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Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results plan view on Figure 3-4. Measurements from December 1997 are used where available. Otherwise, measurements from November 1997 and January 1998 are shown. The predominant direction of groundwater flow is to the northeast(N30E)with a hydraulic gradient of 2.3x10(CH2MHILL 1997). This flow direction and gradient has been relatively consistent since 1976 based on a review of existing data. The flow direction is directly related to a cone of depression created by deep pumping occurring to the east of Stockton. This municipal pumping has caused local changes in groundwater flow direction such that groundwater in nearby aquifers has a tendency to flow towards the pumping location. Groundwater flow in the area is predominantly horizontal,although the deep municipal pumping near Stockton is probably also inducing some downward vertical flow. A downward tendency for groundwater flow may also be caused by recharge from irrigation and seasonal rainfall,which creates local increases in hydraulic head at the surface and thus increase the difference in the vertical gradient. A predominantly downward vertical gradient has been documented in a previous investigation(Kleinfelder 1991). The flow directions and gradients described above are consistent with the groundwater quality results collected during CDM's investigation at ARL. The plume of PCE and TCE trends in a northeasterly direction from the landfill. In addition,water contamination has been found at shallower depths near the landfill,but at deeper depths further from the landfill,indicating that the plume is moving downward as well. The downward migration of VOCS emanating from the landfill may be influenced by the location of sand layers described in Section 3.1 and also by vertical flow conditions. 3.3 Groundwater Quality Summaries of water quality and field parameters collected during this investigation are presented in Tables 3-1 through 3-3. Data for tetrachloroethene(PCE)and trichloroethene(TCE)are presented on the site map (Figure 3-5)and on the cross sections(Figures 3-1 through 3-3). Data presented on Figures 3-1 through 3-4 and Figure 3-5 includes sample results from this investigation as well as those from the Fourth Quarter 1997 Quarterly Monitoring Report(CH2M Hill 1998). Copies of the analytical reports with chain of custody documentation from the current investigation are included as Appendix F. Groundwater samples collected during this investigation indicate the presence of PCE, trichloroethene(TCE),dichlorodifluoromethane,and trichlorofluoromethane. Other compounds detected during quarterly monitoring include chloromethane,1,2-dichloroethene(both cis-and trans-1,2-DCE),1,1-dichloroethane(1,1-DCA),and 1,2-dichloropropane. All compounds besides PCE and TCE were found to be present in concentrations below 20 micrograms per liter(ug/L) and generally lower than 5 ug/L. PCE was found in concentrations up to 69 ug/L(monitoring well MW-11)while TCE was measured up to 30 ug/L(monitoring well MW-14). For reference, maximum contaminant levels(MCLS)set by the State Water Resources Control Board(SWRCB)for PCE and TCE are both 5 ug/L. Because PCE is the compound which has been detected at the highest concentrations,and has the largest areal extent,the following discussion will describe offsite contamination as indicated by PCE contamination. iCDM Camp Dresser&McKee 3-3 W:AREPORTS%STOCKTOMARLPWME.98\SEC3.WPD CS 010771 City of Stockton 190855-0006