EVALUATION OF IN-SITU COLUMNS FOR CHARACTERIZING ORGANIC CONTAMINANT SORPTION DURING TRANSPORT

In situ columns were evaluated for their reliability for characterizin g organic contaminant transport and sorption in the saturated zone. A solute transport model which considers one-dimensional flow, point-sou rce input, two-dimensional mass transport, and nonequilibrium sorption during transport within the cylindrical in situ column was developed and applied to simulate experimental data at two field sites. A nonrea ctive tracer was used to characterize hydrodynamic properties within t he in situ column. Independent measurements of equilibrium and nonequi librium sorption parameters were obtained from laboratory miscible dis placement experiments on the same porous media as for the in situ stud ies. Values of the equilibrium sorption constant, K(p), sorption rate coefficient, k2, and fraction of sorption that attained rapid equilibr ium, F, were determined for three sorptive solutes (benzene, toluene, p-xylene; BTX) in laboratory column studies and fixed in simulations o f the in situ column data. The simulations were in good agreement with the experimentally measured in situ breakthrough curves. The ability to characterize BTX transport in soils, sediments, and aquifer materia ls low in organic carbon content is important for management and remed iation of gasoline-contaminated-groundwater. The experimental and theo retical technique presented here was applied successfully for characte rizing sorptive solute transport at three field sites. With proper mod el validation and appropriate tracer studies, the in situ technique co uld be used to obtain site-specific estimates of equilibrium and noneq uilibrium sorption parameters (K(p), k2, and F).