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.
Low Risk Sites, Transition Assessments, and 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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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!
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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.
Well, today we're going to take off our nerd hats and
maybe talk less about technical issues,
but move into more implementation-oriented issues regarding MNA.
>> Yeah, that's right.
I know it's sort of hard to take those hats off at sometime, but
today we're basically covering key issues that deal with how and when MNA can, or
should be used.
>> Right, we'll talk about low risk closures, and we'll talk about California
and the Air Force, transition assessments, and other topics.
I think maybe the idea is, I had this sort of idea that if we think about in history,
there was this whole debate about economics.
And then Richard Nixon, the President in the 1970s, who said,
hey, we're all Keynesians now, talking about that type of economics.
And there is sort of this maybe tenuous analog which we have today and
there's Dave Adams, and what are you saying?
>> We are all MNA implementers now.
>> And it's sort of this idea that you have these active technologies,
which really can help out sites and do things, but
we can never sort of get all the way there.
And at some point almost all of our sites, we're going to be relying on MNA for
polishing and some sites as the primary one.
But lets go to this sort of MNA Applicability, and we're going to start
thinking about when you apply MNA, and we're going to go back to 1999.
Here's a US EPA Directive, and they have some very specific directions,
okay, that there given ground in our community.
Number one, hey, this is not a default or presumptive remedy,
you really gotta think about this.
And it should be applied, very cautiously as the sole remedy, right.
And then they're else did say that source control is going to be a fundamental part
of anytime you do that.
There's some different thinking now in terms of source zone attenuation, but
this is the way they're laying things out in 1999.
But today, it's being used more frequently as a sole remedy for plumes,
in some cases for sources.
And these states have a lot of these specific criteria that's out there.
So we're going to sort of do a jump around the different people who are talking about
when and how you implement this.
So first, let's go to the ITRC, Interstate Technology and Regulatory Council, and
look at some of the things they talk about enhanced monitored natural attenuation or
enhanced attenuation.
So here is this big flowchart, but basically they're saying that if your MNA
is close, but you're just missing a couple of things, hey,
look at that whole thing and you want to try to improve this enhanced attenuation
by enhancing some of the processes that are out there.
What are some of the criteria that they are looking at?
>> Yeah, I mean, these are the things you have to basically have,
are the risks acceptable, is the plume stable or shrinking,
conditions sustainable, acceptable remediation time frame, and
then does it make sense in terms of the cost benefit?
>> And if you meet all those, then maybe MNA will work for you, but
if you're missing one or two, instead of going to that intensive type treatment,
perhaps there's a couple of attenuation processes you could speed up, so
important guidance here.
Let's go to another one, some real important guidance from San Francisco, and
this is the Regional Water Quality Control Board.
They call it this low-threat site closure, and it's geared at these
chlorinated solvent sites in their area, and so they developed this thing.
It's a process for chlorinated solvent sites, and they had this different
criteria that you made, a total of nine points that you have to check.
And some of them are that this residual pollution is not going to
adversely affect how these ground water plumes are moving.
Not going to sort of affect your ability to get this cleanup standards and
through monitoring you keep people safe and all that.
But then they've closed a number of sites here, and in their guidance,
they talk about 46 different sites were closed under this.
And this is a case of, sort of, the percentile of sites were closed
on the x-axis and then the concentration they closed these at.
And so the basic idea is that some series of sites that they
closed above the groundwater cleanup standards, the MCL.
So what was the median concentration out there?
>> Yeah, just you sort of look at that then of the concentration at
50% of these sites.
Is that what you'd say?
>> Yeah. >> And so you go over from
the y-axis then and you're basically seeing somewhere on the order of,
I don't know- >> 25 micrograms?
>> Yeah, 25, something like that.
So five times above the MCL.
>> So it's sort of this idea the sites can get down to a certain level where they're
not hurting anybody, there's no risk and nature's sort of taking care of things,
and you let nature will go the rest of the way.
Okay, let's go to another example, and this is one the fuels world again,
California.
Some really interesting guidance here.
And this is their low threat closure, but not for chlorinated solvent sites.
But this is for hydrocarbon sites like gas station sites.
>> Yeah, exactly.
>> And this is over all of California.
And they first set up, hey,
there are certain things that you gotta do before we'll let you take this off-ramp,
in terms of things, site must be in a surface area of a public water system.
The site must be petroleum.
The release must have been stopped, so it can't have this continuing source.
Free product must be removed to the extent practicable.
You must have a good conceptual site model.
Secondary source removal has been addressed, and
they want you to make sure you understand your MTBE situation.
>> Yeah, so this is not policy in California for this site.
>> Since 2012, and they have these, I think really neat graphics that sort of
talk about difference scenarios where if you pass all of this points, and again,
your first thing is you want to make sure to be protective.
