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

To put the concept of total storage capacity into perspective, consider a comparison between an
equivalent sized confined and unconfined aquifer with 10 feet of available water storage above
the ambient water level. Assuming an area of 100 square miles, a confined aquifer with a
smaller storage coefficient of 0.0005 can store 160 acre-feet of water while an unconfined
aquifer with a storage coefficient of 0.10 can store 32,000 acre-feet in the same volume. Clearly
for a given volume or unit change in water level, an unconfined aquifer can store or yield
significantly more water. In addition to the volume available for storage, the hydraulic
characteristics of the aquifer will determine the rate at which water can be injected or extracted.

The physical properties that make rocks and sediments good aquifers are the key characteristics
assessed in selecting an aquifer for AR. Excluding issues of water rights and recharge water
sources, a hydrogeologic evaluation of a ground-water basin should consider:

. Surface topography
. Geologic structure and stratigraphy
. Surface soil and unsaturated zone characteristics
. Number and extent of aquifers
. Aquifer hydraulic characteristics (storage coefficient, hydraulic conductivity,
saturated thickness, hydraulic gradient)
. Historic and current water levels
. Aquifer depth and unit thickness
. Water quality

Application of AR technologies is very dependent upon the depth to the top of the aquifer. In
general, water-supply well depths rarely exceed 2,500 feet below ground surface. This is largely
an economic consideration with deep wells incurring increased construction and operation
expenses, as well as a water quality concern as aquifers generally become more saline with
depth. Aquifers that are at or near the land surface are suitable candidates for surface and
subsurface infiltration methods, while deep aquifers can only be recharged by direct injection.
The characteristics of the overlying surficial soil and unsaturated zone materials (porosity,
percolation rates, impeding layers, etc.) must also be considered for surface spreading and
unsaturated zone recharge applications, because these properties determine the rate at which
water will infiltrate the subsurface.

The head freeboard or amount of water level (potentiometric) rise available within the aquifer is
dependent upon the ambient, or static, water level. An aquifer whose water level is at or near the
surface does not have sufficient head freeboard to accommodate much additional storage. Since
recharge produces a rise in water levels, project design must consider the impacts to surface
structure, land use, potential sources of contamination, and increased surface-water discharge.
Because water levels in unconfined aquifers vary seasonally, an understanding of the historic
water levels is critical. The head freeboard, in combination with the area and storage coefficient,
determines the amount of available additional storage an aquifer can handle. Deep confined
aquifers may have hundreds of feet of available head freeboard, while shallow unconfined
aquifers have only tens offeet.

Finally, the rate at which water is transmitted through the aquifer is dependent upon the aquifer's
hydraulic conductivity and the hydraulic gradient. This parameter is strongly dependent upon
the porosity and permeability ofthe material. Hydraulic conductivity is also a function of the

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