Water Quality

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Together with the San Francisco Regional Water System (SFRWS), the MPWD regularly collects and tests water samples from reservoirs and designated sampling points throughout the system to ensure the water delivered to you meets or exceeds federal and State drinking water standards. In 2021, the SFRWS conducted more than 48,320 drinking water tests in the sources and the transmission system. This is in addition to the extensive treatment process control monitoring performed by SFRWS’s certified operators and online instruments.

Drinking water, including bottled water, may reasonably be expected to contain at least small amounts of some contaminants. The presence of contaminants does not necessarily indicate that water poses a health risk. To ensure that tap water is safe to drink, the United States Environmental Protection Agency (USEPA) and the SWRCB prescribe regulations that limit the amount of certain contaminants in water provided by public water systems. The U.S. Food and Drug Administration regulations and California law also establish limits for contaminants in bottled water that provide the same protection for public health. 

This page is available in Spanish here.

Cover image from the 2022 Consumer Confidence Report
MPWD 2022 Consumer Confidence Report (Annual Water Quality Report)

Water quality results are published annually in the MPWD Consumer Confidence Reports (CCRs).
CLICK HERE for the most current CCR with Data Tables (PDF) and HERE for a list of past CCRs

Drinking Water Sources and Treatment

The San Francisco Regional Water System’s (SFRWS) major drinking water supply consists of surface water and groundwater that are well protected and carefully managed by the San Francisco Public Utilities Commission (SFPUC). These sources are diverse in both the origin and the location with the surface water stored in reservoirs located in the Sierra Nevada, Alameda County and San Mateo County, and groundwater stored in a deep aquifer located in the northern part of San Mateo County.

SFPUC Water System Map

To meet drinking water standards for consumption, all surface water supplies from SFRWS undergo treatment before it is delivered to our customers. Water from the Hetch Hetchy Reservoir is exempt from state and federal filtration requirements but receives the following treatment: ultraviolet light and chlorine disinfection, pH adjustment for optimum corrosion control, fluoridation for dental health protection, and chloramination for maintaining disinfectant residual and minimizing the formation of regulated disinfection byproducts. Water from local Bay Area reservoirs in Alameda County and San Mateo County is delivered to Sunol Valley Water Treatment Plant (SVWTP) and Harry Tracy Water Treatment Plant (HTWTP), respectively, and is treated by filtration, disinfection, fluoridation, optimum corrosion control and taste and odor removal processes. In 2020, a small amount of groundwater from five of the eight recently completed wells was intermittently added to the SFRWS’s surface water supply.

Protecting Our Watersheds

Image of the Tuolumne River

The San Francisco Regional Water System (SFRWS) conducts watershed sanitary surveys for the Hetch Hetchy source annually and for non-Hetch Hetchy surface water sources every five years. The latest sanitary surveys for the non-Hetch Hetchy watersheds were completed in 2021 for the period of 2016-2020. All these surveys, together with SFRWS’s stringent watershed protection management activities, were completed with support from partner agencies including National Park Service and US Forest Service. The purposes of the surveys are to evaluate the sanitary conditions and water quality of the watersheds and to review results of watershed management activities conducted in the preceding years. Wildfire, wildlife, livestock, and human activities continue to be the potential contamination sources. You may contact the San Francisco District office of the Stae Water Resources Control Board’s Division of Drinking Water (SWRCB) at 510-620-3474 for the review of these reports.

water quality image of woman drinking water from a glass

Special Health Needs

Some people may be more vulnerable to contaminants in drinking water than the general population. Immuno-compromised persons, such as those with cancer undergoing chemotherapy, persons who have undergone organ transplants, people with HIV/AIDS or other immune system disorders, some elderly people and infants, can be particularly at risk from infections.

These people should seek advice about drinking water from their healthcare providers. USEPA/CDC guidelines on appropriate means to lessen the risk of infection by Cryptosporidium and other microbial contaminants are available from the USEPA's Safe Drinking Water Hotline 800-426-4791 or at epa.gov/safewater

Lead and Drinking Water

Exposure to lead, if present, can cause serious health effects in all age groups, especially for pregnant women and young children. Infants and children who drink water containing lead could have decreases in IQ and attention span and increases in learning and behavior problems. The children of women who are exposed to lead before or during pregnancy can have increased risk of these adverse health effects. Adults can Have increased risks of heart disease, high blood pressure, kidney, or nervous system problems.

