Environmental Engineering Reference
In-Depth Information
9.4.9 Glaciated Appalachians Region
The Glaciated Appalachians region coincides
with the northeasternmost area of the US, cov-
ering most of New England, upstate New York,
and parts of New Jersey and Pennsylvania. The
climate is humid, with up to 1200 mm of annual
precipitation. Bedrock in this region shares
many of the hydraulic characteristics of the
bedrock within the Unglaciated Appalachians
region described in the previous section. The
distinguishing feature of this region is the
stratified drift aquifers that are found in prin-
cipal valleys. These deposits of sand and gravel
are disconnected but similar in terms of geol-
ogy and hydrology. Underlying approximately
12% of the region, these aquifers are import-
ant sources of water in the region (Kontis
et al
.,
2004
). Water tables are generally found at shal-
low depths, often within a couple of meters of
land surface. The aquifers are often hydraulic-
ally connected to perennial streams.
Several sources of recharge can be import-
ant (
Figure 9.6
). Water flowing from adja-
cent uplands, either within streambanks or
as unchanneled flow, is usually an important
source. Diffuse recharge occurs as precipita-
tion that falls on the land surface overlying
an aquifer, infiltrates, and percolates through
the unsaturated zone to the water table.
Interaquifer flow from and to adjoining ground-
water systems also occurs. Under typical natural
conditions, groundwater discharges to peren-
nial streams; however, extraction of ground-
water by humans can reverse this process. If
the water table declines to an elevation lower
than that of the stream, the stream becomes a
source of recharge. Groundwater discharge by
evapotranspiration is also common. High pre-
cipitation rates, permeable sediments, and shal-
low water-table depths result in relatively high
rates of recharge. Kontis
et al
. (
2004
) provided
a more thorough description of recharge proc-
esses in this region.
Streamflow hydrograph analysis methods
are widely used for estimating recharge in this
region, but many other methods have also been
applied. Flynn and Tasker (
2004
) used the stream-
flow recession curve displacement method to
estimate annual and seasonal rates of recharge
recharge rate could be reduced by about 30%,
conceivably impacting springs in the water-
shed that support diverse ecosystems (Hunt and
Steuer,
2001
).
9.4.8 Unglaciated Appalachians Region
The last three regions to be described are in the
eastern part of the United States and all have
humid climates. The Unglaciated Appalachians
region encompasses the Piedmont and Blue
Ridge physiographic provinces (Heath,
1984
).
The Piedmont consists of low, rounded hills
and long ridges that trend northeast to south-
west. The mountainous Blue Ridge area lies to
the west; peaks there rise to altitudes of more
than 2000 m. The mountains are rounded and
are bordered by steep valleys containing well-
graded streams. The region is underlain by crys-
talline and metamorphosed sedimentary rock
of Precambrian and Paleozoic age. Typically, 10
to 20 m of unconsolidated clay-rich material
lies above the bedrock. These deposits, referred
to as saprolite, are derived from in-place weath-
ering of the bedrock. Thin alluvial deposits are
present in some valleys.
Groundwater exists in all of these depos-
its, yet, in general, none of these units can be
called productive because they all have low
permeabilities. Because of the very low poros-
ity of the bedrock, groundwater is stored and
transmitted through fractures. A well must
be screened across fractures to produce water.
Recharge occurs primarily as diffuse recharge
on mountain ridges and in valleys above
streams. Groundwater discharges to streams
and springs and is consumed by evapotranspi-
ration. Most streams in the area are gaining.
Rutledge and Mesko (
1996
) and Nelms
et al
.
(
1997
) used streamflow hydrograph separation
to estimate base flow from over 200 stream-
flow gauging sites; estimates fell in the range
of 250 to 1000 mm/yr. Plummer
et al
. (
2001
) and
Busenburg and Plummer (
2000
) used multiple
gas tracers to determine groundwater ages in
this region. Rugh and Burbey (
2008
) applied
chloride and bromide tracers to obtain quali-
tative information on flow paths and ground-
water travel times through soil, saprolite, and
fractured crystalline rock in western Virginia.
