Vapor Phase Transport as a Groundwater Contamination Process at Arid Landfill Sites

The occurrence of halogenated volatile organic compounds (HVOCs) in groundwater beneath several municipal solid waste landfills in Tucson, Arizona in the absence of significant liquid phase movement has suggested that vapor phase transport may be the principal process transporting HVOCs from the vadose zone into the water table. The results of mathematical modeling of these transport processes were used to evaluate their relative importance under site specific conditions. The processes considered included molecular diffusion and mechanical dispersion through the capillary fringe and the effects of a falling water table on mass transport. Calibration of the mathematical model against soil gas and groundwater monitoring data indicated that mechanical dispersion due to natural groundwater flow is sufficient to result in groundwater plumes of HVOCs exceeding maximum contaminant levels. A falling water table further increases mass transport from the vadose zone to the water table through advective transport of dissolved HVOCs.