CHUCK NEWELL: OK.
We're going to continue with our dilution
week and dilution as an attenuation process sometimes.
So, Dave, let's go back to the year 2000 and think about MTBE,
this fuel oxygenate.
What was the situation back then?
DAVE ADAMSON: Well, MTBE, it was an emerging contaminant and at that time
not really thought to attenuate in groundwater.
But it was present at a lot of sites, almost every gas station
in the United States.
I think there was a realization that most resources of these sources
be relatively small, you know, they're gas-station releases,
so that there would be this new framework
that we would have to think about to describe these sites based on mass flux
or mass discharge.
And that was what we needed to basically supplement
the concentration-based conceptual models we normally deal with.
CHUCK NEWELL: OK.
So this originated back then for these MTBE sites, a lot of them.
But a lot's happened since then, right?
DAVE ADAMSON: Yeah, now we know that MTBE does attenuate
once it's in groundwater, but this mass-flux framework
has gotten increasingly important over the last few years.
CHUCK NEWELL: OK.
So let me just do a quick sort of cartoon example.
Here is this brief overview.
Most are regulations are concentration-based,
where that milligram-per-liter value in the well, the drinking-water,
is important.
And if it's greater than a drinking-water standard
like 0.005 milligram per liter for TCE, it's a problem.
So you can have these two sites.
And if the monitoring wells in both of them-- the one on the left
and the one on the right, those red dots-- have the same concentration,
it would indicate that the risk from both these sites are about the same.
DAVE ADAMSON: But, really, these aren't the same, are they?
CHUCK NEWELL: No, they aren't.
So if you look at the one on the left, it's
got a really big source zone, right?
DAVE ADAMSON: Yeah, I suppose sort of like a megasite.
CHUCK NEWELL: Right.
So that's what we call it.
It's got this big gray area in there.
So what we're thinking about, just an expand a little bit,
it's not just size, but it's also the groundwater velocity
that's going through there.
Let's assume then, on the left-hand plume,
the groundwater velocity is really fast.
It's so fast, it actually snaps off the monitoring wells when you drill them.
DAVE ADAMSON: Really?
CHUCK NEWELL: Sort of a groundwater joke there.
DAVE ADAMSON: OK.
CHUCK NEWELL: But very fast, very wide source on the left.
But on the right, sort of a tiny source, a really stagnant type of a source
that's out there.
DAVE ADAMSON: So what do you call this site on the right?
CHUCK NEWELL: Well, there's a really technical term in Texas
that we use for something like this, and we
call it basically a piss-ant site-- very slow groundwater velocities, very
tiny source, things of that nature.
But if you think about this from a concentration perspective,
these are the same risks.
But if you put in these other factors in there, then
you do see this distinction.
The one on the left is much, much of a bigger
problem than the one on the right.
So talking about these two different types of sites,
let's go on into our framework.
And back in 2000, this really great paper by Einarson and Mackay
was written that talked about MTBE contamination and this new framework
that they had.
So it describes this approach that could be applied for this MTBE problem.
And again the perception where there were
a lot of sources, that this MTBE could show
high concentrations for a short while, do a high solubility,
but overall these sources were relatively
small if you thought about it from this mass-discharge perspective
where you combined both this flow, the size of this concentration.
DAVE ADAMSON: Yeah, I liked the graphic they use in here.
They've got this nice glass of water with the MTBE in it.
But if you look here on the red lettering,
basically what they're saying here-- this
is a new framework for prioritizing environmental site cleanups that
considers interaction with contaminant plumes with water-supply wells.
CHUCK NEWELL: OK.
So let's open up the pages of this paper, go into it a little bit more.
And so we have basically a graphic here.
So let's describe that.
DAVE ADAMSON: Yeah.
And so you're essentially saying that you've got this source zone,
and it's got a plume associated with it moving downgradient towards the supply
well, so that little circle there.
But it's mixing with clean groundwater as it goes there,
and that capture zone, which includes both clean groundwater and the plume
itself, is designated by those dotted blue lines, right?
And then they've got a different view here basically looking
at the aquifer with the sort of plan view,
and you've got a supply well there and showing the plume moving
towards that supply well.
CHUCK NEWELL: So the idea is if you know something
about that well and something about that plume,
you can use this formula where you can figure out
the concentration of contaminants that are coming out
of that water-supply well in milligram per liter if you know two things.
And the first is this mass discharge in mass per time,
something like grams per day, that's leaving that source zone,
and Q, which is the pumping rate from that well.
