Aquifer model development

The valley shape was modelled using profile extrapolation, constrained by well lithologs and geostatistical interpolation. The valley attains a maximum depth of 250 m below ground surface, but typical sediment thickness is about 100 m. The hydrostratigraphy of the aquifer was interpreted from selected high-quality well lithologs, with layering constrained by the Quaternary depositional history of the valley sediments. Approximately 150 well lithologs are used to constrain the hydrostratigraphic model, mostly for shallow groundwater wells (Fig. 2a). The unconsolidated sediments thicken toward the middle of the valley, and have presumed horizontal stratigraphy. The topmost coarse-grained sediments form the Grand Forks aquifer.

Hydrostratigraphic units were modelled in three dimensions from standardized, reclassified, and interpreted well borehole lithologs. Solid models were constructed using GMS software v. 4.0 (Brigham Young University 2002), converted to a five-layer system underlain by solid bedrock (Fig. 2b), and imported into Visual MODFLOW (Waterloo Hydrogeologic Inc. 2004), as is typically done with complex multi-layer aquifer systems (Herzog et al. 2003). Details of model construction are described in Allen et al. (2004b). Representative homogeneous and isotropic hydraulic properties were initially assigned to each layer, based on values determined from pump test data, but later modified slightly during model calibration.

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