WSP09428

• File Number: 8449.926
• Description: South Metro Water Supply Study
• State: CO
• Basin: South Platte
• Date: 2/1/2004
• Author: SMWSS Board
• Title: South Metro Water Supply Study
• Water Supply Pro - Doc Type: Publication

project, including: a review of the SB-74 model, screening of newly acquired hydrogeologic
and well data for inclusion in the new model, results of a comparative modeling analysis that
. was undertaken to evaluate the need for explicit representation of inter-aquifer fine-grained
layers in the regional model, and a detailed plan for updating the SB-74 model with newly
acquired data and infonnation. The model development plan is attached to this report as
Appendix 3B.

. Task 3: Model Development and Application. Following submittal of the model
development plan, Hydrosphere implemented the model development actions outlined in the
plan. With critical support and interim reviews provided by the Peer Review Committee, a
regional ground water model was developed that provides a robust tool for evaluation of
future water supply scenarios for the South Metro area. The remainder of this section
summarizes the results of the calibration efforts, and presents the simulation results for three
future development scenarios, with a detailed evaluation of model results.

3.1.2 Regional Ground Water Model: Assumptions and Limitations - At the direction of the
1996 Colorado legislature, the SB-74 model was originally developed between 1996 and 1998 by
the Colorado State Engineer's Office ("SEO") to evaluate the effect of the 4% replacement and
2% relinquislunent requirements on future water supplies and on existing water rights. One of
the constraints on SB-74 model development mandated in the enabling legislation was that the
SB-5 Denver Basin model with no new data be used as the basis for model development;
consequently, no data gathered since 1985 was incorporated into the development of the SB-74
model. This constraint, together with the lack of sufficiently detailed data, forced model
developers to employ indirect approaches to infer a number of key model parameters, including:
(I) those that describe surface water - ground water interactions, as well as (2) aquifer layering
characteristics that can strongly affect model predictions as the bedrock aquifers transition from
confined to unconfined conditions.

3.1.2.1 Stream Alluvial Aquifer I Bedrock Aquifer Interactions - Due to concerns about the
adequacy of the SB-74 model's ability to project stream depletions caused by pumping in the
Denver Basin bedrock aquifers, a group of peer reviewers, called the Technical Scope Committee
(TSC), was convened. The charge of the TSC was to develop a scope of work and cost estimate
for additional work needed to improve the SEO SB-74 model, particularly as it relates to
characterization of stream depletions. It was generally recognized at that time that data
limitations severely constrained the ability to defensibly quantify such depletions. In December
1998, the TSC recommended a program of seven tasks that included both field and laboratory
data gathering, as well as data analysis and model enhancements. The estimated cost for the
program exceeded $2,000,000 (including the cost of new full-time staff in the SEO) and at least
two years were anticipated for completion of the seven tasks. Since that time, none of the
recommendations have been implemented, although the SEO is committed to implementing
those tasks as part of the South Platte Decision Support System project.

Given the current status of progress made on implementing the TSC's study plan, there remains
insufficient data available to implement the model enhancements recommended by the TSC. An
alternative approach to estimate subsurface flows between the "shallow" hydrologic system
(comprised of the surface water and alluvial aquifer) and the underlying bedrock aquifers would

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