San Luis Obispo recently violated a drinking-water standard for a chemical compound that in elevated levels has been associated with health problems including an increased risk of cancer.
City utilities officials sent a notice to San Luis Obispo residents on Thursday warning them that one sample site tested above the maximum contaminant level for trihalomethanes, a byproduct of the reaction between the chlorine used for disinfecting tap water and natural organic matter in the water.
The violation does not pose an immediate health risk to water customers, according to the notice sent, and residents do not have to use an alternative water supply such as bottled water.
Some people who drink water containing higher levels of trihalomethanes over many years may experience liver, kidney or central nervous system problems, and they may have an increased risk of developing cancer, according to the notice.
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The notice was sent to all water customers, but only households in the neighborhoods near the sample site at Johnson Avenue and Southwood Drive are affected, said Aaron Floyd, the city’s deputy director of water in the Public Utilities Department.
The water sample results for the second quarter of 2015 at that location showed total trihalomethanes at levels of 82.1 parts per billion — above the maximum contaminant level of 80 parts per billion.
The city is required by state water regulators to take quarterly samples to monitor drinking water contaminants. Four sites total are tested each quarter; the levels calculated are an average of the four most recent quarters of sampling results. The three other sites tested did not violate the standard, Floyd said.
A few factors may have contributed to the increased level at the Johnson Avenue/Southwood Drive site, Floyd said.
With increased conservation, water may be sitting in the city’s storage tank reservoirs or pipes longer, giving the trihalomethanes more time to form.
In addition, there may be more naturally occurring organic materials in the drought-stricken reservoirs that the city draws its water from, including Whale Rock, Nacimiento and Salinas (Santa Margarita Lake).
Floyd said city staff is researching ways to alter treatment processes to address the problem. Options may include increased monitoring of the system, according to the notice.
“We’re looking at how to lower any organic materials that are treated in the (water treatment) plant,” he said.
San Luis Obispo water customers use about 5 million gallons a day, but the city has about 25 million gallons stored in 10 tanks throughout town and reservoirs at the water treatment plant to maintain water pressure and for firefighting needs.
The water that tested too high for levels of trihalomethanes came from the storage tank off Bishop Street, but city officials did not know how many customers could be affected.
The testing site is located about a mile south of the tank.
“The problem is if we have too much water stored, we don’t get enough water used, and it can form too many trihalomethanes,” Floyd said.
The notice directs water customers with questions to call Dean Furukawa, water treatment plant supervisor, at 781-7566.