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Piper diagrams of the first through fourth quarter 2006 results for the surface water sampling poitns are included in Appendix H. 0 2.0 Groundwater Monitoring Groundwater samples were collected by Bryan A. Stirrat & Associates during the fourth quarter 2006 monitoring event from the detection monitoring and corrective action wells at the site. Monitoring well AMW-2, which is upgradient from the landfill, is the designated background well. Monitoring wells AMW-6, AMW-7, AMW-12, AMW-13, and AMW-14 are currently designated as detection monitoring wells. Monitoring wells AMW-1, AMW-3, AMW-4, AMW- 5, AMW-10, AMW-11, AMW-18, and AMW-19 are designated as corrective action monitoring wells. The analytical results from samples collected at the corrective action wells are used to evaluate improvements in groundwater quality as a result of previously implemented corrective action measures. Bryan A. Stirrat reported that corrective action monitoring well MW -3 has been destroyed, and that corrective action monitoring well MW -5 could not be sampled during the fourth quarter 2006 due to a nonfunctional pump lodged in the well. Groundwater samples are scheduled to be analyzed quarterly for field parameters (groundwater elevation, pH, specific conductance, temperature, and turbidity) and monitoring parameters (arsenic, barium, bicarbonate, calcium, carbonate, chloride, gasoline -range petroleum hydrocarbons, hexavalent chromium, magnesium, nitrate, oil and grease, potassium, sodium, strontium, sulfate, TDS, and VOCs). 2.1 Detection Monitoring Groundwater samples were collected from the background well (AMW-2) and detection monitoring wells (AMW-6, AMW-7, AMW-12, AMW-13, and AMW-14) during the fourth quarter 2006 monitoring event. The field and laboratory results provided by BC Laboratories, Inc. for the fourth quarter 2006 detection monitoring sampling are summarized on Table 2.2. The historical groundwater data for the detection monitoring wells are presented in Appendix D. The data are graphed on time series concentration plots, included in Appendix E. For VOCs, only those compounds that exhibited detections for the fourth quarter 2006 monitoring event were plotted. The concentration limits presented on Table 2.2 were calculated by Herst & Associates, Inc. from data collected through the fourth quarter 2006 monitoring event. The statistical analysis calculations for detection monitoring wells are included in Appendix E. A copy of the statistical analysis methodology is included in Appendix C. During the fourth quarter 2006 monitoring event, barium at AMW-13, hexavalent chromium at AMW-6 and AMW-12, and field pH at AMW-7 exceeded their respective concentration limits. The fourth quarter 2006 barium concentration at AMW-13 (0.26 mg/L) is comparable to concentrations observed at that well since May 1995, and is well below the National Primary Drinking Water Standard (2 mg/L). The fourth quarter 2006 hexavalent chromium concentration at AMW-6 (0.0061 mg/L) is comparable to concentrations observed at that well intermittently since February 1997. The fourth quarter 2006 hexavalent chromium concentration at AMW-12 (0.015 mg/L) is higher than concentrations previously observed at that well. The fourth quarter 2006 hexavalent chromium concentrations at AMW-6 and AMW-12 are both well below the National Primary Drinking Water Standard for chromium (0.1 mg/L). The fourth quarter 2006 pH value at AMW-7 (8.44) is comparable to or lower than values observed at that well