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.
Analytical Groundwater Models for MNA
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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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Modeling Tools to Support MNA
In this series of lectures, we talk about models and how they can be used to understand MNA. We first start with two key MNA questions: How Long? (will the plume get) and How Far? (how long until the site is clean). Then a review of analytical computer models, remediation timeframe models, the more complex but more powerful numerical models, and even a discussion of the “Fourth Paradigm: Big Data”.
與講師見面
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.
Okay, well let's start with some the simple models that you can go out
there and get and download for free for both the plume and the source zone.
Now Dave just a reminder what is the advection dispersion equation used for?
>> That's for modeling faton transport processes within the plume.
>> Okay and we talked about these simple box models can be used to model the source
>> And one of our themes is that for
MNA modeling, you need to consider both attenuation in the source and the plume.
And many of these analytical models that we're going to talk about have
the capability to model both, but with a number of simplifying assumptions.
>> Mm-hm, okay, well let's start maybe by looking at a list of some of the commonly
used analytical models.
We got the BIOSCREEN AND BIOCHLOR.
REMChlor AND REMFuel then Matrix Diffusion Toolkit and
this slide talks about the first thing as what contaminants are used for.
Some of them are Hydrocarbon specific with BIOSCREEN.
Others are for Chlorinateds like the REMChlor model.
Some you can use for anything.
And then a key thing as this idea that Matrix Diffusion is becoming a more and
more important process particular for MNA.
Day which models can handle matrix diffusion.
>> Well it looks like for REMChlor you can do that in the source.
REMFuel same thing, you can model matrix diffusion in the source.
And then obviously Matrix Diffusion Toolkit,
that's one of the focuses of that.
>> You can do both.
Analyze remediation sort of the answer is no for the BIOSCREEN BIOCHLORs.
MNA only.
The REMFuel REMChlor they can do both.
And then we talk about the platforms, some of these are Excel models and
some of them are stand-alone.
So that's a little bit about this.
Now for us, we're going to, sort of, go through this talk and
really just take one of them, which is this REMChlor model, and really sort of
focus on it to tell you how it works and how you can use it to analyze MNA.
>> Maybe before we do that, one more popped up on here,
this REMChlor-MD at the bottom, you want to describe what that is?
Yeah so that's actually a new version of this REMChlor model that's not out yet.
But it will handle MD is matrix diffusion.
I got two slides in the end, we'll talk about it there.
Okay but here if you get this REMChlor model is built by Dr.
Ron Falta at Clemson University, just Google USEPA and REMChlor.
Ask the Google and they'll send you here, it's a free download and
then you can run this thing.
And so the way this works is, it's got both the source model and
a plume model and they're connected.
So in this one, you can see it's a mass balance type model on the left.
It predicts this discharge.
Including the effects of natural attenuation on the source but
also you can analyze remediation.
Then, it's connected to this analytical model over here and what does that do?
>> Yeah, so, that's the plume model and
you basically can simulate a mass balance you're using advection and
dispersion, retardation, different degradation reactions and you can
also then Seemed that you were doing some sort of remediation in there as well.
>> RIght, so, Dave, just a quick question, maybe a little off topic.
Who's the character in mythology who's got all the snakes in her head?
>> That is Medusa.
>> That's right.
So the connection there is that the way that these models are connected is that
that there are thousands of what are called stream tubes that are coming out of
that orange box there that are then going to the plume.
So the way Dr. Folta wrote this is that each stream tube then has a concentration
he can sort of tune the way these thousands of stream tubes and
thinking of being like thousand of snakes coming out of Medusa's head right?
And then they sort of form that plume but then they will basically, these stream
tubes, then will help you Its a simulate dispersion and absorption and degradation.
So that's the cowder connective in there with the thousands of snakes
from Medusa's head, it's the one way to think about it.
So let's go a little bit more about in terms of a box models we
talked a little bit about that in the last lecture, there's more
power to these guys specifically what's called the power function.
So, let's look at this again, this is the connection of the both terms,
these are some of the key variables but then, I'll do the sources.
You can put in our source to K constant just naturally there's some
degradation occurring in that source material.
But then, you could just, it will calculate how much is naturally washed
away and then, you could actually remove some source.
Then basically in the plume, what are some of the key dials that you can change?
>> We got a whole separate decay rate that you can put in there to
simulate remediation at the source of that lambda,
you've also got dispercivity terms in the 3-dimensional directions x, y, and z.
And so you can incorporate those as well.
>> That's right.
And one note is that you can have natural attenuation in this plume, or
if you're doing some sort of remediation, you can turn up that
first order decay in that plume itself and sort of make all stuff happen.
But let's go to the source and
we'll talk something about a little more flexibility of how you model this stuff.
The graph here has got some different things, but
basically it maps the progression or age of a source starting from the top right,
you see where it says start, and things go down to the bottom left.
The x axis is just how much mass is remaining.
And if you get all the way to the left It's completely cleaned up.
And on the y axis is what is that concentration?
And the key point is that in rem clor and rem fuel you can turn this dial.
There's something called the gamma function and
with that you could sort of get these different patterns.
And so on the top right it's almost like one of those step functions we talk
about the previous lecture if it's really a number that close to zero
the concentration stay the same even though that mass is being depleted and
all of a sudden, it just hurdles down.
And then on the very middle, there gamma equals 1, that's a first order decay.
That's sort of like what we talked about with a first order decay,
slow tail is developed.
>> Sort of middle of the road.
>> That's right, then you can do this.
And then gamma greater than 1, sort of gets into this function where it's
very dominated by this very slow matrix diffusion processes.
