Construction of Three-Dimensional Solids. Data and interpretations from the hydrogeologic cross sections were entered into the GMS program after all of the section line

interpretations were completed. The borehole and cross section modules in the GMS code were used to enter the data into the modeling software, and the solids module was used to extrapolate the

hydrogeologic units throughout the entire model domain. All of the extrapolations required verification to ensure consistency with the hydrogeologic subdomain conceptual model.

Several steps were taken within the GMS program to better constrain the extrapolation in the GMS code. These steps included: (1) extrapolation of all boreholes to the total aquifer thickness; (2) definition of “horizons” in GMS, which were essentially the maximum spatial extent of aquifer unit extrapolation;

(3) specification of the nodal spacing for extrapolation along all of the cross sections and boundaries between the aquifer units, and along the horizon extents and model domain boundaries; (4) specification of the extrapolation method used to build the solids; and (5) refinement of the TIN nodal spacing in areas of high complexity.

The GMS program commonly crashed when performing the solids extrapolation. Extension of all of the boreholes to the aquifer bottom, definition of the horizons within each well on a vertical basis, and definition of the horizontal horizon extents significantly increased the stability of GMS during the solids extrapolation. Even after this was completed, the extrapolation process still produced results that were inconsistent and sometimes problematic. Spurious spikes in the contacts between aquifer sub-units existed with the domain, especially in the area where the Olduavi Lake beds were present. The spikes were generally present where units pinched out or where beds dipped significantly. To overcome these problems, the nodal spacings along the cross-section aquifer sub-units were refined to between 10–50 m, with the density of the overall extrapolation TIN surface increased to a uniform 200 or less throughout the entire model domain. In areas where spikes in the extrapolated surfaces were prevalent, local TIN

refinement was used to minimize the problems.

Figure 2-13a shows the extrapolated solids for all of the aquifer units looking from the southeast to the northwest. The general layering scheme of the volcanic tablelands can be observed along the eastern edge of the model domain, with 3 to 4 hydrogeologic units exposed, depending on the thickness of the aquifer. Along the southern boundary, the hydrogeologic units were representative of a volcanic rift zone, as this boundary lies primarily along the Great Rift of Idaho. The BS_AQ unit was also present in the thickest portion along the southern boundary below at an altitude of approximately 900 m above mean sea level. Figure 2-13b presents the same view of the model domain with the UP_AQ unit removed. This figure clearly demonstrates the location of the prominent volcanic rifts and volcanic centers in the model domain, with the AVH, the Arco Rift, and the Great Rift evident by the lack of aquifer sub-unit MID_AQ.

The basal aquifer unit is shown on Figure 2-13c. The BS_AQ generally filled in the deepest portions of the active aquifer. As stated earlier, the BS_AQ was subdivided into upper and lower portions for convenience with the GMS program. This was primarily necessitated by the presence of the Olduavi Lake sediments, which projected through the basal aquifer unit to the aquifer bottom in the vicinity of the TAN facility (and shown on the figure where the orange and purple solids meet). Figure 2-13d shows only the sediment and sediment-controlled units in the upper portion of the flow system. These aquifer sub-unit solids were primarily located in the vicinity of the Big Lost River and in areas to the east and north, which was consistent with the interpreted evolution of the Big Lost River channel.

2.1.2.4 Constraint of Hydrogeologic Unit Hydraulic Properties. This steady-state phase of the three-dimensional representation of groundwater flow requires characterization of the distribution of horizontal and vertical hydraulic conductivity throughout the area represented by the OU 10-08 model domain. Heterogeneities derived from the basalt architecture (high-permeability interflow zones separated by low-permeability basalt-flow interiors) likely result in a large ratio of horizontal to vertical hydraulic conductivity. Characterization of the storage coefficient is not required for this steady-state

representation; subsequent transient models will require estimates of the storage coefficient.

An inverse numerical modeling technique was used to approximate the areal and vertical

distribution of the horizontal hydraulic conductivity within the model domain as described in the model implementation section of this report. This inverse technique required specification of ranges of hydraulic conductivity for defined hydrogeologic units.

Figure 2-13a. Extrapolated solids for all of the aquifer units looking from the southeast to the northwest.

Figure 2-13b. Extrapolated solids for all of the aquifer units looking from the southeast to the northwest, with upper basalt hydrogeological unit removed to show detail of mid- and base-mid basalt units.

Figure 2-13c. Extrapolated solids basal aquifer unit looking from the southeast to the northwest.

Figure 2-13d. Extrapolated solids for all of the sediment-dominated units looking from the southeast to the northwest.

Most of the aquifer-test data were obtained from wells completed near the water table, but, as seen on Figure 2-14, some of the aquifer tests were conducted in wells with deeper completion intervals.

Aquifer-test data were analyzed to identify trends in the hydraulic conductivity with depth below the water table. The hydraulic conductivity estimates were analyzed by binning estimates into 10-ft-thick bins and assuming that all of the aquifer section exposed to the well contributed equally to the production of water in the well. Hydraulic conductivities for all wells completed over the 10-ft bins were then summed and harmonically averaged. The effect of binning the data and assuming equal hydraulic contribution was to decrease the overall hydraulic conductivity for each interval, but was still illustrative for gaining insight into the property distribution with depth. This analysis, presented on Figure 2-14, shows that hydraulic conductivity generally decreases with depth in most areas. The mean value of hydraulic conductivity near the top of the aquifer is approximately 300 ft/day. The range in the data, prior to binning and averaging, was approximately 10^-2 to 10⁵ ft/day.

LOG Mean Bin Hydraulic Conductivity (ft/day)

Total Wells Open Over Interval

DepthBelowWaterTable(ft)

-5 0 5

0 20 40 60 80 100 120

0

200

400

600

800

1000

Figure 2-14. Aquifer test information presented against depth below the water table. Aquifer test data from all wells were binned on 10-ft-depth intervals and averaged, with the average values shown by red dots. The range of the binned data is presented with horizontal bars, and the number of tests within the bin are illustrated with a blue line and points.