Erica Digap / Feb 27

Is Tap Water in San Mateo County Safe To Drink? 

Erica Digap / Feb 27

Just south of San Francisco is San Mateo County, a sprawling peninsula known for its diverse mix of beautiful beaches, nature reserves and parks, and bustling cities. This beautiful slice of the Bay Area boasts the perfect balance of good weather, gorgeous seaside views, and many thriving industries, making it a great place to both visit and live in 

Whether you’re living in San Mateo County, taking a day trip from the city, or looking to vacation in the coastal beauty that the Bay has to offer, you might be wondering: Is tap water in San Mateo County safe to drink? 

Key Takeaways: 

Where does San Mateo County’s water supply come from 

San Mateo County is made up of several cities and towns including: 

More than 90% of the county’s tap water comes from the Hetch Hetchy Reservoir in Yosemite Valley. The remaining drinking water comes from local sources. For example, cities in the northern half of the county including South San Francisco and San Bruno rely on groundwater sources for about half of their water usage. In addition, Burlingame, Daly City, Redwood City, and Millbrae also use recycled water that returns to the tap after treatment. 

No matter which area of the county you’re visiting, the drinking water in San Mateo County goes through extensive purification processes before reaching your tap overseen by the San Francisco Public Utilities Commission (SFPUC). These sanitation processes vary by water system. For example, water that comes from the Hetch Hetchy Reservoir is treated with chlorine and ultraviolet light disinfection, chlorination to reduce the production of disinfection byproducts, and pH adjustment. Meanwhile, water from local sources in the Bay Area goes through filtration, disinfection, fluoridation, and taste and odor regulation. The county also regularly tests its water and releases yearly Water Quality reports so that all consumers can stay up-to-date on potential issues.  

As a result of this extensive water treatment, the tap water throughout San Mateo County meets all safety standards and is generally considered safe to drink. In fact, many people consider the water quality in the Bay Area to be exceptionally clean and good-tasting. 

Unfortunately, contamination and infrastructure issues can affect even the best-monitored water supply systems. You should still filter your tap water before drinking and consuming it to prevent any possible quality issues.  

Is tap water in San Mateo safe to drink? Possible quality issues   


Lead is a metal that has historically been used to manufacture water pipes. Unfortunately, lead is also highly toxic. It’s been linked to several serious issues like reproductive issues and heart and kidney issues. It’s even worse for children since even small levels of exposure can affect their brain development. 

Fortunately, we no longer use this toxic metal to build our water pipes after the discovery of these serious issues. The government also made lead contamination a high priority and passed bills like the Lead and Copper Rule to monitor the water supply and minimize lead exposure. 

San Mateo County does a thorough job monitoring the water supply to ensure that it adheres to all legal standards. San Mateo County Health also has a Lead Prevention Program that aims to reduce lead exposure, identify high levels, and help families reduce their exposure. 

As a result, lead contamination sometimes feels like an ancient problem. Unfortunately, this toxic metal still continues to plague the drinking water of cities and towns throughout the United States and beyond due to older water pipes remaining in the water distribution system. In San Mateo County, for example, 5 childcare centers in Pacifica were found to have water with lead levels that exceed California’s legal limit in 2023. 

No amount of lead consumption is safe, especially for children. In fact, the Environmental Protection Agency (EPA) has even set its maximum lead contaminant goal at zero since it can be so harmful even in minuscule amounts. So even though lead contamination in drinking water may be less of a problem than it once was, it’s still well worth filtering your tap water to be safe. 


Because of its beautiful weather and fertile location, agriculture remains one of San Mateo County’s biggest industries. Unfortunately, this industry is also linked to potential health risks when it comes to tap water quality. 

Nitrates are a good example of this. Nitrates are a group of compounds that are commonly found in chemical fertilizers. While they help keep crops healthy, there’s also evidence that ingesting nitrates can make it harder for your blood to carry oxygen throughout your body. They’ve even been linked to some cancers. The nitrates that are used as fertilizer can sometimes enter the groundwater, eventually making their way into the drinking water. 

This issue was most recently highlighted in October 2023, when two San Mateo middle and high schools’ water tested for exceedingly high nitrate levels in their water supply. Though the recent bout of contamination made headlines, it turns out that nitrate contamination has been an ongoing issue at these schools, where students have historically relied on bottled water to hydrate rather than taking the risk of drinking their contaminated tap supply. 

Disinfection Byproducts

San Mateo County uses chlorine to treat its water and kill bacteria. Unfortunately, chlorine can sometimes interact with organic matter like decaying plants, and this interaction can lead to the formation of disinfection byproducts like Trihalomethanes (TTHMs) and Haloacetic Acids (HAA5). Both TTHMs and HAA5s are thought to be potentially carcinogenic, especially when someone is exposed to them over the long term.

Because of these health risks, levels of both TTHMs and HAA5s are monitored as part of the San Mateo Water Quality Service Report. The water is also treated with another process called chloramination which is intended to reduce these disinfection byproducts. San Mateo County’s water does test positive for both HAA5s and TTHMs at levels that are technically considered safe to drink by legal standards. 

However, some organizations believe that the legal levels are outdated and that they may still pose a health risk at these limits. For example, according to the nonprofit Environmental Working Group (EWG), San Mateo County’s water supply has 333 times the amount of HAA5 that they consider to be safe. Additionally, the water in the county also tested for 247 times the amount of TTHMs considered safe by the EWG Health Guideline. 

The Bottom Line: Is tap water in San Mateo County safe to drink?  

Because its drinking water is subject to thorough treatment and testing to ensure it meets legal quality standards, the tap water is safe to drink in San Mateo County. However, when it comes to concerning chemicals and toxins like lead and nitrates, it’s better to be safe than sorry. 

So if you’re in the Peninsula, make sure to filter your water before consuming it! Run your tap water through a filtering water bottle like the LARQ Bottle Filtered while you’re on the go to remove common water contaminants like lead, particulates, TTHMs, HAA5 and chlorine. Meanwhile, invest in a filtering watcher pitcher like the LARQ Pitcher PureVis for clean and purified water in your home. You’ll get the best of both worlds — not only will you be able to enjoy some of the best tap water in the region, but you’ll also get peace of mind from filtering out those potentially dangerous contaminants that can still find their way into your supply.