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c7cotogicat Technics Inc. Page 5 �.. Additional Pilot Test Work Plan Project No.770.2 October 30,2008 2.3.2 Groundwater / As stated above, Figures 7 and 8 illustrate the October 2005 CPT data and th February 144, 200 monitoring event groundwater contaminant distribution as superimposed on cross sections A - A' and B — B'. The groundwater data are presented in the diagrams adjacent to r� the respective well's screened interval. �� 3 J Z' Num_ The TPH-D distribution in the shallow wells is illustrated in Figure 9. ell MW-5 has been continuing to contain the highest concentrations of detected contaminants up to February 14, 2008. The contamination levels including diesel have been either non-detect or dramatically ,.. decreased for the last two quarters. The reasoning behind this decrease in contaminant level is either the increase in groundwater level elevation or the laboratory measurement errors/detection limits. It was decided to collect duplicate samples from MW-5 and MW-7 during the 4`h quarter groundwater monitoring and send to different labs to explore the possibility of laboratory procedures contribution to this contaminant level decrease. MW-5 has not been identified as being in a down gradient direction of the former UST location for the monitoring events completed to date by GTI. The reason for this anomaly (high level of contamination at MW-5) remains an enigma. None of the deep wells contained laboratory contaminants concentrations above the laboratory reporting limits and therefore a contour map was not developed. However, the discrete CPT groundwater samples indicate that the high concentrations noted in MW-5 (42,000 µg/1 during March 2006 ground war monitoring event) attenuate very rapidly with depth. The groundwater sample from CPT' l--' 73 feet bgs was obtained just five feet from and three feet below the screen of MW-5. This sample contained only 310 ug/1 TPH-D. �-- 3.0 PILOT TEST OBJECTIVES bw The site presents challenges for the selection of a clean up alternative. Assuming an approximate depth of 15 feet bgs for the bottom of the former UST excavation, a vertical column of 49 feet of impacted soil remains above the water table at 52.5 feet bgs (July 2008). Soil layers of variable permeability are present to inhibit the vertical migration of diesel fuel and cause lateral migration or pooling of the contaminant. This case is demonstrated in the apparent NAPL present at 45 feet bgs in MW-7 where a fine to coarse-grained sand unit lies above a silt unit. In addition, veins and/or fractures in the soil structure has been identified that facilitate vertical migration of contaminants. Removing the contamination from this "" higher permeability zones would be relatively easy using soil vapor extraction (SVE) if the contaminant were gasoline, however, diesel fuel is much less volatile than gasoline. In GTI's September 17, 2003 "Pilot Test for Soil Vapor Extraction" work plan we proposed a pilot test utilizing high vacuum (>25 inch Hg) soil vapor extraction (SVE). In an April 27, 2004 letter the SJC PHS/EHD denied the work plan on the basis that SVE was not applicable �' to diesel contamination and directed that a SCM be developed. GTI staff subsequently met