Transport of tritium and four organic compounds during a natural-gradient experiment (MADE-2)

The transport and fate of aqueous benzene, naphthalene, 14C labelled p-xylene, o-dichlorobenzene, and tritium were investigated during a natural-gradient experiment. The study was initiated with a two-day pulse release of 9.7 m3 of tracer solution into the saturated zone of the heterogeneous alluvial aquifer at Columbus Air Force Base. Tracer plumes were subsequently monitored in three dimensions using multilevel samplers over a period of 15 months. The tritium plume was characterized by extreme skewness in the longitudinal direction resulting from the nonuniform mean velocity along the plume travel path. A longitudinal dispersivity of approximately 10 m was estimated by fitting the spatial moments of the tritium data to the advection-dispersion model for a nonuniform flow field of Adams and Gelhar [1992]. The four organic tracers were strongly affected by natural biotransformation. While the tritium activity recovery remained above 77 percent of injected activity during the experiment, mass recoveries for benzene, naphthalene, and p-xylene steadily declined reaching levels of from 1 to 6 percent after 440 days. A greater fraction (approximately 13 percent) of the o-dichlorobenzene remained after 440 days, consistent with its greater resistance to biodegradation. An aerobic biodegradation process was indicated by an examination of transformation by-products of the ''C p-xylene in groundwater samples near the end of the study which showed greater than 80 percent conversion to 14CO2. Enhanced mixing of oxygen rich groundwater with the organic tracers produced by the extreme heterogeneity of the aquifer may have contributed to the high biodegradation rates observed. Results show that under certain conditions natural biodegradation may effectively reduce aqueous concentrations of organic contaminants