WELLBORE FLOW SIMULATIONS FOR TEST WELLS, FORT IRWIN NATIONAL TRAINING CENTER, CALIFORNIA

Well-bore flow and aquifer-property data were collected from six long-screened test wells at the U.S. Army Fort Irwin National Training Center in the Mojave Desert, California. These data were used with other hydrogeologic data to provide a basis for basin-scale hydrogeologic framework and groundwater modeling efforts. Data collected under pumped conditions were used to estimate the vertical distribution of flow contribution to a well from the surrounding aquifer system. Well-bore flow was simulated for each well by using an integrated flow analysis tool, AnalyzeHOLE, to evaluate aquifer properties and heterogeneity. AnalyzeHOLE simulates a well and adjacent aquifer system with a two-dimensional MODFLOW model. This model assumes an axisymmetric, radial geometry, representing a central pumping well with surrounding aquifer materials. Horizontal layers within the model, (hydrogeologic units - HGUs) were initially defined based on lithostratigraphic-geophysical units (LGUs) established on the basis of interpretations of lithologic and geophysical logs from each well. The HGUs were simulated as radially symmetric, flat-lying, internally homogeneous, and laterally extensive throughout the model domain. The saturated hydraulic conductivity (K) of each HGU was adjusted so that the simulated well-bore flow/drawdown approximated the measured well-bore flow/drawdown. K values derived from the calibrated simulations ranged from <0.01 to 60 ft/day. In general LGUs provided good approximations for the boundaries of each HGU. Some LGU boundaries did not correspond to modeled HGUs; differences may be due to large-scale features such as fracturing or faulting, secondary mineralization/cementation, effects of well construction, or potential gradational changes between units that were modeled as sharp changes in aquifer properties at unit boundaries. Efforts were made to maintain consistent Ks within a given HGU across wells; however, calibrated Ks varied considerably in some HGUs between boreholes demonstrating the potential variability in hydraulic properties within a given LGU. Overall, well-bore flow simulation results show good correlation with the interpretations of geologic, geophysical, and airborne electromagnetic survey data used to generate the basin-scale hydrogeologic framework.