ARCHIVED REPORTS XR0009817 - Laserfiche WebLink

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r rTPH as gasoline and ethylbenzene were detected in MW-2 at concentrations of 120 ppb and 15 ppb Xylenes were detected in MW-3 at 0 57 ppb No petroleum hydrocarbons were detected in MW-4 The analytical results of the groundwater samples are summarized in Table 2 The laboratory report and chain-of-custody are included in Appendix B r5.0 CONCLUSIONS & RECOMMENDATIONS r5.1 Groundwater Flow Direction and Gradient Since March 1993, the groundwater flow direction and gradient have remained consistent r at the site During the same period, groundwater depth has remained within 15 feet of surface grade at the site r5.2 Petroleum Hydrocarbon Impacted Groundwater Petroleum hydrocarbons decreased dramatically in MW-1 between March and June 1994, rfrom 5,100 ppb to 560 ppb Concentrations of BTE&X also showed similar decreases However, TPH as gasoline and ethylbenzene were detected in MW-2 for the first time in June 1994 Previously, no petroleum hydrocarbons have ever been detected in samples from rMW-2 Xylenes were also detected in MW-3 for the first time (0 57 ppb), just above the method detection lima (MDL) of 0 50 ppb The low concentrations of petroleum hydrocarbons detected in samples from MW-2 and MW-3 are probably not representative of site conditions it is unlikely that the hydrocarbon plume is migrating towards MW-2 or MW-3 The petroleum hydrocarbons detected in MW- 2 and MW-3 could have resulted from a laboratory or sampling error Furthermore, the concentrations of toluene, ethlybenzene or xylenes detected at the site do r not exceed the State of California Maximum Contaminant Levels (MCL's) or Action Levels (AL's) The AL for toluene is 100 ppb, the MCL for ethylbenze is 680 ppb, the MCL for r xylenes is 1,750 ppb No MCL's or AL's have been established for TPH as gasoline or diesel Only the concentration of benzene in MW-1 (22 ppb) exceeds the MCL (1 0 ppb) r The overall reduction of petroleum hydrocarbons at the site may indicate that the removal of a significant volume of impacted soil at the site is having a positive effect on groundwater quality The petroleum hydrocarbon plume continues to be sufficiently defined at the site and does not appear to be migrating significantly The down-gradient well (MW-4) remains uncontaminated I At the present time, active groundwater mitigation is not warranted Since impacted soil still remains at the site, remediation systems that target both soil and groundwater should be considered A vapor extraction/air sparging pilot study may be considered as recommended in the March 1994 Quarterly Report, if desired However, based upon the dramatic reduction of petroleum hydrocarbons, further monitoring may be more cost effective c.olepnl Audra S.,I.c. p 661 SRTEWAR—M BPDXON OR 691 8 I