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Pelicans are seen in a sand bar from the research vessel Kerhin, near the Chesapeake Bay. Jose Luis Magana / AP

Residents of towns near Chesapeake Bay have complained about the brackish groundwater in the region for years. Hydrologists now say that an aquifer, about a mile under parts of Virginia and Maryland, is older than dirt, and part of an ancient sea that's stayed trapped for the past 145 million years.

Helium, carbon dioxide and methane bubble out from Early Cretaceous North Atlantic sea water as it emerges from a well bore. Herbert Pierce

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Ordinarily, water from such underground reservoirs drain away through underground channels, but a meteorite that crashed into the Chesapeake Bay 35 million years ago “broke up the plumbing” that would have flushed the pool out, Ward Sanford, a hydrologist at the United States Geological Survey, who was part of the drilling team, told NBC News.

The water is extra salty because it comes from a sea that formed as the American plates ripped away from the Eurasian and African continents, Sanford and his colleagues explain in a study published Wednesday in the journal Nature.

Land-locked seas like this early Cretaceous sea and the Dead Sea today, start out salty but adjust their levels once they connect with the global network, Sanford said.

This ancient water is fizzy when it emerges from the bores, as the image above shows, because carbon dioxide and methane trapped in solution under pressure underground bubble out when the water reaches the surface. It also contains high amounts of helium, released by trace amounts radioactive uranium in the water, which didn't have anywhere to go.

In May this year, scientists announced that they'd found billion-year-old freshwater in a mine in Canada. The USGS team suspects the Chesapeake Bay the oldest body of ocean water to be discovered so far.



In addition to Ward Sanford, the authors of Evidence for high salinity of "Early Cretaceous sea water from the Chesapeake Bay crater" include Michael Doughten, Tyler Coplen, Andrew Hunt and Thomas Bullen.

Nidhi Subbaraman writes about science and technology. You can follow her on Facebook, Twitter and Google+.

