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SS E C 0 R 7-Eleven Store#14117 Quarterly Groundwater Monitoring Report(2Q2003) August 27, 2003 Page 2 Current Quarter Analytical Data (See Figure 3 and Table 1) Maximum TPHg Concentrations MW-3 27,000 ug/L Maximum Benzene Concentrations MW4, 56 u91L _ Maximum MtBE Concentrations MW-4, 170 uq/L Maximum TBA Concentrations MW-1 27,000 ug/L BACKGROUND On July 17, 1998, three 10,000-gallon single-walled USTs were removed from the site The UST excavation was extended to the east, and two new 10,000-gallon double-walled steel USTs were installed The new tanks were installed in a tank pit located east of the original UST locations One ten-inch diameter Schedule 40 PVC conductor casing was installed in the new UST pit between the new tanks for future installation of well MW-2 On September 29 and October 2, 1998, Fluor-Daniel GTI advanced six offsite (OSGP-1 through OSGP-6) and 11 onsite (GP-7 through GP-17) Geoprobe soil borings The offs1te borings did not contain total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and total xylenes (BTEX), or methyl-tert-butyl ether(MtBE) in soil above laboratory reporting limits The onsite borings reported benzene, TPHg, and MtBE at maximum concentrations of 12 5 milligrams per kilogram (mg/kg) (GP-17@20'), 42,900 mg/kg (GP-15@15'), and 234 mg/kg (GP-14@10'), respectively (Fluor Daniel GTI, 1998) On May 13 and 14, 1999, four groundwater monitoring wells (MW-1 through MW-4) were advanced to 51 feet below ground surface (bgs) Benzene, TPHg, and MtBE in soil were reported at maximum concentrations of 2 89 mg/kg (MW-3-15'), 429 mg/kg (MW-3-15'), and 0 119 mg/kg (MW-4-20'), respectively (IT Corporation, 1999) On October 30, 2001, three groundwater monitoring wells (MW-5, MW-6, and MW-7) were advanced to 20 feet bgs TPHg, BTEX, and MtBE in sod were not detected above laboratory reporting limits, except for 0 006 mg/kg total xylenes (MW-7@20') Selected halocarbons also reported in the soil samples indicated possible dry cleaner plume contributions (IT Corporation, 1999) An active USA Gasoline (former Exxon) service station is located east of the site MONITORING AND SAMPLING PROCEDURES The depth to water was measured in monitoring wells MW-1 through MW-7 from top of casing (TOC) 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- 14117 03-2Q Report doc SECOR lntematronal incorporated