Bacteria and Our Beaches

E. coli and beach pathogens

Our public beaches offer vital access points for the community to enjoy our cherished lakes. Unfortunately, unsafe water conditions can prevent their safe use and enjoyment. The two most common reasons for beach closures are elevated concentrations of E. coli and/or cyanobacteria (blue-green algae) blooms. Below, we specifically discuss E. coli and what it means for our lakes and beaches. 

What is E. coli?

Escherichia coli, known as E. coli, is a common bacterium easily grown in a laboratory, making it one of the most-studied bacteria today. There are many different strains of E. coli, some of which can cause serious illness. Humans and all warm-blooded animals have E. coli strains living harmlessly in our lower intestine to aid in digestion. When we talk about the presence of E. coli at the beach, we are usually talking about these harmless strains, but that can indicate the likely presence of disease-causing organisms.

E. coli bacteria, Credit Gerd Altmann (Pixabay)

Why are our beaches tested for E. coli?

All warm-blooded animals have some E. coli living in their lower intestines and found in their feces. While E. coli is not likely to cause illness on its own, it is relatively easy to detect in the lab, making it a useful biological indicator of contamination. If a water sample has high concentrations of E. coli, other more dangerous and infectious organisms may be present. While most E. coli strains are harmless, their presence can mean that illness-causing pathogens are also present.

Because of the risk posed to public health, one of the primary reasons for beach closures is when elevated levels of E. coli are detected. Those who are most at risk to develop illness are children, the elderly, and people with compromised immune systems. Healthy individuals who get an infection will normally not develop serious problems and recover on their own with rest and drinking plenty of fluids. Unfortunately, those at higher risk can develop more severe illnesses.

Note: “Swimmer’s itch” is often incorrectly attributed to E. coli exposure, when in fact it is caused by microscopic parasites that thrive in waterfowl and freshwater mollusks.

Beaches closed
Beach closure sign posted on lifeguard stand

What are the risks of swimming in contaminated waters?

High counts of E. coli indicate that the water is likely to be contaminated by feces, increasing the risk of exposure to pathogens that can cause illness. As mentioned earlier, young children, the elderly, and those with compromised immune systems are considered most vulnerable.

The most common symptoms of exposure are general gastrointestinal discomfort and skin rashes, and eye and ear infections. Some strains can cause other severe and even life-threatening complications, such as bloody diarrhea, kidney failure, and (in rare cases) death. Most cases of illness resulting from swimming in contaminated water are unpleasant but mild, and do not require medical treatment. However, consult with your physician if symptoms are severe or persist.

E. coli symptoms (Image credit JR Bee, Verywell Health, 18April2020)

What causes high E. coli levels at the beach?

High E. coli levels can occur for several reasons. Commonly, large rainstorms will wash dog or bird  (especially geese and seagull) feces into the water, carrying pathogens with them. Raccoons that may be living in storm sewers can be a source of E. coli-containing feces, as can farm-manure runoff. Finally, aging sanitary sewer pipes and other wastewater infrastructure can leak sewage that then makes its way into our lakes.

Complicating the picture, microbes can survive and sometimes reproduce outside of animal hosts, such as in lake and streambed sediments or in beach sand. This makes it difficult to determine how and when a sampling site was contaminated. Genetic sequencing can help identify the animal source, but the tests remain relatively cost prohibitive to be conducted on a regular basis.

What beaches are tested?

The following Madison-area beaches are tested on a weekly basis by Public Health Madison & Dane County: BB Clarke Beach, Bernie’s Beach, Brittingham Beach, Esther Beach, Frost Woods Beach, Goodland County Park, James Madison Beach, Lake Mendota County Park, Maple Bluff Beach Park, Marshall Beach, Memorial Union Pier, Olbrich Beach, Olin Beach, Schluter Beach, Spring Harbor Beach, Tenney Beach, Vilas Beach, Warner Beach, and Hudson Park Lake Access Point.

Madison-area beaches and testing agencies

MendotaJames Madison, Lake Mendota County Park, Maple Bluff, Marshall, Memorial Union, Spring Harbor, Tenney, Warner ParkPublic Health Madison & Dane County
MendotaGovernor Nelson State ParkWisconsin DNR
MononaBB Clarke, Bernie’s, Brittingham, Esther, Frost Woods, Hudson, Olbrich, Olin, SchluterPublic Health Madison & Dane County
WaubesaGoodland, McDanielPublic Health Madison & Dane County
WingraVilasPublic Health Madison & Dane County
KegonsaKegonsa State Park Wisconsin DNR

If a water sample is found to contain E. coli above a specific public-health threshold (1,000 MPN/100 mL), then the beach is closed and sampled every day until concentrations fall below that threshold. At that point, the beach is then reopened.

