Vapor Intrusion from Entrapped NAPL Sources and Groundwater Plumes: Process Understanding and Improved Modeling Tools for Pathway Assessment.

摘要

Factors that contribute to the uncertainty and variability of measured vapor concentrations in buildings and structures resulting from subsurface VOC sources were investigated. Mechanisms controlling vapor generation and subsequent migration through the subsurface in naturally heterogeneous subsurface under various physical and climatic conditions were investigated using laboratory and modeling studies. Experiments were conducted in test systems varying from bench to intermediate scale. Concurrently with the experiments, modeling tools were developed. This research established that subsurface vapor pathways are dynamic and complex, sometimes contributing to counter-intuitive cause-effect relationships. Infiltration was found to affect vapor signals in indoor air, with the time-scales and the strength of the vapor signals depending on the interplay of the intensity, duration of rainfall, and subsurface heterogeneity. Water table fluctuation imparts very complex transport behavior within the capillary fringe that has significant effects on vapor loading from the groundwater plumes that defied simplified models used in screening. Trapped sources in the unsaturated zone are capable of loading significant mass into the unsaturated zone, but the loading rate is a strong function of the moisture distribution in the vicinity of the source. Indoor sampling strategies need to factor in the transients associated with climate and weather. This has to be done in conjunction with subsurface characterization methods to identify potential dynamic pathways.