Lead in drinking water is primarily from materials and components associated with service lines and home plumbing. There are no known lead service lines in our water distribution system. We are responsible for providing high quality drinking water and removing lead pipes, but we cannot control the variety of materials used in plumbing components in your home. You share the responsibility for protecting yourself and your family from the lead in your home plumbing. You can take responsibility by identifying and removing lead materials within your home plumbing and taking steps to reduce your family’s risk. Before drinking tap water, flush your pipes for several minutes by running your tap, taking a shower, doing laundry or a load of dishes. You can also use a filter certified by an American National Standards Institute accredited certifier to remove lead from drinking water. If you are concerned about lead in your water and may wish to have your water tested, call Mid-Peninsula Water District at (650) 591-8941 for lead test. Information about lead in drinking water, testing methods, and steps you can take to minimize exposure is available at USEPA website www.epa.gov/safewater/lead

Mid-Peninsula Water District will be conducting a lead service line (LSL) inventory to identify the materials of service lines connected to the public water distribution system. This inventory will not only include the service line material from the water main to the water meter but will also include the pipe material behind the water meter and extend to the inlet of the home or building. The LSL inventory will be completed by October 16, 2024.

Fluoridation and Dental Fluorosis

Mandated by State law, water fluoridation is a widely accepted practice proven safe and effective for preventing and controlling tooth decay. Our fluoride target level in the water is 0.7 milligram per liter (mg/L, or part per million, ppm), consistent with the May 2015 State regulatory guidance on optimal fluoride level. Infants fed formula mixed with water containing fluoride at this level may still have a chance of developing tiny white lines or streaks in their teeth. These marks are referred to as mild to very mild fluorosis, and are often only visible under a microscope. Even in cases where the marks are visible, they do not pose any health risk. The Centers of Disease Control (CDC) considers it safe to use optimally fluoridated water for preparing infant formula. To lessen this chance of dental fluorosis, you may choose to use low-fluoride bottled water to prepare infant formula. Nevertheless, children may still develop dental fluorosis due to fluoride intake from other sources such as food, tooth paste, and dental products.

Contact your healthcare provider or the SWRCB if you have concerns about dental fluorosis. For additional information about fluoridation or oral health, visit the SWRCB website here or the CDC website www.cdc.gov/fluoridation

Monitoring of Per- and Polyfluoroalkyl Substances (PFAS)

Image of SFPUC's PFAS Fact Sheet

PFAS is a group of approximately 5,000 man-made, persistent chemicals used in a variety of industries and consumer products. In 2021, our wholesaler conducted a second round of voluntary monitoring using a newer analytical method adopted by the USEPA for some other PFAS contaminants. No PFAS were detected above the SWRCB’s Consumer Confidence Report Detection Levels in surface water and groundwater sources. For additional information about PFAS, you may visit SWRCB website waterboards.ca.gov/pfas or USEPA website epa.gov/pfas

For the SFPUC's PFAS Facts Sheet, CLICK HERE (PDF)

For additional information about the latest Health Advisories on Per- and Polyfluoroalkyl Substances (PFAS), CLICK HERE

State Revised Total Coliform Rule

This report reflects changes in drinking water regulatory requirements during 2021, in which the SWRCB adopted California version of the federal Revised Total Coliform Rule. The revised rule, effective on July 1, 2021, maintains the purpose to protect public health by ensuring the integrity of the drinking water distribution system and monitoring for the presence of microbials (i.e., total coliform and E. coli bacteria). Greater public health protection is anticipated, as the revised rule requires water systems that are vulnerable to microbial contamination to identify and fix problems. Water systems that exceed a specified frequency of total coliform occurrences are required to conduct an assessment to determine if any sanitary defects exist. If found, these must be corrected by the water system.

Lead and Copper Tap Sampling Results

Mid-Peninsula Water District conducted its triennial residential Lead and Copper Rule (LCR) monitoring in August 2021, and all the tap sampling results were all below the lead and copper action level. If any Mid-Peninsula Water District customers are interested in participating in the next round of the triennial LCR monitoring, please contact our Office at (650) 591-8941. The next round of the LCR monitoring will be conducted in 2024.

Taste, Odor, or Discolored Water Issue?

water quality image of water glass being filled by kitchen faucet

Opening a faucet or two in your home or business, or an outside spigot, to let the water run for a couple of minutes should resolve it. Remember to capture the water in a bucket to use for watering indoor plants or outdoor landscaping!

Depending upon the water turnover in the mainline serving your connection, or seasonal fluctuations in water treatment by San Francisco Water, a temporary water quality issue could be quickly resolved with a mini-flushing of your service line.

You can now use MPWD's online Water Quality Self Diagnosis Tool to help determine possible causes and solutions of typical water quality issues experienced in a home. If the problem persists, please contact us at 650-591-8941 or online here.