Forensic Analysis of Subsurface Vapor Intrusion to Indoor Air and Background Conditions at a Former MGP Site in New Jersey

Geosyntec's vapor intrusion investigation program includes comprehensive steps to ensure that subslab samples measure only subsurface vapors and are not influenced by ambient air. Geosyntec's vapor intrusion investigation program includes comprehensive steps to ensure that subslab samples measure only subsurface vapors and are not influenced by ambient air.

A former manufactured gas plant (MGP) site was redeveloped into a multi-unit commercial plaza, and an assessment of the potential for subsurface vapor intrusion to indoor air was required to comply with new Guidance from the New Jersey Department of Environmental Protection (NJDEP). The various businesses located with the commercial plaza included a hardware store, restaurant, and beauty salon, all of which were expected to have interior sources of various chemical vapors. In addition, outdoor air in parts of New Jersey occasionally exceeds the target concentrations for benzene, which is an expected constituent of MGP wastes . In order to comply with the NJDEP Guidance, without erroneously attributing background contributions to subsurface vapor sources, Geosyntec designed an investigative program and conducted forensic analysis of the data to distinguish the relative contributions of subsurface, interior and outdoor sources to indoor air vapor concentrations.

Geosyntec's Scope of Services

Geosyntec was retained to design, and interpret data collected from, an investigation of the potential for vapor intrusion. The study consisted of multi-level soil gas sampling, indoor air monitoring during and outside of business hours, and outdoor air quality monitoring. Consumer products and outdoor air contamination were the dominant sources of 40 detectable VOCs in indoor air, 14 of which were detected at concentrations above risk-based criteria. Some compounds detected in indoor air samples are also constituents of MGP wastes, including benzene and naphthalene, so a forensic analysis was necessary to determine the relative proportion from subsurface and background sources. The forensic analysis included graphical presentations of compound ratios, spatial and temporal trends, comparisons to typical background levels, and multi-component analyses.

Notable Accomplishments

Geosyntec prepared a report with a detailed forensic analysis to demonstrate that the primary source of the chemicals in indoor air was interior sources, occupant's activities and outdoor air quality. The contribution from subsurface MGP materials was negligible compared to background sources, and posed no significant health risk. NJDEP concurred with Geosyntec's analysis, and required only confirmatory monitoring.

Project Overview

  • Location: Newton, New Jersey, USA
  • Client: Jersey Central Power & Light Company
  • Project Practice Areas: Contaminated Sites, Air Quality
  • Type of Facility: Former MGP Site
  • Services Provided: Subslab Sampling; Indoor and Outdoor Air Sampling; Risk Assessment; Forensic Analysis; Regulation Liaison
  • Type of Work: Vapor Intrusion Assessment
  • Company: Geosyntec Consultants
  • Governing Regulation: State Regulations
Geosyntec Consultants
Geosyntec is a consulting and engineering firm that works with private and public sector clients to address new ventures and complex problems involving our environment, natural resources, and civil infrastructure.

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