CHUCK NEWELL: Well, we've been talking about dilution as an attenuation process sometimes. Today, we're going to go continue this theme with a discussion about dilution and mixing in groundwater, and how this is a potential attenuation process. DAVE ADAMSON: Yeah. We've said several times that the concept of dilution. It's sort of controversial. But we should really think of the idea of mixing as an integral part of the groundwater business. CHUCK NEWELL: And other businesses. It's an integral part of surface water, think about mixing zones for wastewater treatment plant discharges. It's really part of air pollution. And in some cases, it's a part of this groundwater management of plumes, you can use dilution. DAVE ADAMSON: Yeah. And way back in week one, we talked a lot about the US EPA MNA directive back in 1999. And it talks about attenuation processes, and it says that it prefers destructive mechanisms, but it does list dilution as an attenuation process. CHUCK NEWELL: That's right. So let's go ahead and start, and just give you some examples of where dilution is used in our business. And let's start with a slide from the Texas Risk Reduction program. So here on the screen is a formula for trying to figure out the safe concentration of contaminants in soil, in saturated soil, that could leach into the groundwater. And it accounts for the amount of leachate generated, the amount of clean groundwater that flows and mixes with that leachate once the leachate drops down and enters the aquifer itself. DAVE ADAMSON: So Chuck, what is this U gw term? CHUCK NEWELL: So in here, that's the actual groundwater Darcy velocity. So the more groundwater flow you have, the more it mixes and the higher this leachate dilution factor, that LDF is. DAVE ADAMSON: And this is for Texas, but I know this type of system has been codified in other states, right? CHUCK NEWELL: That's right. So a pretty common use of this type of dilution calculation to manage safe contaminant levels in soils itself. So let's go a little bit deeper, and we'll talk a little bit about submerged sources. I'll talk about this DAFfy graph tool. It's from the American Petroleum Institute. It was developed by Dr Paul Johnson, who's now the big cheese, the president of the Colorado School of Mines. And so while the equation before we saw was for unsaturated zone sources, we're now going to look at a contaminant source that's in the aquifer itself. DAVE ADAMSON: I see this term DAFfy graph. Why is it called DAFfy graph? CHUCK NEWELL: OK. So DAF is the dilution attenuation factor, that's key thing you get out of this document. And well, the fy, maybe something to do with ducks? No, I don't know. But you'd have to ask president Johnson about that. But Dave, let's go to the conceptual model here on this next slide. What do we got going on here? DAVE ADAMSON: All right, well, here on the x-axis is distance. And so we've got groundwater flow moving from left to right in this case. And we've got groundwater then on this y-axis with depth. We've got, on the left hand side, this h, that's this 1 meter thick source, in this case, on the left hand side. And then we've got H w as you move to the right. That is basically the screened interval length, 3 meters in this case. CHUCK NEWELL: And this is, we're looking at a slice in groundwater. We're looking from the side, and so these are all depths into the aquifer, right? DAVE ADAMSON: Mm-hmm. CHUCK NEWELL: But H and H w are these two key things. So let's go ahead and go through how the DAFfy graph works. So it's got a bunch of these tables in here or these graphics. And you get your variables, you put them down in here into the x-axis, then you go up these curves, you take a right hand turn. And the things you get off the y-axis, you plug into this DAF, this dilution attenuation formula right in there. So let's go that a little more detail. Some of the key factors that we have in here, things like the area of the source, the aquifer thickness, distance to the receptor. Hey, there's your screened interval you talked about. There's ground water velocity. So all these things go together to tell you how much of this dilution occurs as well as a few other things, such as biodegradation. But it accounts for this mixing in the well, it accounts for dispersion, things like that. DAVE ADAMSON: So I can see up in the upper right, there's the monitoring well screen length divided by that H w term, dividing through, basically, the source thickness in this case, the H term. And you've got the monitoring well device, that's basically serving to mix ground water, in this case. At if the plume is entering at the upper half of the monitoring well screen, and clean groundwater is moving in at the bottom of the screen, there's dilution that's basically going to affect the concentration that you'd see in that monitoring well, right? CHUCK NEWELL: Just from the instrument itself, just from the monitoring well. So the DAFfy graphs include dilution of the plume with groundwater from infiltration from above and the amount of dilution that occurs as the plume spreads out, moves down gradient. Most groundwater fate and transport models are built with this kind of dispersion term, which acts as a weak force, really, to dilute a plume. Let's go on to another example where surface water dilution's used to manage groundwater plumes. If we go to our next slide here, again, from the Texas Risk Reduction program. This shows the safe groundwater concentration that protects surface water. That SW GW on the left there, that's the concentration in groundwater that is going to protect the fish and things like that there in a stream. And so this is the safe water level, it's called a RBEL, risk-based exposure level. And all you need to do is figure out what that dilution factor is of this groundwater plume entering surface water. DAVE ADAMSON: So how do you establish what the correct dilution factor is? CHUCK NEWELL: OK. Well, you want to know how many gallons of water are entering into that stream. So we're interested in volumes of water. Do we use seepage velocity or do we use a Darcy velocity here? DAVE ADAMSON: Well, if I recall correctly from last time, we're interested primarily in a flow here, so I'm going to say the Darcy velocity. CHUCK NEWELL: So don't use porosity for that. So right. So you remember number six of Doctor Siegel's top groundwater list. So