Researchers are discovering freshwater reservoirs below the coastal seafloor that might someday save dry regions from drought
On a clear September day in 2015, after 10 years of working to get funding, my colleague Kerry Key and I stepped aboard the R/V Langseth, a research ship docked at the Woods Hole Oceanographic Institution in Massachusetts. We were about to lead a 10-day expedition to map a deposit of fresh water, size unknown, hidden 100 meters under the rocky seafloor.
Other questions nagged us. Only about 2.5 percent of all the surface water on this ocean planet is fresh. As the global population grows toward an estimated 10 billion people by 2100, the stresses on our water supply will increase—especially in coastal regions, where 30 percent of the U.S. population now lives. Climate change is also altering rainfall patterns, pollution is compromising extant bodies of water, and agriculture and development are sucking underground reservoirs dry.
A continent does not stop at its shoreline; it extends well offshore as a rocky underwater shelf. The shelf ends at a steep slope that transitions sharply to deep oceanic seafloor. The rock and sediments that make up the world's continental shelves are not dry. Some rocks crack, allowing seawater to penetrate. And most shelves are covered by layers of sedimentary rock, which are like hard sponges with small, interconnected, water-filled pores.
Off the U.S. East Coast the continental shelf extends anywhere from close to shore to more than 300 kilometers out to sea. Perhaps not surprisingly, the geologic layers that form aquifers under land do not stop at the shoreline; they often extend outward as part of the shelf. In the 1970s and 1980s researchers began developing electromagnetic instruments to measure properties of the seafloor, motivated in part by the U.S. Navy's interest in long-distance submarine communications. Through the 1980s and 1990s “controlled source electromagnetic” sensing slowly became more sophisticated.
We have no data for the seafloor between those places, so we do not know whether the two hidden bodies of water are connected or, if so, how. We think there might be fresh water underneath the entire New England shelf, based on surveys and models of aquifers onshore. The water off Martha's Vineyard may have been left there by glaciers more than 12,000 years ago. The water off New Jersey seems to originate in part from rainfall on land.
No one has designed a detailed system to tap a submarine aquifer. Tor Bakken of SINTEF Energy Research in Norway and his colleagues described a general system based on oil-drilling technology. A jack-up rig or a barge would be anchored above a submarine freshwater aquifer. Engineers would drill into the reservoir, and water would flow through a pipeline on the seafloor to a processing plant onshore.
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