09-05-2025
Bacteria forced SLO County to issue boil water notice. What exactly happened?
A large swath of southern San Luis Obispo County was ordered to boil its drinking water last week after bacteria was discovered in Lopez Lake's water distribution pipeline.
Residents of Arroyo Grande, Grover Beach, Pismo Beach, Oceano, Avila Beach and other unincorporated areas of the South County had to to boil drinking water for up to four days depending on where they lived.
The Lopez Water Treatment Plant cleans 4 to 5 million gallons of water per day, which is delivered to 50,000 people, according to the San Luis Obispo County Public Works Department.
The county discovered the bacteria at five routine testing sites in the Lopez water distribution system on April 29. After a second round of tests, the county issued a boil water notice on April 30 — which lifted for some residents on May 2 and others on May 3.
'While Public Works continues to investigate what happened, the department is taking every precaution to ensure your water remains safe,' a Public Works Department news release said.
On Thursday, the county shared additional details of what led to the unprecedented boil water notice for Five Cities residents. Here's what happened.
Every day, water flowing through the Lopez distribution system is tested for contaminants.
The county collected samples from 13 spots in the system on April 28.
Those samples were delivered to the Public Works lab, which typically produces test results in 18 to 24 hours.
'Tests for bacteria take time because they require an incubation period for accurate detection,' the release said.
The test results were ready 24 hours later on April 29, showing that five sites tested positive for total coliform bacteria. One of those sites also tested positive for E. coli.
Total coliform bacteria was detected at sampling sites located near the end of Avila Beach Drive by the entrance of the Diablo Canyon nuclear power plant, along San Luis Bay Drive above the Bellevue-Santa Fe Charter School, the intersection of Highway 101 and North Oak Park Boulevard and the intersection of Highway 101 and North Halcyon Road, a map showed.
E. coli bacteria was found in a sample taken near the intersection of Highway 101 and Price Canyon Road, the map showed.
SLO County residents told to boil water due to bacteria. How dangerous is it?
The county took another sample that day to confirm the findings and 'rule out a false positive or contaminated testing equipment,' the release said.
Those results were ready about 18 hours later on April 30 — showing that two locations still tested positive for total coliform, but none tested positive for E. coli.
The total coliform bacteria was detected at the sampling site above the Bellevue-Santa Fe Charter School and the intersection of Highway 101 and North Fourth Street, the map showed.
That's when the county issued a boil water notice to more than 50,000 residents who receive drinking water from Lopez Lake.
The county alerted residents through AlertSLO, Reverse 911, Wireless Emergency Alerts, the news media and social media, while the SLO County Environmental Health Department notified more than 150 businesses of the hazard, according to the release.
On April 30, Arroyo Grande, Grover Beach, the Oceano Community Services District and the San Miguelito Mutual Water Company paused deliveries from Lopez Dam and used other water sources. After testing produced two rounds of clean samples in the water system, the boil water notice was lifted for those areas on May 2.
Pismo Beach and the Avila Beach Community Services District, however, didn't have another water source — so they continued to draw water from the Lopez water system. They had to wait until May 3, when the Lopez distribution pipe was confirmed clean, to lift the boil water notice.
By May 1, all 13 of the county's testing sites were free of contamination, according to the release. Neither the total coliform or E. coli bacteria entered the pipeline or the distribution systems of local water agencies.
As of Thursday, the county had not yet identified the source of contamination, it said in the release.
Boil-water order lifted for all remaining SLO County areas
Normally, the pipeline is disinfected with chlorinates. But from Friday to May 28, the county will treat the water with a stronger disinfectant known as free chlorine.
'You may notice a stronger chlorine smell or taste in your tap water during this time,' the release said.
The county last used free chlorine in November 2024, and typically swaps it out for chlorinates once per year.
The Public Works Department took a number of measures including retesting the pipeline and water in multiple areas, auditing laboratory procedures and inspecting the Lopez distribution pipeline for cross contamination.
In the release, Public Works said it would continue to:
Perform routine testing throughout the system.
Perform frequent inspections to identify potential problems like leaks, corrosion or damage to pipes and infrastructure.
Monitor system operations for any issues including low pressure or unusual water quality and address them promptly.
Comply with all drinking water regulations.
Conduct an assessment of our response to this specific water notice event and make improvements where needed.
Residents can sign up for emergency alerts at
