CASTLE ROCK — They stand at least 30 feet tall and 10 feet across, eight giant baby blue tanks filled with what is this fast-growing town’s best defense against a glut of industrial cancer-causing chemicals that have been accumulating and percolating for half a century. Called GACs, short for “granular activated carbon” filters, the tanks at Castle Rock’s main water purification facility are effective at absorbing perfluoroalkyl and polyfluoroalkyl substances — toxic and pervasive chemicals known by the shorthand PFAS — and removing them from drinking water. “It just so happens that one of the things the GAC filters are good at is filtering out PFAS,” Mark Marlowe, Castle Rock’s water director, said this week.