Remedial Action Report Construction Phase

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remainder of the treatment system, sediments in the oil/water separator were transferred into the south sparge tank. Sample S-Frac Tank/Sep was collected from the combined sediments in the tank. Both samples were submitted to the laboratory for NWTPH-Gx, NWTPH-Dx, and BTEX analysis. BTEX, diesel, and heavy oil range petroleum hydrocarbons were not detected in the samples (Table 17). Gasoline was detected at concentrations of 61.6 mg/kg and 20 mg/kg in samples N-Frac Tank and S-Frac Tank/Sep, respectively. Based on the analytical results, the sediments were disposed on- site adjacent to the treatment system at a depth of approximately five feet below street grade. 6.0 CONTAMINANT DISTRIBUTION 6.1 Soil Based on the soil analytical results from samples collected during the construction phase remedial actions, the contaminants of concern are gasoline and diesel. The soil matrix cleanup levels for the Courthouse Square property (80 mg/kg for gasoline and 500 mg/kg for diesel) were used to evaluate the extent of petroleum hydrocarbon soil contamination left in place. Of the 13 areas designated with suspected or confirmed subsurface contamination during the construction phase (Areas 2, 3, 6, 7, 9, and 10, and Areas 18 through 24), five areas have petroleum hydrocarbon contamination left in place above the soil matrix cleanup level (Areas 3, 6, 9, 18, and 22). Each of these five areas are discussed below. _ 6.1.1 Area 3 Based on field observations and the analytical results for the Area 3 soil samples, the estimated volume of mineral spirit impacted soil left in place with concentrations above the 80 mg/kg matrix cleanup level is less than four cubic yards. Figure ~ shows the estimated lateral extent of Area 3 impacted soil left in place. The vertical extent of the impacted soil left in place is limited to a one- foot thick layer of soil located between 14.5 and 15.5 feet below street grade. The absence of benzene and low concentrations of toluene, ethylbenzene, and xylenes in soil samples 3-3-14.5 and 3-4-14.5 suggests that the release may have occurred during operation of the former Harbison gasoline station between 1926 and 1961. A comparison between the BTEX concentrations in the Area 3 samples and the applicable RBCA Tier I RBSLs indicates that the impacted soil left in place does not pose a threat to human health or the environment. 6.1.2 Area 6 During pre-construction, approximately 300 cubic yards of gasoline impacted soil with concentrations exceeding the 80 mg/kg matrix cleanup level was left in place in the southeast corner of the former Chevron property (Century West 1998b). The gasoline impacted soil does not appear Construction Phase - Remedial Actions - 21 - 4080600127 CouRhouse Square November 9, 1999