Environmental Engineering Reference
In-Depth Information
from an aquifer; it is defined as the ratio of water that drains freely from the
material to the total volume of water. S
pecific retention
refers to the hygroscopic
moisture or pellicular water.
Porosity
equals the specific yield (effective porosity)
plus the specific retention.
Aquitard
is a saturated formation, such as a silt stratum, which yields inappre-
ciable quantities of water in comparison with an aquifer, although substantial
leakage is possible.
●
Aquiclude
is a formation, such as a clay stratum, which contains water, but can-
not transmit it rapidly enough to furnish a significant supply to a well or a
spring.
●
Aquifuge
has no connected openings and cannot hold or transmit water; massive
granite is an example.
●
Connate water
is the water trapped in rocks or soils at the time of their formation
or deposition.
●
Transient water-table conditions
are a result of groundwater withdrawal for water supply
or construction dewatering, long-term climatic changes such as a series of dry or wet
years, and seasonal variations in precipitation. In
Figure 8.19,
it is shown that for the
period of October through May precipitation at the site substantially exceeds the loss by
evaporation, and recharge occurs. During the period of June through September, evapo-
transpiration exceeds precipitation and the water level drops. The highest water table
occurs in the spring after the ground thaws, snow melts, and the spring rains arrive.
Artesian Conditions
Artesian conditions result from confined groundwater under hydrostatic pressure. If a
confined pervious stratum below the water table is connected to free groundwater at a
higher elevation, the confined water will have a pressure head (see
Section 8.3.2)
acting on
it equal to the elevation of the free-water surface beyond the confined stratum (less the
friction loss during flow). When a well is drilled to penetrate the confined stratum (
Figure
8.18)
,
water will rise above the stratum. The rise is referred to as an artesian condition, and
the stratum is referred to as an
artesian aquifer
.
An example is the great Dakota sandstone artesian aquifer, the largest and most impor-
tant source of water in the United States (Gilluly et al., 1959), which extends under much
of North and South Dakota, Nebraska, and parts of adjacent states. This Cretaceous sand-
stone is generally less than 100 ft thick and is overlain by hundreds of feet of other sedi-
mentary rocks, mostly impermeable shales. The principal intake zones are in the west
where the formation is upturned and exposed along the edges of the Black Hills as shown
in
Figure 8.20.
As of 1959, over 15,000 wells have been drilled into the formation.
Fresh Water Over a Saltwater Body
Fresh water overlying a saltwater body occurs along coastlines or on islands that are
underlain to considerable depth by pervious soils or rocks with large interstices, such as
corals. The fresh water floats on the salt water because of density differences. On islands,
the elevation of the water table is built up by influent seepage from rainwater; it decreases
from its high point at the island center (if topography is uniformly relatively flat) until it
meets the sea. In
Figure 8.21a,
a column of fresh water
H
is balanced by a column of salt
water
h
, and conditions of equilibrium require that the ratio of
H
/
h
should be equal to the
ratio of specific gravity of fresh water to that of salt water, or about 1.0 to 1.03. Therefore,
if the height of fresh water above sea level is 1 ft, then the depth to the saltwater zone will
be about 34 ft below sea level, if the island is of sufficient size.
