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.
Diffusion vs Dispersion
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Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
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Attenuation and Storage in Low K Zones
In these lectures, we will focus on the idea that storage of contaminants in geologic media as a potential natural attenuation mechanism. Under this scenario, certain contaminants diffuse in low permeability media such as silt, clay, and limestone and then eventually reenter the aquifer by a process called back diffusion. Like sorption, this process is not destructive, but attenuates a contaminant plume by sequestering some of the initial loading and then slowly releases it over long time scales. You will learn about diffusion vs. dispersion, see case studies in the lab and field, learn how to sample and model this process, and hear the latest research about degradation processes associated with matrix diffusion.
Conheça os instrutores
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: Today we're going to continue our talk about matrix
diffusion and give a little more historical background
about the changing role of two key contaminant transport processes.
First, dispersion sort of versus diffusion.
DAVE ADAMSON: I think the key point here is for many years
groundwater models and modelers, they sort of focused on dispersion
as the key process that really controls and drives how plumes develop.
Now we're going to talk more about this emerging view,
a real shift in our thinking, that says that we
can explain contaminant transport a lot better by focusing on dispersion
processes as opposed to-- sorry, diffusion processes
rather than dispersion.
CHUCK NEWELL: Exactly, Dave.
And so this is sort of this new idea.
It's sort of catching on, but hasn't.
It is not completely accepted right now.
But let's start with what we call this process of mechanical dispersion.
And it's the idea that if you have this porous media,
these sand grains that are out there, that there are different flow
velocity's going through these different channels as you see in this image here.
There are different paths that these different contaminants or solute
takes as they sort of go through here, sort of like a pachinko
system, these little steel ball bearings are going down.
DAVE ADAMSON: Price is right reference.
CHUCK NEWELL: Exactly.
And then all that causes this plume to spread,
and this idea of matrix diffusion.
The key points that we make in this is that it's
the grain size scale, these different paths that make these plumes spread.
But there's no real theoretical basis for this large scale dispersion.
In some ways it's been used as a fudge factor for computer modeling.
DAVE ADAMSON: OK, well that's dispersion,
maybe then we take a look at diffusion.
So diffusion is a different process, we're
describing the spread of particles to random motion
from regions of higher concentration to lower concentration.
In this case, diffusion is one of those things that's
been studied by a lot of real scientific giants, some of those
are mentioned here, you have Fourier and Fick,
we've got a nice picture here of Fick, and Einstein, and Smoluchowski as well.
CHUCK NEWELL: I don't know if you remember, we were in Denmark,
and sort talking about matrix diffusion and we
said that, well Einstein, who was the second greatest physicist in the world
was involved in this diffusion process.
DAVE ADAMSON: And then I think the Danes,
they said, well Chuck, who is the greatest physicist then,
involved in this field?
CHUCK NEWELL: And I said, well, of course,
the greatest physicist in the world is Niels Bohr, the great Danish physicist.
And we got project, right?
DAVE ADAMSON: Yeah, exactly.
It worked out.
CHUCK NEWELL: So let's talk about Fick's law a bit more, here's this picture.
What's going on with the picture here?
DAVE ADAMSON: Yeah, again, we're talking about the situation
where things are moving in this case from left to right,
from high concentration they're moving into a lower concentration
and there's an example listed here.
CHUCK NEWELL: I started talking about the Motel 6 example,
that if you go into a motel room, people have been smoking a lot of cigars
up there.
Maybe that air has completely been exchanged,
but you can still smell that smoke from those cigars or cigarettes.
It's diffused into the curtains, it's diffused into the carpet,
and it's coming back out.
And so that's one of these ideas on diffusion.
DAVE ADAMSON: OK.
CHUCK NEWELL: But how do you describe this mathematically, Dave?
DAVE ADAMSON: Well yeah, we're usually talking then
in terms of some sort of a diffusive flux.
