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Workplan Addemdum Letter for a Limited Subsurface Site Investigation Moore Petroleum,5491 F Street,Banta, California Page 4 On January 14, 2003, Apex submitted a workplan titled, Workplan for a Limited Subsurface Site Investigation, proposing the installation of five groundwater monitoring wells to delineate lateral and vertical extent of petroleum impacted soil and groundwater. On May 12, 2004, SJCEHD submitted a letter requesting a workplan addendum addressing the need to delineate the vertical extent of contamination at the subject site by relocating one of the proposed wells and adding one proposed CPT location. MONITORING WELL INSTALLATION As noted in the Apex Workplan submitted January 14, 2004, the primary purpose of this task is to determine the vertical extent of soil and groundwater impacted by petroleum hydrocarbons at the site and to assess the extent the site is being impacted by upgradient off-site sources. Well MW-17 will be installed northeast of its original proposed position. This new location is identified on Figure 3. All field work will be performed according to Apex's Standard Operating Procedures (SOP) included in Appendix B. The groundwater level, based on the existing monitoring wells at the subject site, is at approximately 10 to 14 feet below ground surface (bgs). The groundwater monitoring well is proposed to be completed to 20 feet bgs. The boring will be drilled using a hollow-stem auger rig. The boring will be continuously cored below the 5 feet bgs for classification, field screening and for possible chemical analysis. Soil samples will be classified by the Unified Soil Classification System. Field screening will consist of soil headspace analysis using a portable photoionization detector. At a minimum, the well will be constructed according to the standards found in the California . Code of Regulations, Title 23, Section 2647 and 2648 and the California Department of Water Resources Bulletins 74-81 and 74-90. The monitoring well will be constructed of two-inch diameter Schedule 40 PVC with a 0.020-inch slotted casing. The screened interval will extend approximately 5 feet above and 10 feet below the water table (5 feet to 20 feet bgs). The filter pack will be placed from one foot above the screen interval to the total depth of the well. A one- foot bentonite seal will be placed on top of the filter pack. A neat cement sanitary seal will be placed from the top of the bentonite seal to the surface. The wellhead will be secured with a locking, water-tight cap and a traffic-rated wellhead vault box. Construction specifications for a typical groundwater monitoring well are shown on Figure 4. The monitoring wells will be developed, purged, and sampled following installation according to Apex's SOP included in Attachment III. Groundwater from the monitoring wells will be collected at least 24 hours after completion of well development according to the included SOP. The well will be surveyed to an established benchmark, if available, and to the other wells.