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EXECUTIVE SUMMARY

discharge rates of groundwater in the valley. The Regional Model covers the entire area
underlain by the Saugus Formation, plus the portions of the Alluvial Aquifer that lie beyond
the limits of the Saugus Formation. The Regional Model area largely coincides with the
Santa Clara River Valley East Groundwater Subbasin, extending from the Lang stream gage
at the eastern end of the valley to the County Line stream gage area in the west.

The process of designing a modeling analysis to evaluate perchlorate containment consisted
of the following activities:

1. Refining the model grid in and around the areas where impacted wells are located

2. Selecting a period over which to simulate groundwater conditions resulting from
various pumping configurations

3. Defining the pumping plan at the impacted wells and all other wells in the Santa Clarita
Valley, considering the objectives above and the variability in pumping demands that
occur due to cycles of drought and nondrought conditions and year-to-year variations in
the availability of other water supplies

4. Defining the variation in local hydrology (rainfall, streamflows, and groundwater
recharge) on a month-to-month basis throughout the simulation period
5. Running the model to calculate time-varying (monthly) groundwater elevations and
groundwater discharge terms throughout the multi-year simulation period

6. Evaluating the modeling results, as follows:

a. Examining forecasted time-series plots (hydrographs) of water budget terms and
groundwater elevations to evaluate the effects of the pumping plan at the impacted
Saugus Formation production wells and across the basin

b. Analyzing forecasted groundwater flowpaths (using particle-tracking techniques) to
identify the degree of containment provided by the pumping plan for the impacted
Saugus Formation production wells

7. Performing two sets of sensitivity analyses to address the following questions
concerning the selected pumping plan for the impacted Saugus Formation production
wells:

a. Can the containment objectives be met by using lower pumping rates at SCWC-
Saugus1 and SCWC-Saugus2, rather than the rate of 1,200 gpm that has been
selected for each well, based, in part, on the water supply needs of the valley?

b. How would the model predictions change if the degree of connection between the
Alluvial Aquifer and the Saugus Formation is less than the degree of connection that
is simulated by the calibrated model?
Of importance to the analysis was not only the operation of the impacted wells, but also the
operational pumping plan for other production wells in the Santa Clarita Valley. The
operational plan for the Santa Clarita Valley’s groundwater resources has been defined in
the Urban Water Management Plan 2000 (UWMP) for the Santa Clarita Valley (Black &
Veatch, 2000) and in annual water reports that discuss the water demands, water supplies,
and surface water and groundwater resources of the valley (including the Santa Clarita

ES-4 RDD/041840005 (CLR2612.DOC)