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FMS #24 Site Investigation Report <br />California Army National Guard <br />• UST location. The migration of contaminants originating from releases from the former USTs to <br />groundwater has occurred. <br />4.0 CONCLUSIONS <br />Based on information obtained during the previous investigation efforts by Versar and URS, soil and <br />groundwater beneath the former diesel and gasoline UST locations at FMS #24 have been impacted <br />by releases of contaminants from the former USTs. The lateral extent of soil and groundwater <br />contamination associated with the former diesel and gasoline UST location has now been determined <br />to non -detect levels. These soil and groundwater sampling events determined that the soil and <br />groundwater beneath the former diesel and gasoline USTs locations contain concentrations of TPHD, <br />TPHG, and BTEX. The URS investigations in May and October 2007 showed that soil and <br />groundwater contamination extends in all directions from the former USTs, but the lateral extent of <br />contamination has now been determined (to non -detect levels), and the contamination in groundwater <br />appears to be migrating to the northeast (downgradient). A data gap may exist in the area northeast of <br />the former USTs between borings SB 10 and SB 11, which will be addressed following the installation <br />of groundwater monitoring wells and the determination of groundwater flow direction. BTEX and <br />TPH concentrations exceeded regulatory clean-up levels in groundwater. The vertical extent of these <br />contaminants has not been investigated beyond the first encountered groundwater (40 to 45 feet bgs). <br />5.0 RECOMMENDATIONS <br />Based on the information obtained for FMS #24 from the two investigation phases conducted by <br />URS in 2007, the following is recommended. <br />• • Install four 2 -inch -diameter groundwater monitoring wells at the locations shown on Figure 5-1 <br />to monitor groundwater contaminant flow and determine groundwater gradient. The wells will <br />also help assure that contaminated groundwater is not migrating beyond the site boundaries. <br />Analyze groundwater samples from newly installed monitoring wells for TPHG and TPHD, <br />VOCs (including BTEX and oxygenates), and metals on a quarterly basis. <br />• Install one 4 -inch -diameter groundwater monitoring well at the location shown in Figure 5-1. <br />This well will be used to monitor concentrations in the center of the plume. <br />• During the drilling of the 4 -inch well, conduct a vertical assessment of impacted groundwater. <br />Groundwater grab samples will be collected using HydroPunch technology at the approximate <br />depths of 60 feet bgs, 80 feet bgs, and 100 feet bgs, or until the vertical extent is deter rrrined. A <br />mobile laboratory will be utilized to analyze samples on a rapid turnaround time. <br />• The analysis of future groundwater samples should not include metals and oxygenates. <br />6.0 REFERENCES <br />American Cancer Society, 2006. Cancer Facts and Figures 2006. American Cancer Society, <br />Atlanta, Georgia. <br />California Department of Toxic Substances Control (DTSC), 2005. Guidance for the Evaluation and <br />Mitigation of Subsurface Vapor Intrusion to Indoor Air. Interim Final. Department of Toxic <br />• Substances Control, California Environmental Protection Agency, Sacramento, California. <br />Original Issue Date: December 15, 2004; Revised: February 7, 2005. <br />December 2007 <br />