DORADO, Puerto Rico — Every 10 minutes or so, a truck or a van pulled up to the exposed spigot of an overgrown well, known as Maguayo 4, that sits not far from a bustling expressway and around the corner from a Krispy Kreme doughnut shop. Fencing around the area had been torn open, and a red and white “peligro” sign, warning of danger, lay hidden beneath debris and dense vegetation.