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

Water levels at on Arapahoe Aquifer well in Douglas County hove been
dropping at 0 long-term decline rote of almost 20 feet-per-year in
response to increasing ground-water depletions.

drought. However, if the drought cycle continues
through the high demand period, the increased
demands may overwhelm the amount of water avail-
able with the non-tributary water rights. In an extend-
ed drought, the demands can also overwhelm the
physical capacity of the wells and distribution systems.
The disadvantage of non-tributary ground water is
that, since it is disconnected from surface water, it is
not readily replenished. In a sense, non-tributary
ground water is not renewable and is being mined. By
statute in Colorado, non-tributary water rights allow
annual depletions
based on a 100-
year life of the
aquifer, although
several local gov-
ernment entities
extend this life to
300 years_ With
the 100-year life
of an aquifer, a
non-tributary
water right holder
can pump 1 per-
cent of the total
water in storage
beneath their
property in a year.
The net result is
that water levels
in non-tributary
aquifers will
decline over time.
The decline due to
depletions is evident in the hydrograph of a Denver
Basin aquifer well shown on this page_ Dropping
water levels result in lower pump rates and higher
pumping costs. Although this non-tributary water
source provides a level of immunity from the current
drought, in the long-term, it will become more diffi-
cult to meet normal demands as it is depleted.
With these factors in mind, how will ground water
continue to fit in the water supply picture for Colo-
rado, particularly with respect to drought manage-
ment? Tributary ground water is inseparable from sur-
face water in the water rights administration system.
Most surface water in the state has been spoken for
and there is little opportunity for expansion of tribu-
tary sources other than through the exchange of exist-
ing uses and inter-basin diversions_ Examples are
changes from agricultural to municipal use and diver-
sions from the west-slope to the east-slope_ These
exchanges are, at times, quite controversial. Tributary
ground water, however, is renewable. On the other
hand, there is considerable potential for additional
non-tributary ground water supplies, however, this

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Colorado Geological Survey ROCKTALKVol. 5, No. A

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non-tributary ground water is essentially non-renew-
able and reliance on it does not provide a long-term
solution.
The future for ground water lies in how it is man-
aged with surface water. As indicated before, aquifers
provide a considerable volume of storage that can be
utilized as part of the water management scenario.
This storage potential can be utilized in the short-term
season-to-season balancing act between natural
supply and demand. The storage potential can also be
used to provide a cushion for periods of drought.
Referred to as
conjunctive use,
surface water is
used as the pri-
mary source of
water in periods
of abundance,
while ground
water is reserved
for use in times of
scarcity.
When neces-
sary, ground
water recharge
can be enhanced
to take advantage
of peak surface-
water flows.
Ground water can
be recharged
directly using
surface water,
treated waste-
water, or treated municipal water. For shallow
aquifers, ground water can be recharged through infil-
tration ponds, spreading basins, or modifications to
stream channels to improve infiltration rates. For
deeper, non-tributary, aquifers artificial recharge is
accomplished with injection wells. The U.s. Bureau of
Reclamation recently completed the High Plains States
Ground Water Demonstration Program that focused
on the artificial recharge potential in a number of geo-
logic settings throughout the High Plains Aquifer
region from Texas to North Dakota. Included in this
project was an artificial recharge project at a Denver
Basin Arapahoe-Aquifer well in southern Arapahoe
County. This project confirmed that artificial recharge
of the Denver Basin aquifers is technically feasible.
Furthermore, the Colorado Division of Water
Resources has adopted a set of rules and regulations
for the administration of recharge within the Denver
Basin. Expect to see increased application of conjunc-
tive use and artificial recharge throughout Colorado as
the need for more creative water-supply solutions
increases. -Peter Barkmann

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