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WORKPLAN FOR ADDITIONALC�OUNDWATER ASSESSMENT and the City of Stockton Fire Department(#00-91). The former UST location is now beneath a newly paved staff and student parking lot(Figure 2). The soil sampling and analysis completed when the UST was removed identified concentrations of MTBE in the subsurface soil. Upon obtaining soil sample results following the UST removal, SJCEHD requested that UOP conduct an investigation to determine the lateral and vertical extent of soil and groundwater potentially impacted by MTBE. On behalf of UOP,ESA submitted the Soil Investigation Workplan to the SJCEHD on June 8, 2000 for review, and UOP received workplan approval from Ms. Carol Oz(SJCEHD)on June 19,2000. With assistance of a direct- push soil-sampling rig,ESA obtained soil and groundwater samples from various sampling locations on July 5,2000 in accordance with that workplan and prepared the Subsurface Soil and Groundwater Assessment Report,dated August 25,2000. In that document, ESA reported that one soil sample obtained from soil boring TP-4 at 15 feet bgs had a concentration of 11 parts per billion(ppb)of MTBE. Laboratory analysis did not identify concentrations above the detection limits for total petroleum hydrocarbons as gasoline(TPH-g)or benzene,toluene,ethylbenzene and xylene(BTEX). Of the five groundwater samples analyzed, only two concentrations of MTBE were detected: 1,300 ppb in soil boring TP-4, and 1.4 ppb in TP-5. Based on these v findings,ESA concluded that the MTBE impact to groundwater was localized with minimal lateral extent. The SJCEHD responded to the Subsurface Soil and Groundwater Assessment Report in a letter received September 18, 2000 and requested that a workplan be submitted to further evaluate and define the contaminated groundwater at the former UST site. On behalf of UOP,ESA met with the SJCEHD on October 10, 2000 to propose a phased approach to assess the extent of MTBE in the groundwater. ESA has prepared this workplan based on previous discussions with the SJCEHD. SAMPLING AND ANALYSIS PLAN The objective of this phase of investigation,as proposed under this workplan, is intended to further define the extent of MTBE in the groundwater near the former UST. ESA will accomplish this by completing two tasks: 1)a literature search to determine, from previous investigative work,the typical groundwater flow direction and gradient in the site vicinity, and 2)groundwater sampling and analysis. �r The two proposed tasks should provide information to determine the direction of groundwater flow and concentrations of MTBE in a contaminant plume,if one is present. The findings of this additional assessment will determine the need for further investigative work such as groundwater well installation, sensitive receptor surveys, and utility surveys. For example,if the groundwater sampling indicates a significant MTBE plume,then the installation of monitoring wells and a survey of underground utility trenches may be warranted. The need for additional phases of assessment work will be assessed in the report prepared as part of this workplan. v v Environmental Science Associates 2 University of rhe Pacific November 2000 Workplan For Additional Site Assessment 200252 y