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09 January 2017 • • AGE Project No. 00-0769 Page 2 of 9 the potential of off-site soil vapor concentrations, as the intrusion risk to occupants has already been determined as low threat. AGE maintains it is premature to install off-site soil-vapor wells for the following: • Soil gas data should first be collected on-site, in the areas of known petroleum hydrocarbon-impacted soil and groundwater, to determine and evaluate if there is a risk of soil vapor intrusion at the site; • Soil gas samples were previously collected in March 2010 near the residences at 21, 27, and 36 West Jackson Street and 21 East Jackson Street to evaluate the vapor intrusion risk to occupants; total petroleum hydrocarbons quantified as gasoline (TPH-g), benzene, toluene, ethylbenzene, and total xylenes (BTEX) were not detected in any of the soil gas samples collected during the March 2010 investigation (Table 1); and • The SWRCB's Review Summary Report (RSR) September 2016 recommends soil vapor samples be collected near the on-site buildings; the RSR does not recommend collecting off-site soil vapor samples. Based on the above reasoning and technical justification, AGE recommends collecting soil-vapor samples, on-site before collecting additional or duplicate off-site soil-vapor samples; and then evaluating the results of the on-site soil-vapor samples to determine if off-site samples are necessary. If off-site soil-vapor samples are determined to be necessary, AGE will prepare an addendum to this work plan in a timely manner proposing collection of off-site soil-vapor samples. However, because off-site soil-vapor samples were previously collected, and were non-detect, AGE is concerned that if on-site soil- vapor samples are also non-detect, installation and sampling of off-site soil-vapor wells will not be considered a legitimate and necessary cost by the SWRCB and thus the costs for the work will not be reimbursable. The scope of work includes the advancement of four (4) off-site soil borings for collection of multiple grab groundwater samples from each boring; and advancement of two (2) on- site soil borings for the installation of soil-vapor wells to evaluate potential vapor intrusion risk at the site. The work plan provides a description of the scope of work and procedures to be used for soil boring advancement, groundwater sample collection and analysis, soil- vapor well installation, soil-vapor sample collection and analysis and boring abandonment. Additional site background information, historical analytical results of groundwater samples and water elevations, and a summary of results of recent groundwater sampling at the site were presented in the AGE-prepared report, Semi-Annual Groundwater Monitoring Report— Second Quarter 2016 dated 29 August 2016. Advanced GeoEnvironmentsl Inc.