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2.19 Discussion and Estimate of Rennaining CoptWnirrjant Mass in Soil aild Groundwater Versus CoiltaIiii ant_Mass Remove Wright has prepared the following rough estimates for residual contaminants. Residual in soil is assumed to be that revealed in soil samples collected from the B-1 through B4 samples in 2003 and Boring B-5 through B-9 in 2008. Assuming an area about 80 feet wide near the generator building and about 80 feet long north of the former UST and capillary fringe/upper aquifer thickness of about 5 feet or about 32,000 ft', a soil density of 1 l8 lb/ftp from the 2003 study, and average of the five B-5 to B-9 TPHD soil samples from about the 9 to 10 foot depth (assuming 4.4 mg/kg average throughout this soil volume)from Tables 2 and 4,the residual soil estimate is (4.4 mg/kg x 32,000 ft' x 118 lb/ft)�- ib./6.8 gal=2.4 gal. TPI-ID TPHG was not detected in the soil samples and so is assumed to be essentially zero. The laboratory reported that the 2008 data did not exhibit a typical Diesel chromatographic pattern. Wright interprets this as an indication that petroleum degradation processes are operating within the shallow aquifer and have degraded the contaminants, Assuming that the residual in groundwater is measured from the monitoring well data from Table S, an area around the former UST pit and monitoring well of 80 feet long 80 feet wide and 10 feet thick in the shallow aquifer an effective water filled porosity of 25% in that volume, and the M)N"-1 concentration at 51 micrograms per liter (ug/1) of October 22,2009 is present throughout, a 3 51 trail x 64,000 ft x 0.25=-,0.0004 gal.TPHD However TPHD, TPHG and BTEX were not observed in the September 2010 sampling, so the contaminants are minimal and are interpreted to have declined to essentially zero. Wright interprets that contaminants have been removed .from, groundwater by natural petroleum degradation.mechanisms and processes. calcitlationshi any risk assessmata (SOIL VAPOR ASSESSMENT) Neither a soil vapor assessment performed nor soil vapor samples collected for this site. r A high of 33,000 ug/1 TPHD was reported in a grab groundwater sample collected from the site in 2003. A grab sample collected in 2008 from within seven feet of the 2003 sample location was reported to contain only 170 ug/l TPHD. The highest contaminant level observed in soil at the W WTP in 2003 and 2008was ll mg/kg TPHD and 33,000 ug/1 TPHD respectively,and a total of about 26 rag/kg from the 2008 borings. Petroleum hydrocarbon oonstituents were not reported in the last groundwater sampling event. In addition, the UST source was removed in 1998, and the groundwater tlory direction is northwesterly, away from any buildings that might be impacted by soil 4889/CSRJRcpt Page 9 of 17 E c f