INFLUENCE OF AIR-WATER INTERFACIAL ADSORPTION AND GAS-PHASE PARTITIONING ON THE TRANSPORT OF ORGANIC-CHEMICALS IN UNSATURATED POROUS-MEDIA

We investigated in laboratory column experiments, the aqueous-phase tr ansport of four n-alcohols (n-hexanol-n-nonanol), three chlorinated ar omatic compounds(chlorobenzene, o-dichlorobenzene, and o-chlorophenol) , and two alkylbenzenes (ethylbenzene and p-xylene) in a water-unsatur ated porous medium (sand). The influence of gasphase partitioning and interfacial adsorption on solute retardation during steady unsaturated water flow was evaluated over a range of water contents. Air-water in terfacial adsorption was a significant factor for the retardation of n -alcohols. For example, nearly 90% of the measured retardation of n-no nanol could be attributed to interfacial adsorption at a water saturat ion of 34%. Aromatic compounds used in this study were not significant ly affected by adsorption at the air-water interface because of both l ow air-water interfacial area (0-50 cm(2)/cm(3)) generated in the unsa turated porous medium and the small interfacial-adsorption coefficient s. instead, gas-phase partitioning was the primary mechanism responsib le for the measured retardation of most of the aromatic compounds eval uated in this study. Using the batch-measured interfacial adsorption c oefficients for n-octanol and n-nonanol and the column-measured retard ation factors, the effective air-water interfacial areas were estimate d. These values agreed closely with those we reported earlier, based o n displacement experiments with an anionic surfactant as an interfacia l tracer.