Seep and ye shall find: hidden water flow.

Subterranean rivers flowing slowly through the ground carry freshwater into the ocean in quantities far larger than scientists had suspected, according to recent research conducted off the coast of South Carolina. These invisible seeps season the seas with salts, metals, and pollutants.

During July 1994, groundwater oozed out through sediments on the seafloor along the state's coastline at a rate of 30 billion liters per day. This is a little under half the water entering from the region's rivers, says Willard S. Moore of the University of South Carolina in Columbia.

'What we've done is show that the groundwater input to the coastal ocean is extremely important,' says Moore, who described his results in the April 18 Nature.

'This has tremendous implications for understanding everything from pollution to geochemistry,' comments ecologist George M. Simmons of the Virginia Polytechnic Institute and State University in Blacksburg.

Although hidden from view, water stored in porous, underground rocks constitutes 97 percent of the world's supply of liquid freshwater. Like surface rivers, which flow downhill toward the sea, some of this groundwater migrates downward through subsurface formations that open beneath the ocean.

Scientists have made spot estimates of groundwater entering the sea, but Moore's study is the first to gauge this process on a regional scale.

The South Carolina geochemist happened on a technique for measuring groundwater flow while he was studying naturally occurring radium in a salt marsh. Moore found that the amount of an isotope, 226Ra, in the marsh water exceeded that in ocean water. Because no rivers emptied into the inlet, the radium must have come from groundwater seeping into the marsh, he reasoned.

He then used 226Ra to trace groundwater entering coastal waters and again found excess radium that could not have come from the ocean or rivers. 'By process of elimination, the primary source of the 226Ra enrichment must be the discharge of groundwater containing dissolved 226Ra,' he concludes.

Moore suspects that when brackish groundwater penetrates fresh aquifers, it causes sediments to release 226Ra, which can then flow into the ocean. People enhance radium movement by pumping drinking water from coastal aquifers; this reduces pressure in the aquifer and allows saltwater to intrude farther inland than it otherwise would.

He expects that other elements and ions behave similarly, making groundwater an important source for many of the ocean's trace constituents, including nutrients that support aquatic life.

In unpublished studies, geochemist William C. Burnett of Florida State University in Tallahassee also has found significant groundwater movement into the northeast Gulf of Mexico. Burnett's group used dissolved radon as a tracer of groundwater flow.

'When you look at the process, it's a lot larger than people might have thought. We were surprised. It's been one of those things in earth science where people know it goes on, but no one to date had really developed a good way of measuring it, so it's generally not talked about,' says Burnett.

Before these new measurements, some scientists had suggested that groundwater flow into the ocean might equal only one-thousandth of the river flow. But Burnett estimates that groundwater contributions to the ocean total about one-tenth of the amount supplied by rivers globally. In some locations, the groundwater component may dominate, he says.

Measurements off many coasts indicate that subterranean flow can carry pollutants, as well as naturally occurring elements, into the ocean. In particular, nitrates from septic tanks have seeped into seawater, says Burnett.