(CNN) -- — West Virginia Gov. Earl Ray Tomblin asked for patience Monday from about 300,000 people fervently wondering just when it will be safe to turn their water taps on again.
Tomblin's Twitter appeal said state officials were working to restore water supplies after contamination that left parts of nine counties without potable water, but he didn't offer any estimates of when that might happen.
Officials had been expected to announce areas where residents could once again use tap water. A news conference was scheduled for midday Monday, according to CNN affilaite WCHS.
State officials imposed water-use restrictions Thursday after discovering that about 7,500 gallons of 4-methylcyclohexane methanol, known as MCHM, had leaked out of a storage tank a mile upriver of the West Virginia American Water plant. The chemical is used to clean coal.
Residents were told to use bottled water to wash hands, brush teeth and take showers. Police, firefighters and National Guard troops have been helping distribute bottled water to residents. In Kanawha County, the sheriff's office said it had canceled days off for deputies to help escort trucks and provide traffic control at congested distribution points.
The federal Department of Homeland Security sent 16 tractor-trailer loads of bottled water to help. The local water company also provided truckloads.
On Sunday, Tomblin said he thought the crisis could be near its end.
"I believe that we're at a point where we can say that we see light at the end of the tunnel," he told reporters. "Our team has been diligent in testing samples from throughout the affected area."
Once the ban is lifted, residents will be asked to flush their lines before using water again. Customers will be given a credit on their bills to cover the wasted water, the water company says.
The restrictions affect about 300,000 residents in nine counties. By lifting the ban zone by zone, officials hope to keep the water system from getting overwhelmed.
Jeff McIntyre, president of West Virginia American Water, said certain areas will be prioritized, including downtown Charleston, but decisions will also depend on test results.
The medical impact of the crisis was hard to assess.
"We've had a lot of worried-well calls," Dr. Rahul Gupta of the Kanawha-Charleston Health Department said over the weekend. He cited complaints of irritation of the skin, throat, chest and stomach that some residents have linked to possible exposure.
The unknowns made residents anxious.
"They don't even know what the health risks are," Stacy Kirk of Culloden told CNN affiliate WSAZ. "We had bathed, cooked and everything right before the news came on yesterday."
"I don't know anything about the chemical to say too much good or bad about it, so we're all up in the air," Arthur Taylor said. "We're common folks, we're not chemists."
Officials have warned water customers to watch for symptoms of exposure to the chemical such as skin irritation, nausea, vomiting or wheezing.
Karen Bowling, secretary of the state's Department of Health and Human Resources, said Sunday that more than 1,000 people had called the West Virginia Poison Center, concerned about their exposure to contaminated water. There have also been more than 60 animal exposures reported.
A total of 10 people have been admitted to three hospitals, none in serious or critical condition, and 169 patients have been treated and released from emergency rooms, Bowling said.
The goal: Less than 1 part per million
Officials will know that the water is safe for more than firefighting and toilet flushing -- its only sanctioned uses now -- when tests find less than 1 part per million of MCHM in treated water, McIntyre said.
Four laboratories have been set up to measure the levels in a uniform manner. "The treatment plant must consistently produce samples at or below this level before the current do-not-use order is lifted," he said.
West Virginia National Guard Maj. Gen. James Hoyer said Sunday that sampling and testing at the water treatment facility showed a consistent number of below 1 part per million for 24 hours at the outflow of the facility.
"This allows us to move forward to the next phase of sampling and testing throughout the system," he said.
Anxiety about effects of chemical
Linda Jordan of the West Virginia American Water Co. urged people to get medical attention "if they are feeling something ... isn't right."
Many did just that.
"Our emergency rooms have been very busy with individuals unnecessarily concerned and presenting no symptoms," the Charleston Area Medical Center said.
Dr. Robert Maha, chief medical officer for MedExpress, a group of seven medical clinics in the area, said many patients sought treatment for symptoms they worried were tied to the chemical exposure.
He said the water crisis may contribute to the spread of flu because people are having a difficult time finding clean water to wash their hands.
"That's one of our biggest concerns for the community," Maha said.
7,500 gallons leaked
The chemical overflowed a containment area around the tank run by Freedom Industries, which supplies products for the coal mining industry. It moved through the soil into the river.
Mike Dorsey, chief of the Department of Environmental Protection's Homeland Security and Emergency Response group, said officials estimate that 7,500 gallons -- the equivalent of about 10 hot tubs that can accommodate eight people each -- leaked through a 1-inch hole in the tank's stainless steel wall.
"It's an old system," he said about the physical plant, adding that the company had planned to upgrade it.
C.W. Sigman, the deputy emergency manager for Kanawha County, said the tank appeared to be "antique."
"When I see a riveted tank, I know it's an old tank," he said.
Dorsey expressed confidence that the chemical, which smells like licorice, did not start leaking long before Thursday morning, when it was reported. "We would have gotten odor complaints earlier than that if it had been going on longer," he said.
After concluding late Thursday afternoon that the tap water was contaminated, officials issued a stop-use warning to customers in Boone, Cabell, Clay, Jackson, Kanawha, Lincoln, Logan, Putnam and Roane counties.
Since then, the leaky tank has been hauled from the site, officials said.
Freedom Industries President Gary Southern said two Freedom employees noticed material leaking from a storage tank into a dike around 10:30 a.m. Thursday. They contacted authorities and began the cleanup process, including hauling away the chemical still in the tank and vacuuming some from the nearby ground, he said.
"We have mitigated the risk, we believe, in terms of further material leaving this facility," he said.
But Sigman, the emergency management official, said it didn't appear the company was in emergency mode when his team began to arrive Thursday.
"They didn't appear to understand the magnitude of the incident at the time, and we didn't either because we just got there," he said, adding that investigators from the Environmental Protection Agency had also arrived and took the lead in the investigation.
"It took a little bit of time to get a determination how serious it was," he continued. "I never got a good indication from the plant folks how bad the leak was, how much was going to the river, anything else. It was probably a little ways into the incident before we realized how bad it was getting into the river."
-- CNN's Joe Sutton, Tom Watkins and Ralph Ellis contributed to this report.