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Middlesex-London Health Unit

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Beach Water Quality

The following factors can affect beach water quality:1

  • Water and ambient air temperatures
  • Rainfall within 24 and 48 hours or rain intensity
  • Weather conditions (e.g. cloudy, sunny)
  • Wind speed and direction
  • Water clarity/turbidity (e.g. how clear the water is)
  • Wave height
  • Pollution sources, such as waterfowl (birds), industrial wastes, storm water outflows, septic system wastes, algal blooms and agricultural run-off

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Why is heavy rainfall a concern?

Weather is a factor that can affect water quality. Rainfall can wash animal wastes into storm sewers which can then flow into rivers and lakes. This is called run-off. Also, during heavy rainfall, sewage treatment plants can become too full and the extra untreated water can flow directly into rivers and lakes. Run-off, or the outflow of extra water, can dirty the water. Bacteria and other materials may be added into the water and could affect a swimmer’s health and safety.

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Are beaches monitored?

Not all beaches in Middlesex-London are monitored. The Middlesex-London Health Unit works with owners and operators of local beaches to make sure they are safe for the public. Beach usage is reviewed and water samples are taken accordingly. Notice signs are posted on beaches that are not monitored. See Beach Water Monitoring for public beach postings or closures in Middlesex-London and surrounding areas.

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Unmonitored Beach Notice

A picture of a sign indicating that the beach is not monitored.

Sign indicating that the beach is not monitored

Unsafe Bathing Warning

A picture of a sign indicating that the beach is not safe for bathing due to high bacterial levels.

Sign indicating that the beach is not safe for bathing due to high bacterial levels


What happens when water quality is affected?

Depending on the nature and extent of an event that affects water quality, a beach may be posted with a warning sign that high levels of bacteria were found, or it may be closed. See Beach Water Monitoring for public beach postings or closures in Middlesex-London and surrounding areas.

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When are warning signs posted at a beach?

Warning signs are posted at public beaches when results from water tests show high levels of bacteria.1 Levels of bacteria are considered too high when they are greater than the provincial standard that is set jointly by the Ontario Ministry of Health and Long-Term Care and the Ontario Ministry of the Environment, Conservation and Parks. Swimming is not recommended when there are greater than 200 E. coli bacteria per 100 ml of water.1

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When is a beach closed?

According to the Ministry of Health and Long-Term Care’s Operational Approaches for Recreational Water Guideline, 2018 (PDF 596KB) 1 the following are some examples of adverse events that could lead to the closing of a beach:

  • High levels of bacteria
  • Chemical, oil, sewage or other waste spills
  • Waste water treatment plant bypasses
  • Blue-green algae blooms
  • Heavy algae growth
  • Fish or other wildlife die-off at the beach
  • Visible debris, metal, or sharp objects found in the water or beach area

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What are the risks of swimming at a posted or closed beach?

Some risks of swimming at a posted or closed beach are: 

  • Developing illnesses, such as stomach or intestinal illness with symptoms of diarrhea and vomiting.
  • Developing infections, such as lung, eyes, ear, nose, throat or skin infections.
  • Becoming injured due to contact with physical objects, such as rocks, broken glass or driftwood.

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Additional Information

To speak to a Public Health Inspector on the Environmental Health Team about public beaches or beach water quality, please call:

  • 519-663-5317
Date of creation: February 15, 2013
Last modified on: June 16, 2023


1Ontario. Ministry of Health and Long-Term Care. (2018, February 5). Operational approaches for recreational water guideline, 2018 Retrieved from