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.
