Assessment and Remediation of Vapor Intrusion to Indoor Air at South Mesa State Superfund Site, Gilbert, Arizona

James N. Clarke, R.G., MACTEC Engineering and Consulting, Inc., Harry O’Neill, Beacon Environmental Service, Inc., Joseph E. Odencrantz, Ph.D., P.E., Beacon Environmental Service, Inc.

ABSTRACT:

Vapor intrusion to indoor air from volatile organic compound (VOC) contamination in the subsurface is increasingly becoming a more important exposure pathway when developing site conceptual models and ultimately obtaining site closure. Until recently, this exposure pathway was not often considered during site characterization. The direct measurement of the vapor intrusion to indoor air pathway is typically difficult to perform due to sample collection methods and interferences to samples such as ambient air. In order to perform valid measurements, a thorough understanding of the site and use of multiple characterization tools are necessary. A cost effective three-phase approach to assess the vapor intrusion to indoor air pathway at the South Mesa Water Quality Assurance Fund (Arizona State Superfund or WQARF) Registry site in Gilbert, Arizona was implemented.

 

INTRODUCTION

The subject site is a former metal plating facility located within the boundaries of the South Mesa Water Quality Assurance Revolving Fund (Arizona State Superfund) Registry Site (SMWRS). In 1983, PCE was detected in an irrigation well located approximately 500 ft (152 m) downgradient of the site and was immediately taken off-line, though it was periodically sampled. Operated as a containment pumpand treat well from 1994 to 1997, after which the well was permanently taken off-line. In 1985, a sample collected from the irrigation well contained 780 ug/L of PCE. A second irrigation well, located approximately 1.5 mi (2.4 km) downgradient of the site, also had detections of PCE. Preliminary investigation, involving sampling of production wells and the installation of 10 monitoring wells, identified an approximate 1.5 mi long (2.4 km) by 0.5 mi (0.8 km) wide PCE groundwater plume apparently originating from the subject site (Figure 1).

 

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