Researchers have uncovered previously hidden sources of ocean pollution along more than 20 percent of America’s coastlines. The study, published online Aug. 4 in the journal Science, offers the first-ever map of underground drainage systems that connect fresh groundwater and seawater, and also pinpoints sites where drinking water is most vulnerable to saltwater intrusion now and in the future. Audrey Sawyer, assistant professor of earth sciences at The Ohio State University and leader of the study, said that while scientists have long known that freshwater and seawater mix unseen below ground, until now they hadn’t been able to pinpoint exactly where it was happening, or how much.