SITE INFORMATION AND CORRESPONDENCE

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Subject: 503 E 10th St. The EHD has reviewed Soil Vapor Sampling and Human Health Risk Assessment Report, just received. The main concern with the site is a lack of both a true down-gradient- monitoring well and a monitoring well in the former :tank pit, but overall the site doesn't appear to, present'a high risk to groundwater as is discussed below. The vertical extent of impacted soil in the former UST pit was established with soil °boring SB-1 at 15 and 20 feet, the.former 'ND'., the latter had just a bit of xylenes. Most of the sorbed contaminant mass is at or near 10 feet below surface ,grade, coincident with the water table which ranges from approximately 7.2 to 12.6 feet blow the top of the well casing, most commonly near 8.5 feet bsg. Although soil is impacted beneath the former UST pit, it appears to be: of limited lateral extent and very limited vertical extent — the EHD estimates that. it is approximately 5 feet thick in the former UST area and 2 to 3 feet thick laterally away from the former UST area. The EHD made a mass estimate calculation, coming up with approximately 628 lbs TPH (ignoring soil impacted with less than 1000 mg/kg TPHd) and 1.3 Ib benzene - in reasonable agreement with the 813 Ib estimate of July 1999 which probably considered the whole sorbed plume. As noted above, the main concern with this site status is the lack of monitoring wells in the core area and in the apparent direct downgradient direction; there are two wells, MW2 and MW3, which are obliquely downgradient relative to the ' -source area and the inferred contemporary groundwater flow direction. Review of the soil and grab groundwater data from the .ten soil borings advanced in December 1997 shows why the well locations were selected'. MW2 was located 'between the intensely impacted SB3 and S138 locations, which suggest early ,plume migration toward the northeast, so MW2 should have been the downgradient well. MW3 is located near S137 toward the northwest; SB7 was ,moderately impacted, more so than S135 directly northward of the former UST pit. MW 1 was located near SB6, which encountered moderately impacted groundwater and is upgradient of the former UST pit. The contaminant ' concentrations in soil and groundwater suggested that the main contaminant mass was just east of the. former UST pit near SB, and that contaminants migrated primarily toward the northeast and secondarily toward the northwest. Considering these data, the location of MW2 seems quite appropriate for the time of installation. During the first two groundwater monitoring events immediately following its installation, contaminants at detectable concentrations were not encountered in groundwater samples collected from MW2,. despite the high 'concentrations detected in the soil and grab groundwater samples collected from the adjacent S138. This suggests that the plume of dissolved contaminants was 4