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Obtaining defensible and conservative estimates of the nature, extent and concentration of volatile organic compounds (VOCs) in the vadose zone is extremely important when formulating the conceptual model of the site, when performing risk assessments, for estimating contaminant mass, for assessing remedial alternatives, for selecting target areas for cleanup, and/or for making no further action/investigation decisions. Studies have shown that soil gas analytical results provide both a more complete indication of the VOCs present and a higher estimate of their respective concentrations in the vadose zone than the analysis of soil samples alone. For the past several years deep downhole (to 30+ meters) soil gas sampling and analysis has been performed by various consultants during remedial investigations (RIs) and remedial actions (RAs) of the vadose zone at McClellan Air Force Base. A number of these soil gas results have been confirmed by the concurrent collection and analysis (for VOCs) of soil samples (preserved by either refrigeration to 4 degrees centigrade or refrigeration combined with methanol preservation). The use of this VOC sampling and analysis strategy has resulted in the optimization of VOC sampling and analysis procedures, in a better understanding of the relationship between the concentration of VOCs in soil gas and in the soil, and in a more accurate and comprehensive conceptual model for VOC contamination in the vadose zone. The paper will present the methodologies used by the various consultants for sample collection, preservation, and in the analysis of soil gas and soil samples, present the results of a focused QC study on soil gas sampling and analysis, and discuss the correlation between the soil gas and soil matrix analytical results. The current and future strategies for the sampling, analysis, and estimation of VOCs in the vadose zone during RIs and RAs will also be presented.
volatile organic compounds, VOCs, soil gas, soil, sampling, analysis, methanol preservation
Quality Assurance Manager, Northern California Environmental Operations, Jacobs Engineering Group, Sacramento, California