1.    What is a beach closure?
Contamination advisories are posted warning swimmer’s that E.coli counts are too high for body contact with the water by state standards.
2.    Water samples: daily geometric mean vs.  30-day geometric mean
Daily: Below 300 E. coli per 100 mL of water
 30-day: Below 130 E. coli per 100 mL of water
From the Michigan Department of Environmental Quality beach monitoring webpage:
“The daily geometric mean calculated from these (three) samples must be below 300 E. coli per 100 milliliters for the water to be considered safe for swimming. Sometimes one or two of the samples may be above 300, but if the daily geometric mean is below 300, the beach is not in violation of the water quality standard.
After 30 days, a geometric mean is calculated for all the individual samples collected within that time frame. This 30-day geometric mean must be below 130 E. coli per 100 ml for the water to be considered safe for swimming.”
3.    Different organizations can test more often or less often than others for beach contamination—be careful when comparing results.
From the Michigan Department of Environmental Quality beach monitoring webpage:
“County health departments need to take a minimum of three samples each time a beach area is monitored…
A minimum of five sampling events (consisting of at least three samples per event) must be collected within a 30-day period for the results to be considered a reliable indication of water quality.”
But different health departments may test their beaches more often or less often, making the chances of detecting pollution greater or less depending on sampling frequency.
4.    Public vs. private beaches
From Michigan Department of Environmental Quality Beach Monitoring Program Coordinator, Shannon Briggs:
“The definitions are a bit slippery because I am trying to accommodate reality, databases, maps, local, state, and federal regulations.
I have categorized beaches into the following:

• Publicly owned with public access (public beach, state park, sleeping bear dunes, OPEN to the public
• Publicly owned with private access (neighborhood association, subdivision) NOT open to the public
• Privately owned with public access (YMCA, camps, clubs) SOMEWHAT OPEN to the public
• Privately owned with private access (residence) NOT open to the public

5.    Different agencies interpret beach data differently.
The Natural Resources Defense Council interpreted results of a beach report differently than the State of Michigan.
Also check out this article at GreatLakesEcho.org: Report ranks the water quality of Great Lakes beaches.