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The star of the show is the canal's infamous bottom sediment, affectionately known as "black mayonnaise" — a thick, oily sludge that is in some places up to 20 feet deep. It's a lethal cocktail of:
- Coal tar from old manufactured gas plants
- Heavy metals: mercury, lead, copper, arsenic, chromium
- PCBs (polychlorinated biphenyls) — banned for decades, still hanging around
- PAHs (polycyclic aromatic hydrocarbons) — byproducts of petroleum and combustion
- Volatile organic compounds (VOCs)
- Trichloroethylene (TCE) — linked to Parkinson's disease
- Benzene
- Pesticides
- Raw sewage — millions of gallons flow in every time it rains
- Oil, rotting debris, and general industrial horror
In 1999, a reporter described the smell as "like sticking your head into a rubber boot filled with used motor oil and rotten eggs."
It's not just chemicals. Microbiologists have found the pathogens that cause:
- Gonorrhea
- Chlamydia
- Typhoid
- Cholera
- Dysentery
- Tuberculosis
Yes — the canal literally has STDs. A 1974 NYC Community College report confirmed typhoid, cholera, dysentery, and TB. More recent sampling in 2007-2008 found "every kind of imaginable pathogen," including live gonorrhea bacteria after sewage overflow events.
Researchers also found antibiotic-resistant bacteria thriving in the sludge.
The EPA's Superfund dredging has turned up:
- Two whole cars (one possibly a Mercedes), coated in black mayonnaise
- A WWII-era crash boat
- An old-timey wagon wheel
- Porcelain coffee cans
- A glass boat
- Countless tires
- Bricks and textile mill relics
- Sonar scans revealed 36 large objects in the turning basin alone, including two boat wrecks, eight support pilings, a tree, and 25 other items over 5 feet across
In 2021, a dredging barge full of sludge actually sank back into the canal. The toxic soup claimed its own cleanup equipment.
As of January 2026, there's a major concern that goes beyond the canal itself: coal tar plumes from old gas plants have migrated underground to depths of nearly 150 feet, spreading beneath the surrounding neighborhood. The former Citizens Gas Works site (closed 1960) is a primary source. These toxic plumes continue to leach contaminants into the canal and surrounding soil — and the state DEC still has no neighborhood-wide plan to address it, even as luxury development booms on top of partially remediated parcels.
The Gowanus was designated a Superfund site in 2010. As of March 2026:
- The upper segment (RTA 1) was dredged and completed in summer 2024 — but had to be re-dredged because new contaminated sludge accumulated
- The mid-canal segment (RTA 2) began in June 2024; bulkhead installation is ongoing, with dredging to follow
- NYC is building massive CSO retention tanks ($1.6 billion) to capture sewage overflow before it hits the canal — expected completion fall 2026
- There are still tens of thousands of cubic yards of black mayonnaise at the bottom
NYU researchers discovered 455 species of microorganisms in the sludge wielding 64 different biochemical pathways to degrade pollutants and 1,171 genes to process heavy metals. Life, uh, finds a way — even in a canal with gonorrhea.
- Gowanus Faces a Hidden Threat Beneath Its Neighborhood Boom - City & State NY (Jan 2026)
- Developers See Brooklyn's Gowanus Canal as an Alluring New Waterfront - Inside Climate News
- Pollution-Eating Microbes Are Thriving in Infamous NYC Canal - Popular Science
- Long-Overdue Dredging Turns Up 'Black Mayonnaise,' Sunken Cars - Gothamist
- Gowanus Canal - Wikipedia
- The Gowanus Has Gonorrhea - CUNY
- Gowanus Canal - NOAA
- Gowanus Canal Spews Forth Slimy Bounty - Gothamist
- March 2025 EPA Update - Gowanus CAG
- Dredging Barge Full of Sludge Sinks Into the Incredibly Toxic Gowanus Canal - Jalopnik