Late-summer and fall algae blooms in the canals that feed Phoenix's treatment plants release geosmin and MIB, harmless compounds that make water taste earthy or musty. The city says the algae are removed but the odor can linger, and chlorine adds its own taste. Phoenix says the water still meets all health standards.
Why does the taste change in late summer?
The city's Seasonal Taste and Odor FAQ gives the cause plainly. Taste or odor variations are often seasonal, stronger near the fall months, because more algae grow in the canals that carry surface water to Phoenix's treatment plants. About 98 percent of Phoenix's water supply is surface water, mostly snowmelt that travels down the Salt, Verde and Colorado Rivers and then through canals to the plants.
The 2025 Water Quality Report adds the timing: algae that grow in the canals during the late summer and fall are a major source of a "musty" odor and taste noticed by some customers. When the algae bloom, they produce a strong odor.
The compounds behind it have names. The city says the taste comes from geosmin and methylisoborneol (MIB), which are naturally occurring byproducts of algae in lakes and canals. Geosmin is the one people describe as earthy. MIB reads as musty. The city describes them as harmless, aesthetic qualities that some residents may notice.
If the algae are removed, why do I still taste it?
Because the odor doesn't leave as fast as the algae do. The city's report uses an analogy that fits: even though the algae are removed during treatment, the odor may linger, "similar to removing a bouquet of fresh flowers from a room. Even though the flowers are gone, the aroma remains."
Chlorine adds a second layer. The city's FAQ notes that the chlorine used to disinfect your water can also affect taste and odor. In late summer you can end up tasting the two together: a faint earthiness from the canals and the swimming-pool note from disinfection. Our page on why Phoenix water can taste like chlorine covers that half of the story.
### When does it show up, and who notices?
The timing is the useful part. The city's report ties the musty taste to algae growth in the late summer and fall, and its FAQ says the variations are often stronger near the fall months. That means the taste can build through August and September and fade once the algae die back, rather than switching on at a specific date. The report also says these qualities are ones that "some consumers" may notice, which matches what many households find: one person tastes it right away, and another never does.
Water temperature in the pipes plays no official role in the city's explanation, but it's worth knowing the range. The report's data for the distribution system show water temperatures from 47°F to 92°F over the year, so the water reaching your tap in summer really is warmer than it is in winter.
Is it safe to drink when it tastes like this?
Yes, according to the city. Phoenix says its tap water meets or surpasses all regulatory requirements for health and safety, and that more than five million tests and measurements are performed each year across the treatment and distribution systems. It also says that if water quality is ever an issue, customers will be notified.
There's a reason taste and safety are separate questions. Taste, odor and hardness aren't regulated under drinking water health standards. EPA's secondary standards, which deal with taste, color and odor, are non-enforceable guidelines "to assist public water systems in managing their drinking water for aesthetic considerations," and EPA says these contaminants are not considered to present a risk to human health at the guideline level. The city identifies its typical seasonal geosmin/MIB taste as aesthetic. Taste alone does not establish water safety; ask the provider about a sudden, unusual or persistent change.
### What is Phoenix doing about it?
The city says it works to reduce the effects of algae in two main ways. First, it uses activated carbon during treatment to absorb residual odors, much like the carbon in a refrigerator filter. Second, the Salt River Project (SRP) brushes its canals to help reduce the algae that cause taste and odor. The Water Quality Report describes this as a cooperative effort to remove algae from the SRP canals that feed the water treatment plants.
You'll still notice the effect on some days, because algae blooms don't follow a calendar and the odor can linger after treatment.
What can you do at home?
A few simple steps help, and none of them require equipment:
- Chill it. Many people find cold water tastes cleaner, so keep a pitcher in the refrigerator. That's a household tip rather than a city recommendation.
- Use a filter if you want one. The city says filters are typically not necessary unless your physician advises one or you want one for aesthetic reasons. If you choose one, change it according to the manufacturer's instructions. The city's report warns that not following the maker's instructions can result in potentially unsafe water.
- Match the filter to the problem. A carbon-based filter is what people use for taste and odor. Our page on what kind of whole-house filter fits Phoenix covers the options.
### Who to tell if you're concerned
Phoenix invites questions. The Water Quality Report gives an email address for water quality questions, WSDCompliance@phoenix.gov, and the phone line, 602-262-6251, Monday through Friday except holidays, 8:00 a.m. to 5:00 p.m. If you'd like to see the data behind the city's statements, the same report shows the lab results for the year, including the range of hardness, alkalinity, pH and total dissolved solids in the distribution system.
If you already use a refrigerator or pitcher filter, this is the time of year to check the cartridge date. The city's advice covers filters on a tap, refrigerator and similar units alike: change them according to the manufacturer's instructions. An overdue cartridge is the first thing to rule out, and it's the easiest thing to fix.
How can you tell seasonal taste from a real problem?
Seasonal taste is mild, shows up at every tap, and comes and goes with the time of year. Other changes deserve a call to the city or a plumber:
| What you notice | Likely meaning | Who to call |
|---|---|---|
| Earthy or musty taste at every tap, late summer or fall | Canal algae compounds, per the city | Ask the provider if the pattern is unusual; optional taste filter |
| Chlorine or pool smell | Disinfection | Check unusual changes with the provider; see our chlorine page |
| Bad smell only from one drain or hot water | Local plumbing or water heater | A plumber |
| Cloudy, colored or rusty water | Something in the pipes or system | The city, then a plumber |
| Sudden new smell or taste plus illness | Unknown | Stop drinking it and call the city |
Phoenix Water Services takes water quality questions at 602-262-6251, weekdays. For a single fixture with a smell, see why a bathroom drain smells like sewer or ask us about a free estimate.
The city attributes its typical late-summer earthy taste to harmless algae byproducts that can linger after treatment. That explanation applies to City of Phoenix water; another provider or a private well needs its own assessment. See Phoenix tap-water safety for the report context.