HR: 0800h
AN: H11B-0484    [Abstracts]
TI: The Complex Effects of Simplifying Network Structure: A Modeling Evaluation of Chemical Migration in Fracture-Controlled Aquifers
AU: * Wellman, T
EM: twellman@usgs.gov
AF: U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303,
AU: Shapiro, A
EM: ashapiro@usgs.gov
AF: U.S. Geological Survey, 431 National Center 12201 Sunrise Valley Drive, Reston, VA 20192,
AB: It is widely recognized that highly permeable features dominate chemical transport in many fracture-controlled aquifers. Recent investigations also suggest that pervasive fracturing and the primary rock porosity are equally important in characterizing the spatial extent and time constants of chemical migration. Relatively little is known about the significance in which features of various attributes influence chemical transport through fracture networks. To address this issue, a suite of discrete fracture network models is constructed with geological and hydrological conditioning from observations at multiple field sites. The resulting models are used to quantify the changes to transport behavior caused by a systematic removal of aquifer structure. The results indicate that the influence of structural features on transport behavior is often significant but highly variable. The correlation of this behavior to the eliminated features is relatively low, suggesting that transport is jointly influenced by the eliminated feature characteristics and their role within the fracture network in altering hydraulic pathways. Moreover, the effects of eliminating features may be ‘enhanced' or ‘restricted', meaning that the removal of aquifer structure may either slow or advance mass migration, mean travel, dispersion, and tailing behavior. This suggests that correcting for the eliminated features does not invariably follow the classic advection-dispersion equation, where for instance dispersion approximates the effect of the eliminated structure as an invariably additive process to the mean flow. Given that the influence of eliminating structure is not easily approximated, the most useful procedure is to employ ensemble averaging from several structurally simplified models that capture the salient aspects of the fracture network connectivity, which significantly reduce but do not fully eliminate the effects of structural simplification.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1848 Monitoring networks
DE: 1869 Stochastic hydrology
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting