Key Conceptual Model Issues

Several key issues exist regarding the steady-state three-dimensional numerical representation of the conceptual model of groundwater flow within the area of the OU 10-08 model domain. These issues generally are associated with limitations in the hydrogeologic database.

The delineation of hydrogeologic units provides an effective approach to identify discrete geologic volumes within the SRPA that could be represented by a layered model. Uncertainties exist in this delineation approach because of the extremely complex stratigraphic system within the SRPA and

because of limited areal and vertical distribution of borehole data. Additionally, the thickness of the active flow system within the SRPA is not well known because only a few wells fully penetrate that system, particularly in areas near model domain margins.

It is generally agreed that the layered system of interflow zones and massive basalt-flow interiors results in a large ratio of horizontal to vertical hydraulic conductivity. The resulting anisotropy strongly affects three-dimensional flow and must be taken into account in a numerical representation.

The volume of regional groundwater underflow moving into the area of the OU 10-08 model domain is a secondary estimate derived from another numerical flow model (Spinazola 1994). This underflow estimate represents the integration of well-defined inflows and outflows within the area northeast of the OU 10-08 model domain. Although this total inflow estimate is a technically defensible value, uncertainty exists, particularly with depth and the vertical distribution within the layered system.

Additional uncertainty exists regarding the lateral distribution of flows along that cross-sectional boundary.

Transient effects of most inflows (regional and tributary underflow, recharge from infiltration of precipitation) likely are minimal because of the attenuation that occurs because of a thick vadose zone and large permeability of the SRPA. However, transient effects of recharge from infiltration of stream flows along the channel of the Big Lost River locally may be large. These transient fluxes occur in proximity to known contaminant plumes and their effects must be evaluated.

Calibration of the three-dimensional model is dependent in part on use of a water-level data set.

This data set is concentrated in areas of concern. In other areas, limited density of wells limits calibration.

Additionally, these data sets are limited as to the definition of head with depth. Uncertainties require use of additional calibration targets, including chemical tracers and groundwater velocity estimates.

Groundwater flow velocities can be used to constrain the numerical model. These velocities are estimated based on water-chemistry data. Groundwater velocity data sets are limited by water chemistry data largely collected within the upper part of the flow system.

2.4 References

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