A portion of the ground water in the
upper Patapsco aquifer underlying Maryland is more than a million years old. A
new study suggests that this ancient ground water, a vital source of freshwater
supplies for the region east of Washington, D.C., and Baltimore, was recharged
over periods of time much greater than human timescales.
"Understanding the average age of ground
water allows scientists to estimate at what rate water is re-entering the
aquifer to replace the water we are currently extracting for human use,"
explains USGS director Marcia McNutt. "This is the first step in designing
sustainable practices of aquifer management that take into account the added
challenges of sea level rise and increased human demand for quality water
supplies."
This new study from the USGS, the
Maryland Geological Survey (MGS) and the Maryland Department of the Environment
(MDE) documents for the first time the occurrence of ground water that is more
than one million years old in a major water-supply aquifer along the Atlantic
Coast. The oldest ground water was found in the deepest parts of the aquifer,
but ground water even in shallower parts of the aquifer is tens to hundreds of
thousands years old.
Ground water age indicates the length of
time that a sample of water has been in the ground since infiltrating from the
land surface. This study reveals that modern pumping in southern Maryland west
of the Chesapeake Bay and on the Eastern Shore is tapping ground water
resources that have accumulated in the aquifer over multiple cycles of climate
change and are not quickly recharging.
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 United States 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.
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 ground
water production from shallower aquifers to the deeper upper Patapsco aquifer,
which has caused ground water levels to decline.
The findings are being used to help
understand the patterns and rates of ground water movement in the aquifers of
the Coastal Plain. Such information will be used by the Maryland Department of
the Environment to ensure that the management and use of the state's ground
water resources are being carried out to protect its long-term sustainability.
The findings bring into focus that current users are withdrawing ground water
that was recharged eons ago and accentuate the need to review current
water-supply management strategies and develop new tools and models to protect
this valuable resource for the future.
There are relatively few aquifers in the world
in which million-year-old ground water has been documented, including the
Nubian aquifer in the Sahara Desert, Canada's Alberta Basin, and the Great
Artesian Basin in Australia.
