PERSPECTIVE ON HETEROGENEITY, GROUNDWATER MONITORING, AND NATURAL ATTENUATION

Examples of heterogeneous flow and transport analyses in alluvial sedimentary materials illustrate a facies-geostatistical approach to characterization and demonstrate potential importance of heterogeneity. Some of the results are somewhat surprising, because our hydrogeologic interpretive skills have been so strongly influenced by homogeneous conceptual models. As reported previously, high-resolution, three dimensional facies models of heterogeneity in fan and glacio-fluvial sediments in the South Lake Tahoe basin, California, produce simulated MTBE plumes that exhibit significant, century-scale tailing due to diffusion and slow advection into low-K silts and clays. These simulations also indicate the potential for vertical transport of solutes from upper aquifers to lower aquifers via a single extraction well screen, even when the well is pumping, owing to presence of both sedimentary layering and strong vertical hydraulic gradients. Consistent with field data, the simulations indicate extremely large spatial and temporal variations in local solute concentrations. These spatial and temporal variances are sufficiently high, however, that serious questions are raised about the meaning of small numbers of samples (e.g., quarterly) from limited numbers of plume monitoring wells. This sampling problem further calls into question the meaning of field measurements of plume concentrations collected to estimate natural attenuation parameters. Natural attenuation is often declared to be effective when apparent plume concentrations and/or plume lengths are stable or decreasing with time. Simulation results show, however, that such trends can easily be the spurious result of inadequate sampling of a spatially and temporally complex plume.