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at 26 feet bgs The low level soil contamination (2-4 ppm TPH-G) observed in the soil samples from 30 5 and 40 5 feet bgs may be partially a result of cross contamination from the contaminated groundwater in the auger through which the samples were collected The certified analytical laboratory results from the two hydropunch water samples indicate that groundwater has been impacted at historically lower groundwater elevations than are currently be measured in the on-site monitor wells Both water samples from THI (THI-WATER 3T-35' and THI-WATER 43'-45') showed lower TPH-G and benzene concentrations than water samples collected from nearby monitor well MW1 during the most recent quarterly groundwater monitoring round MW 1 is screened between 18 and 32 5 feet bgs The concentrations of TPH-G and BTEX constituents observed in the samples from the three depth intervals are sufficiently different to suggest that the hydropunch water samples represent true contaminant concentrations at those depths rather than cross contamination from shallow groundwater that had entered the borehole prior to sample collection This is further confirmed by the presence of MTBE (a highly soluble constituent) in the water samples from MWI between 18 and 32 5 feet bgs and THI between 33 and 35 feet bgs and the absence of MTBE in the water sample collected from THI between 43 and 45 feet bgs If the groundwater contamination observed in the hydropunch sample from 43-45 feet bgs in THI were merely an artifact from the leakage of contaminated shallow groundwater into the hydropunch bailer (through the hollow rods or disturbed soil at the base of the boring) then the water sample from 43-45 feet bgs should have contained measurable concentrations of MTBE It is more likely that the gasoline discharges that occurred during historically lower groundwater elevations did not contain MTBE as an oxygenant RECOMMENDA'T'IONS 1) Implement Wm J Hunter & Associates "Workplan for Removing Contaminated Soil" dated October, 1997 Remove contaminated soil to a depth of 26 feet bgs 2) Ilistall two additional monitor wells to measure TPH-G, BTEX, and MTBE concentrations at depths belowthe current monitor well slotted intervals Install one monitor well at the center of the groundwater contamination plume following removal of contaminated soil to 26 feet bgs and one well downgradient of shallow groundwater plume Collect hydropunch water samples during the drilling of tile Boreholes for each monitor W611 at 5 foot intervals below 40 feet bgs and screen water samples on-site with a portable chromatograph to establish the required total depth of the monitor wells for measuring historical groundwater contamination Complete the two monitor wells with slotted casing from 30 feet bgs to the total depth of the wells Ms LIMITATIONS The discussion presented in this report is based on the following 1 The observations and data collected by field personnel 2 The results of laboratory analyses performed by a state certified analytical laboratory 3 Our understanding of the regulations of San Joaquin County and the State of California Changes in groundwater conditions can occur due to variations in rainfall, temperature, local and regional water use, and local construction practices In addition, variations in the soil and groundwater conditions could exist beyond the points explored in this investigation State Certified Laboratory analytical results are included in this report This laboratory follows EPA and State of California approved procedures, however, WEGE is not responsible for errors in these laboratory results 5 SANCHEZ SRPT 5/11/98