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.
Natural Source Zone Depletion (NSZD)
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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
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!
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.
Okay, well today we're going to sort of take an expansion of MNA, something that's
called, wait for it, the acronym here NSZD or natural source zone depletion.
Well the phrase M and A has traditionally been reserved for
managing contaminant plumes.
This new term, NSZD, is really used to describe this
attenuation processes that are reducing source zones themselves.
>> This is really just for hydrocarbon plumes?
>> Well it's really for, most of the research has been applied for
these hydrocarbon plumes, particularly their sources, okay.
So we're really talking about the source zones.
But there is an ESTCB project led by Dr. Tom Boyd of the Navy,
who is using some of these same ideas and techniques at chlorinated solvent sites.
Also the SERDP and ESTCP source zone monograph has a whole chapter of this
natural source zone depletion that includes things on the fuels but
also these chlorinateds.
>> All right, well let's get on the treadmill and learn about some NSZD.
>> Okay, well you have a prescience sense of wording choice,
let's look at this conceptual model that we were,
using describing hydrocarbon plume degradation of 1995.
And it is this treadmill that upgrading, have this clean ground water that's going
through the source zone that's feeding in this dissolved oxygen, nitrate, and
sulfate.
And you have the source zone that's slowly dissolving and
making this BTEX compounds and this is fairly fast reaction that occurs in there.
So this was our thinking and we would account for
things like how much dissolved methanes in there and we would map this out.
This is for example a map of methane and
the blue is the dissolved plume and on the right are the methane concentrations at
28 different air force bases where they have these BTEX plumes.
One thing we didn't snap to is there's actually a lot more going on here than we
thought at the time.
And it's really sort of launched very recently, in the past couple of years,
what I call the, wait there's more moment that were out there.
Thinking about TV infomercials, right?
>> Yeah, exactly.
>> And so there's this methane reaction, there's something else going on
that's very important, and the key people working are this
whole series of this US Geological researchers at this Bemidji site.
We'll talk a little bit about that.
Paul Johnson from Arizona State, now Colorado School of Mines.
We're still working with Paul Lundegard at some sites, and then the ITRC, and
all of this is coming out with this, wait, there's more.
There's more degradation out there in these source zones,
at hydrocarbon plumes, then we first thought.
So let's go to this document which talks about natural source zone depletion.
And here they defined this as a quantitative
mass balance assessment of the source zone
that looks of how much of these chemicals are being naturally lost at some rate.
And the whole idea is that in this guidance they provided in 2009,
number one is let's use that treadmill concept and
analyze how much electron acceptors are being consumed and things like that.
So that's the top panel there.
But then the bottom panel says, hey let's also look and see how much of
these contaminants are ending up in the vadose zone and in the surface in there.
And then the way we're doing this is for example for this top
groundwater piece you'll have groundwater wells on the left and right, but then
the key new thing they're really saying is hey let's look at this gas dynamics,
the gas flux that's occurring in this vadose zone and with sampling in here.
So here's the groundwater side.
This is how you get these fluxes.
Pretty common, right?
>> Yeah. >> You're looking at electron acceptors
before the upgradient and downgradient.
And then, this is the new piece.
We're going to look at this.
So the question is how do you measure all of this methane being generated down there
in the subsurface?
And the answer is with this, what's called the gradient method, and so
we have a graph on the right.
What's going on there?
>> Well this has got depth on the y-axis,
starting from the surface up at the top down to about 25 meters in this case.
And then you've got concentration of these gases plotted on the x axis.
And so what we've got is, we've got oxygen, those blue dots, and
oxygen's decreasing as you go deeper.
Carbon dioxide is increasing, but you've also got methane on there,
just a little bit down there at depth.
So that's really what we're interested in, right, is what's controlling that.
>> That's right, so this methane's coming up, and
if it's coming from the saturated zone, it's sort of
bubbling up in these evolution channels >> I mean methane is generated in
anaerobic parts of the vado zone but
then it comes up and then it mixes with this oxygen and gets consumed.
So this methane,
the way you measure it, is by measuring how much oxygen it gets consumed.
You get that gradient multiplied by this effective diffusion coefficient soil, and
you get this NSZD rate.
And this was where some of these remarkable outcomes started showing up at
things like this US geological survey at the Bemidji site, other places.
Is that this groundwater piece which we thought was the big story a long time ago,
it's only 1 or 10% of the whole story that's going out there.
We were missing 90 or 99% of the action, if you look down below,
because of this tremendous flux of these gases through there.
So this really started people wondering a lot.
Hey, we want to know more about at these hydrocarbon sites, what's going on with
the methane, what's going up with these gasses that are coming up through there.
So here is this some images, so just some of the research that I find interesting
about trying to visualize this process, and there's a term called ebullition.
And Dave, what's that?
