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3.5 Types of Aquifers 65
distances results because of marked differences in secondary porosity, which depends on
local conditions. They are second only to sandstones as a source of groundwater.
Limestones are prolific producers under suitable conditions.
Although consolidated rocks are important sources of water, the areas served by
them in the United States are relatively small. Most developments lie in granular,
unconsolidated sediments. Unconsolidated, sedimentary aquifers include (a) marine
deposits, (b) river valleys, (c) alluvial fans, (d) coastal plains, (e) glacial outwash, and,
to a much smaller degree, (f) dune sand. Materials deposited in seas are often extensive;
sediments deposited on land by streams, ice, and wind are less extensive and are usually
discontinuous.
Sands and gravels are by far the best water-producing sediments. They have excellent
water storage and transmission characteristics and are ordinarily so situated that replenish-
ment is rapid, although extremely fine sands are of little value. Porosity, specific yield, and
permeability depend on particle size, size distribution, packing configuration, and shape.
Uniform or well-sorted sands and gravels are the most productive; mixed materials con-
taining clay are least so. Boulder clay deposited beneath ice sheets is an example. Typical
porosities lie between 25% and 65%. Gravel and coarse sands usually have specific yields
greater than 20%.
Clays and silts are poor aquifers. They are highly porous but have very low permeabil-
ities. However, the permeability is seldom zero. They are significant only when they (a) confine
or impede the movement of water through more pervious soils and (b) supply water to
aquifers through leakage by consolidation.

3.4 GROUNDWATER SITUATION IN THE UNITED STATES

Geologic and hydrologic conditions vary greatly in various parts of the United States. To
permit useful generalizations about the occurrence and availability of groundwater,
Thomas (1952) divided the United States into 10 major groundwater regions (Fig. 3.2).
McGuiness (1963) provided an updated assessment of the groundwater situation in each of
Thomas’s regions and has also described the occurrence and development of groundwater
in each of the states. The Water Resources Division (WRD) of the U.S. Geological Survey
is the principal agency of the federal government engaged in groundwater investigations.
The published reports and the unpublished data of the WRD are indispensable to any
groundwater investigation. In addition, many states have agencies responsible for activities
in groundwater.

3.5 TYPES OF AQUIFERS
Because of the differences in the mechanism of flow, three types of aquifers are distinguished:
(1) unconfined or water table, (2) confined or artesian, and (3) semiconfined or leaky.
Unconfined aquifers (also known as water table, phreatic, or free aquifers) are
those in which the upper surface of the zone of saturation is under atmospheric pres-
sure. This surface is free to rise and fall in response to the changes of storage in the sat-
urated zone. The flow under such conditions is said to be unconfined. An imaginary
surface connecting all rest or static levels in wells in an unconfined aquifer is its water
table or phreatic surface. This defines the level in the zone of saturation, which is at at-
mospheric pressure. The water held by capillary attraction at less than atmospheric
pressure may fully saturate the interstices to levels above those observed in wells. Thus
the upper limit of the zone of saturation and water table are not coincident. The