New York City’s tap water is frequently hailed as the “champagne of drinking water.” Sourced from the pristine Catskill and Delaware watersheds, it travels over 100 miles through a massive network of aqueducts to reach the five boroughs. For most of the year, this water is crisp, cold, and neutral. However, as the seasons shift—particularly in the late autumn and early spring—long-time residents often notice a subtle change. The water might develop a slightly “earthy,” “musty,” or “woody” quality.
This isn’t a sign of a failure in the city infrastructure, nor is it a result of local pipe corrosion. Instead, it is a grand-scale biological event known as “reservoir turnover.” Understanding this natural cycle is essential for anyone navigating water quality issues in a city that relies on open-air mountain reservoirs.
The Physics of a Stratified Reservoir
To understand turnover, you first have to understand how a deep body of water behaves during the extreme heat of summer or the deep freeze of winter. Water is a unique substance because its density changes based on its temperature. It is densest at approximately 39°F (4°C).
During the summer, the sun warms the top layer of the upstate reservoirs. This warm water is less dense and floats on top. Meanwhile, the deeper water remains cold and dense. These layers do not mix; they become “stratified.”
In this state, the reservoir is divided into three distinct zones:
- The Epilimnion: The top, warm, oxygen-rich layer.
- The Thermocline: The middle layer where the temperature drops rapidly.
- The Hypolimnion: The bottom, cold, stagnant layer.
Throughout the summer, organic matter—leaves, algae, and microscopic organisms—dies and sinks to the bottom. In the hypolimnion, this matter begins to decompose. Because this bottom layer is cut off from the surface, it becomes depleted of oxygen (hypoxic), allowing minerals like iron and manganese to dissolve into the water, alongside natural compounds like geosmin.
The Great Autumn Flip
As the crisp air of October and November arrives in the Catskills, the surface water begins to cool. Once the surface water reaches that magic temperature of 39°F, it becomes denser than the water beneath it. Gravity takes over, and the heavy surface water sinks, forcing the bottom-dwelling water to rise to the top.
This is “turnover.” The entire volume of the reservoir effectively flips over. The stagnant, mineral-heavy, and organic-rich water from the bottom is suddenly thrust into the upper layers of the urban water systems.
When this happens, the compounds that have been brewing at the bottom of the reservoir for months—specifically geosmin and 2-methylisoborneol (MIB)—are sent down the aqueducts toward New York City. These compounds are harmless to human health, but the human nose is incredibly sensitive to them. We can detect geosmin at concentrations as low as five parts per trillion. To the average New Yorker, this tastes like “dirt” or “old wood.”
Spring Turnover: The Post-Winter Wake-up
A similar event happens in the spring. As the ice melts and the sun begins to warm the 39°F water at the surface, the density shifts again. The melting ice (32°F) is less dense than the water below it. As it warms up to 39°F, it sinks, triggering a second, usually less dramatic, mixing event.
During spring turnover, the “taste shift” is often accompanied by increased turbidity. Spring runoff from melting snow carries fine silt into the reservoirs, which is then stirred up during the turnover process. While the NYC Department of Environmental Protection is world-class at managing this through alum treatment and settling, a fraction of that “seasonal character” often makes it to the taps in Brooklyn, Queens, and the Bronx.
How Turnover Interacts with Your Building’s Plumbing
While turnover is a source-side event, its effects can be amplified by your buildings plumbing.
If you live in a high-rise with a rooftop wooden water tank or a basement suction tank, the residence time of the water increases. During a turnover event, the organic compounds have more time to react with the air in the tank. This can make the “earthy” smell much more pronounced when you first turn on the shower in the morning.
Furthermore, if your building has older iron risers, the slight change in water chemistry during turnover—specifically a marginal shift in pH or mineral content—can cause a temporary increase in “scouring.” This might lead to a brief period of yellow or amber water, as the seasonal water “cleans” the internal scale of your pipes. This is a common point of confusion addressed in our faq.
Why Doesn’t the City Just Filter the Taste Out?
New York City is one of the few large cities in the world that does not use a centralized filtration plant for its Catskill/Delaware supply. Instead, the city relies on the natural purity of the watershed and a rigorous disinfection program using chlorine and ultraviolet (UV) light.
While UV light is excellent at killing bacteria, it does not remove dissolved organic compounds like geosmin. To remove the “earthy” taste during turnover, the city would need to build a multi-billion dollar filtration plant. The Environmental Protection Agency (EPA) allows the city to remain unfiltered because the water meets all safety standards; the “taste” of turnover is considered an aesthetic preference, not a safety risk.
Managing the Seasonal Taste Shift at Home
If you find the “musty” taste of reservoir turnover distracting, there are several simple ways to manage it:
- Chilling and Aeration: Cold temperatures mask the taste of geosmin. Fill a glass carafe and keep it in the refrigerator. Additionally, the act of pouring water into a pitcher aerates it, allowing some of the volatile organic compounds to dissipate into the air.
- Carbon Filtration: A simple charcoal-based filter (like a Brita or an under-sink carbon block) is highly effective at removing the “earthy” notes of turnover. Carbon is designed to “adsorb” organic molecules like MIB and geosmin.
- A Squeeze of Citrus: Adding a drop of lemon or lime juice doesn’t just mask the taste; the acidity actually helps break down the perception of the earthy compounds on your palate.
- The Morning Flush: During turnover season, the water sitting in your building’s pipes and tanks can become particularly “stale.” Running your cold water for 30 seconds to a minute in the morning ensures you are getting the freshest water available from the city main.
Tracking the Seasonal Cycles
Turnover doesn’t happen on the same day every year. It depends entirely on the weather. A warm autumn might delay turnover until December, while a sudden cold snap in October can trigger it early.
We track these seasonal trends and provide updates on source-side water events in our blog. If you notice a sudden shift in your water’s flavor that doesn’t seem to correlate with the seasons, it may be a more localized issue, such as a water main repair on your block or a problem with your building’s internal tanks. In those cases, we recommend checking our contact page to report the anomaly.
Conclusion: Living with a Dynamic System
The taste of “reservoir turnover” is a reminder that New York City’s water is a product of the natural world. Unlike the standardized, highly processed water found in many other cities, NYC tap water is a living, breathing system that reacts to the mountain air and the changing seasons.
While the “earthy” notes of autumn might be a temporary nuisance, they are a hallmark of an unfiltered, high-quality water supply that has sustained the city for generations. Embrace the season, chill your water, and remember that this, too, shall pass as the reservoirs settle into their winter rhythm.