So that's in this equation, this J term, and that's
going to be equal to a diffusion coefficient, something that's
a compound specific type term that describes
that rate of flow via diffusion.
And then you've got this change in concentration
over change in distance terms, that's your concentration gradient
term, dC over dx.
CHUCK NEWELL: Great, OK.
Well now let's go to a more groundwater specific example.
Here's a great paper from 1985 done by Sudicki and colleagues
at the University of Waterloo.
So what we've got is a sand tank here, you
can see that the sand is that white bar on the top left panel, sort
of a silt on either side.
And they ran this thing, they ran solutes through this sand.
And they talked about the effects of this redistribution, of their tracer,
across the strata by transverse molecular diffusion.
So here's what happened when they ran it.
Dave, what's on the axes of this graph?
DAVE ADAMSON: All right, well we're looking
at on the y-axis, the relative concentration, so think about 100%
is at the top.
And then we're simulating days, so time, in this case, on the x-axis.
CHUCK NEWELL: And so the red line, here, is
what they said if you're doing advection dispersion by itself,
that thing sort of gets through real quick, it breaks through real early.
And then you would see everything happen within basically eight days
to flow across there.
But what really happened was this blue line
and it just took longer to get there because some of this contaminant's was
diffusing up and below and you have this sort of much more muted response
that you're looking at.
In some ways like this capacitor you described in week 1.
DAVE ADAMSON: And so that's 30 years ago, that paper came out in 1985.
CHUCK NEWELL: So now let's go to another paper,
this is in the 1990s from John Cherry.
It's another paper about pump and treat, Mackay and Cherry, 1989.
And they are sort of theoretically describing
why groundwater pump and treat cleanup systems-- remediation systems aren't
cleaning up as fast as they can.
So one their conclusions was, they have this drawing here,
but what do they say here?
DAVE ADAMSON: Basically, as plumes are spreading through these aquifers,
the dissolved contaminants move quickly through those permeable zones
and then they slowly invade-- I like that, very slowly invade the less
permeable ones by flow or diffusion.
CHUCK NEWELL: Some good writing, here, in this paper.
Then they talk about what are the implications
that over years and decades, this invasion, as you talked about,
can cause the plume to occupy large volumes of low permeability material
here.
To obtain clean water from wells it's necessary for the lower
permeability parts of the aquifer system to get cleaned up as well
and to clean up those high permeability systems.
So, here, in 1989, they're talking about matrix diffusion.
What's ironic is this paper, they also talk about DNAPL.
And the whole industry really focused on that piece of the paper.
And only much more recently we've realized that these folks
were onto it a long time ago.
DAVE ADAMSON: Yeah, exactly.
CHUCK NEWELL: OK.
So let's now go to 2008.
A great book by Fred Payne, Remediation Hydraulics,
it really goes into this whole thing, dispersion versus diffusion.
And they start talking about heterogeneity's and the scale of these
heterogeneity's.
So here's some pictures.
And if you look at the one on the top right,
you can see that you have these very abrupt interfaces
between high permeability material, on the left 10
to the minus 2 centimeters per second and 10 to the minus 6th material,
just off to the right.
Just on a matter of centimeters there, Dave.
And so most of the flow on that top panel's
going, really, through that highway at the very top.
And then you can see at the bottom, the contaminants are distributed well.
So the subsurface is like this big reactor
with billions and billions of tiny baffles
that prevent this mixing that's going on there.
So what do they talk about though, in terms of heterogeneity
and what homogeneous sites look like?
What's that saying?
DAVE ADAMSON: Well yeah, I mean they're talking--
looking at that top panel of sites that are archetypically homogeneous,
have 1,000 to 10,000 fold variation in hydraulic conductivity at very small
scales, scales of 1 to 10 centimeters.
CHUCK NEWELL: Almost like you can't see it.
So you're not going to see this potentially in a log
and they say it's at the fine-- in micro scales.
It's often below this detection limit that you
have of conventional characterization techniques like taking that core
and plunking it on the table and look at it, you may not see this.
