U of M’s Scott Schoefernacker
Scott Schoefernacker is a senior research scientist at the University of Memphis' Herff College of Engineering. He served on the team that investigated whether or not Tennessee Valley Authority (TVA) wells drilled in the Memphis Sand aquifer posed a threat to Memphis’ drinking water.
That team’s final report was issued last week. Its biggest finding was that the Memphis Sand is hydraulically connected to the contaminated Mississippi River Valley alluvial aquifer, which sits above the Memphis Sand.
Waters from the two are mingling somewhat, Schoefernacker said, and that connection needs further study. But he said the city’s drinking water is not in immediate danger. — Toby Sells
Here are some definitions to help navigate Schoefernacker’s comments:
When water from a shallower aquifer flows into an aquifer below it.
Pump test -
Investigators ran TVA’s wells last year to see how they affected the aquifers.
Tritium testing -
A test to determine the age of water.
Memphis Flyer: Is there is a window between the Memphis Sand aquifer and the Mississippi River Valley alluvial aquifer?
It’s inconclusive. There’s a hydraulic connection between the two.
From what we saw from soil-test boring that we did, there is clay (between them). But when we did the pump test we saw there was a draw-down of the Mississippi River Valley alluvial aquifer. At some place there is a connection between the two, we just didn’t find it.
The biggest draw-down was east of the plant, toward the bluff. That kind of points us in that direction, but we don’t have any what we like to call control. We can infer that there’s something there, but we’re not 100 percent sure.
MF: Is that a huge concern? Is it something that we must explore?
It is something should further explore, obviously. But (the TVA is) not pumping right now.
So, (the connection) is there. But it’s just like all the other windows (in the aquifer).
That was another question that remained. Is it really an erosional surface, where the clay is absent, but you just can’t see it? Or, is it caused by a fault? Is the sediment offset to where somehow the Mississippi River alluvial deposits are in connection with the Memphis Sand?
We think that’s the case. The fault may have dropped us down so much that they’re in connection. But, again, we didn’t actually drill through it, or see it.
Typically, when we find these windows, you’ll drill and then you’ll see your sand and gravel deposits through the alluvial. Then, all of a sudden, you’re in the Memphis Sand. And you’re like, “well, we’re missing clay here.”
They mention in the report the Davis (well field). They actually drilled through a window just south of there. Like at Shelby Farms, you can drill straight through and not hit any clay. Those are the easy ones.
MF: The Mississippi River alluvial is where they found the contaminants from the TVA’s coal ash pond, right?
Yes, the arsenic concentrations. Yes.
MF: The fear is that if that one is contaminated, a draw-down could draw some contaminant into the Memphis Sand. Is that right?
Yes, but it hasn’t. We haven’t seen concentrations like you see north of the plant. I’m sure they’ve done sampling since, and you can do a (Freedom of Information Act) request from (the Tennessee Department of Conservation) for those. I haven’t seen them personally.
From what we saw at the test, the concentrations in the shallow (aquifer) were at at the highest 5.44 micrograms per liter. The (maximum contaminant level) set by the (U.S. Environmental Protection Agency) is 10. They’re half of that. And in the Memphis (Sand) it was even lower than that, an order of magnitude lower than that.
We did see some mixing of water when we did the sampling in the production wells but that was kind of limited to one of the shallower wells where the screens are split. There was a clay layer the ran into in the Memphis Sand and so they had to split the screen on top and below there. The water quality wasn’t as good as the other production wells.
They did the tritium testing and you can see in that well there was a good hit of tritium, which means there was young water in it.
MF: In the study, it said some the findings pre-dated the well pump test.
We did sampling pre-test. Then we did all those wells and then did the pump test. Toward the end of the pump test we did a sampling as well. We did a quick-grab sample for tritium again and and a couple of inorganics.
The tritium dropped, meaning that the water being pulled in during the pump test was primarily what we expect from the Memphis (Sand aquifer). That means the shallow (Mississippi River Valley alluvial) wasn’t really re-charging.
If you had a really good, clear connection…the feeling was the if we ran the pump test you’d see dramatic draw-down and water quality shift. But the fact the tritium decreased…may have indicated that it’s actually pulling water from the Memphis (Sand) and not being recharged in the shallow.
You don’t want it to be recharged from the shallow. That’s where all the crud is, generally.
MF: The “crud.” Thank you for a scientific term I can understand.
That’s my scientific lingo for the day. (Laughs.) When I talk to people and they [don’t understand], I just say, “there’s crud. All the modern crud is in the shallow and then the Memphis is better water quality.”
MF: Would we be in more danger from contamination getting into the Memphis Sand aquifer if TVA officials were running the wells right now?
It really depends on the chemistry of it. Obviously, there was some impact prior to the wells being there. Those aren’t the first production wells in that area. They probably won’t be the last because a lot of the industries have their own private wells.
There’s also an overall downward vertical gradient. So, the hedge between the two aquifers — the Memphis is lower. So, you have a natural flow downwards anyway if there is a conncetion. That doesn’t help. But that’s generally across the area.
MF: The concern, I think, was that the wells were so close to that coal ash pond. I know they’re half a mile away. But folks were concerned that being so close to the contamination…
You have to think about the wells were going to be used for, too. They’re being used to cool the plant. So, it’s not being used for drinking-water purposes. We have our well fields. I think the closest well fields are Davis and Allen.
If this production wells from the TVA went online, they’d probably be pulling contaminants towards (the Allen plant). I’m not saying that’s actually going happen but…it would likely be just pulling it toward (the plant). So, it wouldn’t really impact production wells at the other well fields.
Over time, it may cause contamination influx in there but it’s a guess at this point.
MF: You said the water from the Memphis Sand and the Mississippi River Valley alluvial were mingling before the wells were drilled.
Yes, and the fact the water is already mixing there and what’s the cause of it — it’s more of a planning issue. Do we want to further develop that area and sink production wells there? Or, do we want to pump in water from (Memphis Light, Gas & Water) or some other source? It goes for that area as well as other parts of town.
MF: But our drinking water is not necessarily in danger with the waters mingling now, right?
No, not right now. TVA has already [decided on another path]. That’s already been dealt with. Now that they’re taking water from MLGW and built those large storage tanks on site.
So, unless they can remediate the arsenic contamination and call it clean, I don’t think they’re going to fire up those wells any time soon.