Tracer Migration in a Radially Divergent Flow Field: Longitudinal Dispersivity and Anionic Tracer Retardation

Hydrodynamic dispersion, the combined effects of chemical diffusion and differences in solute path length and flow velocity, is an important factor controlling contaminant migration in the subsurface environment. However, few comprehensive three-dimensional datasets exist for critically evaluating the impact of travel distance and site heterogeneity on solute dispersion, and the conservative nature of several commonly used groundwater tracers is still in question. Therefore, we conducted a series of field-scale experiments using tritiated water ( 3 H 1 HO), bromide (Br − ), and two fluorobenzoates (2,4 Di-FBA, 2,6 Di-FBA) as tracers in the water-table aquifer on the USDOE9s Savannah River Site (SRS), located on the upper Atlantic Coastal Plain. For each experiment, tracer-free groundwater was injected for approximately 24 h (56.7 L min −1 ) to establish a steady-state forced radial gradient before the introduction of a tracer pulse. After the tracer pulse, which lasted from 256 to 560 min, the forced gradient was maintained throughout the experiment using nonlabeled groundwater. Tracer migration was monitored using six multilevel monitoring wells, radially spaced at approximate distances of 2.0, 3.0, and 4.5 m from the central injection well. Each sampling well was further divided into three discrete sampling depths that were pumped continuously (∼0.1 L min −1 ) throughout the course of the experiments. Longitudinal dispersivity (α L ) and travel times for 3 H 1 HO breakthrough were estimated by fitting the field data to analytical approximations of the advection–dispersion equation (ADE) for uniform and radial flow conditions. Dispersivity varied greatly between wells located at similar transport distances and even between zones within a given well, which we attributed to variability in the hydraulic conductivity at the study site. The radial flow equation generally described tritium breakthrough better than the uniform flow solution, as indicated by the coefficient of determination, r 2 , yielding lower α L while accounting for breakthrough tailing inherent to radial flow conditions. Complex multiple-peak breakthrough patterns were observed within certain sampling zones, indicative of multiple major flow paths and the superposition of resulting breakthrough curves. A strong correlation was found between α L and arrival times observed from one experiment to the next, indicative of the general reproducibility of the tracer results. Temporal moment analysis was used to evaluate tracer migration rate as an indicator of variations in hydraulic conductivity and flow velocity, as well as mass recovery and retardation for the ionic solutes compared with tritiated water. Retardation factors for Br − ranged from 0.99 to 1.67 with no clear trend with respect to transport distance; however, Br − mass recovery decreased with distance, suggesting that the retardation values are biased in terms of early arrival because of limited detection and an insufficient monitoring duration. Anion retardation was attributed to sorption by iron oxides. Similar results were observed for the FBA tracers. The assumption of conservative behavior for the anionic tracers would generally result in higher α L values and lower estimated flow velocities.