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S E C 0 R 7-Eleven Store#17647 Quarterly Groundwater Monitoring Report(2Q2003) July 28, 2003 Page 2 BACKGROUND On December 4, 1998, two 10,000-gallon and one 6,000-gallon single-walled steel USTs were removed from the site Six soil samples were collected from beneath the ends of the USTs, and two sidewall samples were collected, at the direction of SJCEHD (IT Corporation, January 1999) Benzene was not detected above laboratory reporting limits in the soil samples Concentrations of total petroleum hydrocarbons as gasoline (TPHg) ranged up to 1,300 milligrams per kilogram (mg/kg) at the base of the excavation, and sidewall samples did not contain TPHg above laboratory reporting limits Concentrations of methyl-tert-butyl ether (MtBE) were less than 0 02 mglkg, and lead concentrations ranged up to 6 1 mg/kg On the basis of these findings, the SJCEHD requested further assessment On June 18, 1999, five push-probe borings (GP-1 through GP-5) were advanced to depths of approximately 15 feet below ground surface (bgs) (IT Corporation, August 1999), at locations adjacent to and farther from the hydrocarbon source area (former USTs/pump islands) Soil samples collected during drilling contained up to 0 026 mg/kg MtBE, and no other detectable constituents Grab groundwater samples ranged up to 19,400 micrograms per liter (dig/L) TPHg and 126 pg/L MtBE Site closure was requested on the basis of these findings The SJCPHS requested groundwater monitoring well installation, and a workplan for this activity was submitted (IT Corporation, October 1999) On December 21, 1999, three groundwater monitoring wells (MW-1, MW-2, and MW-3) were installed to 20 feet bgs (IT Corporation, February 2000) Benzene in soil was not detected above laboratory reporting limits TPHg and MtBE concentrations is soil ranged from 0 005 mg/kg to 0 0233 mg/kg, respectively The highest soil concentrations were detected at depths of 15 to 20 feet bgs A detection of dissolved TBA(111 ug/L, MW-3) is considered anomalous, as TBA has not been detected in this well in subsequent quarterly sampling events On March 20, 2002, two additional push-probe borings (GP-6 and GP-7) were advanced to a depth of 20 feet bgs (IT Corporation, April 2002), as requested by the SJCEHD, to verify lateral plume delineation Groundwater samples were collected using a Hydropunch sampler at depths of 13 feet bgs Soil samples GP-6 @ 15' and GP-7 @ 18', and grab groundwater samples GP-6 @ 13' and GP-7 @ 13', did not contain petroleum hydrocarbons above laboratory reporting limits MONITORING AND SAMPLING PROCEDURES The depth to water was measured in monitoring wells MW-1 through MW-3 from top of casing (TQC) using a water level indicator to within 0 01 feet bgs Dissolved oxygen readings were also collected from each well using a YSI Model 550A dissolved oxygen meter equipped with a down hole sensor Well purging and sampling equipment is thoroughly cleaned prior to purging and sampling each well The sampling procedure for each well includes measuring the water level and checking for the presence of liquid-phase hydrocarbons (LPH), using either an electronic indicator and a clear Teflon® bailer or an oil-water interface probe Wells not containing LPH are purged of approximately three casing volumes of water (or to dryness) using a submersible pump or bailer The equipment and purging methods used for the current sampling event are noted on the field data sheets included in Attachment A During purging, temperature, pH, and electrical conductivity are monitored in order to document that these parameters are stable prior to collecting samples After purging, water levels are allowed to recover to 80% of the original levels prior to collection of the water sample Groundwater samples are collected using a disposable Teflon" bailer, placed into appropriate EPA- 17647 03-2Q Report doc SECOR lntematronal Incorporated