Reactive transport of volatile organic compound mixtures in the unsaturated zone: modeling and tuning with lysimeter data

Petroleum products involve mixtures of different volatile organic compounds (VOCs) that may represent a risk for groundwater after a spill to the unsaturated zone. However, the majority of the published approaches and codes for unsaturated zone modeling handle only either a single or a limited number of compounds in order to cope with the complexity of processes involved in VOC attenuation. This work aimed at simulating complex VOC scenarios using a grouping procedure, which results in the definition of a small number of effective pseudospecies with carefully specified effective properties. Grouping criteria are developed for a hypothetical scenario that involves a mixture of seven VOCs with properties covering a wide range of values. These criteria are assessed through a comparison of group-based calculations with rigorous calculations using the individual components. Groups of species with similar solubility resulted in concentration profiles that compared very satisfactorily with the rigorous results. The approach is validated with data from a large-scale lysimeter experiment. The simulation results compare satisfactorily with the experimental data for several days and for the vast majority of the mixture components. The agreement was significantly improved when temporally varying solubility values were used, justified by the difference in the compound volatility and the concomitant alteration of the VOC mixture composition with time.