The Struggle To Restore Drinking Water After Hurricane Helene

KATHLEEN DAVIS: This is Science Friday. I’m Kathleen Davis. When Hurricane Helene barreled through the Southeast US in September, it dumped an estimated 40 trillion gallons of water onto the region. That resulted in immense flooding, which destroyed roads, buildings, power lines, and other infrastructure.

And it also impacted the region’s potable water supply. For weeks, residents in Asheville didn’t have access to drinking water, which comes from a local reservoir. And we actually heard from two listeners based in Asheville who had some questions about why this purification process can take so long.

SARAH: Hi, Science Friday. This is Sarah–

ALISON: –and Alison–

SARAH: –from Asheville, North Carolina.

ALISON: Unfortunately, the storm destroyed all of the main distribution lines coming from our reservoir, leaving the entire city without running water for weeks. But now that those lines are mainly functional, the reservoir turbidity levels are so high that they cannot run the sediment-filled water through our treatment plant.

SARAH: We’d like to know more about the science behind the turbidity issue and why, even though it has been dry for weeks after the storm, the sediment won’t settle. It feels like this is an intersection of geology, chemistry, even physics that impacts our everyday lives. So we might as well be curious and learn from it.

ALISON: We love the show so much. Thank you. And also, we’d like to say thanks to all the Water Resource Department employees and outside helpers that have been working tirelessly on this issue on behalf of the city of Asheville.

KATHLEEN DAVIS: That was recorded shortly before limited potable water access was restored earlier this week, marking the first time in 53 days that its residents didn’t have to boil tap water. So why can it take so long for cities to clean their potable water after a storm like this? And what could other regions expect from the aftermath of future extreme weather? Here to tell us more is my guest, Dr. Sarah Ledford, Associate Professor of Geosciences at Georgia State University, based in Atlanta, Georgia. She studies the role and impact of water in urban areas. Dr. Ledford, welcome to Science Friday.

SARAH LEDFORD: Thank you so much for having me.

KATHLEEN DAVIS: So can you sum up for listeners what’s happened to Asheville’s water and the region since Hurricane Helene?

SARAH LEDFORD: Yes, absolutely. I think there’s really been three steps in the process after waters receded. The first is the rebuilding of any infrastructure that was washed out. So if pipes were washed away, those had to be rebuilt. We had to get power back up to the water treatment plant.

The second problem is exactly what the callers were talking about, which was the unprecedented volume of water that came in, washed huge amounts of sediment into the reservoir. And in the mountains of North Carolina, that includes a lot of clays. And clays take a long time to settle to the bottom, but we can’t put that water through the drinking water process until those clays are removed.

And so that’s a lot of what Asheville’s water department was working on. It was trying to do what they could to speed up that process of clay settling. And then the final problem is that after the water is potable at the drinking water plant, the entire system has to be flushed so that we can make sure all pipes throughout the whole distribution network have clean and safe water.

KATHLEEN DAVIS: So it sounds to me like city treatment plants are set up to filter certain things, but maybe they’re not set up to filter things that are unusual, like clay.

SARAH LEDFORD: Yeah for many places, that especially ones that pull water from a drinking water reservoir, that reservoir is the assumed place where settling will happen. Other places that might pull water from a stream or river may actually have a tank in their system where they do settling. But when you have a reservoir that usually does this process very well for you, why would you build extra infrastructure that you don’t typically need.

KATHLEEN DAVIS: Yeah. So our listeners mentioned that the water in the reservoir has a high turbidity level. I’m not very familiar with this concept. So what is high turbidity, and what does it have to do with potable water?

SARAH LEDFORD: So high turbidity is basically a measure of how much light gets scattered by water. When you have water that is very clear, light that you put into that water can go on basically forever before it gets scattered or reflected back to the measurement device. When you have water that is full of sediment, especially clays, that light can’t get very far before it reflects back to you. And it looks cloudy to us as humans. And so high turbidity is used as a surrogate measure for high sediment in the water.

KATHLEEN DAVIS: So our listeners, Sarah and Allison, asked why the sediment in the water is negatively charged and flat shaped. How does that impact the purification process?

SARAH LEDFORD: Yeah, clays are a byproduct of a lot of long, long processes of weathering of rock. And clay minerals, which make up most of what we colloquially call clay, are what are called phyllosilicates. And these are a class of minerals that are formed in sheets. And so that’s why the clay is flat is that the way that all of the atoms are bonded together results in sheet-like formations.

