Description
Contamination of groundwater by gasoline is a widely recognized environmental problem. A column study was designed to evaluate the dissolution and transport of MTBE, benzene and toluene from a synthetic gasoline emplaced as a non-aqueous phase liquid in natural formation. The test was conducted under natural groundwater flow velocities using two columns, connected in series. NAPL was emplaced into a restricted portion of the upgradient column only. Concentration data were collected from the effluent port of the upgradient and the downgradient columns. A triplicate sequential dilution batch test was conducted, using an identical NAPL formulation. Column flow rate and applied head were measured throughout the test to determine the hydraulic conductivity of the formation. A chloride tracer was included in the flow-through column influent. A one-dimensional, finite difference numerical simulation was matched to the chloride effluent data to develop a value for dispersivity. The formation was found to have a dispersivity value of one cm, at the 30 cm scale of the experiment. The dispersivity value was incorporated into each numerical solute transport simulation.The sequential batch test provided a baseline of equilibrium dissolution behavior without the effects of dispersion, non-equilibrium dissolution or retardation. The tests revealed that the synthetic NAPL behaved according to ideal dissolution theory, with MTBE displaying an aqueous solubility of 29,700 ppm. The experimentally determined aqueous solubility of each NAPL component was included into an equilibrium dissolution mass balance model. The results of which were coupled to a one-dimensional, finite difference solute transport model and compared to dissolution column effluent data. Comparing the results of this coupled model to the measured effluent data revealed the effects of dispersion and non-equilibrium dissolution. The solute transport column effluent was analyzed using a finite difference solute transport model. The resultant curve matching was simplified to the point where it was reasonably clear that no retardation of MTBE of benzene had occurred. Toluene had been retarded to an extent consistent with its affinity for organic carbon and a 0.001 fraction of organic carbon residing on the granular medium. A mass balance was performed. Each column's effluent curve of concentration versus effluent volume was integrated and totaled. The apparatus was disassembled and contaminant was extracted from the granular medium. The dissolution column recovery was 100% for MTBE, 96% for benzene, 85% for toluene. The solute transport column recoveries were 100% for MTBE, 98% for benzene, 96% for toluene. The missing benzene and toluene mass from the dissolution column is most likely due to small droplets of NAPL that remained in the column after the flow-through test, and were not accurately quantified during the extraction procedure. The close mass balance in the so lute transport column indicates that effluent concentration readings were accurate and little or no evaporative or biologic loss occurred during the experiment.
