Deluge challenges water system
The powerful storm that rolled through Garland County earlier this week was part of the perfect storm that beset operators of the city’s water treatment and distribution systems.
Multiple inches of rain fell in less than two hours Monday, with Hot Springs Memorial Field reporting 3.44 inches. Runoff from creeks and streams that empty into Lake Hamilton poured into the lake, carrying mud and debris with it.
To regulate lake levels, generation ceased at Blakely Mountain Dam, causing mud from the watershed’s runoff to move upstream toward the dam and intake of the city’s Ouachita Plant on Cozy Acres Road. The resulting mud slick degraded the city’s raw water source, creating turbidity levels operators haven’t seen in years.
“We had to stop production at the plant for a little while until the turbidity in the lake cleared up,” Hot Springs Utilities Director Monty Ledbetter said. “We had 57 (nephelometric turbidity units). That’s the highest we’ve seen in at least 10 years. That’s a light measurement that tells us what particles are in the water.”
Ledbetter said the plant, which supplies most of the potable water for the city’s regional system, stopped pumping water into the distribution system for about three hours Tuesday while plant operators waited for sediment in the water to settle at the bottom of sedimentation basins.
“When your raw water quality goes way down like that, you have to add a lot more chemicals to create the floc and get the settlements you have to have,” he said. “There was a time we had to quit production for a little while and let things settle in the basin.”
Ledbetter explained that negatively charged water particles join with positively charged chemicals used in the flocculation process. The flocculant binds with the water and falls to the bottom of the basin.
“When it gets really muddy like that, there’s a lot more particles and it takes longer for them to settle,” he said.
Ledbetter said Owl Creek, which empties into the lake on the opposite side of the Ouachita Plant, gets laden with mud and debris during heavy rains. The mud usually takes about four hours to creep upstream to the intake, but the amount of rain Monday delivered the mud slick in 20 minutes.
“It looks like a wall of mud that’s coming at you,” he said. “It just steadily moves up the river channel when there’s no flow coming from the dam.”
Operators pulled water from the city’s constellation of storage tanks while the plant was
offline. To refill the tanks, the flow had to be increased when the plant started pumping water again Tuesday afternoon. Higher flow rates may have caused dingy-looking water to come out of some taps, Ledbetter said, collecting minerals from the interior of ductile iron pipes in the distribution system.
“If you have a 12-inch waterline, on a normal day you may only be using the center 2 inches of the waterline,” he said. “But when you’re trying to fill tanks quickly you open up the valves wider and you go from a 2-inch normal flow to a full 12-inch flow in that line. You’re moving a lot of material that’s been sitting there for a while. It’s like when the fire department flushes a fire hydrant, it stirs up the system.”
Ledbetter said Monday’s booming thunderclaps may have also upset finished water quality.
“The lightning and thunder that was shaking the buildings was also shaking the waterlines and disturbing the water,” he said. “
Production was increased at the more than 70-year-old Lakeside Plant to make up for the disruption at the Ouachita Plant. Ledbetter said it was operating at its 4 million-gallon a day capacity to keep up with demand.
The Lakeside Plant is at the trailhead of the Northwoods Urban Forest Park and treats water from the city reservoir at Lake Ricks. It feeds the Holly Street tank that serves the north end of the city. The water normally stays there, but operators had to open valves to the twin tanks on Cooper Street while the Ouachita Plant was out of service.
Sending water from Holly Street to Cooper Street reversed the normal flow.
“We were feeding water from Holly Street pressure plain back into the twins pressure plain to put more water into the system,” Ledbetter said. “When you do that you’re reversing flows. That hurts you too a little bit with dingy water.”
Ledbetter said raw water turbidity won’t be an issue at the Lake Ouachita intake the city will build as part of its $100 million water supply project. The intake site is in a cove east of the lake’s main channel.
“The NTUs don’t fluctuate on Ouachita hardly at all, especially where we’re taking it,” he said. “We’re not taking it out of the main channel, and we’re taking it about 40 feet below the water level. The water down there is a lot better quality.”
The city said the California lab that tested samples taken from the site noted it was the cleanest surface water they had tested.
“They made the comment to us that it’s the best water they’ve ever seen,” Ledbetter said.
The intake above Blakely Mountain Dam will feed the Ouachita Plant in addition to the 15 million-gallon a day treatment plant the city will build off Amity Road, giving the Ouachita Plant two raw water sources.
The Arkansas Department of Health rated the Ouachita Plant’s capacity at 21 million gallons a day. The city’s 2005 withdrawal agreement with Entergy allows it to pull up to 30 million gallons a day from Lake Hamilton, but usage cannot exceed a 20 million-gallon-a-day average calculated over a rolling three month period. The Hot Springs Board of Directors exercised the agreement’s renewal option last year, extending it for five years.
The water system produces about 15 million gallons on a day of average demand and more than 20 million gallons during high demand days, according to Health Department surveys.