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.
Application: Does Abiotic Degradation Explain Attenuation Observed at Field Site?
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來自 Rice University 的課程
Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
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Abiotic Degradation Principles
In this series of lectures, we will focus on the principles of abiotic degradation and discuss how these processes support monitored natural attenuation. You will be learning about the key reactions and minerals that are involved in abiotic degradation. We will also be covering what methods are used to determine if abiotic degradation is relevant at a particular site, and the rates at which these reactions may be occurring
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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.
CHUCK NEWELL: This is the last lecture of the section abiotic degradation.
And I know we're going to see an application of these principles,
but what have you got for us today, Dave?
DAVE ADAMSON: Well, we're going to see how
you can determine how much of a reactive mineral is present at your site
and then use that information to see if it explains the containment degradation
rate that you're actually seeing.
CHUCK NEWELL: Cool.
Well, I'm pretty familiar with that approach.
This is something that John Wilson, longtime EPA
researcher now at Scissortail Environmental develops, right?
CHUCK NEWELL: Yeah, with help from others.
But I think it's valuable approach in terms
of supporting M&A, but also showing how these lines of evidence
about degradation can work together.
So to start off, let's take a look at sort of another hypothetical site.
So in this case, consider it as a TCE emanating from some source
there, and forming a down gradient plume.
And in this case, no degradation products detected or very minor
amounts.
They've got several years of groundwater data maybe to evaluate.
And the plume in this case, it appears to be stable and maybe even shrinking.
And you've got that data available in order
to actually estimate a rate coefficient associated with this.
So first order rate coefficient of 0.25 per year.
A half life in this case of 2.7 years.
CHUCK NEWELL: So this is the first signal
is you don't see any of these biological degradation products.
No cis-DC or very little cis-DC out there.
But you have the significant degradation of this plume
going from the source area to the beginning
of the plume, the front of the plume.
And you can do that concentration versus distance rate constant.
And you get this half life 2.7 years.
DAVE ADAMSON: And so from a regulatory standpoint,
you'd have to convince them that is this enough to support M&A
as is the long term remedy.
And I think at a minimum in this case, you'd
have to collect enough lines of evidence to support a abiotic degradation
to make it a good case.
CHUCK NEWELL: And so one other idea-- this is an M&A.
You just can't say I shrink, therefore I am.
This first line of evidence is important,
but you need to know the process.
And we'll tell you how to sort of do the diagnosis
to say this is an abiotic reaction that's working on that plume.
DAVE ADAMSON: And you've a lot of different options.
And some of this is part of your standard data collection thing.
You'd first want to confirm that your geochemical conditions are
consistent with abiotic degradation.
You also could establish if the presence of reactive minerals were there,
and check those against maybe published rates that you'd see.
And then again, you could actually go ahead
and complete your own lab studies.
Try to get a rate information out of it that.
CHUCK NEWELL: Maybe break these down.
We'll do them one at a time.
DAVE ADAMSON: Well the first one again, it's
establishing whether those geochemical conditions are
favorable for abiotic degradation.
Generally talking about anaerobic conditions here.
And these are fairly easy data to get.
Part of most standard protocols for establishing M&A.
CHUCK NEWELL: All monitoring well data, right?
Just taking groundwater samples, and looking
for the nitrate, the sulfate depletion, methane, dissolved oxygen,
things like that.
DAVE ADAMSON: Exactly.
Let's skip over to number 3 then.
So these are lab-based studies.
You go out and get some material from the site
and set up some reactors in order to understand the rates.
CHUCK NEWELL: And those kill controls can
be real valuable to say, yeah, it's degrading even though there are
no bacteria involved in the reaction.
But this requires time and some cost to set these things up.
And the question at the end of the day, will it be convincing?
DAVE ADAMSON: And then there's this second one.
This is the one we're going to spend a little bit about time today.
So this is a fairly new approach, but basically you're
trying to establish the amount of that reactive mineral that's present,
and then use existing correlations in order
to understand if that explains the rate of degradation
that you're seeing at your site.
CHUCK NEWELL: So it's some pretty new hot off the press
stuff here we're showing you.
And we'll show how it works.
DAVE ADAMSON: But the basic approach isn't really that complicated.
Essentially, you're going out to the site,
collecting soil samples from that site, either doing in the field
or probably going back to a lab and measuring the magnetic susceptibility
within that sample.
So it's this magnetic signal that can be related to the magnet type
concentration within there.
And then you're using that basically to compare that measurement, which
is shown on the x-axis of this particular graph versus established
rate coefficients for these types of reactions.
So existing correlations, published data that
will tell you what the rate is, relative to the amount
of magnetic susceptibility that you see.
CHUCK NEWELL: OK, why the focus on magnetite?
It's not one of the most reactive minerals.
The iron sulfite is more reactive.
Why magnetite?
DAVE ADAMSON: Well again, it's something that's pretty prevalent
and it's easy to measure.
We've got these standard methods that people without all that much training
could do it themselves, or a lot of labs that will do this sort of thing for you
as well.
So it's something that we've got correlations built for that we can use.
