SITE INFORMATION AND CORRESPONDENCE

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Lee Higgins -2- 6 May 2011 No Further Action Required Determi&n Mountain House Deveiopment Site#3 Setting: Rural agricultural with some nearby residential properties under development. Source: Historical leak from crude oil pipeline(s) installed in the early 1900's and removed from service by the late 1960's to early 1970's. Actions: Preliminary site investigations were conducted between 2001 and 2008. • In 2001 , sixteen exploratory borings were advanced and soil samples collected were analyzed for TPH constituents of concern. Affected soil was detected in five borings at depths between 5.5 and 48 feet below ground surface (bgs) with TPHc concentrations from 480 to 4,500 milligrams per kilogram (mg/kg). A small area of approximately 100 feet across at 35 feet bgs in the northern part of affected soil contains a limited amount of separate phase crude oil. Benzene, toluene, ethylbenzene, and xylenes (BTEX) were non-detect (ND). Low concentrations of polynuclear aromatic hydrocarbons (PAHs) were detected at concentrations up to 0.7 mg/kg (in sample from 21.5 feet bgs). Groundwater, encountered at approximate depths of 6 and 14 feet bgs was sampled. TPH compounds in one sample were ND, A petroleum sheen was noted in three of the samples; and, PAHs were detected in the samples having a TPH sheen. • During 2004, an additional investigation included nine borings to depths between 35 and 58 feet bgs to obtain samples for analysis to delineate affected soil and groundwater. TPHc concentrations detected in soil samples from depths between 2 and 54 feet bgs ranged from ND to 4,700 mg/kg. BTEX compounds were ND. Concentrations of PAHs were ND in most samples; but, were detected at/or above method detection limits (MDLs) in samples from borings drilled within areas of affected soil and/or groundwater. Analysis of groundwater samples detected TPH as diesel (TPHd) at ND and 480 micrograms per liter (pg/L); remaining sample results reported elevated TPHd concentrations at locations near affected soil. borings containing elevated TPH present in soil. Additional characterization was needed. • In September 2007, ten soil borings were drilled; five downgradient to the northeast and three upgradient to the south-southwest to further define the extent of affected soil and groundwater and evaluate water quality. Results of soil samples were ND for TPH, BTEX and PAH constituents. TPHd concentrations ranged from ND to 98 mg/kg, which is below screening levels. The extent of crude oil affected soil and groundwater was considered adequate. • SAIC completed an HHSE in August 2008 and evaluated analytical results of samples from 0 t o10 feet bgs in accordance with DTSC's PEA Guidance Manual for a hypothetical future onsite resident as the most conservative estimate of risk. The HHSE also included an evaluation of risk to future construction maintenance workers. In accordance with the PEA guidance, the HHSE used analytical results for toluene and certain PAH compounds for 23 samples from 15 borings having crude oil affected soi{ from depths of 2 to 10 feet bgs. The HHSE found that the onsite cancer risk and non- cancer hazard are below the respective de minimus levels. SAIC concluded the TPHc and related TPH compounds detected should not result in an unacceptable health risk for an adult or child onsite resident or construction worker. • In October 2010, SAIC provided a No Further Action Required Request Report (N FAR) for consideration of site closure. The NFAR included documentation of site conditions in accordance with the April 2004 Appendix A -- Tri-Regional Board Staff