The water beneath a large swath of Phoenix isn’t fit to drink. A plume of toxic chemicals has tainted the groundwater for decades, and it’s now at the center of a bitter fight over how the aquifer should be cleaned up and what should happen to the water in the future. At issue are questions about why the cleanup has proceeded slowly, which government agency should lead the effort, and whether the polluted water, which isn’t flowing to household faucets, is releasing chemicals into the air at levels that may pose health risks for people in the area.