Unlike parts of the western U.S., where individuals can enforce private property rights to water, the South has generally held that the region’s water is a publicly held resource. The upside of this is that everybody has equal claim to drinking water without regard to status or income or how long they’ve lived there. The downside is that industry also has the same free claim to the water that citizens do—and they use a heck of a lot more of it.

Currently most of South Carolina gets its water from aquifers, which are basically huge underground caverns made of limestone. Receiving groundwater drip by drip, century after century, from rainfall farther inland, the aquifers essentially act as massive, sterile warehouses for freshwater, accessible through wells.

The aquifers have human benefit only if their water remains fresh. Once saltwater from the ocean begins intruding into an aquifer, it doesn’t take much to render all of it unfit for human consumption—forever. What keeps that freshwater fresh is natural water pressure, keeping the ocean at bay.

But nearly a century ago, paper mills began pumping millions and millions of gallons of water out of coastal aquifers. Combined with the dramatic rise in coastal residential development, that has decreased the natural water pressure of the aquifers, leading to measurable saltwater intrusion at several points under the coast.

Currently, local and state governments in both states are increasing their reliance on surfacewater (i.e., treated water from rivers and creeks) to relieve the strain on the underground aquifer system. But it’s too soon to tell if that has contained the threat from saltwater intrusion.