HR: 14:10h
AN: H23I-03    [Abstracts]
TI: Longitudinal Dispersivity in a Radial Diverging Flow Field
AU: * Seaman, J C
EM: seaman@srel.edu
AU: Wilson, M
EM: wilson@srel.edu
AF: Savannah River Ecology Lab, Drawer E Savannah River Site, Aiken, SC 29802
AU: Bertsch, P M
EM: bertsch@srel.edu
AF: Savannah River Ecology Lab, Drawer E Savannah River Site, Aiken, SC 29802
AU: Aburime, S A
EM: saburime@cau.edu
AF: Clark Atlanta University, Department of Engineering, Atlanta, GA 30314
AB: Hydrodynamic dispersion is an important factor controlling contaminant migration in the subsurface environment. However, few comprehensive data sets exist for evaluating the impact of travel distance and site heterogeneity on solute dispersion under non-uniform flow conditions. In addition, anionic tracers are often used to estimate physical transport parameters based on an erroneous assumption of conservative (i.e., non-reactive) behavior. Therefore, a series of field experiments using tritiated water and several other commonly used hydrologic tracers (Br, Cl, FBAs) were conducted in the water-table aquifer on the U.S. Department of Energy's Savannah River Site (Aiken, SC) to evaluate solute transport processes in a diverging radial flow field. For each experiment, tracer-free groundwater was injected for approximately 24 hours at a fixed rate of 56.7 L/min (15 gpm) to establish a forced radial gradient prior to the introduction of a tracer pulse. After the tracer pulse, the forced gradient was maintained throughout the experiment using non-labeled groundwater. Tracer migration was monitored using a set of six sampling wells radially spaced at approximate distances of 1.5, 3, and 4.5 meters from a central injection well. Each sampling well was further divided into three discrete sampling depths that were monitored continuously throughout the course of the tracer experiment. At various time intervals, discrete groundwater samples were collected from all 18 sampling ports for tritium analysis. Longitudinal dispersivity for tritium breakthrough at each sampling location was estimated using analytical approximations of the convection dispersion equation (CDE) for radial flow assuming an instantaneous Dirac pulse and a pulse of known duration. The results were also compared to dispersivity values derived from fitting the tracer data to analytical solutions derived from assuming uniform flow conditions. Tremendous variation in dispersivity values and tracer arrival times were observed between wells located at similar radial distances and between sampling zones within a given well, with multiple arrival peaks observed for some monitoring locations. However, lower dispersivity values were observed for all data sets when fitting the breakthrough data using the radial flow approximations when compared to uniform flow case. In addition, the 95% confidence interval span for the dispersivity estimate was always smaller for the radial flow solution, indicating a better model fit when compared to the uniform flow model. Additional logistical obstacles in conducting such an extensive set of field tracer experiments will also be discussed.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1846 Model calibration (3333)
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005