FLOOD08081

• County: Statewide
• Basin: Statewide
• Title: Groundwater in Colorado: A Primer
• Date: 10/1/2002
• Prepared For: State of Colorado
• Prepared By: Rock Talk
• Floodplain - Doc Type: Educational/Technical/Reference Information

108'

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108'

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_ Alluvial aquifers .. Precambrian crystalline
rock aquifers

Alluvial and crystalline rock aqujfers of C%rada

and springs. Sand and gravel
deposits, sandstone, limestone, and
fractured crystalline rocks are
examples of geologic units that
form aquifers. The porosity and
permeability of these common
aquifer materials are depicted in
the illustration on page 3. Aquifers
provide two important functions:
1) they transmit ground water
from areas of recharge to areas of
discharge, and 2) they provide a
storage medium for useable quan-
tities of ground water.
Aquifers that are not completely
saturated with water are termed
unconfined aquifers. Unconfined
aquifers provide water to wells by
draining the pores and / or frac-

4

tures of the geologic material sur-
rounding the well, and are re-
charged by water infiltrating and
percolating through the unsaturat-
ed zone. The saturated alluvial
deposits associated with many of
the river systems in Colorado, such
as the South Platte, Arkansas, and
Colorado Rivers, and valley-fill
deposits such as the San Luis and
Wet Mountain Valleys are exam-
ples of unconfined aquifers.
Confined or artesian aquifers
are completely saturated, perme-
able geologic units overlain by rel-
atively low permeability confining
layers, such as clay and shale, that
prevent free movement of air and
water. The water is, thus, confined

104'

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lamar. .. 38'
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0'-

104'
Tertiary igneous crystalline
rock aquifers

under pressure and if tapped rises
to an elevation above the top of the
aquifer, but not necessarily above
the land surface. Confined aquifers
yield water by compression of the
aquifer soil or rock, expansion of
the water, drainage of adjacent
unconfined portions, and leakage
through confining layers. For the
most part, the Denver, Arapahoe,
and Laramie-Fox Hills aquifers of
the Denver Basin are examples of
confined aquifers, where they are
overlain by impermeable layers.

Colorado's Aquifers

Aquifers can also be defined in
terms of their geologic mater-
ials. The geology and geography of

Colorado Geological Survey ROCKTALK Vol. 5, No, .4

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