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Groundwater Monitoring ARCADES Report for the Semiannual Period from January 1 through June 30,2012 Lincoln Center, Stockton, California • Trans-1,2-DCE was detected in one monitoring well(MW124A-L)at an estimated concentration of 2.6 pg/I. • Vinyl chloride was detected in one monitoring well(MW2140)at an estimated concentration of 3.4 pg/I. Distribution of PCE and Total Hazardous Substances in A-Zone Groundwater Groundwater-quality data indicate that PCE and its daughter products were reported in A-zone groundwater at the highest concentrations relative to other detected VOCs (Table 3). Figures 8 and 9 are A-zone groundwater isoconcentration contour maps for PCE and total Hazardous Substances, respectively.The contours shown on these figures were developed from the laboratory analysis of groundwater samples collected from monitoring wells and groundwater extraction wells(GEWs)during the December 2011 and June 2012 sampling events;however,only laboratory analytical results for the samples collected during the reporting period are plotted on the figures. During the June 2012 sampling event,the highest A-zone concentrations of PCE(5,400 ug/1)and total Hazardous Substances(6,220 pgA)were reported in the groundwater sample collected from monitoring well MW-121A-U(Table 2). Because PCE is the primary Hazardous Substance detected,the isoconcentration contours for PCE(Figure 8)are nearly identical to those for total Hazardous Substances (Figure 9). Historically,PCE has been the primary contributor to total Hazardous Substances concentrations, except in the area hydraulically upgradient(west)of Lincoln Center where PCE is generally not present above analytical detection limits. Figure 9 illustrates the extent of Hazardous Substances reported in A-zone groundwater.The upgradient extent of the A-zone plume is defined to the southwest by the non-detectable concentrations of Hazardous Substances observed at wells MWA 10A and MW-001A; however,concentrations of Hazardous Substances(TCE and cis-1,2-DCE)reported at MW-104A during the December 2011 sampling event indicate that there may be some impacts farther upgradient to the west and/or northwest of the Site.The approximate downgradient extent of the A-zone plume is defined by the non- detectable concentrations of Hazardous Substances measured at MWA 17A.The estimated northern crossgradient is bounded by the non-detectable concentrations of Hazardous Substances observed at MWA 06A, and by the low reported concentrations (i.e., less than 1 pg/1)of Hazardous Substances at MWA 05A and MW-107A.The mm Lw[N�(f W_Pu0�2.EM]08)SOMe 7