There has been visual confirmation of ash on the surface of a key reservoir in the watershed, but officials monitoring water quality have seen no change, said Tyrone Jue, spokesman for the San Francisco Public Utilities Commission.
Water is drawn from the reservoir at a depth of 260 feet and there is little chance of contamination, he said.
Hetch Hetchy supplies water to 2.6 million people in the San Francisco Bay Area.
The Rim fire had grown to roughly 179,500 acres, or 280 square miles, and remained at 20% containment, according to an update from Cal Fire on Tuesday. More than 3,700 firefighters were battling the blaze.
The fire is burning in the Hetch Hetchy Watershed and in the vicinity of the reservoir, but there is little vegetation that can be burned around its granite perimeter, Jue said. More problematic than ash on the surface of the water is the potential for runoff to be pushed into the reservoir when rain comes later in the season, he said.
"We're using all the tools at our disposal to deal with any situation that confronts us later on," Jue said.
As part of standard procedure in the event the reservoir becomes unavailable, additional water is pumped to reservoirs in Alameda and San Mateo counties, which also supply Hetch Hetchy customers, Jue said. The amount of water being transferred there has been increased from 275 million gallons a day to 302 million gallons a day as a precaution, according to the commission.
Crews have also repaired one of two hydroelectric power stations brought down by the fire, and they are in the process of assessing the damage to the second unit, Jue said.
Transmission lines from the stations still need to be checked and cleaned of soot and ash before they and the stations are reenergized, Jue said. The lines will not be brought back online until fire crews are finished in the area, though there is little chance of the blaze burning back through the scorched region, he added.
There has been no interruption in power or water to utility customers in San Francisco and around the Bay Area, the commission said. Supplemental power has cost about $600,000 since the two power stations were deenergized Aug. 17, the day the Rim fire started.