It’s become a hallmark of the fall season for metro Atlantans: Before the trees change colors and a chill enters the air, the stench of the algae blooming pours through water faucets.
Although perfectly safe to drink, the smell and taste can be off-putting, if a little concerning. But it’s merely a byproduct of the slow autumn churning of Lake Lanier, the region’s most important source of water, The Atlanta Journal-Constitution reported when water was impacted last year.
The compounds in the lake causing the odor are called 2-methylisoborneol, or MIB, and geosmin. They are released from algae that grow in the cool freshwater of the lake and reach elevated levels most commonly in late summer and early fall, according to Forsyth County’s water department, which put out a notice to residents last week.
Samples from Sept. 1 show the raw water concentration of MIB is 213 parts per trillion, which was reduced to 22.3 parts per trillion when sent out to water customers, the Forsyth water department’s news release says. It’s noticeable in drinking water around 10 parts per trillion, officials added.
The musty, earthy smell and taste has been a problem in or around September for years, as the AJC previously reported. But in 2025, the Chattahoochee River reached a “record high concentration” with 137 parts per trillion that November, according to the Cobb County-Marietta Water Authority.
“For comparison, the highest level recorded in 2024 was 19,” the water authority said in the news release.
Water systems in the region are finding ways to reduce the impacts to drinking water. The Cobb-Marietta Water Authority allocated funding “to begin evaluating advanced water treatment processes in 2026 to improve future taste and odor control.”
Forsyth is using ozone treatment processes, which can make the water taste more like chlorine but is still safe, according to the water department.
“Please be assured your water is safe to use and drink, even if it may smell or taste different than normal,” department officials said in the news release.
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