WSP04188

• File Number: 8449.800.A
• Description: Metro Study SB 96-74
• State: CO
• Basin: South Platte
• Date: 4/1/1998
• Author: CWCB
• Title: Denver Basin and South Platte River Basin Technical Study, Senate Bill 96-074
• Water Supply Pro - Doc Type: Publication

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5. The transmissivity of the bedrock aquifer is a measure of how readily the aquifer transmits
water from one area to another. Transmissivity is another important parameter in the Denver
Basin model that is in need of updating and refinement. This could be done by 1) making
better use of existing aquifer test data which measures transmissivity and 2) through
continued support of cooperative efforts with well owners who drill core holes.

When a large capacity well is constructed it is common practice to conduct an aquifer test on
the new well in order to determine transmissivity (among other factors). Currently these
aquifer test data reside with the well owners or their hydrologic consultants and are not
submitted to the Division of Water Resources along with other well data needed to complete
the well permitting process. If submission of aquifer test results were required, much new
transmissivity data could be collected. If well owners were requested to submit all historical
data, a very large body of additional transmissivity data could be compiled for analyses by
Division of Water Resources staff and incorporation into the Denver Basin model.

Another source of transmissivity data is laboratory analyses of rock-core samples recovered
from core holes drilled into the Denver Basin aquifers. The core samples commonly are
analyzed for horizontal hydraulic conductivity (which can be used to calculate transmissivity),
vertical hydraulic conductivity, specific yield, and porosity. These core samples provide
valuable aquifer data in areas, and in rock units, that may never be accessible from other
sources. The core samples also are a valuable source of information on the specific yield of
the aquifers. Collection and compilation of aquifer test and core-analyses data may require
increased funding to support additional staff within the Division of Water Resources/

6. The specific yield of the bedrock aquifers is a measure of how much water can drain out of
a given volume of saturated rock. Typically a 1-foot water-level decline in an aquifer will cause
about 0.2 cubic feet of water to drain from each cubic foot of rock that has been dewatered,
Specific yield is used to calculate the volume of recoverable ground water in storage in the
bedrock aquifers, and is the basis for administration of water law within the Denver Basin.
Laboratory analyses of core samples are the only means of measuring specific yield in the
confined aquifers of the Denver Basin. Continued State cooperative support for analyses of
core samples from bedrock wells in the Basin is vital. Previously, core analyses studies have
been funded through sources managed by the Colorado Water Conservation Board.

7. Water levels are currently measured and reported annually by the Division of Water
Resources for about 220 wells in the Denver Basin. These data are a vital source of
information on water level changes in the basin, and are important to the Denver Basin
modeling because they provide historical data used in model calibration, and can be used to
verify model simulation results in future years. It is important to maintain this network of
observation wells for historical continuity, and to expand the network by adding new wells as
needed to begin monitoring declining water levels in areas affected by new withdrawal.
Water-level monitoring previously has been undertaken by existing staff of the Division of
Water Resources.

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