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4 ' COPC concentrations that make this risk assessment conservative in nature These conditions will result in a significant overestimation of risk and excessively low RBSLs ' 3.3.2 ASTM RBCA Fate and Transport Algorithms: The ASTM guidelines and the GSI RBCA Spreadsheet System used in this analysis employ a series of simplified fate and transport models for predicting COPC concentrations at points of exposure The simplified analytic nature of these models, ' particularly those used to simulate volatilization and transport of vapor emissions to ambient and indoor air, often result in grossly over-estimated COPC exposure point concentrations (Sanders and Stern, 1994, GSI, 1995 and 1997, AEHS, 1997, Javaherian, 1994 and 1997), in turn,these result in over-estimation of health risks and lower RBSLs In fact, GSI (1995 and 1997)warns against delineation of cleanup levels based on the use of its formulation for vapor intrusion to indoor air Examples of physical and chemical processes ignored by the simplified ASTM vapor transport models include • loss mechanisms -absence of loss mechanisms such as biodegradation and adsorption result in over- estimation of vapor and contaminant flux to ambient and indoor air, • depleting contaminant source-use of a non-depleting, constant source results in an unlimited supply of contaminated vapor and an over-estimation of vapor and contaminant flux to ambient and indoor air over time, and • water movement-absence of water movement through subsurface soils results in an over-estimation of I ' air-filled porosity and vapor and contaminant flux to ambient and indoor air 3.3.3 Representation of Site Conditions: In the absence of site-specific data, the ASTM RBCA process promotes the use of conservative default values for various parameters representing site conditions This RBCA analysis consistently incorporated conservative assumptions for selection of input parameters, while attempting to maintain a reasonable, site-specific evaluation Examples of conservative assumptions ' used to formulate input parameters include The maximum chemical concentrations in groundwater over the past two years are representative of ' current and future source concentrations, The entire soil column is impacted at levels detected in confirmation soil samples, The onsite building exist directly over the locations of maximum groundwater concentrations detected ' over the past two years, Cracks exist in the foundation of the onsite building, and The paved surface has a negligible impact on vapor emission to outdoor air Use of these conservative assumptions result in an over-estimation of health risks and lower RBSLs 3 3.4 Standard Exposure Factors Standard exposure scenarios evaluated in this RBCA analysis incorporate the most likely site-specific exposure pathways and represent the greatest potential for exposure to contaminants at the site Conservative assumptions consistent with state and federal ' guidelines were used to conceptualize the exposure scenarios These methods and procedures contribute to an overall overestimation of potential exposure Numerous conservative exposure assumptions serve as the basis for exposure parameters adopted by ASTM(1995) Duration, frequency, and other input parameters were selected to represent the maximally exposed individual and are not an accurate portrayal of time spent at a place of business or residence The quantitative effect of these uncertainties would be considered significant in overestimating overall potential health risk ' 13