Baltimore Public Works officials said 40,000 people in Northwest Baltimore County who have been without water since a main break early Saturday will remain without water overnight and are likely to have minimal water or none through part of the day Monday.

Residents north of Pleasant Hill Road, including Reisterstown and Glyndon, remain affected. Immediately after the break at 2 a.m., about 100,000 residents along Reisterstown Road were left without water. As of 6 p.m. Sunday — 40 hours after the initial disruption — water still had not been restored to a fifth of that geographic area, according to Kurt Kocher, a spokesman for Baltimore's Department of Public Works.

Workers are trying to find a way to accelerate the slow process of filling water tanks that were drained after the break.

Kocher said the main Pleasant Hill tank was filling at a foot an hour. It needs to gain another eight feet before Public Works will be able to pump water to residents in the community who have been without water for almost two days.

Kocher said residents who live south of the area that is still affected, such as those in Pikesville, should be sparing in their use of water to help their neighbors to the north.

The process of restoring water could take longer, Kocher said, if the residents to the south don't cooperate. "Every time someone uses the water unnecessarily that does really slow the process down," Kocher said.

"You can do the basic things for personal hygiene like brushing your teeth, flushing toilets and washing your hands. And if you take a shower, try to keep it as brief as possible. Waiting until the end of the day would certainly be helpful," Kocher says. "Please let the dishes pile up in the sink or in the dishwasher. Today is not a laundry day."

The break occurred in a wooded area under a stream, making it difficult to locate the source of the break. Officials don't know what caused the break in the 36-inch concrete pipe, which dates back to 1956 or earlier. The cause won't be known until the pipe is dug up and taken to a lab, Kocher said.

"It's an older pipe. It's getting toward the end of its useful life," he said.

Water is being re-routed through a 16-inch pipe, but it will be a slow process to refill the seven or so tanks that lost millions of gallons in a short time. In fact, two tanks nearest the break lost 1.3 million gallons in 36 minutes.

Some worry water won't come fast enough.

Scott Bass, a Social Security Administration contractor who lives in the Hathaway development near Reisterstown Road, says he's frustrated by a lack of online updates from the city. He's starting to consider hotel options for Sunday night, as "I would pay 70 bucks to be able to clean up and shower."

Reisterstown resident Mark Simon said he noticed that his water had dropped to a trickle at 6 p.m. By Sunday morning, he had no water. Simon, 50, said he has a disability and it's difficult for him to get to the store to buy water. "I called everyone and anyone I can possibly call" to find someone to buy him water, he said.

But Sunday morning Simon had no luck, and he said a local grocery store told him that there has been a run on water and it's flying off the shelves.

Shawan Dillard of the Reisterstown Village Food Lion confirmed that water has been going quickly, but she said that customers have been very civil, with no pushing or shoving that she's seen. At midday Sunday, the store had just received a new shipment of bottled water that would probably last "not even until the end of the day, I'm sure."

Residents in Northwest Baltimore County without water will be able to get water until 7 p.m. at certain county fire departments.

Water not meant for consumption but that can be used for flushing toilets and other household purposes can be obtained at the Pikesville Volunteer Fire Company, 40 E. Sudbrook Lane in Pikesville; and the Boring Volunteer Fire Company, 14711 Old Hanover Road. Residents must bring their own containers for this water, officials said.

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The Baltimore Sun's Patrick Maynard contributed to this article.