And so it's really difficult to map and interpret
when you have that variability that's really high.
So here's this potential paradigm shift, this ITRC training that's coming out,
the DNAPL characterization.
Great work from the ITRC DNAPL team there.
This talks about sort of a new paradigm, a new thinking about transport that's
sort of based on this.
And the first is heterogeneity really replaces homogeneity.
Almost all these systems we deal with out there in nature
have a lot of heterogeneity to it, so that's the first one.
DAVE ADAMSON: Yeah, and then also, this idea
that you can't describe these things with sort of the classical Gaussian
way.
You have to move to more sort of the lognormal ways of thinking
about how things are being transported.
So look-- think about those longer tails.
CHUCK NEWELL: Exactly.
The diffusion causes this contaminant storage is a key point.
DAVE ADAMSON: And then that storage may be considered an attenuation mechanism.
And that's what we talked about and the last thing that we'll continue
to talk about throughout this week.
CHUCK NEWELL: Your capacitor idea, and that basically there's
this one idea, this paradigm, that says we should replace dispersion
with diffusion, and that's this key point.
And so all of these are really important,
because you sort of put all these together.
And originally, I'm a chemical engineer, was thinking that all we have to do
is just get these chemicals into this reactor, which are these sand beds
and that's all you had to worry about.
But there's a lot more.
The stratigraphy, the geology becomes lot more important.
And in some ways, we get to the point now,
where I'm saying that in terms of our field, it's-- what do we see?
DAVE ADAMSON: Ah, the revenge of the geologists.
Here's a geologist looking happy that people are finally
listening to him after all these years of him
telling about these sorts of processes.
CHUCK NEWELL: These are important.
These are all the chemical engineers trying to learn about this,
becoming a very important part of our remediation story
that this whole diffusion piece is all about the stratigraphy,
about the geology.
So geology becomes much more important here.
OK.
Dispersion out, diffusion in, there's this emerging argument.
And so there's this idea that based on our increasing knowledge
and controlling role of geologic heterogeneity
and-- I'll let you pronounce this one, Dave.
I can't do it.
DAVE ADAMSON: Anisotrophy.
CHUCK NEWELL: You're so cool, you're good.
In the subsurface, these are controlling the subsurface containment
fate and transport.
So let's go to two panels in the old idea, the classical view,
is all about dispersion.
And what's going on here?
DAVE ADAMSON: Yeah, so if you look at this dispersivity model that's
shown in this panel, it's sort of classically looking
as things transport down.
If you were to go at any particular point,
you would see these nice symmetrical curves
and sort of easily predictable ways of describing where the contaminants are.
CHUCK NEWELL: Just beautiful symmetrical cloud in some ways that goes out.
But now this new idea, if you have to account
for this stratigraphy, this geology, it looks something like this.
You have these transports occurring in these poor space channels,
this mobile zone that's in there.
And it's surrounded by these lower permeability zones
that may occupy most of the aquifer.
And it will store stuff and then they will release it slowly.
So that's this key idea-- maybe we should be thinking about our sites
more like the panel on the right.
And even change some of our models and think about our whole set up.
So this idea of this paradigm shift, still not
completely accepted by everybody, sort of an emerging idea, right?
DAVE ADAMSON: Yeah, I think more people are still probably more
towards the left than to the right.
CHUCK NEWELL: OK.
But wanted to bring up some new thinking that's
going on, particularly in this idea of dispersion versus diffusion.
Well let's wrap up, there's this new conceptualization
of contaminant transport that abandons dispersion
as a basis for accounting for local hydrogeneities and aquifers
and as an explanation for dilute concentration in wells.
DAVE ADAMSON: And then at the same time, sort
of embracing diffusion and slow advection as a fundamental governing
processes at a lot of contaminated sites.
CHUCK NEWELL: And just to wrap up that diffusion and dispersions really
important to understand how this natural attenuation works, how transport works,
but a big deal for M&A.
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