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Attachment B Site History Previous Environmental Work December 1999 Monitoring Well Installation and DWR Well Search by IT Corporation On December 9 and 10, 1999, IT Corporation installed four groundwater monitoring wells, MW-1 through MW-4, to delineate the lateral and vertical extent of hydrocarbons in soil and groundwater. Soil samples were collected every five feet from each boring during installation and groundwater samples were collected following development of the wells. No hydrocarbons were detected in any soil samples. No TPHg was detected in any groundwater samples. Benzene was detected in MW-3 at a concentration of 0.61 µg/L. Methyl tertiary butyl ether(MTBE)was detected in groundwater samples from wells MW-3 and MW-4 at concentrations of 3 µg/L and 5 µg/L, respectively. IT Corporation conducted a file review on Department of Water Resources (DWR)well logs to identify any wells within a 2,000-foot radius of the site that could be potentially impacted by groundwater conditions at the site. One well was located approximately 1,200 feet north and crossgradient of the site. The construction details or current use of the well were unable to be determined. Further information of site assessment activities and the well search are available in IT Corporation's February 25, 2000, Site Assessment Report. July 2000 Soil Borings by IT Corporation On July 20 and 21, 2000, IT Corporation advanced four soil borings, GP-1 through GP-4, to delineate the vertical extent of hydrocarbons in soil and groundwater. GP-1 through GP-3 were advanced to approximately 60 fbg and GP-4 was advanced to approximately 30 fbg. Soil samples were collected every five feet and grab-groundwater samples were collected at select intervals from each boring. TPHg was detected in soil samples at a maximum concentration of 4,570 mg/kg in GP-2 at 40 fbg. Benzene was detected at a maximum concentration of 21.1 mg/kg in GP-3 at 60 fbg. Toluene, ethylbenzene, and total xylenes were detected at maximum concentrations of 121 mg/kg, 74.5 mg/kg, and 270 mg/kg, respectively, in GP-2 at 40 fbg. No MTBE was detected in any of the soil samples. Maximum hydrocarbon concentrations in groundwater were detected in GP-2 at concentrations of 128,000 µg/L for TPHg, 9,490 µg/L for benzene, 17,700 µg/L for toluene, 3,700 µg/L for ethylbenzene, and 17,400 µg/L for total xylenes. MTBE was only detected in GP-3 at a concentration of 12.8 µg/L. Further information is available in IT Corporation's August 30, 2000, Additional Site Assessment Report. November 2000 Monitoring Well Installations by IT Corporation On November 27 and 28, 2000, IT Corporation installed three groundwater monitoring wells, MW-5 through MW-7, to further evaluate the lateral and vertical extent of impacted soil and groundwater at the site. All three wells were installed to approximately 80 fbg and screened from 70 to 80 fbg. Soil samples were collected every five feet during well installation and groundwater samples were collected following well development. TPHg was detected in soil samples from MW-5 and MW-6 at a maximum concentration of 4,780 mg/kg in MW-6 at 60 fbg. Benzene and toluene were only detected in soil GHD I Well Installation Work Plan Addendum I Attachment B 111193145(05)I Page 1