Virginia Beach Hydrogeology EarthCache
Virginia Beach Hydrogeology
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Located in a City of Virginia Beach neighborhood park
At the posted coordinates you will find an
observation well that is operated by the United States
Geological Survey (USGS) in cooperation with federal, state
and local agencies. The well is nearly 33 m deep and the land
surface altitude is less than 2 m above sea level. The station
is part of a national network for obtaining water-resources
information. The recorded water levels from this well and
others are used for many hydrologic investigations.
Virginia Beach Hydrogeology
The City of Virginia Beach is underlain by numerous water-bearing
geologic formations, called aquifers. In Virginia Beach, the
uppermost hydrogeologic units, defined as the Shallow Aquifer
System, consist of the Columbia Aquifer, the Yorktown Confining
Unit and the Yorktown-Eastover Aquifer. Confining units are layers
that inhibit vertical movement of water and they typically consist
of clays. A conceptual model of the Shallow Aquifer System at
Virginia Beach is shown below (Smith and Harlow, 2002).
The shallow aquifer system at Virginia Beach is recharged by local
precipitation. That is, rainwater or (sometimes) snowmelt can
infiltrate into the ground to recharge the water table. Where
enough recharge water can infiltrate to increase the water table,
ground water begins flowing towards the nearest surface water.
Ground water recharge underneath much of Virginia Beach is limited
by various factors, including the flat terrain, low altitude and
the proximity of open waterways, tidal channels, and wetlands.
Infiltration of water falling on the land surface is further
hindered by shallow layers of clayey soils and the limited depth of
permeable sediments beneath the soils. Extensive systems of ditches
have been built throughout Virginia Beach to lower the water table
beneath poorly drained soils. Ground-water recharge is also
inhibited by paved surfaces, drains, and stormwater sewers,
particularly in the urban areas of Virginia Beach.
The Columbia Aquifer
The uppermost aquifer at Virginia Beach, the Columbia Aquifer
(elsewhere also referred to as the "surficial" aquifer), is defined
as the predominantly sandy surface layers above the Yorktown
confining unit that were deposited mainly during the last 1.8
million years (Holocene and Pleistocene epochs). Because the
Columbia Aquifer is generally an unconfined (watertable) aquifer,
it is vulnerable to contamination from various land uses. The
Columbia is mostly used for irrigation or, where no other sources
of freshwater are available, for domestic drinking water. Droughts
can cause shallow wells in the Columbia aquifer to go dry. In the
summer of 1985, after nine months of dry weather, a number of
shallow private wells failed within the city. Declines in water
levels of 1.5 to 3 m had been observed since 1980. The declines
followed urban growth and an increase in the construction of small
irrigation wells and ground-water heat pumps, and possibly,
increased draining of borrow pits. In addition, recharge to the
water table probably decreased after the construction of improved
drainage systems, parking lots and the paving of streets. Those
declines in the water table were amplified by the dry
conditions.
The Yorktown Confining Unit
The Yorktown confining unit is a series of discontinuous clay
layers on top of a geologic formation (Yorktown unit) deposited
during a succession of marine advances in the Early and Late
Pliocene Epoch (5.3 to 1.8 million years ago) The top of the
Yorktown confining unit ranges from approximately 7 to 18 m below
sea level in the southern watersheds of Virginia Beach.
The Yorktown-Eastover Aquifer
The Yorktown-Eastover aquifer is defined as the predominantly sandy
deposits above the confining clays of the St. Marys Formation. In
the southern watersheds of Virginia Beach, the top of the
Yorktown-Eastover aquifer varies from about 23 to about 37 m below
sea level. The aquifer's thickness ranges from about 30 to 60 m.
Freshwater is limited to the upper part of the Yorktown-Eastover
aquifer. Excessive ground water withdrawals from wells have
caused intrusion of saline water at several locations in the
city.
Tasks
To log this Earthcache, you must complete the following tasks:
- Visit the
homepage for the observation well and retrieve the available
information on water levels. Post in your online log the difference
between the water levels on the day of your visit and one year
before.
- Take a picture of yourself or your caching team at the posted
location and include it in your log.
- Email me the answers to the following questions. Do
not post
them in your log.
- Name some types of hydrologic investigations performed using
the recorded water levels from observation wells.
- Which phone number should you call to obtain more information
about the observation well and its uses?
- Which aquifer does the bottom of the well penetrate? (You can
find the answers for this and the following questions by careful
reading of this page).
- Name two natural events and two human activities that can
affect the water table in an aquifer.
- What could happen to the quality of the ground water when sea
water levels rise?
References and Further Reading
Smith B.S. and Harlow G.E. Jr. (2002) Conceptual
Hydrogeologic Framework of the Shallow Aquifer System at Virginia
Beach, Virginia. U.S. Geological Survey (Prepared in cooperation
with the City of Virginia Beach, Department of Public Utilities).
Water-Resources Investigations Report 01-4262. Richmond,
Virginia.
Smith B.S. (2003) Ground-Water Flow and Saline Water in the Shallow
Aquifer System of the Southern Watersheds of Virginia Beach,
Virginia. U.S. Geological Survey (Prepared in cooperation with the
City of Virginia Beach, Department of Public Utilities).
Water-Resources Investigations Report 03-4258, Richmond,
Virginia.
Smith B.S. (2005) Simulated Changes in Water Levels Caused by
Potential Changes in Pumping from Shallow Aquifers of Virginia
Beach, Virginia. U.S. Geological Survey (Prepared in cooperation
with the City of Virginia Beach, Department of Public Utilities).
Scientific Investigations Report 2005-5067, Reston, Virginia.
McFarland E.R. and Bruce T.S. (2006). The Virginia Coastal Plain
Hydrogeologic Framework. U.S. Geological Survey (Prepared in
cooperation with the Virginia Department of Environmental Quality).
Professional Paper 1731. Reston, Virginia
Additional Hints
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