Analysis
of tracer migration in a diverging radial flow field
J.
C. Seaman, F. Majs, J. Singer, S. Aburime, S. O. Dennis, M. Wilson and
P. M. Bertsch
Savannah River Ecology Laboratory, The University of Georgia, Aiken, SC
29802
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 ("alpha"L)
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 "alpha"L
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 Br-
data without accounting for sorption reactions, indicating that chemical
interactions with the geologic matrix may be misinterpreted in terms of
a physical transport process.
SREL Reprint #3022
Seaman, J. C., F. Majs, J. Singer, S. Aburime, S. O. Dennis, M. Wilson and P. M. Bertsch. 2007. Analysis of tracer migration in a diverging radial flow field. Georgia Water Resources Conference, Athens, Georgia, The University of Georgia.
