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Groundwater Monitoring Report for the Semiannual ARCADIS Period from January 1 through June 30,2012 Lincoln Center, Stockton, California from well to well during a single monitoring event,reflecting the uncertainty in water level conditions in this area.Typically,only the wells(i.e., MW-2038-2 and MW-203B-3)that are consistent with regional groundwater elevation data are used for contouring. Figure 5 presents groundwater elevation contours for the C-zone aquifer based on water levels measured at the Site on June 11 and 13,2012.The potentiometric surface map indicates that C-zone groundwater beneath the Site generally flows to the northeast, and that the magnitude of the horizontal hydraulic gradient in the C-zone aquifer was approximately 0.0017 ff/ft.This flow direction and gradient are consistent with previous monitoring events. 2.2 Groundwater Sampling Methodology and Quality Control/Quality Assurance During the reporting period,26 samples were collected from the Annual Event Wells in accordance with the RWQCB-approved groundwater monitoring plan presented in Section 1.3. In addition, a sample was collected from MW-212B outside of the RWQC®- approved groundwater sampling schedule, in order to further assess the extent of the plume. All groundwater samples were collected using passive diffusion bag (PDB)technology. Use of PDBs for periodic groundwater monitoring was approved by the RWQCB in a letter from Ms.Marie McCrink,dated April 17,2006. Each PDB sampler was filled with delonized water by the manufacturer and shipped to the Site in a sealed plastic bag.At each groundwater monitoring well,a pre-measured,factory-prepared tether fitted with a weighted suspension cord was suspended in the monitoring well casing from the well cap.A PDB sampler was fitted to the tether at the midpoint of the screened interval and was left to equilibrate for 14 days,at a minimum. PDB samplers cannot be used to measure field water-quality parameters such as pH,temperature,and redox potential. However, as discussed with the RWQCB,these parameters are typically not necessary to meet the goals of a sampling event where the use of PDB samplers is appropriate. In general, pH,temperature,and redox potential are measured to document that a well has been adequately purged before sample collection,or for use in evaluating natural attenuation. However,well purging is not an issue when PDB samplers are used. ARCADIS installed the PDBs in the groundwater monitoring wells on June 11 and 13, 2012.The 27 PDBs were subsequently retrieved,and groundwater samples were collected on June 27,2012, at least 14 days after the installation of the PDBs. Each groundwater sample was collected by inserting a discharge tube into the clear polyethylene membrane of a PDB.Once the membrane was pierced,the groundwater amym2cn'ucrz-exnamso.aee 5