ARCHIVED REPORTS_SECOND SEMIANNUAL AND 2014 ANNUAL GROUNDWATER MONITORING REPORT

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3.ANNUAL REPORT FOR 2014 concentrations,coarse versus fine-grained lithology,shallow versus deep screens) during ' the September 2011 semiannual sampling event. The two sets of results were found to be comparable. The results are presented in the Second Semiannual and 2011 Annual Groundwater Monitoring Report(CH2M HILL,2011). ' Although the current MRP has provided important data,the sampling frequency is now considered to be excessive given the approximately 24 years of monitoring results that are ' continuing to show a long-term trend of decreasing concentrations of hydrocarbon constituents and decrease in plume size. The concentrations are currently monitored on a semiannual basis, as described. Review of the long-term concentration trend charts in ' Appendix E indicate that annual monitoring would be sufficient to track the continued long-term trend of decreasing concentrations of hydrocarbon constituents and decrease in plume size. ' Therefore,it is recommended that the following changes be made to the current MRP: • Continue water level measurements on a semiannual basis (March and September), ' because the groundwater elevations are seasonally high in the winter and spring, and seasonally low in the summer and fall. • Collect and analyze groundwater samples on an annual basis in the spring(March), when water levels are seasonally high. The concentrations are generally higher in the spring compared to the fall. In addition,the likelihood for having insufficient water to collect samples is lower in the spring when water levels are seasonally high. Occasionally,samples cannot be collected during the fall due to seasonally low water levels. , • Analyze the groundwater samples for TPHg,TPHd, and BTEX constituents. • Prepare an annual report to document and interpret the year's monitoring data. ' 3-4 ES101214202053SCO