We're here on set for the last of about 55 lectures,
about monitored natural attenuation.
I'm Elizabeth Pina, an engineer at GSI Environmental.
A recent graduate of Rice University and a manager for this project.
Today, we'll have a short discussion where I ask three question to each of our
instructors and we'll all have a brief discussion following each question.
So we'll go ahead and get started with our first question.
After about 20 years of applying MNA is our underlying philosophy still valid?
Are there any other factor that might makes us less
likely to do MNA in the future?
Pedro, you want to get started?
>> Yes, absolutely the philosophy is valid.
There is no question that some priority pollutants at some sites will degrade
faster than they migrate and that makes monitor natural attenuation,
the most logical and most cost effective approach in those cases.
But we have to remember, that the burden of proof that these process
are proceeding at an acceptable rate lies on the deeper opponent.
And we have learned organized last nine weeks or so, many techniques and
scientific principles to demonstrate that is the case and
enhance your decisions to select or reject natural attenuation.
>> Yeah, that's a great answer I'm thinking about the first part
of your question was is the philosophy still valid?
And I think back to a earlier lecture where I went on record saying that we're
all MNAers, so, yeah I believe the question to that is yes as well.
And then in terms of other factors to overcome, one thing that comes to
mind is this idea that there are still emerging contaminants out there and
it's PFAS, these per- and polyfluoroalkyl substances,
are of particular concern because I don't think we've really got the proper
conceptual model built for those in terms of how they behave in the environment.
So I think we're still a few years in terms of using MNA for
those types of sites.
>> Okay.
>> Well, good points, Pedro and Dave but Emily did
your question about what might makes use MNA less frequently in the future.
And it sort of taught three things.
So, they going back in time, 20,
maybe 25 years ago when the MNA was first being developed.
Some people said, there's going to be a failure, well, people get sick and
that's going to really reduce the use of this.
And I don't think that happened yet, but that's still possible in the future.
And that can be one constraint of big event where if people get sick,
because the emanate failed and, hey, we're going to use less of it.
Number two is changing exceptual models.
And this is happening a little bit things like vapour intrusion,maybe we're going to
have to reopen sites, you talked about those perfluorinated compounds.
If those are in a lot of sites and they're not amenable to MNA,
maybe we can't use it, because we've got to think about those compounds.
Number three, what about the silver bullet.
An intensive,
active remediation technology that can really remove most of these mass.
In that case, maybe we wouldn't need MNA.
So those are the three things that I thought about.
>> Great. Well, let's go ahead and
move to the second question.
Under what circumstances are we confident enough in MNA processeses and
our predictions about MNA,
that we can close sites before they meet drinking water standards.
Chuck, you want to go ahead and get us started?
>> Well, sure as I think many of you guys know that
there's many regulatory programs now that have these conditional closures out there.
That under certain circumstances,
you can watch this and understand, disseminate and then close this site.
But I think maybe with some of the new stuff that's coming out, and
we've talked about this in one of our lectures.
There's a lot of vary building in groundwater data and
instead of just having two years of data,
maybe we need longer records to really get that confidence in there.
So that's what I was thinking.
>> Yeah, and I think that I agree with you because we also went
through in one of the lectures there's policy, there's state policy out there
that allows you to actually sort of designate things as low risk sites.
Where concentrations might be above the MCL but
if you can demonstrate he distance to the receptor, and the concentrations,
are low enough that you can, actually, go out there and get these sites closed.
So, I think we're definitely transitioning that way.
>> Yeah, that's right.
I mean, nothing beats field data, showing that or
long terms, the plume is no longer expanding, that it might be stable,
or shrinking, and that's highly unlikely to ever reach a receptor.
But, we gotta be weary of unintended consequences when for
example, you try to accelerate Sorenson bioremediation and
in doing so, you end up either hindering microbial activities or
mobilizing some priority pollutants faster.