GUAYAMA, PUERTO RICO — The coastal communities of Guayama and Salinas in southern Puerto Rico feature acres of vibrant green farmland, and a rich, biodiverse estuary, the protected Jobos Bay, which stretches between the neighboring townships. But this would-be tropical paradise is also the home of both a 52-year-old oil-fired power plant and a 22-year-old coal-fired power plant, which local residents say contaminate their drinking water and air, and harm people’s health.