Arkansas Basin_Water Supply and Needs

• Basin Roundtable: Arkansas
• Title: Water Supply & Needs Report for the Arkansas Basin
• Date: 6/1/2006
• Author: CDM
• Basin Roundtables - Doc Type: Needs Assessment Documents

Section 7
Availability of Existing Water Supplies in the Arkansas Basin
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independent of the Prior Appropriation System. Permits
limit annual usage to depleting a certain percentage of
the computed aquifer volume, usually 1 percent.

In many cases the groundwater supplies are limited
either by their physical or legal availability. The physical
availability is the amount of water an aquifer can
produce. The legal availability is the amount of
groundwater that can be extracted from an aquifer under
the water rig hts system that is present for the specific
groundwater basin.

The amount of groundwater that each of these aquifers
can produce is difficult to determine. This is due to
several factors including uncertainty about the
transmissivity, porosity, thickness of an aquifer, its
extent, and locally, the effects of pumping that draws
down the groundwater supply.

The transmissivity of an aquifer describes its potential to
provide water. An aquifer with high transmissivity can
provide a large amount of water per foot of aquifer
drawdown. Transmissivity is a product of the aquifer
saturated thickness and its water-bearing properties.
Both of these aspects vary naturally throughout an
aquifer. The aquifer saturated thickness and the extent of
an aquifer usually are estimated based on a review of
driller's logs of the subsurface and mapping of the
permeable aquifer zones. An aquifer is composed mostly
of soil or rock particles, with the groundwater existing in
the porous void spaces in between. Soil and rock strata
of both aquifer and non-aquifer materials change in
composition due to how the strata were deposited, so the
void spaces also vary. The water-bearing properties of
an aquifer, defined as its hydraulic conductivity, are
related to the size, number, and interconnectedness of
the void spaces. It can vary by several orders of
magnitude due to natural variations in the aquifer
materials. Estimates of hydraulic conductivity can be
made from the aquifer grain size and from aquifer
pumping tests. The natural variation in porosity affects
the ability to accurately estimate the amount of
groundwater in storage in an aquifer. The range in
porosity also can be up to several orders of magnitude
for consolidated bedrock deposits and by a factor of 2 or
3 and for unconsolidated deposits. Due to the natural
variations of these aquifer properties, any estimates of
the amount of groundwater in storage and its availability
will have a larger amount of uncertainty associated with
them than will estimates of surface water availability.

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7-2

The groundwater resources in each basin have been
characterized based on published reports and data for
the major aquifer systems.

7.2.2 Denver Basin Bedrock Aquifers

The Denver Basin contains four major aquifer units.
These cover an area of approximately 6,700 square
miles extending from Greeley south to Colorado Springs
and from Limon west to the edge of the foothills. The
aquifers consist of layers of sedimentary rocks that are,
from youngest to oldest, the Dawson, Denver, Arapahoe,
and Laramie-Fox Hills. Figure 7-2 shows a cross-section
of the aquifer through the center of the basin.

There have been several estimates of the available water
in storage. One of the early estimates of water availability
was from the USGS (Robson 1987). In this study,
information from driller's logs, laboratory tests of core
samples, aquifer pumping tests, water level
measurements, and groundwater flow modeling were
used to delineate the configuration and storage
coefficients of each aquifer, from which estimates of the
available volume were made. The USGS study
concluded that approximately 467 million AF of water
existed in the Denver Basin aquifers, and of this
approximately 269 million AF of water could be
recovered. In 1985 the Colorado General Assembly
promulgated Senate Bill 5, which set forth criteria for
management of these bedrock aquifers. As part of this
Bill, the storage coefficient was determined for each
aquifer. The total amount of recoverable groundwater
was estimated to be 295 million AF.

The aquifer storage coefficient has a strong influence on
the estimated volume of water contained in an aquifer.
Detailed studies conducted on core samples from a
borehole located in the center of the basin near Kiowa
(Lapey 2003) indicated that the storage coefficient might
be as much as 30 percent lower than previously thought.
This translates into a possible 30 percent reduction in the
amount of recoverable water in storage, to approximately
206 million AF.

Even the lower estimates of the amount of available
water in storage in the Denver Basin aquifers are quite
large. Unfortunately, the sediments that make up each of
the aquifers tend to be relatively fine grained and include
many interlayered clay and shale units that have very low
permeability. As a result, the water-bearing ability of the

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