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SECTIONTHREE T®chnlcal Approacd • documented The purpose of the exposure assessment is to arrive at conservative, yet realistic, estimates of potential exposures and,therefore, approximate real-life exposure c ircurnstances The chemical concentrations at exposure points will be calculated for average and reasonable maximum scenarios sure point concentrations (both average and RUE) for the assessment of Expo P � � ) present risks will be derived, where possible, from existing chenucal data for soil and groundwater samples Environmental fate and transport modeling for groundwater transport and potential volatile emissions will be conducted in order to assess future potential exposure point concentrations The modeling will be performed in accordance with U S EPA modeling guidance, California Department of Toxic Substances Guidance, and ASTM guidance Once receptor point concentrations have been estimated, chemical intake by the receptor can be estimated Depending on the complete exposure pathways identified for the site, receptor point concentrations may be required for air, groundwater, and soil If only exposure routes to be analyzed are soil ingestion and dermal absorption of chemicals due to contact with soil, the only receptor point concentrations required are the concentrations in soil For Shop 10, since groundwater concentrations, will need to be assessed for future potential concentrations and related health risks, fate and transport modelling will be conducted There are two approaches for the fate and transport modeling at the site that are being proposed 1)ASTM Risk Based Corrective Action modeling, which uses supple analytical models, and/or 2)a more complicated fate and transport model, like the Jury Behavior Assessment model for the unsaturated zone linked,with a groundwater transport model ASTM RBCA model allows for alternative points of exposure(i e a potential receptor point can be set at different distances from the source) Alternative groundwater well distances can also be set to estunate what the future groundwater concentrations of chemicals of concern may be under conservative worse case assumptions ASTM RBCA is based on a tiered evaluation of sites that have been designed to match the site evaluation effort to the relative risk or complexity of each site Tier 1 of the RBCA process compares generic nsk based screening levels with maximum data concentrations in different media. Tier 2 of the RBCA process calculates site-specific target levels to protect human health at the source zone or at a separate point of exposure using site-specific data input Tier 3 of the RSCA process calculates site-specific levels to protect human health or ecological resources at complex points of exposure using more detailed site-specific data on transport mechanisms and • receptors With each tier upgrade the degree of uncertainty and conservatism involved in the cleanup standard calculation is reduced Because of the concern regarding the impacts to groundwater and � � 0 T%I907W7300DNA D=7-AUC;-p 73MMAMA 3-5