When a Portland teen reportedly urinated in the local drinking-water supply last month, city officials said they had to get rid of the water. Those were the rules.

But when an unidentified person recently broke into a giant water tank in Marin County and may or may not have slipped something into it, the decision about what to do with some 900,000 gallons of water — perhaps tainted, perhaps not — wasn’t quite clear.

After all, the region is gripped by a serious drought and the water is needed. But what if something deadly was slipped into the tank?

Those were the issues that faced Chris DeGabriele, general manager of the North Marin Water District in Novato. On Friday, DeGabriele told The Chronicle that despite some criticism over his plan, he couldn’t in good conscience let the water flow to customers.

The tank — about 3 percent of the district’s stored water supply — was drained.

“The decision we made was precautionary, to error on the safe side,” DeGabriele said. “Whoever this intruder was really wanted to get into the tank. That’s why we were so cautious.”

On the evening of March 17, the water district’s security system alerted managers that a locked hatch on a hillside south Novato water tank had been opened, according to the district. When officials responded, they found three locks broken — one on the fence into the property, one on a ladder on the side of the tank and one on the hatch at the top.

“What he was after I don’t know,” said DeGabriele. “We’ve seen break-ins before but never this extensive.”

The intruder was not found.

Follow-up tests on the water — for bacteria — showed no signs of contamination. However, the list of things that someone could have put into the tank is a long one, DeGabriele noted. DeGabriele decided it was futile to spend more time or money testing for dozens of chemicals.

The district initially planned to move the water into its recycled water reserves, to be used primarily for irrigation. But those supplies were full so the water was ultimately dumped.

Neighbors told television station KTVU that they’re not sure the district did the right thing. To be dumping water when residents are being asked to reduce water use by 20 percent because of the drought seems inconsistent, some said.

“I always hate to second guess the experts, but they did test it and did say it was OK,” said Mike Holland.

The district, which gets its water from a local reservoir and the Russian River, serves about 61,000 people. District officials say that even after dumping the water, they expect to have enough supplies to make it through summer.