So there a little bit of mass but it's coming out really slowly.
So some people try to connect this to source architecture and
what actually happening out there.
So here is just some ideas that if you have sort of NAPL and
it mostly it needs high conductivity zones or it pulls in homogeneous media, well,
maybe you want to use a gamma that's less than one.
Then this NAPL in the right-hand side,
if it's mostly in the low permeability zones and it's a lot heterogeneity,
well maybe you want to use something that's closer to one.
But it's hard to actually connect this to real field data.
So this, as you said, the middle road I think most people might simulate
their sites as being this gamma of 1 and
sort of use this classic first order k type process in there.
>> Okay, just a couple examples how this gamma might actually be used.
Let's look at data from three different TCE sites.
And what's on the y axis?
>> Well, that's the normalized concentration.
So that's looking at concentration at sort of times zero and
then plotting the concentration at other time divide by that.
>> Okay, if so we're going to try to use one of these models the x axis is this
time we're looking at the concentration versus time things.
Here's how the different gammas might be.
The green line, hey, let's use a gamma < 1,
because it looks those concentrations aren't changing very much.
And the blue line here, the sort of these concentrations taking a dive,
but then it sort of leveled out, and so then you might use this gamma > 1.
And then the red, sort of a straight line, it's this first-order decay.
So that's a little bit about how the source works.
Let's go a little bit into the plume.
And Dave, quickly, what's going on here?
>> Well these can incorporate the fact that you see degradation going on
within the plume.
So the y-axis is your normalized concentration then.
We're starting with TCE as the parent compound here, but
you see it degraded as you move down in distance from the source.
That radiant from the source can we see that DCE line that blue line there have
pop up and then VC cord as well >> Okay so theres a first order decay rate
for each one of this but what REMChlor does says you can do this not just
one place but actually >> In three different places.
So here, you take that plume and you [SOUND] chop it up into three pieces.
And in each piece, you can have separate decay coefficients.
And sort of the thinking is is that in that red zone,
maybe that's something where there's high anaerobic degradation going on.
Maybe you don't need remediation in there.
Then you get to this zone 2, maybe it's more aerobic, or
actually they've sparging something to get rid of the vinyl chloride.
Then zone three, maybe those little background rates.
But then there's in each one of these boxes then you can put separate
decay rates.
>> So separate reaction zones basically.
>> That's right, but actually it's more complicated than that.
You can see here.
This is our whole Plume Remediation Models and there are these nine boxes and
the key ideas that each one of these space time zones can have a different decay rate
for each of the chemical specie.
Okay.
>> And so it's sort of like, I like the term space time,
it sort of reminds me a little bit about Star Trek.
>> I know you are a big Star Trek fan so yeah.
>> And so when you go home today, you can tell your daughter and
your wife that you were dealing with space time issue at work.
But if you see this today, the Y-axis is time, the X-axis is distance and
so you can say in the first say the bottom part of that graph,
the first basically 30 years, all natural attenuation in all three of the zones.
Then you go in there from 2005, 2025, they're doing this remediation.
And you can put these separate terms in there.
And then basically you can populate all these different boxes with
different amount of degradation going on in each one of the different boxes.
>> Pretty neat, lots of options of time and space there.
>> Yeah, let's just take a quick look at what the input data look like.
This is for this REMChlor model again.
It's got these different to zones in here but
you're going to basically do the same thing.
We talked about in the last lecture, there's some hydrogeology,
there's some transport and there's all this degradation, the top right stuff,
you'd see the nine boxes there,
that's the space time stuff that you're going to put in there and
you fill one of those and out for each one of the different constituents.
Now one node is sort of focused on REMChlor
which is this a very powerful model.
You can use it to analyze sort of mediation or you can use it for
analyzing MNA.
But there's a new one out called REMFuel and so
it handles a basically the different chemicals that are associated with
the fuel degradation and particularly MTBE will degrade to what compound?
>> MTBA.
>> Exactly, so let has that decay change in there again, with the nine boxes,
the nine space times kind of thing.
Okay, let's talk about coming attraction.
Okay here is the REM Chlor-MD model.
Again Dr. Falta is working on this and brilliant guy,
his is the guy who thought about all the snakes, the medusas, and
these stream tubes, but now his reading back in all these sort of really great
papers from long time ago about heat conduction.
And so he's reading these things and
he says I can take those same mathematical equations and
approaches and put them into REMChlor and so that's what he's doing there.
>> Mm-hm. >> Or he does that because
heat conduction works a lot like chemical diffusion, right?
>> Same type of equations.
Like the Tice Equation or
the Teem Equation, those are all based on heat transport type stuff.
So it's a similar idea that heat's translating this stuff.
This model is being funded by ESTCP, should be out sometime in 2016.
But with it you can get some really nice flexibility in modeling
not only the matrix fusion and the source, which REMChlor does now, but
also in the plume itself.
And you can model two-layer things on the right, fractured media the bottom right,
multiple layers, so quite a bit of power on this thing.
Coming attraction, so we'll watch out for that.
But let's sort of wrap up what we did, just try to give you a quick overview of
powerful but relatively simple groundwater models that can be used to evaluate MNA.
The key one that we looked at here was REMChlor.
>> Yeah, REMChlor is nice, it's gotta source term sort of innovative that you
can control when and how the contaminants leave the source on this gamma term.
>> And then the plume has a Space-Time zones to model
>> Different times and
places that biodegradation occurs.
>> And then we touch a little bit on REMChlor-MD, and
this should be an important tool because it includes matrix diffusion.
