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pack around the well screen. A typical method for well development is to surge or jet water or air in and out ofthe well screen openings. A properly developed gravel pack keeps fine sediments out ofthe well and provides a clean and unrestricted flow path for groundwater. Further defining the vertical extent ofgroundwater contamination will aid in supporting the argument for low ` risk site closure criteria#2 and#3 and will also aid in construction of the new downgradient well,i.e., determining the required depth of the well and location of the well screen. 4.3.4 Risk Based Corrective Action The ASTM's standard for Risk-Based Corrective Action (RBCA), ASTM E-1739-95, details a ` framework and provides a methodology to perform a three-tiered risk analysis at petroleum release sites. This methodology incorporates EPA risk assessment practices to determine non-site specific and site ` specific cleanup levels that are protective of public health and environmental resources. Traditional assessment and remedial action uses a"forward calculation"approach to evaluate the carcinogenic and non-carcinogenic health risks that maybe posed from exposure to chemical concentrations in specific �. media at a site. The RBCA approach starts with accepted risk and"back calculates"allowable levels of chemicals in the impacted soil and groundwater. The calculation is done using accepted risk assessment and fate and transport protocols. Tier 1 ofthe RBCA process consists ofthe following main components: l)initial site evaluation to identify potential human and ecological receptors and potentially significant transport pathways;2)classification of the site to evaluate the need for immediate response; and 3)comparison of media-specific chemical concentrations detected at the site against conservatively calculated Risk-Based Screening Levels(RBSLs). Should site maximum chemical concentrations exceed the Tier 1 RBSLs,then a Tier 2 evaluation is appropriate for those constituents. A Tier 2 evaluation may calculate Site-Specific Target Levels(SSTLs)using site-specific data and/or recommended alternative compliance points. In calculating SSTLs,the fate and transport equations used for the Tier 1 evaluation or other accepted models may be used. A Tier 3 evaluation uses site-specific data and more complex fate and transport models and/or statistical simulations. The RBCA approach should be taken at the site to address low risk site closure criteria#5 and#6. For �- this analysis,the groundwater ingestion pathway does not need to be considered as the groundwater is not currently used as a source ofdrinking water or projected to be used as a source ofdrinking water within the life ofthe plume. In addition,RBCA does not have any specific guidance on evaluating environmental ` risk, although the basic framework is appropriate if site specific exposure pathways and ecological receptors are included. The site does not have the potential to significantly impact surface water,wetlands, ` or other sensitive receptors, and therefore can be considered for low risk closure. 16