SITE INFORMATION AND CORRESPONDENCE

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r • • Mr. Paul Supple Arco Station No. 6100 25775 Patterson Pass Road Page 4 of 6 potential receptor of contaminants released from the UST system, the well may actually be supplying a transport mechanism to facilitate migration of contaminants laterally and vertically. Prudence, however, dictates that sampling of the domestic well should continue at this time. If this model is valid, each sand unit intersecting the domestic well filter pack below 40 feet bsg or other recharge source is a potential migration pathway. Other potential sources would include the storm water retention basin and possibly the septic tank along the northern margin of the site. The occurrence of free product in MW-1 appears to be the result of free product migration under the same lateral migration mechanism described above for ground water. The well is screened through a sand interval extending downward into a fine-grained unit. Free product may be migrating as a thin layer or thick film along the sand/silt contact or interface under the influence of gravity down slope in the direction of MW-1. An important factor to consider to optimize capture of the product would be to characterize the attitude of the sand/silt contact and strategically place recovery well(s) to intercept the migrating material. Another consequence of the model presented above, if valid, would be that wells with screened intervals through more than one sand interval could be providing a vertical conduit for contaminant migration. Vapor well VW-15, located roughly between the dispenser area and MW-1 connects two, possibly three sand units, bottoming in the sand unit penetrated by MW-1 (Cross section B-B', SAWI Report). The possibility of this well being the conduit to the deeper sand unit should be investigated. As VW-15 bottoms in the sand unit, this model predicts that the well will be dry. Any liquids in the bottom cap would be informative, however W-15 was reported as having dirt in it in 1QR02. EHD recommends that the ideas presented above be tested and modified as appropriate by your consultant, or that another model be constructed. EHD's analysis of the data is cursory and EHD acknowledges that it may be possible to construct other models that can account for the data obtained to date equally well or better. Additional cross sections that include all borings and screened intervals should be prepared that correlate as many sand units as possible. If sufficient data is available, it may be possible to contour the sand/silt contacts and position wells for more efficient recovery of free product and impacted ground water. A site conceptual model prepared by your consultant should form the basis and provide the justification for future site investigatory activities. EHD is concerned that borings east of the UST pit may not be deep enough to show that the lateral extent of the plume of impacted soil and ground water has been defined. The lower sand that apparently conducted free product to MW-1 appears to have not been penetrated, except by B-33, which had low concentrations of benzene in samples collected below the sand unit. As drawn by Delta on cross section B-B'