Three public beaches in the region are closed, while the Windsor Essex County Health Unit has posted warnings at another four.
Colchester Beach, Holiday Beach, and Sandpoint Beach all had E.Coli counts of 1000. The safe level is 200 per 100 mL of water.
It posted warnings at Cedar Beach, which had an E.Coli count of 349, Lakeshore Lakeview Park West, Point Pelee North West Beach, where it was 435, and Seacliffe Beach, where the water contained 347 ppb.
Swimmers should stay out of the water at those beaches.
Only Cedar Island Beach had levels safe enough for swimming.
The health unit samples water at public beaches every Wednesday.