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 taste and odor problems by the time it reaches your tap, depending on the time of year or the source of the water coming into our treatment plants. The predominant tastes and odors are “chlorinous,” caused by the disinfection process. However, at times, drinking water may have a musty or earthy taste and odor, which is caused by algae or a “rotten-egg” smell caused by a lack of oxygen in the source water.

In Santa Clara County, musty or earthy tastes and odors are more likely to occur in late summer and fall from algae blooms caused by warmer temperatures and increased sunlight. Although the Santa Clara Valley Water District’s modern treatment plants are able to remove the algae, some of the taste- and odor-causing compounds may be left behind. Despite temporary taste and odor problems in water supplies, your drinking water is safe.

Starting in 2006, residents in parts of the county served by Penitencia and Santa Teresa treatment plants (generally, East Valley and South San Jose areas) should notice better tasting water because ozone rather than chlorine, and carbon filters will become part of the standard treatment process. A residual amount of chloramine will still be added to the water to keep it disinfected as it travels through the distribution system.

Monitoring and treatment
In addition to laboratory analyses which measure the concentrations of taste- and odor-causing compounds, the district also conducts “flavor profile” analyses on a regular basis. Flavor profile analysis uses a group of trained panelists to identify flavors and/or aromas in raw and treated waters. Current methods for treating taste and odor problems include using powdered activated carbon or potassium permanganate, neither of which leaves the treatment plant in the treated water. As mentioned above, the district’s water treatment plants are undergoing significant upgrades. These upgrades include the installation of a new ozone system and granular activated carbon filter media. Another tool, hydrogen peroxide, can be used to combat high levels of musty or earthy compounds when needed, which will significantly improve the taste of your tap water.

Chlorinous
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 all the way to homes and businesses. Chloramines in the drinking water may have a chlorinous, or bleach-like taste or odor to some water users. Chloramines, rather than chlorine, are used to maintain a disinfectant residual in the water delivered to the users because they are more stable, form fewer disinfection byproducts, and tend to produce less offensive tastes and odors.

Musty or earthy
An earthy or musty smell, particularly in hot water, may be the result of an algae bloom in the untreated water supply. Algae thrive at different times of the year in reservoirs. Taste- and odor-producing algae typically bloom in the late summer or fall. Although algae are removed during the treatment process, some of their metabolites may be left behind. 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 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 the natural or man-made reservoir storing water before it is treated. It is caused by bacterial decomposition of algae and organisms, which, when no oxygen is present, produces hydrogen sulfide. Hydrogen sulfide emits a rotten-egg smell. The district’s water treatment processes can oxidize the hydrogen sulfide in the water and removes the rotten egg odor. Although unlikely, some residual odor may remain.

Other
Other common odors you may occasionally notice in your tap water are fishy, grassy or marshy odors. 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.

For more information about your water quality, contact Senior Water Quality Engineer Bruce Cabral at (408) 265-2607, ext. 2796.