Written by John Boyle, Asheville Watchdog.
Asheville’s North Fork Reservoir will get another turbidity-reducing mineral treatment next week, as the lake’s murkiness continues to decline.
At Friday’s daily Tropical Storm Helene briefing, Asheville Water Resources spokesperson Clay Chandler said the key turbidity level dropped again since Wednesday, falling below 17 Nephelometric Turbidity Units (NTUs) by Friday morning, about a 1 point drop from Wednesday. Also, the amount of lake water the city is able to push into the system for filtration rose from 17 million gallons to 19 million, also a good sign as the city slowly heads toward providing potable water again.
The ultimate goal is to get turbidity down to 1.5-2.0 NTU, which would allow for optimal filtration.
Helene devastated the city’s water system Sept. 27, knocking out transmission lines from North Fork, which provides 80 percent of the city’s drinking water, and leaving the 350-acre lake a murky mess. Chandler said North Fork’s output capacity hovered between 17 million and 19 million gallons Thursday, until a 16-inch water line on Patton Avenue broke in the afternoon.
“And if you were anywhere in the area of Patton Avenue yesterday afternoon, you saw water on the street, but that’s been fixed,” Chandler said. “So that’s a good example that our ability to push treated water into the system is going to depend, obviously, on turbidity continuing to fall and no major breaks in the system.”
The city is pursuing a two-pronged approach to restoring potable water: reducing the reservoir’s turbidity through the installation of turbidity curtains to still the water, coupled with a four-day application next week of aluminum sulfate and caustic soda; and pursuing the installation of an Army Corps of Engineers mobile treatment facility at North Fork to treat high-turbidity water.
“They have verbally awarded the contract to a contractor for that project,” Chandler said of the Army Corps of Engineers. “That contractor will be on site early next week for a site visit, and the build-out will begin shortly after that.”
The timeline for that project’s completion remains late November or early December. Chandler explained that while the equipment, which allows filtration of high-turbidity water, will come in on trucks and is considered mobile, it still requires some construction, as well as determinations on what size of equipment is most appropriate.
“There is a lot of piping that’s going to have to be installed,” Chandler said. “Just right now, our estimate is that the piping is probably going to take up most of the build-out time. It’s not just a few feet — it could be as much as a couple of thousand feet.”
Additionally, the units have to be laser-leveled when set into place, as even a small amount of unevenness could cause problems or even damage.
Asheville Watchdog asked Chandler why the city initially said it could not process North Fork water until the turbidity level had dropped to 1.5 to 2 NTU but is now filtering 19 million gallons a day of water with 17 NTUs.
“We’re able to push treated water into the system due to the change not only in the overall level of turbidity, but also in the density of the turbidity-causing material,” Chandler said after the briefing. “Initially, the turbidity was thick mud, which would obviously wreak havoc on our filters. Since then, the overall level has lowered and the particles have ‘slimmed up,’ so to speak, so they can be washed out of the filters.”
By comparison, the water in the city’s Bee Tree Reservoir in Swannanoa remains a thick mud with a turbidity in the 140 NTUs range, down from more than 200 initially, he said.
The city needs North Fork to reach 27 million gallons of water per day being treated and sent out to fully pressurize and flush the system. That flushing and repressuring could take two to three weeks, Chandler has said previously, so a return to potable water could still not come until mid-December.
Chandler also noted that North Fork’s 20,000-plus acre watershed sustained “significant tree loss” during Helene, including from a possible tornado that twisted off trees half-way up.
“We don’t expect the quality of the water to decline,” Chandler said. “The tributaries that are pushing water into the North Fork Reservoir now are crystal clear.”
Starting next week, a contractor will begin the city’s third application of aluminum sulfate and caustic soda at North Fork. The materials help clay particles coagulate and sink, leaving clearer water to filter.
Asheville Watchdog is a nonprofit news team producing stories that matter to Asheville and Buncombe County. John Boyle has been covering Asheville and surrounding communities since the 20th century. You can reach him at (828) 337-0941, or via email at [email protected]. The Watchdog’s local reporting during this crisis is made possible by donations from the community. To show your support for this vital public service go toavlwatchdog.org/support-our-publication/.