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

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Hvdrol!eo1ol!ic Conditions Favorable for Recharl!e
Hydrogeologic studies are the critical element, and typically most time-consuming component of
an AR feasibility assessment. Careful evaluation of an area's hydrology and geology can lead to
the identification of aquifers suited for AR, available sources of recharge water, selection of
treatment options, and application of recharge technologies. Aquifers are classified as either
unconfined or confined. The top of the saturated interval, or water table, in an unconfined
aquifer is at atmospheric pressure and is free to move up or down as water is added or withdrawn
from the aquifer. Unconfined aquifers are recharged by deep percolation from the land surface
or by streambed infiltration. The water in a confined aquifer is under pressure, as the aquifer is
sandwiched between impermeable layers. The water level in a well completed in a confined
aquifer rises above the physical top of the aquifer. Confined aquifers are recharged at their
outcrop areas where the aquifer has become unconfined and by minor vertical leakage through
the confining layers.

Aquifers provide two important functions: they transmit ground water from areas of recharge to
areas of discharge, and they provide a storage medium for usable quantities of ground water.
The means by which confined and unconfined aquifers yield water is also an important
distinction. Unconfined aquifers yield water from storage by vertical drainage of water within
the pore spaces. Injection or withdrawal of water in an unconfined aquifer results in a change of
the saturated thickness of the aquifer. Confined aquifers yield water from storage from the
compressibility of the mineral skeleton and the expansion of pore water. In the case of a
confined aquifer, the saturated thickness remains constant.

When the water level of an aquifer changes, water will either be stored or expelled. The quantity
of water that will either be stored or expelled per unit surface area per unit change in water level
is defined as the storage coefficient. Because of the physical differences between confined and
unconfined aquifers, the storage coefficient of an unconfined aquifer is orders of magnitude
higher than that of a confined aquifer.

Significant quantities of water can only be stored when an aquifer is of suitable extent and
thickness, and has sufficient porosity and permeability. The amount of volume in storage can be
expressed as:

v.. = SA.dh

Where S is the aquifer's storage coefficient, A is the aquifer's area, and LJh is the change in water
level or hydraulic head.

For AR considerations, the amount of water-level rise available within the aquifer is dependent
upon the ambient or static water level. For example, if water levels in an alluvial aquifer are 10
feet below ground surface, the available head or freeboard may only be five feet without
impacting surficial structures (i.e. flooding basements). These five feet of available head,
however, may translate into a large storage volume if the areal extent of the aquifer is large.
While an aquifer's area is an important factor in the evaluation, it must be considered in the
context of the objectives of the proposed recharge project. The geographic extent of an aquifer is
a primary factor in computing the amount of water in storage. Implementation of a recharge
project, however, generally occurs on a local scale and depending upon recharge rates, far-field
effects may not be realized for decades. As such, the impact of an individual recharge project
may be dwarfed by the regional storage capacity present in large, extensive aquifers.

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