Lake Ontario carries a significant pollution burden, though the situation has improved dramatically since the 1970s when industrial dumping was largely unregulated. Today, the lake faces a mix of legacy chemical contamination in its sediments, microplastic pollution at the highest levels of any Great Lake, nutrient runoff that fuels toxic algal blooms, and a steady stream of urban contaminants from surrounding cities. The water is safe to drink after municipal treatment, but fish consumption advisories remain in place, and beaches close periodically due to bacteria.
Legacy Chemicals in the Lakebed
Decades of industrial activity left PCBs and mercury embedded in Lake Ontario’s sediments. While concentrations have declined substantially since regulatory changes began in the 1970s, the contamination hasn’t disappeared. As of 2018, PCB levels in surface and suspended sediments ranged widely, from less than 10 nanograms per gram up to 357 nanograms per gram. That enormous range reflects the persistence of legacy hotspots, areas near old industrial sites where contamination remains concentrated, alongside ongoing diffuse inputs from urban areas.
These sediment-bound chemicals matter because they work their way into the food chain. Bottom-dwelling organisms absorb them, small fish eat those organisms, and larger predatory fish accumulate the chemicals at higher concentrations. This is why fish consumption advisories persist even though the lake’s water itself tests clean at treatment plants.
Microplastics: Worst in the Great Lakes
Lake Ontario has the highest surface water concentration of microplastics among all five Great Lakes, averaging over 230,000 particles per square kilometer. Some samples have recorded more than 1.3 million particles per square kilometer. For comparison, Lake Erie, the second most contaminated, averages roughly 45,000 particles per square kilometer.
Lake Ontario’s position at the downstream end of the Great Lakes system helps explain this. Water flows from Lake Superior through Huron, Erie, and their connecting rivers before reaching Ontario, carrying accumulated plastic debris the entire way. Combined with direct inputs from cities like Toronto and Rochester, the lake essentially collects plastic pollution from the entire basin. These particles are too small to see individually but include fragments of packaging, synthetic clothing fibers, and degraded larger plastics.
Nutrient Runoff and Toxic Algal Blooms
Phosphorus from agricultural fields, lawn fertilizers, and wastewater feeds algal blooms in Lake Ontario’s bays and nearshore areas. Sodus Bay, a shallow embayment on the lake’s southern shore, illustrates the problem well. Tributaries and direct drainage deliver an estimated 6,385 kilograms of phosphorus per year, and an additional 5,854 kilograms comes from internal loading as phosphorus stored in bottom sediments gets recycled back into the water. The result: summer phosphorus levels that consistently exceed New York State guidelines.
Starting in 2010, Sodus Bay began experiencing repeated toxic cyanobacterial blooms in late summer. Water sampling in 2011 and 2012 found the toxin microcystin at concentrations up to 24.9 micrograms per liter, well above levels considered safe for recreational contact. These blooms produce visible green scums on the water surface and can cause skin irritation, nausea, and liver damage if ingested in sufficient quantities. Shallow, warm bays with high nutrient inputs are most vulnerable.
Urban Stormwater: A Cocktail of Contaminants
Every time it rains, stormwater washes a remarkable variety of pollutants off roads, parking lots, and lawns into Lake Ontario. A recent study of stormwater ponds detected 200 organic compounds in runoff, including pesticides, pharmaceuticals, industrial chemicals, and vehicle-related pollutants. Common lawn chemicals like the herbicide 2,4-D and the insect repellent DEET were found at 100% of sites tested, alongside the antidepressant mirtazapine, the pain reliever lidocaine, and caffeine.
Traffic-related metals, including copper, zinc, lead, barium, and manganese, were consistently elevated compared to reference sites. For seven metals, every single stormwater pond tested higher than the unpolluted reference stream. A particularly concerning finding was the universal detection of 6PPD-quinone, a chemical that forms when a rubber additive in tires reacts with ozone. This compound is highly toxic to certain fish species, particularly coho salmon, at very low concentrations.
Chloride from road salt and fecal contamination were also widespread. Wastewater treatment plants contribute their own share, and Ontario’s 2024 drinking water report confirmed that nearshore water quality is “significantly influenced” by rivers and discharges from wastewater facilities.
Is the Tap Water Safe?
Yes. Municipal treatment plants that draw from Lake Ontario produce water that overwhelmingly meets safety standards. Ontario’s 2024 drinking water report found that 99.9% of more than 524,000 tests met provincial standards. Over 80% of Ontario residents get their drinking water from the Great Lakes, so this system is heavily monitored.
One emerging concern is PFAS, the group of persistent synthetic chemicals sometimes called “forever chemicals.” The good news: PFAS concentrations in Lake Ontario’s nearshore drinking water intake zones dropped between 2006 and 2023, following restrictions on their manufacture. Total PFAS levels measured in 2023 were well below Canada’s drinking water objective of 30 nanograms per liter. Ontario has set its own guidance value at 70 nanograms per liter for 11 specific PFAS compounds and is reviewing whether to adopt the stricter federal number.
Beach Closures and Swimming
Lake Ontario beaches close periodically when bacteria levels spike, most often after heavy rainstorms that flush sewage overflows or contaminated runoff into the lake. New York State parks test for E. coli in freshwater, and any reading above 235 colonies per 100 milliliters triggers a response. Depending on the beach’s history of contamination, this either means immediate closure or rapid retesting with a closure decision within 18 to 24 hours.
Beaches with a strong track record of clean water get the benefit of the doubt and stay open pending retest results, provided conditions look otherwise safe. Beaches with a history of repeated problems close automatically. Cold temperatures, high waves, strong winds, and recent thunderstorms also factor into closure decisions. If you’re planning a swim, check posted signs at the beach or look up current water quality results before going in.
Fish Advisories Still in Place
You can eat fish from Lake Ontario, but with limits. New York updated its guidelines in 2021, loosening some restrictions as contaminant levels have declined. Women under 50 and children under 15 can now eat up to four half-pound meals per month of lower-risk species: brown bullhead, rainbow smelt, rock bass, white sucker, and yellow perch. For salmon and several other species that accumulate more contaminants, the limit drops to one half-pound meal per month for the same group.
The advisories exist primarily because of PCBs and mercury that bioaccumulate in predatory fish. Larger, older, fattier fish tend to carry higher concentrations. The trend is moving in the right direction, with some species showing steady declines in contamination since the 1970s, but the chemicals persist long enough that advisories will likely remain for years to come.
Cleanup Progress at Toxic Hotspots
The U.S. and Canadian governments designated the most contaminated sites around the Great Lakes as “Areas of Concern,” committing to remediate them. Two Lake Ontario hotspots on the U.S. side have been officially delisted after completing cleanup: the Oswego River in 2006 and Rochester Embayment in October 2024. Delisting means the site met all restoration targets for contaminated sediments, fish health, wildlife habitat, and water quality.
Across the entire Great Lakes basin, 23 U.S. Areas of Concern remain on the list and are still undergoing restoration work. The pace has accelerated in recent years, with six U.S. sites delisted since 2013. For Lake Ontario specifically, the trend is encouraging: the most recent delisting at Rochester came after extensive sediment remediation and habitat restoration along the Genesee River, which empties into the lake at Rochester, New York.