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

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involve quantifYing all diversions and return flows from the system (i.e., both surface and
alluvial aquifer subsurface), integrating this data with surface gaging data, and use of a mass
balance model to compute gains and losses from the shallow system. This also would be a
significant effort involving new data collection, and the computed losses/gains would likely be
swamped by uncertainties in the various components of the mass balance model (unless those
components are directly and accurately gaged).

Finally, even if significant new data was available, the scope and budget for the current study's
regional ground water model did not contemplate expending project resources on resolving the
stream depletion issue. Nonetheless, as described in Section 3.2.4, this issue was addressed via
sensitivity analyses in which streambed conductance values were varied over a range.
Consequently, adaptation of the SB-74 model for this study is subject to the same caveats as the
original model with respect to estimates of stream depletions.

3.1.2.2 Consideration of Aquifer Interbeds in Regional Ground Water Model - The other
concern related to representing the aquifer units as single, vertically homogeneous layers rather
than explicitly accounting for the fine-grained interbeds that are universally recognized to occur
within the bedrock aquifer units. The Peer Review Committee raised a valid concern that the
long-term net effects of the layering remains highly uncertain due to lack of experience in
observing the response of the aquifers as they transition from confined to unconfined conditions.
In the past decade, there have been some observations (e.g., Parker Rowley Downs well) that the
. pumping capacity of some wells are reduced much more significantly than theory would predict
as the aquifers start dewatering. This extreme reduction in well pumping capacity probably
results from a number of factors (including aquifer hydraulic disconnection in the well-bore
vicinity and related air-blocking of formation permeability) that may be associated with the
aquifers' low-permeability interbeds, and there was concern that not accounting for them in the
regional model may result in an overly optimistic (e.g., less drawdown) picture of well
performance over time. The potentially detrimental effects of the layers were not ignored, but as
described in Appendix 3B, a number of scoping modeling analyses suggests that it is in the local
well-field modeling, and not the regional model, that the aquifer low-K interbeds need to be
explicitly accounted for.

3.2 Overview of Model Set-Up and Calibration

Development of a ground water model involves a number of elements. Among the items that
need to be specified are:

I. The model domain.
2. The model geometry-aquifer tops, bottoms, etc.
3. The parameter distribution-transmissivity, storage coefficient, leakance values for each of
the model units.
4. Boundary conditions for the model-this includes the stream/aquifer interactions.
5. Aquifer stress-recharge, pumping with time.
6. Aquifer response-some history of how the system responds to stress.

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