Groundwater Volatilization Model. Evaluation of the inhalation exposure routes requires estimations of indoor and outdoor air concentrations based upon the amount of chemical contamination present in groundwater. For each groundwater COPC, the Xxxxxxx and Xxxxxxxx Volatilization model (EPA, 2000b) was used to estimate indoor air concentrations and a volatilization factor (VF), taken from the ASTM methodology (ASTM , 1995), was used to calculate the fraction of each chemical present in the contaminated media that would evaporate to the air (outdoor). Details of the Xxxxxxx and Xxxxxxxx model can be found in the USEPA Users Guide (EPA, 2000b). The EPA Xxxxxxx and Xxxxxxxx (EPA, 2002) spreadsheet cannot be utilized due to the number of chemicals and number of xxxxx that must be evaluated to generate the risk isopleths. Consequently, only the Tier 1 Infinite Source equations from Xxxxxxx and Xxxxxxxx (as implemented by EPA, (EPA, 2000)) were extracted and. used to develop a database model capable of handing the large amount of site groundwater data. The ASTM VF equations for outdoor air were also incorporated into the database model. Assumptions required for use of the building parameters employed in the volatilization model are detailed in Table 14. It was assumed, based upon projected ]and use, that the typical structure that will be constructed would be a three story, 90,000 square foot (30,000 square feet per floor) office/research building. Nominal dimensions were 200’ by 150’ by 40’ tall. Planning factors for office buildings typically allocate a total of 250 square feet person (including personal office space as well as common areas such as conference rooms and break rooms). ASHRAE Standard 62-1999 (ASHRAE, 2000) sets a ventilation rate of 20 cubic feet per minute (cfm) per person for office buildings. Multiplying the ASHRAE ventilation rate times the floor space requirements, and converting units, results in a ventilation rate of 3,400,000 cm3/sec. VFs were calculated based on models and recommendations presented by the American Society of Testing and Materials (ASTM) in Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (ASTM, 1995). ASTM makes use of a simple, yet conservative, approach to the prediction of outdoor ambient air concentrations by the use of a “box model.” The box model is a simple mass-balance equation that uses the concept of a theoretically enclosed space or box over the area of interest. The model assumes the emission of compounds int...
