classes
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From the course by Rice University
Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
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Rice University
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From the lesson
New Directions for MNA
This final series of lectures will cover a mélange of interesting MNA topicsl We start with the brand new ESTCP BIOPIC tool, then talk about the broad universe of MNA that extends to a lot of different contaminant classes. Then some new developments in the LNAPL world: Natural Source Zone Depletion, both with conventional methods and a new method based on thermal monitoring. Then we talk a little bit about low threat closures and transition assessments, and end up with a brief conversation with the three instructors. And then you are done!
Meet the Instructors
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.
Okay, well today we're going to break free of our chains,
our chlorinated solvent decay chains, that is.
And discuss monitored natural attenuation applied to other contaminants besides
these chlorinated solvents and fuels.
So Dave, we covered this in this ESTCP Frequently Asked Questions About MNA
document that we recently published, right?
>> Yeah, this was actually Frequently Asked Question number 8.
>> Okay, well let's go to that FAQ number 8 here, and
the actual question is, can I apply MNA to metals, inorganics, and radionuclides?
So there were actually,
the foundation of this is the EPA came out in 2007 with this first of three different
volumes that talks about how you do this for this basically.
And then first one is sort of the key points on how would you do
sort of apply this.
Then we got actual specific information about I think it's 19 different ones so
that are in there but Dave, how does it work?
What are the key things they want you to do?
>> Well, I mean, this approach is really similar to other contaminants that we've
already talked about and that you've got these tiered lines of evidences that
you're trying to sort of gather.
And they're listed right over here, but
basically you're looking at plume is not expanding and that sorption is occurring.
So sorption is one of the key processes that we're looking here for things like
metals then you need to establish that that's the attenuation mechanism,
estimate the rates then you're determining the capacity and sustainability of those
types of reactions and then just developing the proper monitoring.
>> Now the sustainability is not really about carbon dioxide but
it's about will that stuff stay sore?
>> Right, so is that sort of a stable process,
going to keep those things sequestered in place.
>> Okay let's go through the key points maybe their documents versus that this
primary attenuation pathway for many of these inorganics is this transformation
to less mobile forms through a co-precipitation of sorption, right?
>> Yeah, and then generally these are a little bit more complex reactions and
they're definitely influenced by the sorts of geochemical conditions that you'd see
at these sites.
>> Right, so those three volumes are sort of the science on how this works and
the reactions and how you actually do these measurements.
But just very recently 2015, EPA came out
with the directive that sort that gives some of the policy implications for this.
>> Mm-hm. >> This is one you can do it.
This is maybe one you should really look at it carefully and
things that made you so well.
So wanted to nurture that let's go in and
talk about some of the specific chemical classes, contaminant classes themselves.
So here we've got a series of contaminants,
what's on that first row there?
>> That first row, for example, we're looking at nitrate, and so
you can think of that as a typical inorganic contaminant.
And we're looking at whether it's biologically degraded where there's
abiotic reaction or aequestration, so we're trying to say which one is relevant.
So for something like Nitrate, yes degraded anaerobically,
not degraded aerobically, it is degradable abiotically with reactive iron and
there's no evidence for sequestration or something like that.
>> Great, so we got the same information here for chloride and
you have all the metals in there in that second to bottom row and
then some of this radium nuclides from the very bottom, right?
So talk about this biological reactions as anaerobic or aerobic, abiotic or
sequestration.
Now one thing that we did, I was part of the team from the Department of Energy
that took those three volumes of scientific information about MNA for
this inorganics, we tried to use this scenarios approach.
So let's look at this slide here a lot going on here.
But, on the left, we said, and this is working with Mike Turek's, from PNNL, and
Pat Brady, from the Sandia Lab, they know a lot about metals, hey,
let's break up all these metal plumes into, sort of, six main buckets,
that are dominated by these three types of parameters.
>> Mm-hm >> And then you just put three parameters,
the ORP, this oxidation reduction potential, cannot exchange capacities,
number two, and then how much iron is on the soil.
And you can see here that on the left is you say take your site, and
which one of these buckets best sort of accommodates or fits your plume?
And then once you know which bucket you're in, you can go to this
big chart on the right and then you can see the list of contaminants, Dave,
give us some examples of what's on the contaminant list.
>> Well the top one on there,
we've got trivalent chromium, hexavalent chromium below that.
>> Then we have technetium, we have plutonium, uranium, cadmium, copper,
lead, zinc, arsenic, selenium it's a whole list of things.
But then if you know which bucket you're in, you could tell, basically,
the mobility of that plume in groundwater.
What's the difference between the red and the grey boxes?
>> So the reds are for anything that would be considered high mobility based
on those characteristics, medium is a yellowish color I guess and
then that grey is sort of low mobility with the idea that
maybe the mobility in those cases is controlled by the ph, right?
>> In some cases yeah.
So you'd see different ones,
they each chart depending on the slope of those curves.
How pH sensitive it is, but let's just go,
let's say that you've got high ORP, low cation exchange capacity, high iron.
We're in scenario 5 for this chrome 6, what's the color?
>> Well, if I look over here, it's a- >> Looks like it's a red one, right?
>> Yeah, yeah.
>> So highly mobile on the other hand, arsenic, for this scenario three,
which is high ORP, high cation exchange, high iron, it's great.
Which means that as a retardation factor of maybe 1,000 or more.
So anyway, just trying to condense down all that knowledge into one, so anyway,
that's about metals and inorganics.
Let's go on to another one coming from the ESTCP document,
and let's look at frequently asked question nine, and what's that question?
>> In this case it's saying I can MNA to BTEX, but how about oxygenates?
