Safe Swimming: Bacteria in Ponds & Lakes
Public swimming beaches are routinely sampled to check bacteria levels to make sure the water is “safe” for body contact. Sample results and beach advisories/closures are posted on the Department of Environmental Quality’s Beach Guard.
Is my pond or lake safe for swimming?
Surface Water Sampling Procedures
- Samples must be taken at least 1 foot below the surface, in water that is 3-6 feet deep.
- The sample bottle should be inserted into the water upside down, and then righted when it is about 1 foot below the surface of the water.
- The form that comes with the bottle should be completed and attached to the bottle with a rubber band. If sampling multiple locations, be sure to note the different sample points (site 1, site 2, etc.).
- Provide an accurate phone number on the form so you can be contacted about an unsafe result.
- If the sample is collected from water that you believe is contaminated with raw sewage or other pollutant, make a note of this on the sample form.
- The samples need to be kept chilled, and they MUST be returned to the laboratory on the same day they were collected. Samples should be brought to the lab as soon as possible after collection.
- It is not recommended to sample a surface water body during or within 24 hours after significant rainfall. Water quality is usually poor after rainfall due to run-off, where rain flows over the surface of the ground into the body of water, carrying contaminants with it. Limiting full body contact after a significant rain event is recommended.
- If the water appears cloudy or murky, chances are high that the sample will be unsafe.
- Multiple sampling sites are recommended for larger ponds, lakes with multiple residences or a “commons” area and a broad beachfront (over 100 feet), or multiple swimming areas.
- Laboratories do not test surface water for “swimmers itch,” which occurs in some local lakes as the water warms in late June to early July.
Surface Water Sampling Results
The E. coli bacteria test takes 1-2 days to complete. Per the State of Michigan Bathing Beach Guidelines, a beach is considered “safe” for full body contact (i.e. swimming) as long as the result is:
- Less than 300 colonies of E. coli per 100ml for a single sample.
- A geometric mean of less than 130 colonies of E. coli per 100ml for a minimum of 5 sample events collected over a 30-day period.
In some cases, Environmental Health has received requests to sample pond or lake water to help people decide if it is safe to consume fish from a particular body of water. Our office does not offer testing for this purpose. More information on this subject can be found in the Michigan Fish Consumption Advisory.