The other thing that clays have is they have a high static electricity on the outside of the clays because those oxygen atoms that are at the outside of those bonds result in those sheets of clay having a negative charge on the outside. And then when negative comes close to negative or two clay particles come near each other, they’re going to repel each other because they’re both negative.

KATHLEEN DAVIS: So our next question is also from our listeners.

SARAH: They’re using aluminum sulfate and caustic soda to try and bring the turbidity down. How do those chemicals interact with the sediment.

KATHLEEN DAVIS: So yeah, how do you even begin to clean this.

SARAH LEDFORD: So what they’re using are actually techniques that are typically used at wastewater treatment plants, because this is a process we do when we treat wastewater at the other end of the system. They’re trying to get the sediment to form into flocs. And what that means are just larger particles.

So the chemicals that they’re adding are trying to overcome that negative repelling action and cause clay particles to start to stick together. And then the larger and larger, the particle becomes, the easier it is for gravity to pull it to the bottom of the lake. And then they can pull the clean water from the top of the lake into the drinking water plant to be treated.

KATHLEEN DAVIS: So our most recent update from Asheville is that the water is safe to drink, but it’s not totally clean yet. And city officials are recommending to not use large amounts of the water for landscaping and long showers. How long might it take for things to get completely normal?

SARAH LEDFORD: The good news is that if the city is saying it’s safe to drink, that means that there’s no bacteria or pathogens in the water. And as humans, that’s really one of our biggest concerns about safe drinking water is we don’t want anything that’s going to make us sick in the water. From my reading, it sounds like the turbidity is still a little bit high, but it’s coming down very quickly and much closer to the pre-Helene conditions. And so I don’t know that we really have a clock at this point because the city has said it’s safe to drink.

The other question, though, I think really has to do with the volume of water, and that’s probably why they’re encouraging folks not to water their lawns. They want to make sure that the water is being used for the most important purposes and for human consumption because it may still be waiting for other parts of the lake to have the turbidity come down. And we don’t want to pull too much volume of the lake into the treatment plant.

KATHLEEN DAVIS: So after the hurricane, there was a lot of news around Asheville that it could no longer be considered a climate haven. We’re going to continue to see more extreme weather events like this in the future. Can other regions expect this kind of problem with their drinking water if it’s based in an open-air reservoir that maybe they haven’t experienced before?

SARAH LEDFORD: I think, absolutely. I think what we know about climate change is that it’s increasing the ability of the atmosphere to hold water vapor, which means extreme precipitation events are going to become more likely. The other compounding factor that we get in regards to drinking water reservoirs and climate change is wildfires.

One of the things that helps keep sediment from flowing off the environment into a drinking water reservoir is the vegetation that holds all of that material in place. I think another really big risk that we have coming forward is if areas that have had wildfires come through, get large rain events on them afterwards that are areas that are source water for drinking water reservoir, we’re very likely to have a similar problem, potentially even with less rain.

KATHLEEN DAVIS: Is there a way for local governments to prepare for this?

SARAH LEDFORD: Continuing to try and make sure your source water area is protected, that it’s not being developed, because that’s going to be the best chance that we have for keeping vegetation in place. I think that’s a really key piece of the puzzle. Another one is continuing to use controlled burns to mimic the natural wildfire patterns, which will then hopefully decrease the ability for a very extreme wildfire to come through and do as much damage, is another key piece.

KATHLEEN DAVIS: So you study community engagement in urban water management. Do you have any thoughts about how residents of Asheville and other cities can get more involved in their water systems in this kind of context?

SARAH LEDFORD: Yeah, I always encourage everyone to try and do what they can to understand where the money that they pay in their bills is going. So you’re paying a water and a sewer bill every month. If you’re getting drinking water from a centralized system like this, how is that money being distributed?

I also encourage people to continue to try and understand the double whammy that many drinking water facilities are facing, which is the potential that they’re going to have to treat new and more difficult chemicals, for example, PFAS concerns, while also making up for the fact that across the United States overall, we’ve underinvested in our infrastructure and we’re having to play catch up to repair it and get it back up to the best quality.

KATHLEEN DAVIS: Well, that is all the time that we have for now. Thank you so much for explaining this to us.

SARAH LEDFORD: Thank you for having me.

KATHLEEN DAVIS: Dr. Sarah Ledford, Associate Professor of Geosciences at Georgia State University based in Atlanta. And thank you to our listeners, Sarah and Allison from Asheville, North Carolina, for writing in. If you have a science question about an issue in your city can reach out to us at SciFri@ScienceFriday.com. That’s SciFri@ScienceFriday.com.

Copyright © 2024 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/