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Workplan for Additional Assessment and Subslab Gas Sampling Tracy Office Plaza 324 E. 11th Street, Tracy,CA November 22,2013 INTRODUCTION On behalf of Tracy Office Plaza LLC, Pangea Environmental Services, Inc. (Pangea) has prepared this Workplan for Additional Assessment and Subslab Gas Sampling(Workplan).This Workplan was requested by a San Joaquin County Environmental Health Department(SJCEHD)letter dated August 21,2013(Appendix A). The Workplan was requested to define the lateral and vertical extent of contamination identified in Pangea's Phase II Environmental Assessment Report dated July 23, 2013. Pangea proposes soil borings to assess the veritcal and lateral extent of contamination and subslab gas sampling to evaluate potential vapor intrusion into indoor air for the onsite building. SITE BACKGROUND Site Description The subject site is an office building operated by Tracy Office Plaza,LLC located at the intersection of East Street and E. 1 Ith Street in Tracy,CA(Figure 1).The site consists of offices and a large parking lot.Land use surrounding the site is predominantly commercial with residential land use present 275 ft southwest of the site (upgradient).Additional site background information is provided in the Phase I Environmental Site Assessment report.The Phase I identified concerns regarding the property being the location of a truck/auto station from the 1950s until 1973. In 1973, a permit was obtained to demolish the station and remove six USTs. No documents confirming the removal of the USTs have been found to date at the San Joaquin County Environmental Health Department or the Central Valley Water Quality Control Board. The Phase I also identified petroleum hydrocarbons north of the site at Tracy High School. The estimated groundwater direction at the Tracy High School site was northerly,indicated the subject property is upgradient of Tracy High.The Tracy High School case was closed in March 2009.The locations of four of the former six monitoring wells is shown on Figure 1. Monitoring well data and groundwater flow maps are included in Appendix B. The Phase I also recommended a subsurface investigation to determine if the source of hydrocarbons at Tracy High is the subject property. As part of a Phase II Environmental Assessment, Pangea coordinated a geophysical survey utilizing ground penetrating radar(GPR),electromagnetic utility locating techniques, and a metal detector to survey the area where the former USTs were potentially located. Geophysical surveying did not identify any USTs but did identify a potential 20-ft long underground pipe anomaly near the potential former tank location.Following the completion of the geophysical survey,Pangea coordinated the installation of six soil borings(B-1 through B-6) onsite.Hydrocarbon contamination was detected in soil and/or groundwater in borings B-1,B-4 and B-6.The investigation results were reported in Pangea's Phase II Environmental Site Assessment dated July 23,2013. 1