Thee are also two State-owned beaches, located at Lake Kegonsa State Park and Governor Nelson State Park, that are tested for E. coli by the Wisconsin Department of Natural Resources on Monday through Thursday.

Water sample threshold

Threshold typeLevel of E. coli presence (CFU of E. coli/100 mL)Gastrointestinal illness cases per 1,000 water usersSafe to swim?Action taken by professionals in the field
Low235~8Caution warrantedPost advisory signs
Medium235-1,0008-14High caution warrantedPost advisory signs
High (Critical)>1,00014No swimmingImmediately convey the closure of the beach
(dnr.wisconsin.gov and US EPA, 1983, 1984, 1986, 2002)

Standard sampling methods take several hours to complete, so it is not possible to report E. coli levels on the same day the sample is collected. This delay presents a challenge for public health officials and beach managers who have to balance the risk of potential exposure against lost opportunities to access the beach. Models can be developed for individual beaches to predict the next day’s E. coli levels (based on variables like rainfall, temperature, and previous bacteria counts), but they do not always provide acceptable levels of accuracy.

How can I avoid illness?

Take the following precautions to reduce your risk of exposure once you are at the beach:

  • Look for signage indicating water quality advisories or beach closures
  • Never swim at a beach that is closed
  • Avoid swimming in murky water where you cannot see your toes at knee-depth
  • Wait 24 hours after heavy rain before heading to the beach
  • Check beach closures and water clarity reports at lakeforecast.org
  • Never swallow lake water and make sure children understand this as well

Where should I go to find out whether a beach is closed?

Visit lakeforecast.org (also a free app for your smartphone) or the Public Health Madison & Dane County website to find out which beaches are open or closed. You can also subscribe to PHMDC email alerts on beach closures.

If one beach is closed, should others be closed?

Not necessarily. Beach water quality problems are often localized because our lakes are so large. A number of beaches tend to experience high E. coli after a large rainstorm when contaminants on land can be quickly delivered to the lake.

Some beaches may be more vulnerable to contamination because of water circulation patterns, beach design, surrounding land use (such mowed grass fields that may attract geese), and nearby stormwater outfalls. Since our lakes are very dynamic, even two nearby beaches can experience different water quality conditions at any given point in time.


Public Health Madison and Dane County beach closure sign

What can be done to reduce E. coli levels at beaches?

The best solution is to prevent contaminated runoff from entering our lakes in the first place. This means addressing problems at the source: picking up pet waste, discouraging seagulls and geese from congregating near the water, preventing manure runoff, and detecting and fixing leaky sanitary sewers.

Beach managers often take action to deter congregating waterfowl. These actions include replacing lawns with taller native grasses and wildflowers near the water’s edge, erecting geese barriers, and raking sand to remove feces and expose bacteria to sunlight. Diverting potentially contaminated stormwater into rain gardens or retention ponds is another good practice. This helps the water soak into the ground than runoff into the lakes.

Clean Lakes Alliance promotes these actions, partners with local organizations to help farmers reduce manure runoff, and advocates for smart beach design, including features like native vegetated buffers. Native plantings help absorb and filter runoff before it reaches the lakes, and are effective at discouraging geese that favor clear sight lines to the water.

Dane County has also installed “Clean Beach Corridors” at several county parks. These chemical-free treatment systems filter lake water through sand, expose it to pathogen-killing ultraviolet light, and then return the water to the designated swimming area.

Pick up pet waste to help our lakes
Pick up pet waste to help our beaches and lakes

How can I help?

  • Pick up after your dog and encourage others to do the same
  • Build a pollinator-friendly rain garden to catch stormwater runoff at your home
  • When at the beach or a park, refrain from feeding wildlife like geese and ducks. Feeding encourages them to congregate and disrupts their natural foraging behaviors.
  • Keep sanitary sewers in good working condition by only flushing toilet paper. If you have a septic system, schedule regular maintenance. 
  • Support community plans to incorporate “green infrastructure,” such as rain gardens and water-permeable hardscapes, into street reconstructions and park designs

To report a suspected problem:

  • Public Health Madison & Dane County at 608-243-0356 for the following beaches: BB Clarke Beach, Bernie’s Beach, Brittingham Beach, Esther Beach, Frost Woods Beach, Goodland County Park, Hudson Park Lake Access Point, James Madison Beach, Lake Mendota County Park, Marshall Beach, Memorial Union Pier, Olbrich Beach, Olin Beach, Schluter Beach, Spring Harbor Beach, Tenney Beach, Vilas Beach, and Warner Beach.
  • Wisconsin DNR Office of Great Waters at 608-266-1926 for the following beaches: Governor Nelson State Park Beach and Lake Kegonsa State Park Beach.
  • Village of McFarland at 608-838-3153 for McDaniel Beach.
  • Village of Maple Bluff at 608-244-3048 for Maple Bluff Beach Park.

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