>> That's just the fact you've got so much methane forming, building above
the saturation of it in water, that you're actually forming bubbles and
that those bubbles then are sort of moving, moving out of the groundwater.
So you see an off-gassing effect.
>> So on the left is this chart with some of the bubbles in there and
then on the right is this image of these methane channels that are coming up.
And so looking at day one and two at the top right,
you can see one of these methane channels are starting to emerge and
in this methane gets in the vadose zone, it hits that oxygen front and
then methadone oxidizers will decay these things.
But here's this image of sort of those ebullition process on this
off- assing that's going on.
So there's this idea that all of this gas is being generated, that the methane is
being converted to CO2, then some researchers went out there and at this
Bemidji site in Minnesota, and used what's called this dynamic closed chamber method.
This was actually used for things like global warming studies,
where they're seeing how much carbon dioxide's coming off of rice paddies or
cow dung or whatever.
But in our case, what they did is they would go to the surface and they would
measure this amount of carbon dioxide coming out there and calculate some of
these rates and that sort of its key essence that you can measure the carbon
dioxide flux at the top and you'll know this natural source on depletion rate.
Now Dave, we're not talking about plumes here, we're talking about sources, right?
>> Yeah, yeah.
So, we're putting these things into a source and
really trying to see what's going on there.
>> Yeah. With the whole picture,
you can see there's this idea that there's what I call a methane blanket down there,
and then there's CO2 coming up.
So it's a sort of a two step process of things that are happening.
But, these USGS researchers and their whole series of [INAUDIBLE] and then,
University of British Columbia researchers are involved in this.
This is a recent paper sort of culminating about 12 different papers out
there where they sort of take this pipeline spill that occurred in 1979.
They break it up by the different chemical classes and
they tell you what happened over 30 years.
And there's some remarkable things,
most of the toluene went away, about 60% of the benzene went away.
But the key point is, they said this natural degradation is still humming
after 30 years of being out there.
And the other key point was if they do their mass balance,
85 or 90% of this carbon degradation is being out gassed coming out the top.
>> Yeah, so that's what they would've missed if they wouldn't of been able
to account for it.
>> That's right. So when we did BioScreen,
I was involved in that, that's what I didn't know.
And now this stuff's coming up.
So pretty remarkable paper from a Ng et al 2015 that sort of puts
together a lot of research at this Bemidji Site from the US Geological Survey.
But there's other ways to sort of measures this carbon efflux.
Lets go to that and it's the whole idea of these carbon dioxide traps.
And so you put these things in there and
it's sort of like the carbon traps they used in Apollo 13, right?
The carbon, you remember that movie?
>> Yeah.
>> Same type of substance, and
if there's any carbon dioxide coming from the bottom, they can trap it.
So I think we actually got a movie that sort of explains some of this.
Let's see if we can see that's what these things look like.
You're out there, you put them out there for 14 days.
But here it is, the traps.
And just one remarkable thing,
what is amazing is where we put these things out you hear this music.
So I think that's a great feature of these things.
But we're seeing these different things, there's some natural carbon dioxide,
which is the purple, and the green is the contaminate.
There's ways to split this up, using isotopes.
But here's all you do is you have a receiver and you put these traps right in
there, and then set them up the tops a little wind screen and
then you leave it in there for 14 days.
And he's going to walk out here and then come back in 14 days, ship it to the lab,
and you get this number.
And the number is gallons per acre per year of degradation
that's occurred at this site.
So what type of rates are being observed out there?
So, here's just some of the different studies and
the map shows different places where these traps have been employed.
But the rule of thumb here is that if you think about this flux, which is mass
per area per time, rates of the hundreds to thousands of gallons per acre per year.
>> Yeah, it's a pretty big range, obviously,
but even at the lower end of that, that's a lot of attenuation that's going on.
>> If you do it metric units, what have we got?
>> Liters per hectare.
>> Okay, thousands to ten thousands, about a conversion.
So quite a bit of degradation that's going on here.
So I guess here is this new conceptual model, and
this has got a submerged LNAPL source in the yellow, it's generating methane and
carbon dioxide but they're coming up through these ebullition channels.
It's getting in the vadose zone,
and then there's some oxidation where the methane gets converted to CO2.
So at the top, at most of these sites, it's only the CO2 coming out.
You can measure that and get this natural source zone depletion.
Okay, let's wrap up.
So this is in this LNAPL world, there some just amazing research regarding LNAPL
source zone attenuation, this NSZD process.
>> Just remember that the key process,
were talking about here as methanogenesis, followed by ebullition and
off-gassing, then followed by methane oxidation in this unsaturated zone.
So that's the under-appreciated process that we're dealing with.
>> All right, so those three conventional methods that you can use to measure this
NSZD rate.
There's the gradient method, there are these dynamic closed chambers, and
then these carbon traps right now.
And then the typical rates you get, 100 to 1000s of gallons per acre per year.
