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Taste and Odor Facts


Does your drinking water taste or smell funny?
Drinking water imported from the Sacramento-San Joaquin Delta or captured as rainfall in local reservoirs can occasionally have noticeable taste and odor, depending on the time of year or the source of the water coming into our treatment plants. The predominant tastes and odors are musty or earthy (caused by algae), “salty” (caused by high salinity levels), or “rotten-egg” (caused by a lack of oxygen). The “swimming pool” taste or odor often encountered in drinking water is caused by the chlorine used in the disinfection process. All of our tap water meets federal and state standards for safety.


During the treatment process, chlorine is added to water as a disinfectant. Before the water leaves the treatment plant, ammonia is added to form chloramines to keep the water disinfected while it is distributed to homes and businesses. 

Chloramines, rather than chlorine, are used to maintain a disinfectant residual because they are more stable, form fewer disinfection by-products, and tend to produce less offensive tastes and odors. However, they may impart chlorinous or medicinal taste and odor to your drinking water.

Musty or earthy
Earthy or musty tastes and odors, particularly in hot water, are often the result of an algal bloom in the untreated water supply. Algae thrive when water is warm and there is abundant sunlight. Thus, taste- and odor-producing algae typically bloom in the late summer or fall.

Although the Santa Clara Valley Water District’s modern treatment plants remove the algae, some of their taste- and odor-causing metabolites may remain. The two most common metabolites are geosmin and 2-methylisoborneal (MIB). Even though these compounds are harmless, the human senses of taste and smell are extremely sensitive to them and can detect them in the water at concentrations as low as 5 parts per trillion

Rotten egg
A rotten-egg smell typically occurs from a lack of oxygen in reservoirs, lakes and wells. It is caused by bacterial decomposition of algae and organics which, when no oxygen is present, produces hydrogen sulfide. Hydrogen sulfide emits a rotten-egg smell. The district’s treatment process re-oxygenates the water and removes the rotten-egg odor. Although unlikely, some residual odor may remain.

About half of the water used in the county originates hundreds of miles away in the Sierra Nevada and is delivered through the Sacramento-San Joaquin River Delta or San Francisco’s Hetch-Hetchy system. Most of the remainder comes from local rainfall, much of which is captured in the water district’s 10 local reservoirs. Some areas of the county rely on groundwater.

When California experiences a drought, salinity levels in the Delta typically increase, sometimes significantly. This may result in a very slight salty taste in your tap water.

You may occasionally notice other common odors in your tap water such as fishy, grassy or marshy. These are also caused by compounds produced by different types of algae. Although these compounds may impart an odor, they do not adversely affect the safety of your drinking water.

Monitoring and treatment
In addition to weekly laboratory analyses which measure the concentrations of taste- and odor-causing compounds, the district has a Flavor Profile Analysis (FPA) panel that meets on a regular basis. The FPA panel consists of a group of persons trained to identify flavors and/or aromas in untreated source water and treated drinking water.

The district’s current methods for the removal of taste and odor include the use of powdered activated carbon, ozone and peroxone (a combination of ozone and hydrogen peroxide). The district’s Penitencia and Santa Teresa water treatment plants currently have ozone and peroxone systems. The district’s third and oldest treatment plant, Rinconada, is undergoing major upgrades, including the installation of an ozone system, which will significantly improve the tap water produced at this plant.

Contact Us
For general information about water quality, contact Water Quality Unit Manager Peter Zhou at (408) 630-2365, or [email protected]

You can also submit your water quality inquiries or comments on-line using the district’s “Access Valley Water” system.

For specific questions regarding your tap water, contact your water service provider. If you’re not certain who provides your water, you can find out here:

Related Files
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Taste and Odor Fact Sheet.pdf 625.23 KB