SLO County to temporarily change water disinfection after Five Cities contamination
The San Luis Obispo County Department of Public Works is temporarily changing how it disinfects water in the South County after residents were put under a boil water notice late last week.
Around 50,000 residents in the Five Cities area were told on April 30 to boil their water before use after a sample from the Lopez Lake water system tested positive for E. coli, the county said. It was the first time such a sample had prompted a boil water notice for the distribution system, which feeds much of the South County region.
The notice was lifted for all impacted residents as of Saturday.
Now, Public Works is expected to temporarily change the disinfectant used in the Lopez Project distribution system from chloramine disinfection — which uses a blend of chlorine and ammonia — to free chlorine, according to a news release.
The change will last from Wednesday through May 28.
According to the release, the switch will impact the water supply for Arroyo Grande, Pismo Beach, Grover Beach, Oceano, Avila Beach and Port San Luis.
The process, commonly referred to as a “free chlorine burn,” is an industry-standard practice, Public Works said in the release.
“Free chlorine is a stronger disinfectant than chloramines and is particularly effective at targeting potential bacterial contamination within water mains,” the release said.
The county typically does this annually, most recently in November 2024, according to the release.
So why not use free chlorine all year round?
According to the release, though it is “effective for microbial control, it is not suitable for continuous use in our system due to the increased potential for forming disinfection by-products, which are regulated compounds that can result from treating surface water sources like ours.”
“Chloramines remain our primary disinfectant under normal conditions to minimize DBP formation while still maintaining water quality,” the release said.
During the change, some people may notice the taste or smell of chlorine in water, similar to that of a swimming pool.
To mitigate that, residents were told they can “boil the water, use a carbon filter or let the water sit uncovered overnight to allow the chlorine to dissipate.”
This story was originally published May 6, 2025 at 5:59 PM.
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