Analysis of Tracer Migration in a Diverging Radial Flow Field

Abstract

Hydrodynamic dispersion is an important factor controlling contaminant migration in the subsurface environment. However, few comprehensive data sets exist for critically evaluating the impact of travel distance and site heterogeneity on solute dispersion. Therefore, a series of field-scale experiments using tritiated water (³H₂O), and bromide (Br-) as tracers was conducted on the U.S. Department of Energy's Savannah River Site. For each experiment, tracer-free groundwater was injected at a fixed rate of 56.7 L min-1 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 six sampling wells radially spaced at approximate distances of 2.0-, 3.0-, and 4.5-m from the injection well. Each sampling well was further divided into three discrete sampling depths that were pumped continuously throughout the course of the experiments. Longitudinal dispersivity (αL) and travel times for ³H₂O were estimated by fitting the field data to analytical approximations of the advection-dispersion equation (ADE) for uniform and radial flow. Dispersivity varied greatly between wells located at similar transport distances and between zones within a given well. The radial flow equation described ³H₂O breakthrough better than the uniform flow solution, yielding lower αL values while accounting for breakthrough tailing inherent to radial flow conditions. Temporal moment analysis confirmed the retardation of Br-, generally considered to travel in a conservative manner, despite data truncation due to extensive tailing that biased retardation estimates when compared to ³H₂O. Despite retardation and incomplete mass recovery, both ADE models were able to reasonably describe the Brdata without accounting for sorption reactions, indicating that chemical interactions with the geologic matrix may be misinterpreted in terms of a physical transport process.