How We Monitor Beach Water Quality

Beach water sampling

We monitor water quality at Dane County area beaches for E. coli bacteria and blue-green algae. We do this to keep harmful bacteria and toxins from making you sick while you enjoy our lakes and beaches.

Want to know how the water quality is at your favorite beach? You can check conditions on our webpage or sign up for email alerts.

Beaches We Monitor

Lake Mendota:

James Madison, Lake Mendota County Park, Maple Bluff, Marshall, Memorial Union, Spring Harbor, Tenney, Warner Park

Lake Monona:

BB Clarke, Bernies, Brittingham, Esther, Frost Woods, Hudson, Olbrich, Olin, Schluter

Lake Waubesa:

Goodland, McDaniel

Lake Wingra:


Stewart Lake:

Stewart Lake


Troll Beach


Fireman's Park Beach

What we Test Water For

E. coli Bacteria

  • The most common germs found in water are bacteria, viruses, or parasites from human or animal feces. These germs can make you sick.
  • Testing for all the germs in the water that could make you sick is time consuming and expensive. We test for Escherichia coli (E. coli) because it is the best indicator if there are germs in the water that can make you sick.

Blue-Green Algae (Cyanobacteria)

  • Blue-green algae can form blooms that can be toxic to people and animals, causing a variety of symptoms.
  • Blue-green algae blooms are created when there is sunshine, high levels of nutrients in the water, and other factors. 

When We Collect Water Samples

E. coli Bacteria

  • We collect water for E. coli testing at least once a week at each beach.
  • If bacteria levels are high, we close the beach for swimming. We test again each weekday until the sample meets an acceptable bacteria level, then re-open the beach for swimming.

Blue-Green Algae (Cyanobacteria)

  • We look for blue-green algae blooms at least once a week at each beach.
  • If a lifeguard or beach user reports an algae bloom, we will visit that beach as soon as possible to decide if it should be closed. 
  • If a blue-green algae bloom is present, we will test water daily (M-F) for blue-green algae toxins.
  • The beach will stay closed for swimming until levels of blue-green algae are low enough to re-open for swimming. 

Beach Closures for Swimming

  • A beach water quality closure means that the water is unsafe for swimming.
  • Closures happen when levels of E. coli or blue-green algae toxins are higher than safety limits.
    • For E. coli, this limit is 1,000 MPN/100 mL.
    • For blue-green algae, this limit is 8 ppb or higher of toxin-producing algae and/or microcystin toxin.
  • You can still enjoy the sandy area of the beach and other park facilities, but don't go in the water.

Conditions Can Change Quickly

Many things can affect water quality:

  • Recent wind or rainy weather
  • Time of day
  • Birds and other wildlife at the beach
  • Number of people at the beach

Conditions can vary a lot through the day and from day-to day. Always look at the water around you before you get in. Conditions can change quickly, and a test result may not always reflect the current water quality. 

A few things to remember:

  • You cannot always see or smell when water conditions are bad.
  • Avoid swimming after a heavy rainfall because bacteria levels in the water may be high.
  • People and pets should stay out of the water if there is a blue-green algae bloom or the water is murky. Blue-green algae blooms are usually blue-green, but can also be reddish-purple, or brown.
  • Do not drink lake or river water.
  • Wash your hands before eating if you have been in the water.
  • Wash your pet after swimming.
  • Stay out of the water if signs say it is not safe for swimming.
  • Email us if you think you see a blue-green algae bloom.

How we communicate about beach water quality

Request Testing for Your Water Event

If you are hosting an event where people will be in the water, you can request additional testing for E.coli and blue-green algae. See our fact sheet for options and costs. Contact us at (608) 243-0357 to schedule testing.

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