The oldest water was found in the deepest parts of the upper Patapsco aquifer, with shallower groundwater tens of thousands to hundreds of thousands of years old, according to a new study by the USGS, the Maryland Geological Survey and the Maryland Department of the Environment. The aquifer provides freshwater for the region east of Washington and Baltimore.
The analysis shows that water flowed from the land surface into the deep aquifer during cooler periods in earth's history, when glaciers covered much of the northeastern U.S. and sea level was about 125 meters lower than it is today. During warmer periods in earth's history, such as in modern times, higher sea levels slow recharge of fresh water to the aquifer, due to a lower gradient between the recharge and discharge areas.A 2007 state report found that the groundwater supply might not meet the future needs of southern Maryland.
Modern-day pumping rates have lowered water pressures and changed water chemistry, affecting the aquifer's ability to provide freshwater for drinking and other uses. Concerns over saltwater intrusion in some areas have led water managers to increasingly move groundwater production from shallower aquifers to the deeper upper Patapsco aquifer, which has caused groundwater levels to decline.
Worldwide, there aren't many aquifers containing million-year-old groundwater. Two of them might surprise you: the Nubian aquifer in the Sahara Desert and the Great Artesian Basin in Australia. Another is the Alberta Basin in Canada.
The USGS has a "groundwater watch" database containing records from about 850,000 U.S. wells over the past 100 years.