So that's sort of the rationale for why magnetite.
CHUCK NEWELL: OK, well, let's go to the field.
What's it look like?
DAVE ADAMSON: OK, well let me just again, highlight the key point on here.
This magnetic susceptibility can be used to estimate the abiotic degradation
rate.
So that's what we're going to try to do here.
There's a lot more detail on this.
This is if you look up the ESTCP report that's highlighted here,
as well as the biopic tool for more information.
CHUCK NEWELL: Now we go to the field?
DAVE ADAMSON: Now we go to the field.
So again like I said, fairly standard methods.
You can collect these.
Get a geoprobe out there, collect some soil cores.
And you don't really need that much in terms of the amount of material.
20 to 200 grams, basically.
Not any special preservative requirements.
And holding time is not really a big issue on these things as well.
You will see some decrease in magnetic susceptibility over time,
but you can actually do this in archive samples with some success too.
So once you've gotten that material, you're
then again, relying on these magnetic susceptibility meters.
This is something manufactured by Bartington
that you can get right off the shelf.
Same thing is used at commercial labs.
Pretty cheap.
We're talking about less than $100 a sample for most cases.
And it's a pro bass instrument again, that you
can teach just about anybody to use.
You're collecting data within minutes.
It's not like you have to wait a long time for a GC to run,
or something like that.
And again, there's fuel portable options for this.
So if you were to send this off to a commercial lab for example,
you might get this lab report back from them
where they summarize the amount of magnetic susceptibility
that's seen in these various samples.
And so this is an example of some of that data
that we got out from Hilaire Force Base.
And then you get some sort of representative measurement.
So in this case, we're using a geomean of all those measurements.
And we get an answer that says it's 2.6 times 10 to the minus 7 meters cube
per kilogram magnetic susceptibility.
CHUCK NEWELL: OK, so let me just get this straight.
You can either take the sample, send it to a commercial lab.
They'll charge you $100.
They'll give you the number.
Or you can buy the gizmo, and your field team go up
and do these measurements in the field.
What do we do with hill?
DAVE ADAMSON: This was sent off to a commercial lab.
CHUCK NEWELL: Got it.
DAVE ADAMSON: And then your next step in this process
is basically compare the existing data.
So these are again, correlations that have already
been set up with existing rate coefficients that
have been found at these sites based on the amount of magnetic susceptibility
that they saw in the samples.
So this blue area in there sort of shows the area of fairly high confidence.
The area that you might be able to work in,
in terms of developing a correlation at your site.
CHUCK NEWELL: So some pretty new information out here.
Looks like there's quite a bit of uncertainty in some ways of this point.
But what's the thinking behind sort of applying this type of relationship?
DAVE ADAMSON: We are talking about a fairly limited number of data points
at this point.
But the idea behind these is that more people are going to be measuring this,
and so we sort of keep populating this data, this database,
and get more points in there so we can strengthen up this correlation,
and maybe tighten up the blue area little bit.
But let's take a look at maybe the data that we had collected in here.
And so there is our measurement of magnetic susceptibility
and so I've drawn a dotted line there that goes up from the x-axis that
shows where that would fall.
And then remember we said that our rate coefficient that we
established at this hypothetical site was it was 0.25 per year.
So that's moving from the y-axis horizontally.
It falls within that particular area.
And obviously then there's a sort of an intersection point.
And so that would suggest that at this site
that this correlation that we're doing falls within that blue shaded area.
So it falls within sort of what you would
expect to see based on existing correlations
that abiotic degradation by magnetite in this case,
would be responsible for what we're seeing in terms of our rate coefficient
for there.
So no bacteria, but the minerals themselves, or the magnetite
itself is sort of giving you this rate.
What is that, what is that, about a three year half life?
Something like that.
And that's contributing this degradation.
And that's one of these it processes that's controlling that plume.
So this might be a very, valuable line of evidence
but it's also good to sort of explore what other things you
might have seen, for example, you might have data that
would show that other mechanisms besides abiotic pathways
were more responsible for that.
So if you fell up the way in this part of the plot, where you've
got a fairly high rate coefficient, but maybe
not that high magnetic susceptibility.
Probably not abiotic degradation.
CHUCK NEWELL: More likely biodeg.
Biodegradation.
DAVE ADAMSON: You also might find yourself maybe down
in this part of the graph.
Sort of lower where you've got a lot of magnetic susceptibility,
but not a lot of degradation being shown.
And there could be a lot of reasons why this might happen.
One case, you might not actually be estimating your rate coefficient
all that well.
So let's summarize some of the key points from this lecture.
Magnetite in this case, was something we looked at closely.
And it's an example of a reactive mineral species
that you can actually use, measure its abundance in field samples.
CHUCK NEWELL: Now there's this whole emerging idea
these abiotic degradation rates for chlorinated solvents they correlate well
with this magnetic susceptibility.
Go out there, take those soil samples and get that susceptibility number.
DAVE ADAMSON: And once you've got that data,
you can use that in terms of existing correlations
to sort of determine whether the amount of degradation that you're
seeing at your particular site is actually
consistent with abiotic degradation pathway.
