2007-11-20_REVISION - M1982015 (14)

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5.1 Quarry Pond #1 (Sunset Lake) On January 25, 2007, benzene was detected in the sample collected at the staff gauge near the southeast corner of the pond with a concentration of 20 micrograms per liter (ug/L). Due to the static and stagnant nature of the water body, this impact was considered to be localized. Low levels of benzene and other compounds were detected in subsequent samples until air-sparging was initiated in the pond on February 27, 2007, to mitigate this impact. No BTEX compounds were detected after pond sparging was performed. Water samples were collected from the southwest, southeast, and northeast corners of Quarry Pond #1 to verify the absence of BTEX compounds in preparation for pumping of water from this pond into the St. Vrain River. No BTEX compounds were detected during any of the samplings. 5.2 Quarry Pond #2 (Big Lake) Quarry Pond #2 was almost completely drained for construction during the time of the release and subsequent work. A staff gauge was placed in the southwest corner of this pond where groundwater was seeping into the pond from the direction of the St. Vrain River. No BTEX compounds were ever detected from the seep. However, on February 27, 2007, during the completion of the slurry wall near sm 23+00, approximately 10 cubic yards of soil excavated from the slurry wall trench was inadvertently pushed over the pond bank. Based on sampling of this material discussed in Section 4.4, it was possible the soil contained BTEX compounds. ' Work was stopped on the slurry wall to remove this material from the corner of the quarry pond. A small sump was excavated in the southwest corner of the pond to capture watery portion of the material that flowed down the steep embankment to the ' pond. Water samples were collected from the sump and a rivulet that formed from the sump. Both samples were found to contain low levels of benzene and xylenes. However, during subsequent samplings of the sump, no BTEX was found in water samples and the rivulet was dry. A soil sample was also collected from the pond bank where the soil spilled into the pond. No BTEX compounds were detected in the sample. The laboratory reports from these samples are included in Appendix 4. 5.3 Quarry Pond West The unnamed quarry pond located to the west of Sunset Lake was to be pumped for construction purposes. The pond and an effluent sample during pumping to the St. Vrain River were sampled. No BTEX compounds were found in the samples. The results of the sampling are included on Table 3. The laboratory reports are included in Appendix 4. 5.4 St. Vrain River Sampling The St. Vrain River has been sampled repeatedly at areas upgradient and downgradient to the release area. No BTEX compounds have been found in any of the samples collected since sampling was started on December 18, 2007. The results of the sampling are included on Table 3. The laboratory reports are included in Appendix 4. 7