Maryland’s Ancient Groundwater

Maryland Groundwater Dated to More Than 2 Million Years Old


Glaciers that melted more than two million years ago deposited layers of sediment around what is now the Chesapeake Bay. Underground rivers run between those layers, tapped by wells and recharged by rainfall over time. The water flowing in them is tens of thousands to more than 2 million years old, according to recently published research, a fact that had been theorized but never proved.

The finding stacks Maryland aquifers up against the few to have been dated so old, including those underneath the Sahara Desert and the Australian outback. And it shows that a resource many take for granted cannot, in fact, be renewed on a human time scale, geologists said.

While in much of the Baltimore area, drinking water is stored in reservoirs created by dammed rivers and streams, for the Coastal Plain to the east and south of the Interstate 95 corridor, aquifers are the primary source of water for more than 1 million people. The massive stores of water come to the surface near the fall line, roughly along the length of I-95, and deepen toward the southeast under the Chesapeake and Eastern Shore.

Layers of clay and sediment separate the aquifers, which vary in depth from as little as 20 feet to more than 1,000 feet underground. Where they meet the surface, what rainfall is absorbed into the ground slowly fills the aquifers. There are about six major systems containing 40 to 60 aquifers in the Coastal Plain, stretching across parts of Southern Maryland and the Eastern Shore.

“These layers were laid down millions of years ago by streams or estuaries,” said Jack Eggleston, a geologist with the U.S. Geological Survey who co-wrote the water-dating study. “It’s all a very tight package.”