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<br />SOI~h'\1~o Groundwaler Model Developmenl Reporl <br /> <br />Oel. 1/. 2001 <br />Page 4 <br /> <br />"shallow" hydrologic system (comprised of the surface water and alluvial aquifer) and the <br />underlying bedrock aquifers would involve quantifying all diversions and return flows <br />from the system (i.e., both surface and alluvial aquifer subsurface), integrate this data <br />with surface gaging data, and use a mass balance model to compute gains and losses from <br />the shallow system. This also would be a significant effort involving new data collection, <br />and the computed losses/gains would likely be swamped by uncertainties in the various <br />components of the mass balance model (unless those components are directly and <br />accurately gaged). <br /> <br />') <br />.' <br /> <br />Finally, even if significant new data was available, the scope and budget for the SMWSS <br />regional groundwater model did not contemplate expending project resources on <br />resolving the stream depletion issue. Nonetheless, as described in section 4, we do plan <br />to address this issue via sensitivity analyses in which streambed conductance values are <br />varied over a range. Consequently, our adaptation of the SB-74 model for the SMWSS <br />will be subject to the same caveats as the original model with respect to estimates of <br />stream depletions. <br /> <br />2.2 <br /> <br />Consideration of Aquifer Interbeds in Regional Groundwater Model <br /> <br />) <br /> <br />The other concern relates to representing the aquifer units as single, vertically <br />homogeneous layers rather than explicitly accounting for the fine-grained interbeds that <br />are universally recognized to occur within the bedrock aquifer units. Some peer <br />reviewers raise a valid concern that the long-term net effects of the layering remains <br />highly uncertain due to our lack of experience in observing the response of the aquifers as <br />they transition from confined to unconfined conditions. In the past decade, there have <br />been some observations (e.g., Parker Rowley Downs well) that the pwnping capacity of <br />some wells is reduced much more significantly than theory would predict as the aquifers <br />start dewatering. This extreme reduction in well pwnping capacity probably results from <br />a nwnber offactors (including aquifer hydraulic disconnection in the well-bore vicinity <br />and related air-blocking of formation permeability) that are associated with the aquifers' <br />low-permeability interbeds, and these peer reviewers worry that not accounting for them <br />in the regional model will result in an overly optimistic (e.g., less drawdown) picture of <br />well performance over time. We do recognize the potentially detrimental effects of the <br />layers, but as described in Section 4, a number of scoping modeling analyses suggest to <br />us that it is in the local well-field modeling, and not the regional model, that the aquifer <br />low-K interbeds need to be explicitly accounted for. <br /> <br />J <br /> <br />Hydrosphere Resource Consultants <br />1002 Walnut SUilc 200, Boulder, CO 80302 <br />PO Box 44S. Socorro, NM 87801 <br />