An organization representing mayors of more than 120 cities in the Great Lakes region is challenging a decision to let Waukesha, Wisconsin, draw water from Lake Michigan. The Great Lakes and St. Lawrence Cities Initiative this week requested a hearing before a council representing the eight states adjoining the lakes. The council in June granted Waukesha permission to tap Lake Michigan, even though a regional compact prohibits diverting Great Lakes water outside the natural watershed with rare exceptions. Waukesha is barely outside the watershed boundary. Its groundwater is contaminated with radium. The mayors group says Waukesha didn’t meet rigorous standards for water diversions outlined in the compact. The group says the decision sets a bad precedent and could lead to a flood of similar requests from other cities.