Tuesday, November 6, 2007 : 9:30 a.m.

Using Indoor MTBE Measurements for Soil Vapor Intrusion Investigations

Bernard Bloom, M.S., CIAQP, Building Sciences & Engineering Associates Ltd. and Charles Wilkes, PE, Ph.D., Building Sciences & Engineering Associates Ltd

Soil vapor intrusion (SVI) investigations that follow multi-tiered approaches often include the measurement of VOCs inside buildings over suspected gasoline-contaminated subsurfaces as one of the initial steps. Where gasoline leakage is suspected, screening analytes will include BTEX. Risk-based analysis is usually driven by benzene. Ambient benzene concentrations have fallen to less than 0.30 ppb in much of the U.S. This fact, coupled with very low analytical detection limits, permits resolution of benzene SVI increments inside homes to a risk equivalent of ~4x10-6 lifetime cancer risk.

Interpretation of indoor VOC measurements is confoundable by the presence of BTEX in consumer products, including benzene from indoor cigarette smoking and stored gasoline.  MTBE is unique: it is not present in consumer products. MTBE was phased-out in California (post-2002), and in the Northeast, the Mid-Atlantic, and Texas (in 2006) and therefore appears indoors primarily from SVI.  We present ambient data that show a 10 to 40-fold decline in ambient MTBE during the course of 2006. Now, indoor MTBE measurements can be used as an additional tracer for gasoline-sourced SVI, where MTBE had been the RFG oxygenate.

We illustrate the utility of MTBE as a SVI tracer, using a study of about 100 homes that we conducted during the summer of 2006. Fifty six of these homes were judged to have some degree of benzene intrusion (without using MTBE to reach that conclusion). However, using a 3:1 indoor/outdoor MTBE ratio, we found approximately 45% of these 56 homes were also experiencing MTBE intrusion. Before the MTBE phase-out, the comparable percentage would have been approximately 15%. 

Indoor MTBE measurements can now be used as an additional analytical tool in areas in which MTBE had been the RFG oxygenate, provided the analyst carefully takes into account the relative subsurface properties of MTBE and other chemicals of concern.

Bernard Bloom, M.S., CIAQP, Building Sciences & Engineering Associates Ltd. Mr. Bloom is a principal in Building Sciences & Engineering Associates. His career has been divided between government (federal and local) and the private sector, and has addressed ambient and indoor air quality topics and aerospace contamination control. He has a broad range of indoor air quality technical capabilities, including experience in low-level VOC sampling in vapor intrusion circumstances. He was a principal investigator in the 2006 Riggs Park Air Quality Study in Washington, DC, a vapor intrusion study of benzene and perchloroethylene. His work has included schools, commercial, and residential buildings. and chemical sensing for building protection systems.

Charles Wilkes, PE, Ph.D., Building Sciences & Engineering Associates Ltd Dr. Charles R. Wilkes, PE, PhD is a principal in Building Sciences & Engineering Associates. He has extensive experience in indoor environmental exposure assessment, and IAQ problem investigations, including soil vapor intrusion. Dr. Wilkes is both involved with a practical indoor environmental consultancy and in development of a comprehensive model for assessing human exposure/dose to indoor pollutants in multi-zone indoor environments. Earlier work for USEPA’s OPPT involved development of the windows-based version of the Multi-chamber Concentration and Exposure Model (MCCEM), which models emissions, transport and decay of chemicals from various household consumer products.


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Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation® Conference