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Suggested No Adverse Response Level(S. NARL)for2hthalene. Based on that result, the soils . remaining at the site would not constitute a designated waste, provided average water levels . remained at about 54 eet bgs. The average water level in DMW-1 between August 1990 and March 2007 has been approximately 47 feet bgs. y 2.4.2 Distribution of Site Groundwater Constituents of Concern The indicator parameter naphthalene, discussed above, has not been identified in groundwater ' samples. With the exception of low concentrations in samples from three wells in December 2006 , and one well in March 2007, TPH/D has not been reported during the last two years.. The chemical 1,2-DCA is used as an industrial solvent, and has been reported in samples collected from monitoring wells at and downgradient from the former waste oil UST (Tank 18820005) (Figure 7). Concentrations of 1,2-DCA have been reported iri groundwater samples at up to 36 µg/l_ (DMW-9, June 2005). The California Environmental Protection Agency(Cal-EPA) maximum contaminant level (MCL)for 1,2-DCA is 0.5 µg/L. F The distribution of 1,2-DCA is limited vertically to the shallower site monitoring wells. Samples collected from DMW-8, located downgradient from the waste oil tank and screened between 70 and 80 feet bgs, have contained detectable concentrations only twice out of 12 sampling events, with a maximum concentration of 1.1 µg/L. Table 3 summarizes historical groundwater sampling analytical data. - 2.4.3 Trends in 1,2-DCA Concentrations Time-versus-concentration plots for 1,2-DCA in monitoring wells DMW-2,DMW-5, DMW-6, and DMW-9 are provided on Figures 8 and 9. The data represent quarterly monitoring results for samples collected and analyzed beginning in August 2000, when 1,2-DCA became a constituent of concern at the site. A linear regression trend line and equation are shown for each wells data set. The equation of the trend line is in the form of y =mx + b,where m is the slope of the line. A positive value for m indicates a rising trend, while a negative value indicates decreasing concentrations of the plotted constituent. A downward trend for 1,2-DCA is shown for wells DMW-2, DMW-5, and DMW-6:!iBy extrapolation, 1,2-DCA concentrations in these wells would become non-detectable by approximately 2014 or earlier. Although 1,2-DCA concentrations in DMW-9 have been decreasing over the most recent four quarterly monitoring events, overall, an upward trend"is - calculated for this well. The combination of the declining concentrations at DMW-6 and the rising trend shown for DMW-9 suggest a 1,2-DCA contaminant plume is moving away from the source area represented by the former waste oil tank and downgradient,through the vicinity of DMW-9. 3.0 DATA GAPS AND PROPOSED ADDITIONAL WELL LOCATIONS Figure 10 provides isoconcent ration contours for 1,2-DCA based on the March 2007 groundwater monitoring event. The contours exclude the data for.the deeper wells, DMW-7 and DMW-8. The contours show a 1,2-DCA plume originating in the vicinity of the former waste oil tank at the east, end of the former maintenance shop,and extending downgradient to the north. The plume is reasonably defined to the south and west of the source area by DMW-1 and DMW-2, respectively. However, data gaps exist to the east and southwest of the former waste oil tank, and downgradient to the north of DMW-9. 5