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FORMER AMERICAN FOREST PRODUCTS 3.0 Groundwater Closure Surnmary Report July 2009 URS Project 17322736 Page 3-4 3.4 GROUNDWATER CONDITIONS This section discusses the source areas of groundwater impacts. It also provides modeling results showing the effect of remaining source constituents on groundwater. The distribution of groundwater impacts and concentration trends are also discussed. 3.4.1 Source Areas of Groundwater Constituents of Concern As discussed in Section 22 (two USTs, a 10,000-gallon diesel tank at the west end of the former maintenance shop, and a 1,000-gallon waste oil UST at the east end of the shop)were removed in 1988. Following their removal a total of approximately 2,000 cubic yards of impacted soil were removed from the two excavations,based on a remedial objective of 10,000 milligrams per kilogram(mg/kg) of TPH as diesel (TPHd). Based on analytical data from soil borings Dames & Moore concluded(Dames & Moore, 1994) that the removed soil represented approximately 47% of the TPHd in soil at the site. They also concluded that 88% of the remaining soil had a concentration of less than 5,000 mg/kg TPHd. 3.4.2 Distribution of Site Groundwater Constituents of Concern This section presents historical groundwater monitoring results (Table 4)through June 2009, and maps the extent of 1,2-DCA in groundwater(Figure 7). Data in Figure 7 include the results of five quarters of groundwater monitoring subsequent to the installation of additional five additional groundwater monitoring wells for the purpose of plume delineation. Figure 8 provides isoconcentration lines of 1,2-DCA for June 2009. With the exception of low concentrations in samples from three wells in December 2006 and one well in March 2007, and a few detections near the reporting limit, TPHd 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 2). Concentrations of 1,2-DCA have been reported in groundwater samples at up to 36 pg/L(DMW-9, June 2005), but by June 2009 had decreased in samples from that well to 7.9 pg/L. The California Environmental Protection Agency(Cal/EPA)maximum contaminant level (MCL) for 1,2-DCA is 0.5 }tg/L. Vertical Delineation of 1,2-DCA—Figure 3 shows the distribution of 1,2-DCA is limited vertically to the shallower site monitoring wells. Samples collected from DMW-8 downgradient from the waste oil tank and screened between 80 and 90 feet bgs have contained detectable concentrations only twice out of 18 sampling events, with a maximum concentration of 1.1 pg/L. The vertical extent of 1,2-DCA contamination is also defined by the decline in concentration with depth from 8.8g f.. (June 2, 2009} in DMW-6 (screened from 32 to 52 feet bgs)to 1.0 pg/L (June 2, 2009) in DMW-5 (screened from 51 to 71 feet bgs), and non-detect (June 2, 2009) in DMW-14 (screened from 90 to 100 feet bgs). 1,2-DCA has been non-detect in samples collected from DMW-14 for the five quarters since it was installed in the spring of 2008. Lateral Delineation of 1,2-DCA—Based on four quarters of monitoring data subsequent to plume delineation well installation during the second quarter of 2008, data continue to show good lateral and vertical definition of the extent of 1,2-DCA contamination. Lateral definition to the northwest of the former source area is provided by consistent non-detect results for 1,2-DCA for monitoring well K.Nkp, ,r s-:"_:.7x6 Americen Fores�.Cinsurc-Swn:nan-Report-RevOl.doc