WSP01846

• File Number: 8449.926
• Description: South Metro Water Supply Study
• State: CO
• Basin: South Platte
• Date: 2/1/2004
• Author: SMWSS Board
• Title: Technical Appendices - Appendix 3A - Hydrosphere's Model Requirements
• Water Supply Pro - Doc Type: Publication

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S. Metro GW Model Requirement~. Memo

January 8, 2001
Page 3

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2.1 Conceptual Model for Denver Basin Bedrock Aquifers

Recalling project objectives, it is important that the model be capable of evaluating transient
performance of the four major aquifers in response to development stresses in the basin in
general and the South Metro region in particular. Although most of the groundwater
development occurs in the Arapahoe aquifer, the other three aquifers (Dawson, Denver, and
Laramie Fox Hills) also store potentially significant water supply sources.

As described by Robson (1987), based on decades of observations and data collected throughout
the development history of the Denver Basin, the four major bedrock aquifers are composed of
areally extensive sandstone and conglomeritic permeable units containing fine-grained interbeds
and separated by thick, low permeability confining units. While the confining units likely allow
some leakage between the aquifers in response to potential gradients, at pumping-test time scales
drawdowns have been observed to remain restricted to pumped aquifers (John Halepaska, Ph.D.,
personal communication, 11/17/200]). This suggests thaI/he model needs to account for a very
low vertical conductance between the aquifers.

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Within aquifcr units, however, the interbedded fine-grained materials are less areally extensive
and exert less impedence to pressure propagation and flow between the permeable beds (HRS
Water Consultants, 1997). At the regional (mile and larger) scale, the aquifer units thus appear
to behave as vertically homogeneous layets. At the local (well and well-field) scale, analyses by
HRS (1997) suggest that the effect of fine-grained interbeds can be adequately accounted for by
using analytical approximations (Trescott et aI., 1976; Lerner, 19&9) and/or a radial
axisymmetric well module (named the MCW module; HRS, 1997) for MODFLOW. The ability
of the MCW module (coupled with a precursor to the SB-74 model) to simulate drawdowns
observed during well pumping tests in the Denver Basin bedrock aquifers (HRS, 1997) provides
particularly compelling evidence of the robustness of this approach. The model needs to
adequately capture expected local drawdown behavior in response to long-term pumping.

As described by Robson (1987), the bedrock aquifers receive natural recharge around the basin
perimeter where they intercept deep percolation in the highland areas between stream channels
and where they contact saturated alluvial aquifer materials. In the central portions of basin,
leakage from the shallower bedrock aquifers recharges the deeper aquifers. The model needs to
adequately account for all natural recharge sources.

The permeable units in the basin are underlain by a thick sequence of Pierre Shale, which
represents an essentially impermable basement to the economically developable water resources
of the Denver basin aquifers. Around the basin margins, the aquifers are truncated at the ground
surface or buried beneath shallow alluvial aquifers in the stream and river valleys. The model
needs to adequately account for all these natural hydrogeologic boundaries.

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