SITE INFORMATION AND CORRESPONDENCE_1994-1996

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Wendy L. Cohen -2- 5 May 1994 7. Separate phase hydrocarbons (SPH) extraction systems should be installed to remove SPH in MWs 8, 10, 11, 13, 15, 16, 17, and other areas or wells where SPH exists. 8. Appendix A of the RAP did not provide the basis for the extent of the SPH plumes in SSA 1 and 2. For example, Pacific assumed that the SPH plume covers 10% of SSA 1. Based on the presence of SPH in MWs 10, 11, and 15-17, the SPH area appears to be closer to 30% than 10% of SSA 1. 9. In Appendix B, the RAP used ground water velocities of 0.016 ft/day'.and 0.19 ft/day, which were obtained from Harding Lawson Associates' (HLA) 3 May 1991 Preliminary Hydrogeologic Assessment, to estimate the distance traveled by the unit volume. These values, which were determined from slug tests, provide estimates of aquifer characteristics only for a small area since the test is highly dependent on well efficiency. If the well screen is clogged or corroded, measured values will be inaccurate. Conversely, if the well is developed by surging or backwashing prior to testing, the measured values may reflect the increased conductivities induced in the gravel pack around the intake. A pumping test, on the other hand, provides aquifer properties which are averaged over a large and representative aquifer volume. Results of aquifer tests for the site are presented in HLA's 3 February 1992 Evaluation of Groundwater Extraction and should be used in all the mass transport calculations. Using the average hydraulic conductivity (k) of 19 ft/day obtained from the pumping test will result in an average ground water velocity of 0.5 ft/day. Consequently, the distance traveled by the unit volume from the holding pond will be 8,000 feet instead of 3,000 feet which is way beyond the boundaries of Hooper Drive and James Drive. I concur with Appendix B's statement that biodegradation has inhibited the south/southwesterly expansion of benzene. However, since the vertical extent of contamination has not been defined, downward migration of benzene and other contaminants also needs to be investigated. 10. The RAP evaluated three alternatives and recommended Alternative 3 to remediate the site. Alternative 3 proposes to establish an effective treated ground water disposal method, implement a soil vapor extraction (SVE) system, continue ground water extraction using the existing system, and continue ground water monitoring. No ground water extraction or treatment has taken place since April 1993. Therefore, the ground water extraction and treatment system must be restarted. The existing three-well extraction system will not capture the southern extent of the dissolved plume represented by hydropunch samples 3, 6, 9, 14, 20, and 25. Also, it will not entirely capture SSA 3. Shell should evaluate more thoroughly the merits of air sparging to effectively remediate contamination in the saturated zone. Air sparging would supplement the SVE system which is a cost-effective method of removing volatile organic compounds from contaminated soils in the vadose (unsaturated) zone. Plume Characterization Investigation Report Due to unresolved issues pertaining to the cause of increased TPH concentrations west of Hooper Drive, we requested in our 10 February 1994 letter that Shell proceed to remediate only the areas east of Hooper Drive. These are areas which have clearly been impacted by Shell's activities, and their remediation should not be hindered by uncertainties regarding contaminated areas whose sources are not clearly identifiable at that time.