BOARD00363

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

Artificial Recharge of Ground Water in Colorado
A Statewide Assessment

The northern end of the valley, where there is no external drainage of surface water, is referred to
as the "Closed Basin." A ground-water divide separates the Closed Basin from the alluvial
aquifer underlying the Rio Grande and, for water management purposes, ground water in the
Closed Basin is not considered to be in hydraulic connection to the surface flow ofthe Rio
Grande. This separation has affected how irrigators in the Closed Basin use their water. In this
area, surface water is imported from the Rio Grande for irrigation and recharged via five
principle ditches shown in Figure VI-3.

Water is recharged at many individual sites throughout the irrigation season, with cumulative
rates averaging over 120,000 ac-ft per year when water is available from the canals. The farmers
then pump from wells as needed while the individual ditch companies maintain an account of
recharge and pumping totals (Steve Vandiver, oral commun., 2003). The accounting system in
the San Luis Valley is much simpler than that in the South Platte River Basin since it is held that
pumping ground water from within the Closed Basin does not cause depletions to the Rio Grande
and the ground-water usage is not administered with augmentation plans.

In addition to the conjunctive use within the Closed Basin, water is also recharged during the
winter by the Rio Grande Water Users Association (RGWUA) using six principal ditches over a
larger area covering the west central portion ofthe valley that includes the Closed Basin (Figure
VI-3). Reported annual recharge totals have been as high as 20,000 ac-ft. Recharge by the
RGWUA offsets depletions to the Rio Grande caused by pumping outside of the Closed Basin
and thus allows pumping to continue when surface flow is low.

Denver Basin Aquifer Storage and Recovery
The Denver Basin is a structural basin that encompasses much of the Denver and northern
Colorado Springs metropolitan areas (Figure VI-I), home to almost 60 percent of Colorado's
population. The administrative ground-water portion ofthe basin underlies a 6,700 square mile
area and is subdivided into the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers.
These aquifers consist of interbedded sandstone, conglomerate, shale, and claystone of Tertiary
to Upper Cretaceous age. The basin is asymmetrical with the center just west of the town of
Parker where the base of the Laramie-Fox Hills is approximately 3,000 feet deep. Water levels
are quite variable depending on geographic location as well as well depth. However, much of
the ground water in the basin is under confined conditions and is considered non-tributary. The
most prolific aquifer is the Arapahoe, which has a stratigraphic thickness ranging between 400
and 700 feet and contains up to 400 feet of saturated sand and conglomerate. Transmissivities in
the Arapahoe aquifer range between 500 and 5,000 gpd/ft with pumping rates in excess of 1,000
gpm common in the deepest part of the basin where there is sufficient available head above the
top of the aquifer.

In 1987, Robson estimated that the Denver Basin aquifer system has a storage capacity of
approximately 470 million ac-ft of water, however the actual amount ofrecoverable water may
be closer to 200 million ac-ft due to physical and practical limitations (Topper and others, 2003).
Still, this volume is a tremendous resource with over 1,000 times the volume of storage in Dillon
Reservoir, Denver Water's largest surface reservoir. The DWR estimated that nearly 445,000
ac-ft of water were withdrawn from the Denver Basin aquifer system in the Denver metropolitan
region in 1995, and the annual withdrawal has certainly increased since then.

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