E. coli (Escherichia coli) is a type of bacteria commonly found in the intestines of humans and animals. While most strains are harmless and play an important role in healthy digestion, their presence in lake water can indicate that fecal contamination has entered the system through stormwater runoff, wildlife, or other sources. Because some strains can cause illness, E. coli is used as a key water-quality indicator to help determine when swimming or recreational contact may pose a health risk. GLEA regularly monitors E. coli levels around Geneva Lake to help ensure safe and informed lake use.

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Lake water naturally contains many types of bacteria beyond E. coli, including those that break down organic matter, support nutrient cycles, or arrive through runoff after rainfall. Most of these microbes are harmless and play essential roles in a healthy aquatic ecosystem, but certain conditions—such as warm temperatures, heavy storms, or high nutrient levels—can cause some bacteria to increase. 

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While many people associate E. coli with foodborne illness, E. coli naturally lives in the intestines of humans and all warm-blooded animals, including dogs, geese, ducks, gulls, deer, cattle, and other wildlife.

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After a rainfall, stormwater runs across the landscape before entering streams and lakes. Along the way, it can pick up fecal material from many sources, including:

• Pet waste left on lawns, parks, and trails

• Waterfowl droppings from beaches, shorelines, and parks

• Wildlife such as deer and raccoons

• Agricultural runoff from livestock operations

• Human sewage from leaking sewer lines, failing septic systems, or sanitary sewer overflows

 

That runoff carries bacteria into the lake, which is why E. coli levels often increase after heavy rain. The E. coli test itself does not identify whether the bacteria came from a person, a dog, a goose, or another animal, it simply tells us that bacteria is present.