So if you've done all this stuff, and the risks, there's no active risk to anybody.
So here's one scenario where you can close a site.
So what do we got here is that we got the gas station up top, and so up there and
the little buildings, the fatal zone, you have the plume and the ground water.
And they sort of say under number one, if your plume is less than a hundred foot in
length, and that the nearest water supply or surface water discharge for
that aquifer is more than 250 feet away, and
you meet all the things in the previous slide, hey, you can close that site.
>> Okay. >> So let's just go to a couple more,
Dave, the scenario here, what does it look like?
>> Well, this is a little bit longer plumes so
we're talking about a 250 foot plume here,
and the distance to the receptor as wide as of greater than a thousand feet, right?
>> Right. >> And so they've just got sort of this
idea there, so into this decision mix, it's well how long that plume is.
And then, in this case, how far away is that nearest receptor?
That's sort of your insurance policy in some ways, right.
We know these plumes don't get very long, but this is this insurance element to it.
And in this fourth scenario, they actually add one more.
>> Yeah, these are longer plumes, we're talking about a thousand foot long plumes
in this case, also looking at the concentration, right?
So we're talking about dissolved concentrations of benzene and
MTB less than 1 ppm.
>> Right, okay, so anyway, some important guidance from them.
But them I'm going to sort of, well, I can't help it, but
to go back a little bit technical.
And there's this idea that some of these low-threat closures especially on
the chlorinated site world is related to this concept of the plume lifecycle and
what happens.
And so they talk about both for hydrocarbons and for
chlorinated solvent sites, you might see something like this.
The plume expands, it's stable, shrinking.
Then it has this exhausted plume and some of the plumathon studies,
we talked about there's a certain amount of sites in those buckets.
>> And then so under this thinking about closures and low risk, those things
are low-risk sites, and we'll go through this explanation maybe on this slide.
Is if you're thinking about a site it's a late stage or it's an exhausted plume,
you may have a different source process than you had originally.
Maybe originally it's all NAPL, maybe this one's the key thing,
it's matrix diffusion is we have this sort of conceptual model here.
So if the matrix diffusion percent of that source is high,
maybe it's not a principal threat waste.
And that sort of opens the door for thinking, well, lower risk and
maybe we can keep going.
And then you have those conceptual model that if you have basically a site that is
dominated by these matrix diffusion processes,
maybe you don't have much potential for source migration, right?
>> Mm-hm.
Further source remediation is difficult.
It's hard to remove some of that mass, but
maybe those are some of the candidates that the sort of do this.
So we sort of expanded some of that idea.
And this is some guidance we did for the US Air Force, is that if you have some of
these sites that are these low threat, there are certain indicators you can do to
try to see if you fit into that low-risk budget.
And basically it means MNA the rest of the way.
And here's some examples of what is a low-risk site and what's not.
>> Well, anything with mobile NAPL would not be considered a low-risk site under
this- >> NAPL still potential to expand,
then maybe you need to do something about that.
But matrix diffusion, that stuff's in there.
It's just coming out slowly.
So this particular guidance here, we're showing a flowchart here, where you go
through this series of checks, and you say, do I have mobile NAPL?
>> A little bit like taking the SAT test here.
>> That's right, fill in the blanks, and then you sort of add these things up, but
it's a way decision point to say, are you this,
in sort of this low risk sort of approach or not.
The actual guidance is called LoRSC, low risk at the top.
>> Low-risk site closure, all right.
>> You got it. Okay, the acronym actually stands for
something.
But so this sort of this approach and
let me just wrap up with one last vignette, I guess, about closures.
And this is coming from Mount Olympus out there,
the key scientist in the National Research Council.
They came out with this document talking about complex sites.
And one of the key things they talk about is, you get to a certain point where you
should start thinking about transitioning to natural attenuation or
some other active or passive management.
>> And I think the key idea here is that when you get to a point of diminishing
returns with active remediation,
then you do this transition assessment to see if it makes sense to do MNA.
There's actually an ITRC now that's talking about complex sites, and they're,
sort of, weighing this idea of transition assessment.
So we're going to see what comes out of that.
But, a lot of interesting concepts about thinking about, as these sites age, and
as they get lower and lower risk, how do we manage those things, okay?
Well, let's just wrap up, I guess, in terms of this session here, or
this lecture.
First, MNA is likely to be a component of almost all remedies at some time during
the site life cycle.
>> Yeah, and then Chuck talked about several different great examples
of California Low Threat Closure, Air Force, and
National Research Council Transition Assessments.
>> So we talked about those.
And finally, that in some ways it's not a matter of if,
but when MNA is applied in the life cycle of your site.
