SITE INFORMATION AND CORRESPONDENCE FILE 1

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Adequate plume definition is illustrated by data from wells MW-1,MW-2,MW-3,MW-4,and MW-7. In combining its knowledge of contaminant transport mechanisms, contaminant source location, and the consistent direction of groundwater flow, Chevron is able to make certain inferences about the areal distribution of the contaminant plume. For instance,the width of the plume perpendicular to the prominent flow direction should be shorter than the downgradient migrational distance. This inference is supported by actual groundwater results. Before their screens became submerged, the concentrations measured at cross-gradient wells MW-1 and MW-2 appeared to be lower than those measured at downgradient well MW-7. The inferred plume shape is demonstrated by there being higher concentrations at MW-7 which, is a greater distance from the source area than wells MW-1 and MW-2. Additional support for the inferred plume shape is the stable-decreasing trend in contaminant concentrations observed at wells MW-1 and MW-2. This indicates the contaminant plume is no longer migrating in the cross-gradient direction and that the migrational distance is limited. On the other hand, the results obtained from well MW-7 (and MW-5)suggest the plume may not be stable and that downgradient migration may still be occurring. Therefore, the migrational distance is greater and no limit has been established. Another inference that can be made about the plume shape/contaminant distribution is that there should be no groundwater contamination upgradient from the known source. Considering the consistent flow direction and source location(former UST complex), wells MW-3 and MW-4 would be located upgradient. Again, the inference is supported by site-specific data as historic groundwater data indicates no groundwater contamination at either of these wells. The groundwater investigation data for the subject site does not allow Chevron to determine the most cost- effective plan of remedial action. This is mainly due to there being no information available concerning the location of any sensitive receptors or the downgradient extent of contaminant migration. Both deficiencies should be corrected upon the implementation of Pacific's work plan and work plan addendum. It should be pointed out that the deficiencies noted do not relate to the monitoring and reporting program at the subject site. The historic groundwater monitoring data from the subject site supports inferences that were made based on site specific information and on contaminant transport theory. The fact that actual field data supports scientific theory indirectly suggests that there is adequate definition. Therefore, considering the above arguments Chevron requests that your agency reconsider the following, scientifically-based changes to the monitoring and reporting program. To assist with your reconsideration, rationale are provided along with each recommendation. -Gauge and sample wells MW-1,MW-2,MW-5,and MW-7 semi-annually (lstQ,4thQ) Wells 5 and 7 will assist in tracking downgradient plume migration. Wells 1 and 2 will track any plume variations in the cross-gradient direction. Monitoring wells 1 and 2 will also confirm plume stability in those directions. The semi-annual frequency is more than enough considering that the plume is adequately defined and that contaminant migrational velocity is less than 3 ft/yr. -Discontinue monitoring wells MW-3,MW-4,and MW-6 Wells 3 and 4 have consistently been non-detect. The repeated measurements of such are redundant and would provide no further value to understanding plume conditions. Well 6 is further cross-gradient than well 1. Since cross-gradient plume migration has been shown to be stable/limited, data from well 6 would be redundant to data from well 1 and unnecessary. -Analyze all samples for TPHG,BTEX,and MTBE Theca nnnlvtac raflart the rnntnminnnte of rrnnrrarn