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66 Chapter 3 Water Sources: Groundwater

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2 Alluvial basins
3 Columbia lava plateau
4 Colorado plateau
5 High plains
6 Glaciated central region
7 Unglaciated central region
8 Unglaciated appalacians
9 Glaciated appalacians
10 Coastal plain
Figure 3.2 Major Groundwater Regions of the United States (After Thomas and McGuinness of USGS)
capillary fringe may be significant for sediments with small interstices and low perme-
ability, such as clay.
More than one zone of saturation occurs when an impervious or semipervious layer or
lens in the zone of aeration supports a less extensive zone of saturation above the main
water table, giving rise to the so-called perched water table.
If a porous stratum in the zone of saturation dips beneath an impervious layer, the
flow is confined in much the same way as in a pipe that drops below the hydraulic grade
line. There is no free surface in contact with the atmosphere in the area of confinement.
The water level in a well tapping this confined or artesian aquifer will rise, under pres-
sure, above the base of the confining layer to an elevation that defines the piezometric
level. If the recharge areas are at a sufficiently high elevation, the pressure may be great
enough to result in free-flowing wells or springs. An imaginary surface connecting the
piezometric levels at all points in an artesian aquifer is called the piezometric surface
(Fig. 3.1 depicts some of these terms). The rise and fall of water levels in artesian wells
result primarily from changes in pressure rather than from changes in storage volume.
The seasonal fluctuations are usually small compared with unconfined conditions.
Aquifers that are overlain or underlain by aquitards are called leaky aquifers. In nat-
ural materials, confining layers seldom form an absolute barrier to groundwater move-
ment. The magnitude of flow through the semipervious layer is called leakage. Although
the vertical permeability of the aquitard is very low and the movement of water through it
extremely slow, leakage can be significant because of the large horizontal areas involved.