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Groundwater Monitoring Work Plan 1950 West Fremont Street,Stockton, California (PEA) investigation in January 200 (first phase)and August 2006(second phase)to address the findings and issues originating from DTSC's site inspection. The results of PEA investigation indicated elevated levels of chemicals exceeding the regulatory criteria, such as California Maximum Contaminant Levels (MCL), in the groundwater beneath the site (Figure 3). As presented in the Final Preliminary Endangerment Assessment Report(Final PEA Report)dated August 15,2007, the groundwater analytical data indicated the following: • Dissolved nickel was detected in groundwater at concentrations up to 0.794 milligrams per liter (mg/L), which is above the 0.1 mg/L California MCL for nickel. Groundwater with dissolved nickel concentrations exceeding the MCL is localized to the site and has not migrated from below the building. • N-Nitrosodimethylamine (NDMA) was detected in one grab groundwater sample at a concentration of 8.8 micrograms per liter (µg/L) (or 8.800 nanograms per liter Ing/L]) in January 2005. Using a more appropriate NDMA analytical method with ultra-low detection limits. subsequent investigation at the same location in August 2006 showed a significantly lower concentration of 37.8 ng/L. The maximum NDMA detection in grab groundwater samples in August 2006 was 103 ng/L. The California Notification Level for NDMA is 10 ng/L. • Methyl tert-butyl ether (MTBE) was detected in several grab groundwater samples collected in August 2006 at relatively low concentrations (maximum of 16 gg/L). The California Primary MCL for MTBE is 13 pg/L. In a letter dated October 4, 2007, DTSC suggested that quarterly groundwater monitoring be conducted for a minimum period of one year and that the work plan for groundwater monitoring be submitted for review and approval. 1.2 INVESTIGATION PURPOSE AND OBJECTIVES The purpose of this investigation is to further evaluate the quality of the groundwater at the site. where chemicals have been previously found at concentrations exceeding regulatory criteria. The proposed groundwater monitoring program is designed to meet the following primary objectives: • Verify that groundwater with nickel. NDMA and MTBE at concentrations exceeding regulatory criteria is not migrating off-site, and • Evaluate the trend of nickel, NDMA and MTBE concentrations in groundwater over a period of four consecutive quarters to assist with evaluating site closure. Project 102790 3 December 2007