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S E C ® R 7-Eleven Store#21756 Sampling Reduction Request December 2, 2003 Page 2 r analytical data indicated dissolved concentrations of benzene (up to 0 32 ug/L), TPHg (up to 55 ug/L), MtBE (up to 230 ug1L), and tert-butyl alcohol (TBA) (3,710 ug/L) The reported TBA detection appears to be anomalous, as TBA has not been detected in subsequent quarterly samples Down- gradient limit of MtBE plume is defined by MW-4, which was not installed until 2002 On April 1-3, 2002, monitoring well MW-4 was installed to 30 feet bgs and screened from 15 to 30 feet bgs (Shaw, August 2002) Dual-nested sparge/vent wells (SPV-1 and SPV-2)were advanced to 29 feet bgs, and screened from 12 to 22 feet bgs as a soil vapor extraction (SVE) well, and 24 to 29 feet bgs as a sparge well A Geoprobe sod boring (GP-6) was advanced to a depth of 40 feet bgs Soil samples collected from GP-6 and MW-4, and a grab groundwater sample from GP-6, did not contain BTEX, TPHg, and fuel/oxygenates above laboratory reporting limits Historical groundwater analytical data is included as Table 1 An SVE test was conducted on April 24, 2002, by application of vacuum to well SPVA Magnehelic gauge measurements of wells MW-1, MW-2, MW-3, and SPV-2 indicated an estimated radius of vapor extraction influence of 25 to 30 feet around the extraction point A second SVE test was conducted October 17, 2002 Vapor extraction was performed on SPV-2 and MW-1, which resulted in an estimated radius of influence of 25 to 30 feet • GEOLOGYIHYDROGEOLOGY } The shallow stratigraphy in the area consists of Quaternary alluvium deposits Materials underlying the site consist largely of fine to medium-grained sands, which are interbedded with clayey silt below about 13 feet bgs Depth to groundwater (DTW)as first encountered was approximately 13 to 19 feet bgs During the current groundwater monitoring event, equilibrated depth to water ranged from 20 44 to 23 82 feet bgs Groundwater consistently flows to the northwest with an average gradient of 0 002, as shown in Table 2 and Figure 3 CONCLUSIONS AND RECOMMENDATIONS The USTs and associated fuel lines and dispensers were removed from this site in December 1998 Monitoring wells were installed in January 2000, and dissolved hydrocarbon concentrations in groundwater have been shown to be stable or decreasing in all wells Groundwater has been shown to flow consistently to the northwest, and groundwater collected from the downgradient well (MW-4) has been below the laboratory reporting limit for all analytes since the installation of the well in April 2002 (except for 0 78 ug/L of total xylenes in July 2002) Residual concentrations of dissolved hydrocarbons in groundwater have decreased to near or below the laboratory reporting limit in samples collected from MW-2, MW-3, and MW-4, and are stable in MWA (Graphs MW-1 through MW-4) Based on the data presented above, SECOR recommends annual groundwater monitoring and sampling If, at any time, the data indicates an increase in hydrocarbon concentrations, or any concentrations above the action level are detected in groundwater collected from MW-4, SECOR will immediately resume sampling on a quarterly basis 21756 Request for Sample Freq Reduction doc SECOR Intemational Incorporated