you multiply that Darcy velocity by this vertical cross-section, that area, as this plume discharges into the stream. And you get some flow rate in something like cubic meters per second. Then you get the flow rate in the stream itself or the flow rate in your mixing zone and have that in cubic meters per second. And that can give you this amount of dilution that occurs, the amount of contaminated stuff coming in from the groundwater plume, clean groundwater coming upstream from the stream, and you get this DF factor. DAVE ADAMSON: OK. So is this used frequently? CHUCK NEWELL: In a lot of state regulatory programs it is, yes. There's also something similar in the Federal RCRA regulations called ACLs or alternate concentration limits. So this slide here talks about this. This is that alternate concentration limits. There are these three surface water specific rules when you're applying ACLs, and I've got them down here. So the guidance is on the left. The idea is that, if you have a regulated unit-- you see that red-- that's the plume. The plume must have already reached the stream itself. So that's rule number one. Number two, you can't have any statistically significant increase over background concentrations of that contaminant in the stream. So sometimes you might be able to use a detection limit as your control out there in the stream. And of course, you've just got to protect all the receptors that are out there. DAVE ADAMSON: So this ACL approach, how often is this used? CHUCK NEWELL: Not that much. There's, maybe, I could find 20 or 30 examples. The requirements were really tightened up in a 2005 US EPA policy document. So it's not used that much. I think the US Environmental Protection Agency, they prefer not to rely on dilution from surface water to manage groundwater. But other regulatory programs do use it. So we have a few slides about how this dilution factor can be calculated, a little bit more about the surface flow, if you want to know something like that. DAVE ADAMSON: I know an important concept then, ends up being mixing zones in this case. So here, next slide, we got a little bit about that. And a lot of this is the state of Oregon, right? And a lot of these regulations deal with wastewater discharges. CHUCK NEWELL: That's right. So again, this is wastewater. This is not for groundwater. But just to show you how some of this mixing zone works, this is their NPDES guidance, and it talks about acute and chronic mixing zones on the left, in terms of the toxicity of this thing to the fish and other things in the stream. And it shows you how the mixing zone is defined by a length and a width. If you go onto the right hand side, the length is a function of the string width. And the mixing zone is always less than 25% of the stream, it's only a narrow band to make sure that the fish can go around that mixing zone, that they can avoid it, things like that. So that's one example. Let's go to another approach is to use a seven day, 10-year low flow as a stream flow, that flow rate when you do your dilution factor calculations. So how does this work? Well, you take the average weekly flow over many years, then figure out, based on statistics, the value that you might expect to occur every one year. So here's one graph. Dave, what's on the y-axis here? DAVE ADAMSON: So we're talking about surface water discharge with units of cubic feet per second. And looking at time, then, on the x-axis. So looking over the course of about a year. CHUCK NEWELL: About a year, right. So if you're looking at this, this is the flow. They're measuring the flow in that stream. And it's pretty high in the middle. It gets over 2000 cubic feet per second. And then the low is over here on the right hand side. So let's look at that in a little more detail. So right there, I think that's maybe the seven day low-flow period. And if you average those seven days, you can get this flow rate. And then that gets used in the statistical calculations. So now let's look at this. So now, we have a different graph. Again, it's this average minimum, seven day flow in cubic feet per second on the left. So each one of the xes on the lower graph is a year. And so our value falls in right here. But then use a special type of paper or a computer program to look at a non-exceedance probability. And with that, you can figure out how often these low flows, these really drought periods occur. So for the green one, every two years, what type of flow might you expect as being the lowest flow in there? DAVE ADAMSON: So you'd have to use that green arrow right there, and you're going to what, about 25 cubic feet per second in this case? CHUCK NEWELL: That's right. So you get that as your low flow about every two years. Hey, let's look at the blue one. This is about a 0.1, that occurs every 10 years. So this is the droughts getting worse, that flow rate. DAVE ADAMSON: It'd be a little bit lower then. In this case, it's the blue line going to about 10 cubic feet per second. CHUCK NEWELL: OK. So that's how you might get this number. And then again, those numbers are used if you're doing some sort of mixing. Regulatory programs will pick a particular value. The Texas Risk Reduction program uses the green line, basically, a seven day, two-year frequency to use for these groundwater to surface water dilution equations. DAVE ADAMSON: One other thing on this graph. I see this is USGS Boggy Creek, Texas. So I'm not familiar with that one. Where is that? CHUCK NEWELL: OK. So this is actually data from somewhere else. I just put over some other labels. But that's actually the creek that goes through my grandparents' ranch out there in West Texas, actually called Boggy Creek. So hats off to them in Albert, Texas, Throckmorton county. OK. Let's wrap it up. Let's go through some of the key points. First is that mixing is used in a lot of different risk assessment studies, a lot of the different risk assessment programs. DAVE ADAMSON: And then you've got to identify and evaluate these parameters, things like infiltration and groundwater mixing zone thickness and dispersion. CHUCK NEWELL: Right. And then surface water mixing is allowed in some cases. Low-flow surface water techniques sometimes come into play, such as this seven day, 10-year or seven day, two-year low-flow thing that we talked about, along with something like base flow estimation. DAVE ADAMSON: Mm-hmm.
