BOARD00363

• Board Meeting Date: 5/24/2004
• Description: Artificial Recharge of Ground Water in Colorado - A Statewide Assessment
• Board Meetings - Doc Type: Memo

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Artificial Recharge of Ground Water in Colorado
A Statewide Assessment

VII. Statewide Potential for Artificial Recharge/Underground Water Storage

Colorado's AQuifer Systems

The occurrence and distribution of Colorado's water resources are inherently linked to the state's
geography and underlying geology. Geologic units and hence, aquifers, consist of either
unconsolidated sediments or consolidated rock. Ground water is simply water that fills the pore
spaces between rock grains in sedimentary rocks or in crevices such as fractures and faults in
crystalline rocks. A geologic unit's ability to store and transmit water is dependent not only on
the amount of pore space (porosity) within the rock or sediment, but also on the size and degree
of interconnection (permeability) of those openings. A geologic unit containing interconnected
pore spaces or crevices that are filled or saturated with water is termed an aquifer. Not all rocks
make good aquifers. In fact, some geologic units actually impede the flow of water, as their
porosity and permeability are very low.

As a result of Colorado's complex geology, a multitude of aquifers in various areas of the state
are suitable for AR projects. Colorado's principal aquifers are categorized into the following:
(I) unconsolidated Quaternary-age alluvial aquifers associated with major river systems; (2)
poorly consolidated or unconsolidated sediments such as valley-fill deposits; (3) consolidated
sedimentary rock aquifers; and (4) volcanic and crystalline rock aquifers. Alluvial deposits
associated with the state's major river systems consist of unconsolidated silt, sand, and gravel
that have been deposited during recent geologic time by water transport. The statewide
distribution of mapped modern, Quaternary-age alluvium is illustrated in Figure VII-I. As in
other western states, the aquifers with the highest yields in Colorado are composed of
unconsolidated sand and gravel deposits adjacent to major river systems. Wells completed in
alluvial deposits can yield from hundreds to thousands of gallons per minute (gpm).

Unconsolidated to poorly consolidated sediments are largely derived from erosion of
surrounding highlands. These sediments were deposited by wind, water, and gravity, and
represent the accumulation of alluvial fans, landslide deposits, glacial drift, and eolian deposits.
The principal aquifers of this type include the Ogallala Formation of the High Plains aquifer, and
the valley-fill deposits of the San Luis and Wet Mountain Valleys. Valley-fill deposits have
hydraulic characteristics similar to the alluvial aquifers, though with slightly lower permeability,
and they can be hundreds of feet thick.

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