Let water sit around for 24 hours -- stagnating in an old tire, say, or in a birdbath in the backyard -- and some pretty nasty things can start breeding in it. After Mt. St. Helen's erupted in Washington State in 1980, newly formed oxygen-deprived pools became breeding grounds for Legionnaires' disease. Then there's cholera, typhoid, dysentery, and other illnesses that can be contracted from untreated bathing and drinking water. Yet, while swimming in a bacteria- and pathogen-filled pool poses clear health threats, chlorine is not a risk-free solution. What lies beneath? Some very harmful substances -- dioxins and furans, …