Boil Water Notice Summary and Next Steps

Where Does My Water Come From?
Source water for Lopez Project comes from Lopez Lake, located approximately 10 miles east of Arroyo Grande. Lopez Lake water is conveyed 3 miles by pipeline to the Lopez Terminal Reservoir adjacent to the WTP. Water remains in the Terminal for over a month. During that time, particles naturally settle out of the water, and exposure to sunlight helps reduce the risk of bacterial and viral contamination from human activity in Lopez Lake.

After this natural settling process, the water enters the Lopez Water Treatment Plant, where it undergoes several treatment steps to make it safe to drink:

After water leaves the Lopez WTP, it is gravity fed through a 25-mile pipeline where it can be delivered through multiple turnouts for the Lopez Project participants who are retail water suppliers, and they provide water directly to their customers.

What Happened?
As part of routine water testing, samples were collected on April 28, 2025, from thirteen (13) locations in the Lopez distribution system. Test results became available the next day, April 29, and showed that five (5) sites tested positive for total coliform bacteria. One (1) of those sites also tested positive for E. coli, a bacterium that can sometimes cause illness. A summary of the test results is explained in detail below.

Public Works immediately notified the State Water Resources Control Board Division of Drinking Water (Division of Drinking Water) and Lopez Project participants. Public Works began retesting the system.

On April 30, the next set of test results showed that two (2) locations still tested positive for total coliform and zero (0) positive for E. coli.

Because these follow-up results still showed the presence of coliform bacteria after one (1) earlier sample tested positive for E. coli, the Division of Drinking Water required Public Works to issue a Boil Water Notice to the Lopez Project participants in order to protect public health. It is important to note that all local water agencies that tested their distribution systems also reported no contamination during the event.

Working with the Lopez Project participants, Public Works and County Office of Emergency Services (OES) notified impacted communities. Nearly 50,000 residents were notified through AlertSLO, Reverse 911, and Wireless Emergency Alerts (WEA). As these systems do not reach 100% of the population, news media and social media were also utilized. Additionally, County Environmental Health was on-site in the impacted communities doing outreach to restaurants and other food establishments. They notified and assisted over 150 businesses.

Public Works continues to work with the Department of Drinking Water in an attempt to identify the source of contamination. At the time of posting this document, the source has not yet been identified.

Daily Summary of Test Results:
As required by the Division of Drinking Water, the County continued daily sampling of the pipeline until there were two (2) consecutive sets of samples over a 48-hour period showing no total coliform or E. coli. This was the minimum regulatory time period necessary to remove the boil notice.

What Are Coliform Bacteria?
Coliform bacteria are naturally found in the environment and are used to indicate possible contamination in a water system. While not all coliform bacteria are harmful, their presence can mean other pathogens like E. coli may also be present.

E. coli in drinking water can indicate recent sewage or animal waste contamination. While most E. coli strains are harmless, some can cause illness. Ingestion of E. coli-contaminated water can lead to symptoms like diarrhea, stomach cramps, and nausea. Public water systems are required to notify consumers if their water may be unsafe.

When these bacteria are detected, follow-up testing is required according to Division of Drinking Water regulations to ensure the water's safety.

Why Did It Take Two Days From the First Test to the Issuance of the Boil Notice?
The testing process for total coliform and E. coli takes time. Laboratories performing this testing are certified by the California Environmental Laboratory Accreditation Program and may use a range of approved methods. Most water systems, including the Public Works water lab, select methods that provide results within 18- 24 hours. These test methods can detect a single viable coliform or E. coli organism per sample.

Tests for bacteria take time because they require an incubation period for accurate detection and identification, if present. Routine water samples were collected on the afternoon of April 28, and the results were available approximately 24 hours later on April 29. A second set of bacteriological samples was collected on the afternoon of April 29 to confirm the first set of results. These test results were available approximately 18 hours later on Wednesday, April 30. Once Public Works received confirmation, the department notified the Lopez Project participants and issued the Boil Water Notice.

To rule out a false positive or contaminated testing equipment and not unnecessarily issue a boil notice, the second test is required to verify the existence of the bacteria. The entire process was monitored and guided by the Division of Drinking Water under their regulations.

What Are We Doing to Fix the Problem?
Although the water treatment plant continues to meet all State and Federal standards,
Public Works is taking extra precautions. Here's what Public Works has done and is continuing to do:

They began extensive testing within their water system and confirmed two (2) rounds of clean samples, allowing the Division of Drinking Water to lift their boil water notices on May 2.

Other areas, including Pismo Beach, Avila Beach Community Services District, and CSA 12 continued to take water deliveries from the Lopez water system because they did not have another source of water available. Although these communities also performed extensive testing and confirmed clean samples in their distribution systems, they had to wait for the Lopez distribution pipeline to clear before the boil water notice could be cancelled, which occurred one day later on May 3.

What Are We Doing to Keep Your Water Safe?
Public Works will continue to:

While Public Works continues to investigate what happened, the department is taking every precaution to ensure your water remains safe. Public Works will share more information as soon as the Division of Drinking Water completes their review. Public Works will post notifications and use press releases for any updates or changes to the water system.

How Do I Get Future Emergency Alerts from the County of San Luis Obispo?
Information on the County's alert and notification systems can be found here. It is highly recommended that you sign up for Reverse 911 and AlertSLO to receive future messages.

Thank you for your patience and understanding.