For nearly a third of the residents of Boston and some of its suburbs, it was a weekend without clean tap water after a water main ruptured Saturday in the suburb of Weston and rendered water undrinkable.Gov. Deval Patrick declared a state of emergency on Saturday, and emergency officials drove through neighborhoods using bullhorns to warn residents that water was not safe. Residents in affected areas also received reverse 911 calls telling them to boil their water before drinking it. The alert set off a frantic rush for the last available bottled water in area stores, a supply that dwindled in mere hours. The problem began Saturday about 10 a.m., when the pipe, which was 10 feet in diameter, broke in Weston, about 15 miles west of Boston, sending about 8 million gallons of water an hour gushing into the Charles River at the high point of the spill. The rupture affected water service to nearly two million people in 30 cities and towns.