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2.0 Groundwater Monitoring Program Groundwater samples were collected by ATC and Associates from two background monitoring wells (MW-6A and MW-7A), three detection monitoring wells (MW-9A, MW-913, and MW- l0A), and one evaluation monitoring well (MW-2) during the fourth quarter 2006 monitoring event. ATC and Associates reported that detection monitoring well MW-8A could not be located and evaluation monitoring well MW-IA was dry during the fourth quarter 2006. The analytical results from samples collected at these wells are used to determine potential landfill-related impacts. The well screens for MW-IA and MW-2 are located in waste material. The analytical results from MW-lA and MW-2 are used to evaluate the changes in water quality within the landfill. During the fourth quarter 2006 monitoring event, analysis for field parameters (pH, specific conductance, temperature, and turbidity) and monitoring parameters (barium, bicarbonate, calcium, carbonate, chloride, magnesium, nitrate, potassium, sodium, sulfate, and total dissolved solids [TDS]) was performed for the background monitoring wells, detection monitoring wells, and evaluation monitoring wells. 2.1 Results for Detection Monitoring Wells Fourth quarter 2006 field and laboratory sampling results from background and detection monitoring wells are summarized on Table 2.2, along with the concentration limits determined from background monitoring well data. The historical groundwater data for the background and detection monitoring wells are presented in Appendix D. The data are graphed on time series concentration plots in Appendix E. For VOCs, only those VOCs with fourth quarter 2006 detections in background or detection monitoring wells were plotted. The concentration limits were developed using data collected through the fourth quarter 2006. The statistical analysis calculations are presented in Appendix E. A copy of the statistical analysis methodology is presented in Appendix C. As presented on Table 2.2, the following constituents exceeded their respective concentration limits: barium at MW-913, calcium at MW-913, chloride at MW-913, magnesium at MW-10A, potassium at MW-9B and MW-10A, and temperature at MW-9A, MW-913, and MW-10A. The chloride concentrations (with high specific conductance values) could indicate a saltwater intrusion to the groundwater. The French Camp Landfill is within the central Stockton area impacted by saltwater intrusion (Fourth Quarter/Annual 1999 Monitoring Report, French Camp Landfill, CH2MHILL,January 2000). Historical data from the background and detection monitoring wells were evaluated for temporal trend using Sen's Slope trend analysis. Graphs of the Sen's Slope analyses are included in Appendix E. There were no constituents that both exceeded their concentration limits and exhibited increasing trends according to Sen's Slope analysis. The following constituents exhibited an increasing trend, but did not exceed their respective concentration limits: barium at MW-9A, chloride at MW-l0A and sulfate at MW-913. Sen's Slope analysis identified decreasing trends in the following constituents: barium at MW-913, pH at MW-6A, nitrate at MW-8A and MW-10A, specific conductance at MW-7A, sulfate at MW-7A, and turbidity at MW-7A, MW- 9A, and MW-10A. As noted above,MW-6A and MW-7A are background monitoring wells, and trends in the groundwater quality at these wells are not indicative of an impact from the facility. 2