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.
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, the team calculated what fraction of the water is carried to the sea by rivers, and what fraction sinks into the ground.
They then focused on the small wedges of coastal land, sandwiched between river drainages, where groundwater drains directly into the fragile ecosystems near shore. Nationwide, this subterranean flow of freshwater accounts for just 1% to 2% of total land runoff, according to the study, which is published today in Science. But despite its relatively small volume, fresh SGD can carry huge contaminant loads, Sawyer notes.
The team found that SGD was highest in places like the Pacific Northwest, where there is both heavy rainfall and steep topography that leads the groundwater to the ocean. Many of these regions of high SGD—12% of the overall coastline—overlapped with areas of heavy development, and Sawyer says these areas are the most vulnerable to contamination. “That’s where we’re growing food, paving cities and towns, and digging septic tanks that leach nutrients and contaminants,” she says.
But extremely low rates of groundwater discharge can also pose a problem. Along the coast of southern California, for example, low outflows increase the likelihood that saltwater could encroach into an aquifer, particularly as sea levels rise. That puts local freshwater supplies in danger, Sawyer says. “It really only takes a tiny fraction of saltwater invading groundwater to render it nonpotable.”
The study can help researchers identify which variables of topography and climate drive submarine groundwater discharge, says Robert Buddemeier, a geohydrologist at the Kansas Geological Survey in Lawrence. The West Coast’s rocky, steep terrain, combined with periods of intense rain and drought, makes it intrinsically different from the coastline along the Mid-Atlantic, which receives steadier rain and slopes more shallowly to the ocean, he says.
A previous attempt to model fresh SGD found flow rates that were an order of magnitude greater than those from the current study, says Willard Moore, an emeritus professor of geochemistry at the University of South Carolina, Columbia. However, Moore says the current study is “a big leap” forward, because it uses better topographic and climatic data, and matches well with 18 localized measurements conducted by other groups.
Sawyer says she hopes researchers and environmental groups will use the new, publicly available database to identify potential points of contamination in their own communities. Armed with local knowledge of where farms, paved surfaces, and septic fields may be contributing to toxic runoff, she says, “people will be able to make more powerful estimates of where risk of contamination might be higher.”