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4.4.2.1 Model 3: Pure Pilot Point Approach. For the thin aquifer scenario, the

three-dimensional flow model was treated in a similar manner to the thick scenario. Pilot points were distributed within the six layers, hydraulic conductivity fields for individual layers were inversely solved using PEST, and the goodness of fit was checked against the June 2004 head observations. The final near optimal pilot point distributions within individual model layers for the thick scenario were also used for the thin scenario, except for additional truncations of pilot points by the aquifer base of the thin scenario.

In general, for this scenario the same pilot point distribution was used as in the final thick aquifer scenario.

Figure 4-16 depicts the head distribution, residual error bars for all observation wells, and error statistics for the thin aquifer scenario as found from inverse solution with pure pilot point approach. In general, the thin scenario provides the same level of match between simulated and observed heads as does the thick scenario. The majority of the observation wells have mismatches less than 1 m. There is also no systematic bias on the residual distribution. The simulated head contours for the thin scenario are very similar to the thick scenario, as expected since both thin and thick models are calibrated with the same set of head measurements. Figure 4-17 shows the simulated residuals for all model layers.

Figure 4-12. Simulated heads (m) in the top layer of the three-dimensional flow model, thick scenario, calibrated with pilot point approach using sedimentary layer bounds (base case model 2). Simulation residuals shown with green bars are less than 2 m and those with yellow bars are from 2 to 3 m.

Residual (m)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

Figure 4-13. Head residuals (simulated minus observed) for all model layers, thick scenario, calibrated with pilot point approach using sedimentary layer bounds (base case model 2).

Figure 4-14. Conductivity distribution (m/day) within six model layers for thick aquifer scenario estimated using the pilot point approach with sedimentary layer bounds (base case model 2).

Layer 1 2

3 4

5 6

Model 2

1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09

1 25 49 73 97 121 145 169 193 217 241 265 289 313 337 361 385 409 433 457 481 Pilot Point Number

Estimated K (m/d) and 95% Confidence Interval

Figure 4-15. Confidence bounds of model parameters estimated by PEST for base case model 2.

Figure 4-16. Simulated heads (m) in the top layer of the three-dimensional flow model, thin scenario, calibrated with the pure pilot point approach (base case model 3). Simulation residuals shown with green bars are less than 2 m and those with yellow bars are from 2 to 3 m.

Residual (m)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

Figure 4-17. Head residuals (simulated minus observed) for all model layers, thin scenario, calibrated with the pure pilot point approach (base case model 3).

Figure 4-18 shows the resulting hydraulic conductivity fields of individual model layers for the thin aquifer scenario for the pure pilot point approach. As shown in Figure 4-18, the thin aquifer model also shows similar spatial features of the hydraulic conductivity field, particularly in the upper layers, for example, a relatively more conductive zone in the area of Arco rift and a low conductivity in Quaking Aspen Butte rift area. The bottom two layers of the thin scenario show the overall lower hydraulic conductivity values imposed from the conceptual model. However, there are some important differences on the final hydraulic conductivity distributions between the thick and thin scenarios, particularly for the top model layers of the two scenarios. Within the Arco rift area, the top layer of the thin scenario appears to be more conductive than that of the thick scenario. This is important since this area is downgradient from INTEC and adjacent to RWMC; therefore its hydraulic conductivity will have significant impact on transport predictions, which is particularly germane since most contaminants reside in the upper portion of the aquifer.

For both scenarios, the top layer is always 35 m thick everywhere in the model domain. Thus, the differences of the K field for this layer are not caused by the variation of the grid layer thickness. It is necessary to investigate which case is more reasonable. Although the spatial patterns of the hydraulic conductivity field of the lower model layers, for both thick and thin scenarios, are similar, the two scenarios do have variations in magnitude, mainly due to the changes of the model layer thickness between two scenarios. The bottom layer of the thin scenario also shows larger hydraulic conductivity values, similar to the thick scenario, due to the high inflow flux from the northeastern boundary assigned to this layer. This is a natural result of using the similar boundary conditions and observation heads with a thinner aquifer.

Figure 4-18. Conductivity distribution (m/day) within six model layers for thin aquifer scenario estimated using the pure pilot point inverse solution approach (base case model 3).

Layer 1 2

3 4

5 6

Figure 4-19 shows the 95% confidence bounds of estimated K values at all pilot points for base case model 3. As before, the graph shows increased sensitivity of the objective function to some model parameters.

Estimated K (m/d) and 95% Confidence Interval

Figure 4-19. Confidence bounds of model parameters estimated by PEST for base case model 3.

4.4.2.2 Model 4: Sediment-Constrained Pilot Point Approach. Similar to the pure point approach, which was applied to both thick and thin aquifer scenarios, the same sediment-bounded pilot point approach was applied to the thin aquifer scenario. Figures 4-20, 4-21, and 4-22 depict the simulation results of the sediment-constrained pilot point approach for the thin scenario. In general, the bounded pilot point approach also provides satisfactory fit to the measured heads, with slightly higher residuals due to the additional constraints to the model parameters. Table 4-5 summarizes the root mean square errors for the two aquifer thickness scenarios and two inverse simulation approaches.

Similar to the thick scenario, the estimated hydraulic conductivity maps for the thin scenario (Figure 4-22) with the bounded pilot point approach also show essentially the same large-scale spatial patterns as shown in Figure 4-18 (which is obtained from the pure pilot point approach); however, there are significant local variations in magnitude of the estimated hydraulic conductivity values obtained by two inverse simulation approaches.

Figure 4-23 shows the 95% confidence bounds of estimated K values at all pilot points for base case model 4. Like the other three base cases, this graph shows some sensitivity to model parameters as indicated by the variable bound width.

Figure 4-20. Simulated heads (m) in the top layer of the three-dimensional flow model, thin scenario, calibrated with pilot point approach using sedimentary layer bounds (base case model 4). Simulation residuals shown with green bars are less than 2 m and those with yellow bars are from 2 to 3 m.

Residual (m)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

Figure 4-21. Head residuals (simulated minus observed) for all model layers, thin scenario, calibrated with pilot point approach using sedimentary layer bounds (base case model 4).

Figure 4-22. Conductivity distribution (m/day) within six model layers for thin aquifer scenario estimated using the pilot point inverse solution approach with sedimentary bounds (base case model 4).

Layer 1 2

3 4

5 6

Model 4

1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10

1 25 49 73 97 121 145 169 193 217 241 265 289 313 337 361 385 409 433 457 481 Pilot Point Number

Estimated K (m/d) and 95% Confidence Interval

Figure 4-23. Confidence bounds of model parameters estimated by PEST for base case model 4.