The study identified 12 percent of the continental U.S. coastline — including the northern Gulf Coast from Mississippi to the Florida Panhandle, northern Atlantic Coast, and Pacific Northwest — where the once-hidden drainage systems make the ocean most susceptible to freshwater contamination from septic tanks and fertilizer runoff.
In contrast, another 9 percent of coastline — including confirmed locations such as southeastern Florida, southern California, and Long Island — are especially susceptible to the opposite threat: contamination from sea to land, the study found. In these areas, saltwater intrudes inland and infiltrates the fresh groundwater supply.
Overall, more than 15 billion tons of freshwater flows through invisible underground networks into the ocean along the continental United States coastline every year, the researchers found.
That sounds like a lot of water, but it’s less than 1 percent of the total amount that flows from the continental United States into the ocean, pointed out study coauthor Cédric David of JPL. The other 99 percent comes from rivers and surface runoff.
Still, David explained, the study is significant because it provides the first continental-scale high-resolution estimate of that 1 percent — a portion which, when compared to the other 99 percent, can be particularly rich in nutrients and other contaminants.