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Study on exposure assessment for vaporization of subsurface contaminant

Volatile contaminants can present in the subsurface with a variety of types, including free and/or residual nonaqueous phase liquid (NAPL), sorbed, vapor and dissolved phases. Contaminant vapors emanating from NAPL, sorbed and dissolved phases pose a human health and environmental risk and should be assessed by a rigorous way. Environmental Protection Agency in Taiwan provided a guideline on risk assessment and methodology for contaminated subsurface and groundwater. Human exposure to the vapors are catalogued into three pathways: vapors emanate from soil (surface, shallow or deep soils), from dissolved phase plumes and dissolved phase plumes transport to downstream and then vaporize to ground surface. Exposure amount of the vapors from these pathways, as suggested in the guideline for the first and second tiers of risk assessment, were estimated by some empirical or theoretical methods without considering complexity of environmental system. Some analytical and numerical models were also suggested for the second and third tiers of risk assessment. These methods and models need to be assessed for their theoretical background, limitations and restrictions to adequately apply them to contaminated sites. This project aims at assessing these methods and models for analyzing exposure pathways and amount of vapors emanating from contaminants in the subsurface. Analytical models, including Jury-finite source, Farmer, BIOSCREEN (or BIOCHLOR), and a numerical model, MOFAT, will be used to assess emission rate of contaminant vapor and depletion of contaminant source from different sources and pathways. Several sets of soil gas data obtained from different contaminated sites and provided by MWH will be used to adjust model parameters, calibrate these models and assess model performance. Model parameters will also be tested for their sensitivity on model result to assess sensitive ones required from field study. This work can give valuable insight into model application to the exposure assessment of contaminant vapors emanating from contaminated subsurface and groundwater