Scientific Investigations Report 2005-5161
and Ground-Water Availability in the Carbonate Aquifer System of Frederick
By George E. Harlow, Jr., Randall C. Orndorff, David L. Nelms, David J. Weary, and Roger M. Moberg
The carbonate aquifer system of the northern Shenandoah Valley provides an important water supply to local
communities, including Frederick County, Va., which depends on ground water as a source of water supply.
The county and surrounding area are undergoing increased urbanization, and increased demands on the carbonate
aquifer system are expected. A study was conducted between October 2000 and March 2004 by the U.S. Geological
Survey (USGS), in cooperation with the County of Frederick, Va., to describe the hydrogeology and ground-water
availability in the carbonate aquifer system underlying the county. The study area encompasses about 25 percent
(105 square miles) of the county that is underlain by carbonate bedrock.
The carbonate aquifer system of Frederick County is in the Shenandoah Valley region of the Valley and Ridge
Physiographic Province. Approximately 10,000 feet of folded and fractured Middle Cambrian to Upper Ordovician
sedimentary rocks are exposed and are overlain by Pleistocene(?) and Holocene surficial deposits. All geologic
units in the study area are considered to be aquifers. The geologic units are generally unconfined, fractured-rock
aquifers that are recharged by precipitation and discharge locally to streams and springs, and by
Stream density in the carbonate study area is less than in the remainder of the county, which is underlain
by siliciclastic rock units. Most streams flow normal to strike (from the northwest towards the southeast)
across the study area. These streams are characterized by shallow incisement and are usually limited to a
single stream channel. In the southern third of the study area, streams flow parallel to strike (from the
northeast towards the southwest) towards the deeply intrenched Cedar Creek. Springs are commonly located
at the start of flows for all streams in the carbonate study area, and spring discharges are often a large
portion of the streamflow (especially during drought conditions).
The general direction of ground-water flow is from the hills in the west of the study area into and across
the carbonate valley. A ground-water divide may occur north of Round Hill in the vicinity of the Apple Pie
Ridge fault where the North Mountain fault zone cuts out the resistant Silurian and Devonian sandstone units
and results in surface drainage from the carbonate rocks toward the west and out of the carbonate valley.
Estimates of effective ground-water recharge for 2001–02 range from 5.8 to 6.2 inches in the Cedar Creek Basin,
with base flow accounting for between 60 and 64 percent of streamflow, and from 3.2 to 3.8 inches in the Opequon
Creek Basin, with base flow accounting for between 86 and 92 percent of streamflow.
Water budgets calculated for 2001, a year of below-normal precipitation (33.1 inches), and 2002, a year of
above-normal precipitation (41.2 inches), include a streamflow of 9.0 inches in 2001 and 9.2 inches in 2002
in Cedar Creek. Evapotranspiration ranged from 25.9 to 30.7 inches, and ground-water storage decreased 1.8
inches in 2001 and increased 1.3 inches in 2002. Streamflow was 3.7 inches in 2001 and 2002 in Opequon Creek.
Evapotranspiration ranged from 29.8 to 37.5 inches, and ground-water storage decreased 0.4 inch in 2001 and
did not change in 2002.
the report (PDF, 2.2 MB)
Purpose and Scope
Description of Study Area
Relation of Geology to
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questions or comments about this report to the Virginia Water Science Center (804)
more information about USGS activities in Virginia, visit the USGS
Virginia Water Science Center home page.