So they have this formula, but you can also think about this graphically.
Here's a nomograph in the paper they use,
this solute-concentration nomograph.
Dave, what's on the X-axis?
DAVE ADAMSON: We got pumping rate, liters per minute-- so that's
pumping rate from that water-supply well in this case--
and then mass discharge in grams per day.
And those people that are more comfortable in English units,
I guess you've got pumping rate is gallons per minute
on the top part on the X-axis.
CHUCK NEWELL: So if you know this flow and you know this term, mass discharge
in grams per day of that plume, you can find where you are out here
and get, hey, this is this concentration coming out of this well.
And you can see, is it risky or not?
Now, have a lot of cautions in here.
They sort of talk about it.
We'll go to the next one.
The method considers dilution of clean water
with contaminated water in a water-supply well.
But they do say, we do not advocate reliance on in-well blending
to maintain water-supply standards.
So there's this distinction between using the method for screening
or prioritization versus using it in real life
to meet treat drinking-water standards.
But some systems do just that as a matter of course.
So it's a complicated question and a key thing
just to make sure you protect the users of water coming out of the well.
DAVE ADAMSON: This all sounds pretty good,
but where do you get the mass discharge number?
Where do you get the grams per day that's associated with this plume?
CHUCK NEWELL: OK.
So that's this key thing, the grams per day.
What we're really looking at, here's one method
that we'll talk about right now real briefly.
You get this transect going across that plume,
and then you take these measurements in different pieces.
This is some high-resolution sampling.
So see on the top-right panel.
Then you divide everything up into these boxes
and then you multiply these concentrations by the area of the box
and then by the Darcy velocity.
We're interested in how much flow is going through there.
And then add up all the boxes, and you get the mass discharge for that plume.
But we'll talk more about that next week.
DAVE ADAMSON: Yeah, and if you know the numbers,
you can learn a lot about how the particular releases might behave
and how risky they are.
And so let's look at a few of the conclusions from this paper.
CHUCK NEWELL: You know, they say, because there are many potential MTBE
sources, it could be extremely valuable to gather
this reliable mass-discharge values for gas stations,
the most common type of release site, and various hydrogeologic settings.
Such an effort could help to find the range of these expected
mass-discharge values, which would be very useful in this first approximation
of risk management for groundwater resources
that are known or suspected to be affected by these gas stations.
DAVE ADAMSON: OK.
Well, I know that these guys' work inspired some other people's work,
and maybe you wanted to spend a minute talk about that.
CHUCK NEWELL: Yeah, so let's go on to the next one.
This is actually a tool that I wrote for the American Petroleum Institute.
It's called the Groundwater Remediation Strategies Tool.
And with this particular document, there are a couple pieces in here.
But number one, it sort of tells you how this mass discharge framework works,
how you get some of these numbers.
And one thing to note is we called it mass flux back then when
we talked about grams per day.
Some of the terms have changed.
But you have this, and so what I'm showing here
are a series of these model runs.
This is something like the type sites we saw in the matrix-diffusion piece.
We're looking at the top left or different source-type things
where there's a decaying source over time.
Each line is a release at a different year.
Then each one of the other five graphs is the different combination
of biodegradation in the aquifer and things of that nature.
And so with this, you can see, if that release happened,
this is the sort of pattern that occurs as you
move into time, as you move away from the source, the distance from source.
But in these, of course, the Y-axis is what, Dave?
DAVE ADAMSON: The Y-axis is the actual mass flux
that you express as a percent of the original flux, sort
of a normalized mass flux in this case.
CHUCK NEWELL: Right.
Well, let's go ahead and, I think, wrap up now, right?
DAVE ADAMSON: Sure.
CHUCK NEWELL: And some of the key things that there's this concentration
paradigm versus mass-discharge paradigm, and I think both of them
are used together.
We're not saying get rid of the concentration standards,
but mass discharge can really help you understand these sites.
DAVE ADAMSON: Yeah, and one of things that it can help you do you, basically,
is use mass discharge to estimate the potential impacts
to pumping wells and surface water.
CHUCK NEWELL: And a key thing is, if you want
to know that estimated concentration in that well or a stream,
you take that mass discharge, divide it by that flow rate in the well
or the flow rate in that mixing zone, and you get this concentration.
DAVE ADAMSON: Yeah, and this whole approach
is really good for a first-order estimate of impacts
and basically allowing yourself to prioritize sites.
