Belle Isle Beach closed for swimming on Thursday after a routine test conducted last week showed high levels of bacteria. The closure, ordered by the Detroit Health Department, is the first in almost two years, according to the state’s beach monitoring website.
Rangers retested the water early Friday morning and should have results by Saturday, at which point they may reopen the beach, said Tom Bissett, who manages Belle Isle Park for the Department of Natural Resources.
While test results normally come back in fewer than 48 hours, Bissett said they were delayed because the office was closed during the Memorial Day holiday weekend, and then by another day because of miscommunication between the testing company and park rangers.
In a story last week, Outlier Media explained why experts said it’s safe to swim at Belle Isle Beach — most of the time. The flow of the Detroit River keeps out most industrial contaminants and flushes out bacteria within hours or days.
Experts suggested a rule of thumb to avoid a bacterial infection while swimming: Stay out of the water after a heavy rain, which can wash bird poop from the shore into the water.
But Detroit has had little rain for a month, and none since May 20.
So, what gives? Is there another problem we should be keeping an eye on?
“Things we would look for include a dirty shore line, heavy bird activity, (or) tons of people in the water when we sample,” Bissett said.
Park rangers who collect water for testing didn’t notice any of those things.
What are the other possible culprits?
The usual sources of bacterial contamination are birds, and in rare cases, human waste.
A study published in 2022 found that geese and gulls are the most likely sources of bacterial contamination at Belle Isle.
Water samples are taken at three points along Belle Isle Beach. Last week, one had a low level of E. coli, one had a higher-than-usual level, and one was drastically higher than the level considered safe for swimming.
Could a goose have spent some time near that sampling site just before the water was collected?
Perhaps, but we can’t know for sure, said Shannon Briggs, a toxicologist at Michigan Department of Environment, Great Lakes, and Energy.
“Anything is possible, and people often wonder if a fresh dropping had skewed one of the three samples,” she said. “This could be the case for the results at Belle Isle.”
Sewage dumping by cities is also a possibility, but in this case, it’s apparently not the culprit. While Detroit and Farmington Hills reported dumping sewage into local waterways shortly after a half-inch of rain fell May 19, it entered the Detroit River downstream of Belle Isle.
The 90-degree temperatures that hit Detroit this week may have contributed to the problem because E. coli and other dangerous bacteria thrive at higher temperatures.
The bottom line?
Swimming in freshwater involves some risk. We’ll continue to look for answers from park officials about how these bacteria got into the water, but this episode is an uncomfortable demonstration that when experts say swimming is typically safe except after a rain, that’s a rule of thumb — not a guarantee.
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More about bacteria at freshwater beaches
- While water tests look for E. coli only, high levels mean there are multiple kinds of bacteria present. E. coli is an indicator species, meaning when it thrives, other bacteria are also likely thriving.
- Bacteria are more likely to be found in shallower, warmer water.
- Bacteria levels are generally lower in the afternoon because many microbes can’t survive long in direct sunlight. To be conservative, water tests at Belle Isle are taken in the morning.