AQUIFER TEST ANALYSIS UTILIZING NUMERICAL MODELS AND INVERSE METHODS

Analysis of aquifer tests has been expanded greatly by the use of numerical models and inverse methods. Numerical models facilitate the analysis of complex geohydrologic conditions such as multiple aquifers with complex geometries and boundary conditions. Inverse methods allow for the estimation of multiple hydraulic properties simultaneously and, just as importantly, identify properties that cannot be estimated. In addition, isolating small, pumping induced drawdowns from large, background water-level fluctuations has been made tractable with inverse methods.

Analyzing aquifer test results along the Colorado River illustrates the advantages of simultaneously interpreting multiple aquifer tests with a single MODFLOW model and applying inverse methods. The aquifer system at the site consists of alluvial fan deposits that overlie a sloping bedrock surface that are incised by fluvial deposits of the Colorado River. The complex geohydrology of the site was simulated with a 26-layer MODFLOW model. Aquifer test results were complicated by management of the Colorado River for electrical power generation that results in ground-water levels fluctuating between 0.1 and 1 m on a daily basis. Inverse modeling of the water levels allowed detection of drawdowns of less than 0.01 m. More than 50 drawdown and recovery series were matched simultaneously while more than 20 parameters were estimated using inverse methods. The analysis of the multiple aquifer tests identified a permeable interval of 5-m thickness in a 30-m thick sequence of undifferentiated alluvium. This permeable interval may be important to understanding contaminant transport at the site.