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Quik Stop Market No. 132 January 6, 1993 Project No. F7132.34 Page 6 aquifer beneath the Quik Stop site, were not detected in wells MW-6 and MW-7, which monitor the upper beneath the Taco Bell site (see Plates 3 and 4 and Table 2). Laboratory analysis of soil samples collected from MW-6 and MW-7 did not detect any TPHG or BTEX constituents (see Table 1). Analysis of soil samples collected during the installation of monitoring wells MW-1R and MW-5 (Exceltech, July 1991) on the Quik Stop site, did not detect any TPHG or BTEX constituents. This data would indicate that the observed petroleum hydrocarbon constituents are confined to the aquifer and have not affected the overlying soils. Prior to groundwater sampling, depth to groundwater measurements were obtained. ' The resultant groundwater elevation contour maps generated from these measurements differs from historical measurements (Plates 5 and 6). The reason for these variations ' may be related to the fact that during the time of the well installation, development and sampling, the area was experiencing heavy winter rains. The presence of asphalt covered areas and open planted areas as well as the locations of subsurface drains and utility lines can affect the infiltration of rainwater into the groundwater. This can cause short term variations in the apparent groundwater flow directions. However, the groundwater flow direction has historically been to the east (RESNA, June 1992; October 1992; November 1992). Groundwater samples were collected from monitoring wells MW-IR, MW-2, MW-5, MW-6, and MW-7. TPHG and BTEX were not detected in the groundwater samples from wells MW-1R, MW-6, and MW-7. As in previous quarters a discrete nonpetroleum hydrocarbon peak was detected in the samples from MW-l R and MW-5. In the September quarterly analysis (RESNA, November 1992) this peak was identified as methyl-tert-butyl-ethane (MTBE), a non-metallic (lead free) antiknock compound. As MW-IR and MW-5 are located downgradient from previous excavation activities at ' this site (see Site Background, Appendix G) the presence of such a compound is not unexpected. Detectable concentrations of TPHG and BTEX have never been detected in groundwater samples collected monitoring wells MW-IR, MW-3 and MW-4. In fact, analysis of groundwater samples collected from these wells have yielded eight consecutive quarters in which TPHG and BTEX have not been detected (see Table 2). However, in the case of MW-1R, MTBE was detected in September 1992 (Table 1). MTBE is a volatile, highly mobil gasoline related compound which can result in it preceeding the arrival of ' other, less mobile, gasoline constituents. Due to this situation, we recommend continued quarterly sampling and analysis of MW-1R. However, on the basis of an ' established history of nondetectable concentrations of gasoline related compounds in MW-3 and MW-4, RESNA recommends that Quik Stop Markets be permitted to discontinue sampling and analysis of these two wells. This recommendation is made in compliance with the State of California, LUFT Manual, October 1989, page 61. Low concentrations of the petroleum hydrocarbon related constituents benzene and toluene were detected in the groundwater sample from MW-2. These compounds were detected in low concentrations in March and September 1992. MW-2 is located