A San Francisco woman who tested her tap water with a store-bought kit and got a positive reading for pesticides, then posted the results to social media, has prompted the city to step up water testing not just near her home in the Sunset District but across the city.

Officials at the San Francisco Public Utilities Commission insisted Tuesday, for the second day in a row, that municipal supplies are safe to drink. However, they said they would take the precaution of performing additional inspections of 23 locations, many where groundwater is added to the water system.

City officials also acknowledged that their own lapse in communication might have contributed to the minor panic in the Sunset District after the Monday post on the networking site Nextdoor. Before residents’ warnings to neighbors to avoid the water started blasting across the internet, an operator at the Public Utilities Commission apparently confirmed or at least suggested to one resident, and likely others, that pesticides were found in the water, officials said.

“I think there was some misinformation-slash-miscommunication presented to the customer,” said Tyler Gamble, spokesman for the Public Utilities Commission. “There’s no evidence that we have that the water isn’t safe.”

Still, Gamble said that city water officials were meeting with the Sunset woman whose test kit detected pesticides in their effort to restore confidence in municipal supplies.

The resident, Mel Scardina, who tested the tap water into her home on 25th Avenue, said she became concerned because of an “odd” taste, like it had come “out of a hose.” The home kit she purchased through Amazon showed the water contained Atrazine and Simazine, she said. She said she called the city Monday, and a representative confirmed the presence of pesticides.

At least one other Sunset resident used a home kit over the weekend, this one purchased at Home Depot, and also detected pesticides, Scardina said. That finding was at a home in the Parkside area of the Sunset.

Several neighbors also posted on Nextdoor to say that they, too, called the city to find out what was going on and were similarly warned of problems.

Scardina said even after receiving a visit from city water officials Tuesday afternoon, she’s concerned about safety at her tap. The officials took water samples from her home, which will be tested, and assured her that her supplies were fine.

“I’d love to say I’m convinced, but I’m not at this point,” Scardina said. “Whether or not (the first person I talked to) spoke out of turn, I don’t know. But he was very detailed (about the pesticides). I have a hard time imagining that he was wrong to that level.”

Gamble, with the Public Utilities Commission, suggested that the problem could be an error with internal plumbing or the test kits themselves. The relatively inexpensive kits are prone to false readings.

He said the results of his agency’s more reliable testing won’t be known until next week. Water samples are being collected across the city and sent to an independent lab, he said.

The Public Utilities Commission regularly monitors the water it delivers to households in the Bay Area. Last year, the agency did 101,900 drinking water tests on its delivery systems, according to city records. More than 80 percent of the city’s water comes from reservoirs in the Sierra Nevada, with smaller portions coming from local reservoirs and groundwater supplies.

Water to the Sunset District is mostly a blend of surface water and groundwater, though some is purely surface water, according to city records. The blended water typically contains less than 1 percent groundwater.

The Public Utilities Commission began adding groundwater to its Sierra supplies in April of last year in an effort to diversify its water portfolio.

City officials say routine tests of the groundwater supply are done every two weeks and that results from Sept. 17 to Sept. 30 showed no problems. Tests specific to pesticides are performed quarterly. The latest, done in July, also showed no issues.

Sunset resident Rob Schaezlein, who has been following the back-and-forth on Nextdoor, said he and his wife remain concerned about their water because they’ve also noticed it’s “smelled weird.”

“We’ve always bragged about how good our water is and look what’s happened now — they’ve messed with it,” he said, suspecting the recent addition of groundwater may be the problem.

“Maybe there’s nothing here, and maybe there is something here and the city is trying to cover it up,” he said. “I don’t know how to make sense of it, to be honest. I guess we won’t know until the city comes back with a more accurate test.”

Kurtis Alexander and Evan Sernoffsky are San Francisco Chronicle staff writers. Email: kalexander@sfchronicle.com esernoffsky@sfchronicle.com Twitter: @kurtisalexander @EvanSernoffsky