Laboratory and field measurements of non-equilibrium transport in the Borden aquifer, Ontario, Canada

Development of practical techniques to predict the mobility of halogenated hydrocarbons in aquifers is an important step toward protecting the quality of groundwater resources. This study involved the use of laboratory-column and field-column experiments to estimate retardation coefficients of five halogenated hydrocarbons (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2-dichlorobenzene and hexachloroethane) in the Borden aquifer. These compounds were selected because they were used in the highly-monitored, natural-gradient field-tracer experiment conducted in the unconfined sand aquifer at Canadian Forces Base Borden in southern Ontario, Canada. The field-column experiments were conducted in the vicinity of the natural-gradient tracer experiment, and the laboratory experiments were conducted using aquifer material collected from the field site. The retardation coefficients were found to increase as the average pore-water velocity in the columns decreased, suggesting the presence of non-equilibrium conditions in the reactive transport process. Analysis of the data with a two-site reactive-solute transport model, that allows for time-dependent interaction between solutes and a fraction of reaction sites, provided significant improvement over analysis with a reactive-solute transport model that assumes local equilibrium. For experiments conducted at similar velocities, the mobilities were in close agreement for both the laboratory-and field-column experiments. Retardation factors calculated from batch experiments also agreed closely with the column results and were consistent with those obtained from previous studies at the Borden site. While the retardation factors calculated from the present small-scale methods were in close agreement with those observed during the first month of the large-scale natural-gradient experiment, large deviations were observed for longer travel times.