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1a' Q d WORK PLAN FOR 1 GROUNDWATER EXTRACTION PILOT TEST �09 VALLEY MOTORS 3 2 800 East Main Street Stockton, California 1.0 INTRODUCTION Investigation of soil and groundwater contamination at 800 East Main Street (Figure 1) began in 1994, when several borings were drilled to collect soil samples for laboratory analysis of petroleum-range hydrocarbons. Further drilling took place in 1995, and again in 1996,'1998, 1999, and 2003. At the completion of that work, eight groundwater monitoring wells, one vapor extraction well, one air sparging well, and numerous soil borings and vapor probes had been installed(Figure 2). The results of the investigation were subsequently summarized in a Problem Assessment Report and Soil Remediation Plan that was prepared in October 2003 and submitted to the San Joaquin County Environmental Health Department (SJCEHD). A thermal oxidation vapor'extraction system was installed at the site in mid-2005, and soil remediation began in December of that year. The remediation system operated until November 2006, when it was shut down for reasons of cost effectiveness. In the Fourth Quarter 2006 Report, we recommended removing the vapor extraction unit and replacing it with a regenerative blower and granulated activated carbon canisters for vapor destruction if further soil remediation was deemed necessary by SJCEHD. We also recommended installing a low-volume pump in monitoring well VM-2 and conducting a short-term groundwater extraction test "to determine whether further reductions in contaminant concentrations are feasible". In February 2007, SJCEHD directed Valley Motors to "install a properly screened air sparge well and conduct an air sparge/vapor extraction test". EHD also cautioned that " Other UST sites have found that GWE(i.e. groundwater extraction) is not cost effective after the city requirements were factored in to the proposal." A work plan to conduct the air sparging test was submitted in April 2007 and approved by EHD in May. The test was performed in August, and the results were reported in September.. Upgradient concluded that air sparging would not be effective at this site because the contaminated aquifer lies below a low-permeability silt-clay bed at a depth of 45-55 feet and vapor wells are unable to withdraw the sparged.vapor through the low- permeability bed. EHD concurred with that conclusion and directed Valley Motors to prepare a feasibility study of at least two other alternative remedial methods. Upgradient Environmental evaluated groundwater extraction and in-well air stripping in a report entitled Groundwater Remediation Feasibility Study in January 2008. The report identified extraction as the preferred alternative, and in February SJCEHD directed Valley Motors to submit a work plan to test the feasibility of groundwater extraction.