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Geologfcol Tec"Cs Inc. Page 5 Site Conceptual Model Project No.989.2 1 December 17,2003 ' PHS/EHD since 1995. The groundwater analytical data are summarized in Table 2 of Appendix A. TPH-G and BTEX TPH-G and BTEX concentrations have declined exponentially since monitoring began in ' August of 1995. TPH-G concentrations measured during the May 2003 monitoring event are 2 orders of magnitude less than TPH-G concentrations measured in March 1996. BTEX constituents show ratios of declining concentrations similar to those of TPH-G. Chart 1 ' shows the decrease in constituents analyzed in MW-1 over time. The TPH-G and BTEX plumes appear to be depleting plumes. Analysis indicates the plume ' is decreasing in volume and not migrating downgradient of the source area. Measurements from MW-1, the well nearest the former USTs, indicate the average annual concentrations of BTEX and TPH-G constituents significantly decreasing over time. Average annual concentrations measured at MW-1 and wells directly downgradient of MW-1 indicated the plume is not migrating downgradient from the site of the former USTs (Chart 3). The most recent laboratory analysis of the groundwater samples, collected during the September 2, 2003 groundwater-monitoring event, show the presence of BTEX and TPH-G constituents in MW-1 and MW-4. A trace of xylene was detected in MW-3. BTEX constituents were not found in any other well during this monitoring event. TPH-G was below detectable limits in all remaining monitoring wells. 1 � TPH-G mass estimate calculations indicate there is 0.1 gallons of TPH-G remaining in the groundwater at the VWDC location (Table 6). See Section 4.6 for details on contaminant ' mass estimate calculations. MtBE ' Six of the site's 10 groundwater-monitoring wells showed the presence of MtBE during the most recent groundwater-monitoring event. The concentrations ranged from non-detect <1 ug/L to 93 ug/L (Table 2). MtBE in groundwater was not monitored during the initial stages ' of site investigation at VWDC. The collection of MtBE groundwater data began in March 1996 and there is seven years of available information. MtBE data indicates- the concentration of MtBE found inroundwater is a function g of groundwater elevation at the time of sample collection as shown in Chart 4. Overall ' concentrations of MtBE appear to be decreasing in the MW-6, the well with the highest concentrations. Chart 5 shows the historical change in MtBE concentrations found in MW-6 since the well installation in August 2000. The MtBE plume seems to be stable with measured concentrations showing slight decreases over the last 28 quarters. 1