Almost 2 million Washington-area residents will endurean even drier holiday weekend than expected thanks to water restrictions implemented with another failure of the area's aging water pipes.

The Washington Suburban Sanitary Commission, which supplies water for 1.8 million people in Montgomery and Prince George's counties, banned customers from using more water than necessary — at least through the weekend.

The restrictions include not flushing toilets after every visit, no lawn watering, and running only full dishwasher and washing machine loads.

Early Thursday, the agency's fiber-optic equipment identified multiple "ping" sounds — snapping wires and a precursor to a bursting pipe — inside a 40-year-old water main near Tuckerman Lane and Gainsborough Road in Potomac.

The failed 96-inch main is the largest in the system and is a major water distributor.Had it burst, it could have done more damage than a similar, well-publicized2008 incident, when a smaller Bethesda pipe spewed freezing water onto River Road — stranding residents who were rescued by boat and helicopter.

On the eve of a fireworks-filled weekend, with no rain on the horizon, Washingtonians hardly needed a reminder of the area's archaic water-delivery network. But it came Thursday, when just hours after the Potomac incident, a water main broke near Old Town Alexandria and shut down traffic there throughout the day.

Nearly a quarter of all WSSC pipes are at least a half-century old, and there were 1,534 water main breaks or leaks in 2009, according to the most recent data. Virginia and District water pipes are an average of 80 years old.

However, WSSC officials pointed to the fiber-optic technology as proof they aren't in the dark about the state of their pipes.

"This proves the system works and helped us to prevent what could have been a much more serious situation," said WSSC General Manager Jerry N. Johnson.

Counties have raised sewage and water rates continually to address their outdated water infrastructure. The WSSC is installing the fiber-optic equipment for the largest water mains.

Montgomery and Prince George's customers aren't in danger of losing too much water, WSSC officials said, adding the restrictions were implemented to ensure suitable water pressure for fire protection, especially over the July 4 holiday.

Crews Thursday afternoon began digging near the pipe, which is expected to take at least through the weekend to repair.

Water outlaws face up to a $500 fine if caught using a sprinkler or washing their car, but police say they won't be on water patrol.

"We're a busy county," said Prince George's Police Cpl. Henry Tippett. "I don't think we have the resources to patrol just for water violations."

Marylanders who rely on municipal or well water are not subject to the restrictions. Montgomery residents can report violations by dialing the county's 311 information number.

  Water restrictions and tips

»  No outdoor use, including watering lawns, washing cars and topping off swimming pools.

»  Use dishwashers and washing machines with full loads only.

»  Limit shower time.

»  Don't flush the toilet after every use.

»  Turn off water while shaving or brushing teeth.

»  Check all faucets, pipes and toilets for leaks.