Remedial Action Report Construction Phase

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5.9 Drywells During the construction phase, five drywells (labeled DW-3 through DW-7) were encountered throughout the Courthouse Square property. Each drywell was approximately three-feet in diameter, constructed with bricks, and extended to approximately 13 feet below street grade. Figure 4 shows the location of the drywells. No visual or olfactory evidence of contamination was noted in the soil surrounding any of the drywells. No groundwater was observed in the excavations. After the drywells were removed, soil samples DW3-1-13.0, DW4-1-13.5, DWS-1-13.5, DW6-1-12.5, and DW7-1-13.5 were collected beneath drywells DW-3, DW-4, DW-5, DW-6, and DW-7, respectively. The soil samples were analyzed for NWTPH-HCID with quantification, PCBs, HVOCs, and leachable cadmium, chromium, and lead. No petroleum hydrocarbons, PCBs, HVOCs, or leachable metals were detected in the soil samples collected beneath drywells DW-4, DW-5, and DW-6 (Table 14). Diesel was detected at a concentration of 25.3 mg/kg in soil sample DW7-1-13.5, significantly below the soil matrix cleanup level of 500 mg/kg. Methylene chloride, a common laboratory contaminant, was detected at a concentration of 0.21 mg/kg in soil sample DW3-1-13.0. The EPA Region IX preliminary remediation goal formethylene chloride on industrial sites is 18 mg/kg. No record of the historical on-site use of this compound was identified in the ESAs. Based on this information, the detection of inethylene chloride in the soil sample is attributed to laboratory contamination. Low levels of leachable cadmium and lead were also detected in sample DW3-1-13.0. 5.10 Discharge Monitoring In accordance with the NPDES permit and City of Salem requirements, effluent water was monitored prior to discharge into the storm drain. Discharge monitoring included discharge flow measurements and effluent water sanlpling. Petroleum hydrocarbons and BTEX were not detected at concentrations exceeding the permit cleanup limits. Table 16 summarizes the analytical results for effluent water sampling. A total of approximately 1,413,645 gallons of water was discharged into the storm drain. Based on the discharge monitoring data, Marion County was in compliance with the NPDES permit. Copies of the monthly discharge monitoring reports are provided in Appendix B. 5.11 Treatment System Sediment Disposal Approximately two cubic yards of sediments accumulated in the oil/water separator and the sparge tank during operation of the treatment system. Prior to decommissioning the north sparge tank, sample N-Frac Tank was collected from the sediments in the tank. Prior to decommissioning the Construction Phase - Remedial Actions - 20 - 4080600127 Courthouse Square November 9, 1999