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Mr. M. Scott Mansholt-CE• • ' Addendum to the Work Plan for Additional Site Characterization October 5,2006 Mountain House Development Site#3 Page 2 of 3 ' Two water supply wells exist within 2,000 feet of the PAH plume(Figure 1, Well ID Numbers 2 and 3). The tops of the screened interval for these wells are both 75 feet below ground surface (bgs), respectively, and as the vertical extent of affected groundwater at Site#3 is undefined to a ' depth of approximately 50 feet bgs, there is the possibility that the water-bearing zone for the wells and the affected groundwater are hydraulically connected(Table 1). The water well uses are currently unknown. However, the use of the wells as drinking water sources is not likely because the Total Dissolved Solids(TDS) level in groundwater at Site#3 is approximately 1,500 milligrams per liter(mg/L), which is well above the U.S. Environmental Protection Agency (EPA)and California Secondary Maximum Contaminant Levels of 500 mg/L. A third water well was located at 18500 Henderson Road, approximately 2,000 feet north of the PAH plume(Figure 1, Well ID No. 1). However, the home at that address has been demolished and the land is now owned by Trimark Communities,LLC(Trimark), which is developing the ' 251-acre parcel for future residential home construction. According to Trimark's environmental consultant, Condor Earth Technologies, Inc., the well was likely destroyed,plugged,and abandoned. WORK SCOPE CHANGES In order to define the extent of dissolved-petroleum constituents in groundwater and identify any ' potential health risks associated with Site#3, the following additional site characterization activities are proposed: • Sample the water supply well(Well ID No. 3) located on Assessor Parcel Number(APN) 209-150-32 to which CEMC has a Site Access Agreement(SAA), and submit the samples to a laboratory for the following analyses: o Volatile Organic Compounds by EPA Method 8260B using a 25-milliliter purge ' for HPP Risk Assessment Detection Limits; o TPHd(CIO-C25)by EPA Method 8015 Modified with and without silica-get cleanup; o Semi-Volatile Organic Compounds by EPA Method 8270 C large volume injection for HPP Risk Assessment Detection Limits; and ' o TDS by EPA Method 160.1 Modified. Additionally, SAIC will verify the current presence, locations,and uses of wells No.2 and No. 3 with the site owner(s)by means of telephone interviews and/or conducting "door-to-door"visits. Note, a SAA to APN 209-160-03 that would enable the sampling and analysis of groundwater from Well No.2 will be pursued,pending the water well survey results. • Advance three up-gradient borings—MH3-10 through MH3-12(Figure 2)—from which groundwater samples will be collected for laboratory analysis of the above-listed ' constituents of concern and methods. • Advance five down-gradient borings—MH3-13 through MH3-17(Figure 2)—from which groundwater samples will be collected for laboratory analysis of the above-listed ' constituents of concern and methods. Because water well sampling is involved and to facilitate the use of groundwater analytical results in a Human Health Risk Assessment(HERA) (only if the groundwater is determined to be