All but one public swimming beach in the state is open for the last holiday of summer. Some beaches were closed for days or weeks, and a few for most of the summer, because of bacterial contamination. Bureau Chief Steve Pennington says while septic tanks and feedlots often get the blame for fouling waterways, bacteria in lakes can’t always be tracked to specific causes.The beach at George Wyth State Park near Waterloo in Black Hawk County is closed due to high bacteria levels, while warnings are posted for swimmers at Clear Lake and Backbone State Parks. Pennington says though the beaches have been cleared, they’re still natural environments and aren’t guaranteed to be germ-free.High bacteria levels closed many swimming beaches after this spring’s long rainy spell. The DNR hopes to have findings later this fall about research it’s done on beaches that had bacteria problems.