WHAT DO PUMPING TESTS MEAN? EVALUATION OF A HETEROGENEOUS ALLUVIAL AQUIFER, LAWRENCE LIVERMORE NATIONAL LABORATORY, CALIFORNIA

Analytical methods for evaluating pumping test data are based on several assumptions, many of which are not valid in a heterogeneous fluvial setting. These methods can produce results that are misleading unless evaluation of the site heterogeneity is conducted. To illustrate this point, we evaluate pumping test results at the Lawrence Livermore National Laboratory site, California, using both an analytical and a numerical modeling approach. The numerical modeling approach is based on high-resolution, geologically-constrained realizations of the alluvial aquifer generated with transition probability geostatistics in a stratigraphic framework. This modeling approach produced results that reasonably mimic the observed pumping test data for this site. These results show that the cone of depression follows coarse-grained channel deposits; thus, drawdown at a set distance from the pumping well is greater in wells screened within the central portion of the channel than those screened in adjacent deposits that contain only a thin, finer-grained portion of the channel deposit. Estimation of hydraulic parameters using analytical approaches (e.g., type-curve matching) misidentifies the thin, finer-grained sediments as having higher conductivity since drawdown is lower in these wells. Conversely, the thicker, coarse-grained channel deposits are indicated to have low hydraulic conductivity. By development of realistic models of heterogeneity and numerical modeling of groundwater flow, we are able to optimize modeling results to better estimate hydraulic parameters of various facies in the unit. Results of this study suggest that caution be used when evaluating pumping test data in heterogeneous systems.