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0 0 Stockton Avenue Groundwater Extraction System Shutdown Request July 7, 2010 Former Nest16 USA, Inc. Facility, Ripon, CA The report concluded that: • TCE and other COCs in the B-zone at Stockton Avenue have been sufficiently reduced, • Additional mass removal via groundwater extraction is unlikely to result in further reductions in COC concentrations within the B-zone, and • The presence of TCE and other COCs in the C-zone at Stockton Avenue is attributable to migration of COCs from the City of Ripon sewage treatment ponds. The following is a brief summary of the analysis presented in that report, as well as additional information to justify the shutdown of the Stockton Avenue System. B-Zone ECM completed installation of two B-zone monitoring wells (M-526 and M-53B) on December 22, 2009 at 519 South Stockton Avenue (Figure 1). During drilling of the boreholes prior to well installation, ECM collected depth-discrete groundwater samples using Simulprobe technology which were analyzed for TCE and its daughter products. The results of these analyses were compared with the concentrations reported for samples collected from nearby monitoring wells. The depth-discrete groundwater sample data collected in December 2009 from wells M-52B and M-536, and submitted in the Installation Report, indicated the presence of relatively low levels of TCE and cis-1,2-DCE (all sampling results were less than or equal to 10 pg/L), consistent with other B-zone wells in the vicinity of the 519 South Stockton Avenue site. The most recent groundwater monitoring data from the April 2010 monitoring event are provided in Table 1 and further illustrate the point that a continuing source of TCE is not present in the B-zone due to low concentrations which are very close to or below the applicable MCLS. A review of historical data from the extraction wells and nearby monitoring wells indicates that the Stockton Avenue System has successfully remediated the target area surrounding this location and TCE concentrations have reached asymptotic levels in E-2 and M-86 (Figure 2). Since the TCE concentrations in the Stockton Avenue area have reached asymptotic levels, further extraction at well E-2 is unlikely to significantly reduce concentrations in this area. Furthermore, the remaining low-level concentrations in the target area indicate there is no remaining TCE source present from the historic sewer line release. As such, containment is no longer needed to protect groundwater in this area. Therefore, ECM requests RWQCB concurrence to cease extraction from well E-2. C-Zone ECM competed installation of one C-zone monitoring well (M-54C) on December 9, 2009. The well is located south of the Stockton Avenue System and north of the City of Ripon sewage treatment ponds on Fifth Street(Figure 1). ECM collected groundwater samples and analyzed for TCE and its daughter products immediately after installation. As discussed in greater detail below, the sampling results from M-54C, submitted with the Installation Report, are consistent with the chemical analyses from other C-zone wells closer to the City of Ripon sewage treatment ponds. This indicates that COCs in the Intermediate Aquifer under the Stockton Avenue site are attributable to CDCs migrating from the City of Ripon sewage treatment ponds, which is consistent with the SCM. The most recent data, collected in April 2010, are provided in Table 2. 2