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Elmet. ' VAPOR INTRUSTION PATHWAY EVALUATION An evaluation was conducted to determine if concentrations of monitored constituents in groundwater Present a risk to human health via the vapor intrusion pathway Several constituents have been monitored at the site for over ten years Most of these constituents are often not detected or detected at low levels Therefore, this evaluation has been conducted to determine whether any of the wells or constituents can be eliminated from further monitoring This section describes the methodology and presents the results of the evaluation Derivation of Screening Levels Groundwater screening levels were derived using the California Environmental Protection Agency (CalEPA) version of the Johnson and Ettinger Model'(HERD, 2005) The model is used to derive the concentration of a constituent in groundwater below which no adverse human health effects would be expected from the vapor intrusion pathway The following assumptions were used in the modeling • Construction type is slab-on-grade (depth below grade to bottom of enclosed floor space = 15 10 cm) o Depth below grade to water table = 34 5 feet (1,051 cm) This is the shallowest depth to groundwater measured between July 2004 and April 2005 • Soil type directly above water table to the ground surface is silt © Because the area is non-residential, standard occupational exposure parameters were used o Exposure Frequency=250 days (USEPA, 1991) o Exposure Duration = 25 years(USEPA, 1991) + Hexane was used as a surrogate for Total Petroleum Hydrocarbons as gasoline (TPHg) This is a conservative assumption as hexane is likely more volatile than TPH S All other default assumptions in the model were used The model calculation spreadsheets are included as Attachment B The following groundwater-to- indoor air screening levels were derived J%ProjeWati08940Z098 2931WorWng%Reportz%Letter 1 Juty 2005 Requesting Reduction in SamphngWapor Intrusion Pathway Evaluation-Letter doc