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Twenty soil samples were collected during monitoring well installation at depths of 5, 10, and 15 feet bgs. Each sample was analyzed for TPHd and one sample was also analyzed for MtBE and BTEX (Table 0U5-P, soil samples 34-1 through 34-6 and 34-16 through 34-18). Results are all ND. TPHd laboratory reporting limits was below the Industrial Threshold and Protection of Groundwater SSL. Ten Hydropunch groundwater samples were also collected during monitoring well installation. Each sample was analyzed for TPHd and one was also analyzed for MtBE and BTEX (Table OU5-P, Hydropunch samples 34-1-HP through 34-6-HP and 34-16-HP through 34-18-HP). Hydrocarbons with chains greater than approximately 10 were noted as TPHueh in all but one sample at concentrations ranging from 110 to 580 ug/L. Concentrations exceed EPA Tap Water and Tier 1 Groundwater ESLS, however MtBE and BTEX were reported ND. These wells were sampled and analyzed for petroleum-related compounds during four events over an 8-year period: 1997, 2001, 2003, and 2005 (Table 0U5-P, MW-15 through MW-24). Hydrocarbons with chains greater than approximately 10 were noted as TPHueh at concentrations ranging from 360 to 3,200 ug/L in 1997; as TPHueh at concentrations ranging from 529 to 868 ug/L in 2001; and as TPHueh at concentrations ranging from 760 to 840 ug/L in 2003. No hydrocarbons were detected in 2005; however, laboratory reporting limits were 500 ug/L. Moreover, MW-24 was only sampled in 1997 and 2001, as it could not be located 2003 or 2005, and hydrocarbons were never detected in MW-23 samples during the monitoring periods. When MW-23, MW-24, and 2005 data are removed to correct for these inconsistencies, average concentrations decrease from 1,830 ug/L in 1997 to 816 ug/L in 2001 and 725 ug/L in 20033. Because laboratory reporting limits were 500 ug/L in 2005 when no hydrocarbons were detected, concentrations further declined to at least just under 500 ug/L. Regression analysis indicates a logarithmic trend (R2 = 0.92), which, if projected beyond the monitoring period, indicates concentrations would have reached 0 ug/L sometime around 2008 (Chart 0U5-P). This average declining concentration is consistent with expected natural attenuation; however, the last ND of less than 500 ug/L may still represent concentrations exceeding the TPHueh Tier 1 and EPA Tap Water ESLs of 100 ug/L, and while the R2 indicates that time alone can explain about 92% of the variance in average concentrations, the regression is based on limited data. MW-22 through MW- 24 were also analyzed for VOCs with TICs and Semi-VOCs with TICs in 2001 and 2005. All results were ND. Petroleum: Recommendations Based on the hydrocarbons detected in groundwater at concentrations exceeding the EPA Tap Water and Tier 1 Groundwater ESLs, Kleinfelder recommends field exploration to confirm that petroleum-related groundwater contamination is naturally attenuating as expected. Kleinfelder also recommends investigation to ensure contamination does not pose health risks via soil vapor. Existing data indicates potential natural attenuation over the previous observation period, but it does not clearly demonstrate that (Chart 0U5-P). Petroleum-related compounds naturally attenuate. Kleinfelder therefore generally anticipates that exploration should indicate natural attenuation; however, there is potential for unanticipated results, including TPHueh that may be more resistant to biodegradation-caused natural attenuation, or solvent contamination that may pose soil vapor health risks. 3 If MW-24 data are included, average concentrations decrease from 1,340 ug/L in 1997 to 720 ug/L in 2001, marginally increase to about 725 ug/L in 2003, and then decline to at least 500 ug/L by 2005. 20173951.001A/SAC17R68420 Page 10 of 26 November 2, 2017 © 2017 Kleinfelder www.kleinfelder.com KLEINFELDER 2882 Prospect Park Dr., Suite 200, Rancho Cordova, CA 95670 P I 916.366-1701 f I 916.366-7013