Austin officials told the American-Statesman on Thursday that the city's boil water rules could be lifted as soon as Sunday.

Austin Water Director Greg Meszaros said Sunday was the "operating target" date for ending the boil water notice issued early Monday. Debris and sediment that washed into the Highland Lakes from Hill Country floodwaters have strained operations at the utility's water treatment plants. Bacteria, parasites and viruses could attach to those particles, experts said, though tests haven't revealed any such bacteria so far.

With the water utility struggling to get back on its feet, Mayor Steve Adler on Thursday signed a disaster declaration for Austin, as state and county officials already have done. The move allows the city to seek reimbursement for expenses related to recent flooding, officials said.

Meszaros and Adler first mentioned the Sunday goal at a news conference Tuesday, but officials were careful to couch it in terms of the variables at play and the work required.

After the city's tap water failed state standards in turbidity tests late Tuesday because too many particles were found in the water, the city utility must follow state protocols to lift the notice.

Meszaros said the utility also must ensure its plant capacity is adequate. Output remains about one-third of normal, he said, but it is slowly ratcheting up.

“We want to see the river water quality continue to improve,” he said, adding that Wednesday’s rains didn’t seem to downgrade river water quality.

Austin Water is working to refill its tanks and reservoirs. The utility normally stores one day’s worth of drinking water to maintain pressure and keep up firefighting reserves.

“We drained down the bulk of that water, and we’re at 50 percent storage now,” he said. “We want to get to at least 75 percent storage — and really closer to 100 percent. As we do that, the system will continue to strengthen.”

He said the utility is in contact with the Texas Commission on Environmental Quality about what the state needs to see before allowing the utility to lift its boil water notice.

“We do bacteriological testing throughout the system every day, but they’ll want us to do that in an enhanced way,” Meszaros said. He said the state agency will tell the water utility about its testing expectations.

“They’ll want us to demonstrate adequate chlorine residuals” — the amount of chlorine in the drinking water — “to be protective. I don’t anticipate that to be an issue.”

He said the commission also will require that water samples meet state turbidity standards — and perhaps even more stringent standards.

“We’re moving ahead gingerly,” Meszaros said. “The system is still fragile, and we don’t want to push it too hard.”

Commission spokeswoman Andrea Morrow said it is providing technical and operational assistance at the city's Emergency Operations Center.

According to agency rules, a water utility violating state standards must correct the underlying problem and show that the water "does not pose an acute health risk." Sometimes the entire system must be flushed and disinfected.

The system also must return to normal operating parameters — power restored, required pressure, no excessive turbidity — and maintain required minimum disinfectant residual levels at all parts of the distribution system. A system also must collect bacteriological samples and obtain negative results of coliform bacteria, which are a standard indicator for the sanitary quality of water and food.

Only then can the boil water notice be lifted.

Regional experts pointed Thursday to the recent history of drought and rain in the Hill Country to explain the overwhelming debris that gummed up Austin’s water treatment plants.

“My gut sense, my ecological sense, is that it’s because it’s been a long, long time since we’ve had a real gully washer out here,” said Tom Arsuffi, director of Texas Tech University's Llano River Field Station in Junction, where he teaches courses in stream ecology. “We haven’t had a significant flood event here since the big drought ended in 2011."

"Dust settles, soil, you get debris, you get all kinds of things," he said. "All that stuff gets flushed.”

Arsuffi said big flood events, like this month's on the Llano River, scrub the river and the land.

“River-bottom rocks are normally slimy, covered with algae and biofilms. You pick up stones and rocks in the river after something like this — they’re like sandpaper," he said. "You’ve now got kayaks and paddle boards and things up in the trees. That gives you an impression of the erosional potential at work here.”

Katherine Romans, executive director of the nonprofit Hill Country Alliance, said that “like the 2015 Blanco Flood, it is hard to pin the exact cause of the incredible magnitude of flooding (or of siltation in this case) on anything other than the perfect storm of circumstances that dumped more than 15 inches of rainfall on the headwaters of the river in a very short amount of time.”

She said of the Llano flooding: "More than 10 inches of rain in the upper reaches of that river basin meant incredible flash flooding that had a big capacity for moving sediment.”

Meszaros of Austin Water said it’s “hard to say precisely” why this flood led to boil water orders — as opposed to previous Hill Country floods.

“I don’t think the Llano River has flooded like this in at least 30 years," he said. "A whole lot of silt and sand and debris had built up over last 30 years. This scoured that entire system out."

The utility's staff never experienced anything like that, Meszaros said.

"It moved how many millions of tons of silt and debris downstream? And a lot of fine particles – it’s especially hard to get them to settle," he said.

Meszaros said the question of why other cities that get their water from the Colorado River, like Cedar Park, have not had a boil water notice would be something Austin Water would be examining. He said the difference could be a combination of the depth of the intake pipes, their positioning relative to river dams, and the method of treating water.

Referring to Statesman aerial photos that show how muddied the Colorado River has become, he said, “Oh, my God, how can we make that into drinking water?”