Groundwater Secrets

By Kathy A. Svitil|Sunday, September 01, 1996
Nearly all of Earth’s freshwater--some 97 percent--consists of groundwater. Yet surprisingly little is known about the movement of all that water. In particular, no one has carefully measured how much of it enters the sea. Willard Moore, a geochemist at the University of South Carolina in Columbia, has addressed that oversight. He has found that more groundwater reaches the oceans than anyone suspected.

Most groundwater nestles in underground pores in rock layers that may be thousands of feet thick. This water leaks into the sea primarily in two ways: either as coastal springs bubbling directly into the sea or by a process called tidal pumping. At high tide, salt water, which is denser than freshwater, washes into groundwater-saturated sediments on the continental shelf; when the tide ebbs, the brackish seawater and groundwater mixture is sucked into the ocean. New groundwater then flows into the sediment and is pumped out with the next tide.

Tidal pumping is how most groundwater enters the oceans and is the input that Moore measured along a 200-mile stretch of South Carolina coast. He determined the groundwater flow indirectly, by sampling the concentration of a radioactive isotope.

The idea, says Moore, is that both rivers and groundwater carry radium 226 eroded off rocks and soil to the ocean. Because freshwater becomes diluted in the sea, seawater near a continent contains more dissolved radium than does seawater farther offshore.

Knowing the amount of radium typically carried by rivers, Moore realized that rivers alone couldn’t explain the high radium levels he was finding, especially in areas like Myrtle Beach, where no river empties into the sea. In such places the radium could only be supplied by an influx of groundwater.

Moore estimates that as much as 8 billion gallons of groundwater may flow into South Carolina’s coastal waters daily--40 percent of the state’s total river discharge. That’s significant for a number of reasons. First, researchers have assumed that most freshwater was pumped into the ocean by rivers. Although Moore didn’t specifically measure groundwater pollutants, his work raises the possibility of an unacknowledged source of pollution that could harm marine life.

Additionally, the chemistry of river water and groundwater is fundamentally different: groundwater holds a higher concentration of dissolved materials than rivers or streams can. Why? River water contains more oxygen, which combines with metals such as iron in sediments. These oxidized metals have a large surface area and a strong electric charge that helps them bind up other elements in the water. Those bound elements become trapped in sediments that either remain stuck in the riverbed or that ultimately settle to the seafloor.

Since groundwater has less oxygen, its metals seldom oxidize and don’t efficiently bind to trace elements. So more of those materials remain dissolved when the groundwater reaches the sea. Typically we find dissolved trace elements in groundwater to be literally orders of magnitude higher than in river water, says Moore.

River water also usually passes through estuaries on its way to the ocean, where plankton, plants, and animals absorb trace elements and pollutants. Groundwater, on the other hand, flows right into the sea. It is largely bypassing this huge filter that keeps things out of the ocean, says Moore.

Moore’s findings have another implication: they could significantly alter our understanding of ancient oceans. Paleo- oceanographers study elements found in cores drilled from the seafloor to try to piece together ancient ocean environments. Cadmium, for example, is an element fixed in the shells of small creatures called foraminifera. When they die, the cadmium they leave behind in sediments gives a measure of the abundance of life in oceans past. Cadmium, however, is common in groundwater, and given Moore’s findings, researchers may have to reassess their estimates of ancient ocean productivity. I’m raising a flag, says Moore. It’s up to the people who use these chemicals as indicators to try to determine if this is a problem or not.
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