SITE INFORMATION AND CORRESPONDENCE_1985-1997

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5606 Pacific Ave. page 2 feet south of the former dispenser islands and were not in the area previously over- excavated in 1988. MW-10 and MW-13 are both north and border the up-gradient areas of the previously impacted area and are consistently reporting non-detect for the groundwater contaminants. MW-11 and MW-12 are south and border the down-gradient area of the previously impacted area and are consistently recording the highest levels in the groundwater for this site. My field notes during MW-12's installation indicated that petroleum odors and discoloration of the soil were noted to begin at 59.5 feet and continued down to 71 feet bgs. Soil data for the three closest borings in between these borders comes from B-4, B-5, and MW-2. Unfortunately, the deepest soil data for all of these borings is from only 26 feet bgs for B-4 and B-5, and 41 feet bgs from MW-2, as depicted in Table 2 that you included in your last correspondence. All of the data indicated non-detect for TPH-G and the BTEX constituents. With this situation presented, you can now see that some further investigation in the area in between the borders is needed in order to verify if a source is present. You mentioned a Risk Based Corrective Action (RBCA) approach for this site. In preparing this site for acceptance, it is important that certain aspects of the site classification are successfully addressed. In ASTM ES38-94 Tier 1, the user is required to identify contaminant sources, obvious environmental impacts, if any, the presence of potentially impacted humans and environmental resources (for example, workers, residents, water bodies, etc.) and potential significant transport and exposure pathways (for example groundwater flow, atmospheric dispersion, vapor migration through soils and utilities such as sewers and electrical conduits etc.). In addition, a vertical delineation of any potential contaminant source is to be included in the ASTM-RBCA approach. With documented contamination at 71 feet bgs in the area of MW-12 and no soil analyses from the in between area, up-gradient of the impacted monitoring wells, your next phase of investigation should be aimed at identifying and evaluating the vertical limits of any soil that could be impacting the groundwater. Accurate and correctly screened groundwater monitoring wells are also required to successfully evaluate the RBCA approach. Monitoring wells not screened above and below the groundwater they were constructed to monitor, are considered to be inadequate and will be required to be replaced.