SITE INFORMATION AND CORRESPONDENCE

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Ivi. Scott Mansholt _ 2 Mountain House Site #1 . 26 March 2017 PAH concentrations in 25 of 30 borings reported detections at or slightly above the method detection limits for the respective compounds, Phenanthrene and pyrene were the two most commonly reported PAHs and were present in 32 of 43 samples analyzed with concentrations ranging from 0.0004 to 4.7 mg/kg. Benzo(a)pyrene and dibenzo(a,h)anth racene, at concentrations ranging from 0.0003 mg/kg to 0.12 mg/kg and from 0.0007 mg/kg to 0.074 mg/kg, respectively, was reported in 20 of the 30 borings completed. Similar to BTEX compounds, elevated PAHs are commonly found with elevated TPHc concentrations. Groundwater Depth to groundwater at the project is approximately 12 to 18 feet and the gradient is north- northeast. TPHd was detected in groundwater samples collected for analysis (approximate sample depths were between 12 and 22 feet) with values ranging from 1,200 pg/I to 4,200,000 pg/I (boring MH1-19); TPH as gasoline (TPHg) concentrations ranged from 380 pg/I to 6,400 pg/l (boring MH1-10). Benzene, with one exception of 0.1 pg/1, was not detected; toluene, ethylbenzene, and xylenes reported concentrations at or slightly above the detection limits. Generally, the elevated petroleum hydrocarbon compounds concentrations in groundwater correspond to samples having elevated TPH in soil. Concentrations of PAHs were detected at or slightly above the laboratory reporting limits in each of the 17 groundwater samples submitted for analysis. The highest PAH concentrations were reported in groundwater sample MH1-24 (TPHd @ 100,000 pg/1). PAHs were detected in MH1-24 at concentrations ranging from 7.5 pg/I benzo(a,h) anthracene to 130 pg/I pyrene. Benzo(a)pyrene and dibenzo(a,h)anthracene were not detected at the respective method detection limit of X1.9 pg/I. Comments Based on review of the January 2005 report, Regional Water Board staff concurred in our 8 March 2005 response to Chevron, the extent of affected soil and groundwater had been adequately delineated. According to the report, affected soil is limited in extent with approximate dimensions of 150-feet by 200-feet. The extent of affected groundwater generally coincides with the outline of affected soil. Both the affected soil and groundwater occurs within the Byron Road and Union Pacific Railroad (UPRR) right-of-ways, primarily beneath Byron Road, the UPRR tracks, and two existing pipelines located next to the tracks. The data indicates the majority of affected soil and groundwater occurs adjacent to both sides of the OVP and beneath Byron Road; the former OVP is along the southwest side of Byron Road. Vertically, soil and groundwater impacts are defined by non detect for TPH compounds in samples obtained at 48 feet from boring MH 1-1. Additionally, in our March 2005 letter, Regional Water Board staff concurred that removal of impacted soil is not generally feasible given that the majority lies beneath the railroad, Byron Road, a portion of Mountain House Creek, and the new highway bridge located within the Mountain House Development site # 1 project.