So those fuel oxygenates that may be present as part of a fuel.
So this is a sort of interesting story, right?
>> It sort of says that early on we said no this MTBE really doesn't look like
it's degrading.
It's going to have these real long plumes but then the science started accumulating
and now the answer is yeah there is a lot of attenuation processes going on in here.
So lots of research and lab work and
field work in the following five to ten years and it sort of ended up with this,
hey we know a lot more it's different than we first thought.
So let's go through some examples of this.
This is a nexium plume maps from a paper called exceptionally long MTBE plumes.
Where these were identified five to ten years ago as being
pretty long MTBE plumes.
And, the question that McDay and
all these authors came up with is, how have they changed over time?
If you look at the left panel, here's a plume in New York, and on the left panel,
the far left, it says this is what it looked like early on and
then four years later, 2007, what happened to it?
>> You can see just the decrease in the green, the plume, and
they are estimating a plume length in here that changed between 2003 from 1,270
metres down to 530 metres in 2007.
>> So they talk about in the paper that this is a combination of sum remediation,
at this side, only source remediation, there was no plume remediation, but
also attenuation process, so both thing are going on here.
But there is this remarkable progression.
Here's another one looking on the right Hampton Bays New York 2003.
It's about 820 meters long and then here it is in 2011 it's just a lot shorter 150.
>> Yeah 81% decrease and a 99% decrease in the concentration.
>> And so just some more plumes here we're looking at.
Here's Lindenhurst, New York going from a long plume to almost all gone right there.
>> Yeah.
>> And then Riverhead, New York, sort of the same picture,
just a much smaller plume.
So it's a little unusual, we talk about matrix diffusion and
things of that nature.
But this, because of its strong tendency to degrade aerobically,
I think this really can take care of a lot of these long plumes over time.
So just this example of some of these attenuation processes.
But then there was this question,
are MTBE plumes that they're looking at now where they're expanding.
So this is a paper from Kamath et al.
Does this Mann-Kendall analysis.
And so what was the results of this question?
Are they expanding or are they shrinking?
>> Well this is benzene and MTBE, MTBA at these 30 to 40 sites,
basically, that were in this study.
And so based on this, the Mann–Kendall trend analysis was shrinking for
the majority of all of those compounds at these sites.
>> We really sort of looking at the green lines were there which are the MTBE ones
and so 73% of them were down there.
The TBAs, that's this degradation product, so there is this degradation occurring and
some of those 26% of those plumes were expanding.
But overall it just looked like these MTBE plumes were about the same a their
BTEX sort of companions.
Okay, well let's go on a little more about MTBE in torbutrol alcohol,
where do they degrade?
And so let's go through some of this.
Let's start with MTBE, does it degrade biologically?
>> Yeah, I think that there's a lot of evidence for
both aerobic and anaerobic degradation.
Anaerobic in most cases would be associated with some sort of acclimation
period to get it started.
TBA then can be a breakdown product of MTBE.
Again, this is something that can be degraded both anaerobically and
aerobically.
It's generally faster under aerobic conditions.
In anaerobic conditions it may be difficult to
have happen in strictly methanogenic conditions.
And then finally, ethanol down here,
another oxygenate that's obviously very, something that bugs a lot, so
it's degraded both under aerobic and anaerobic conditions.
And oftentimes preferentially over BTEX.
>> Okay, so we can consume it and it will degrade it, and so
can the aquifers themselves, right?
>> Sure. >> Okay,
well let's go to emerging contaminants, and
I think we had a frequently asked question about that.
Which emerging contaminants are MNA candidates and we come in there and the we
listed another maybe a Trichloropropane, NDMA, Phthalates and maybe others, right?
>> Yeah and let's remember that a lot of this emerging contaminants are something
that the people are still getting a handle on.
So there's maybe not as much data out there to establish that and so
we're getting more and more data each day in order to sort of address
whether these things are actually subject to MNA but.
I think we've put together a chart or I mean a table on these shown on here on
this next graph so just to take an example, something like 1,4-Dioxane in
this case so we do see evidence for aerobic degradation.
Not yet on the anaerobic side,
that's sort of a holy grail to see that happening but for that compound then,
abiotic degradation not documented yet and sequestration not really.
>> Not going to sorb very well, right?
>> Yeah.
>> Okay, next we talk about these perfluorinated compounds,
just not much information yet, right?
>> Yeah.
>> And then moving down the list the NDMA maybe yes is on those and
then finally the trichloropropane.
You got some yes' under the biological, but not much on the abiotic.
>> Yeah, yeah. And
so I think if there's promise in terms of some of the MNA associated with these it's
probably on the biological side in terms of the relevant processes.
>> Yeah, hey, let's go back to the perfluorinate.
I know it isn't Brice University working on a reaction with iron and sunlight?
>> Yeah, yeah, and that's actually proven to be a viable reaction.
But in this case it's using the power of sunlight in order to promote this
photocatalytic reaction.
And it's shown to be fairly effective for something like PFOA.
>> Not superfast but just something interesting that you get the light to it
which is hard to do in ground water.
>> Yeah. >> Then
perhaps there's this reaction can be used in certain applications I guess.
Okay so that's our sort of list of these things.
Let's go ahead and wrap up just some key points, the USEPA for example has detailed
guides for MNA of inorganics which includes metals and radionuclides.
>> And this is a good example sort of how our
thinking has advanced as we go through the years so MTB,
originally not thought of being all that subject to attenuation now it is.
>> And then lots of research on MNA for
these emerging contaminants, some look promising, others not so much.
A lot of research.
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