The rain barrel collects water of your roof. The roof  collects debris falling off trees and from animal activity.  There is plenty of animal feces and it is possible a bird died on your roof or eggs cracked open and some of the bacteria can wash into your barrel. Or perhaps there is an abundance of oak droppings and pollen that have collected and drain into the barrel. Either way in hot temps this can become an incubator for bacteria. While the Great American Rain Barrel is thicker than most barrels and UV protected this occasionally happens. To eliminate the odoryou may want to use ¼ of Clorox to disinfect the water. Swab down the inside of the barrel as  you drain the barrel down. And clean it when empty. For maintenance we recommend using stones as an added layer of filtering and clean your screen occasionally. Also wipe down the inside of the barrel once in a while.