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Most raindrops find their way to the oceans via streams and rivers. But some of them also wind up as a part of a hidden underground flow that seeps into the ocean through seafloor fissures. When this water, called submarine groundwater discharge (SGD), trickles through contaminated soil and rock, it can pick up and transport a variety of ions, nutrients, and chemicals to the sea—including pollutants that contribute to coastal dead zones and toxic algal blooms. Now, a new study provides the first high-resolution map of the freshwater flow along coastlines in the continental United States, revealing pollution “hot spots.” The study finds that 12% of the U.S. coastline is particularly vulnerable to contamination, including parts of the northern Gulf Coast, the Pacific Northwest, and the northern Atlantic coast, where high rates of seepage overlap areas of human development.
“This freshwater is right underfoot, flowing around leaky gas and septic tanks,” says Audrey Sawyer, a hydrogeologist at The Ohio State University, Columbus, who led the study.
Researchers - Waders - Wetsuits - Points - Groundwater
Typically, researchers don waders or wetsuits to hunt for individual points of groundwater seepage along the coast. They then use the flows measured at these points as representative of wide stretches of coastline, an extrapolation fraught with assumptions. Moreover, these hand-collected samples are limited to easily accessible places, such as the shallow, sandy shorelines of the Atlantic coast. The resulting data are extremely spotty, making it difficult for scientists to accurately predict the location of seeps, their flow rates, or where pollution will occur, Sawyer says.
Recently, public databases on U.S. rivers, streams, and coastlines have made more sophisticated analyses possible. In the new paper, Sawyer and colleagues tapped the National Hydrography Dataset, which contains realistic topographic models of riverbeds, streams, and coastlines across the United States. Combining these models with data on local rainfall and snowmelt,...
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