ARCHIVED REPORTS XR0010073 - Laserfiche WebLink

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The seven reconnaissance groundwater samples from the April 19, 2001 borings collected at depths of 20-26 feet ranged from 1,700 to 5,600 ppb (see Table 4) The highest concentrations were in PB-16 (downgradient of the former pit), PB-17 in the former tank pit) and PB-18 (about 5 feet from MW-9) Benzene concentrations were low, with the highest at PB-15, 16 and 19,just downgradient of the former tank pit The laboratory reports that these appeared to be degraded contaminants and in some cases did not match the laboratory standards Data Interpretation Data from the April, 2000 work initially appeared to show the most elevated concentrations of both soil and dissolved contaminants occur near the urigation pipeline Also, on the basis of the soil data, there does not appear to be a deep contaminant penetration;and contaminants were not detected in the underlying confining strata One unvenfied report of surface spillage of fuel near MW-9 was relayed to Wright however, the shallow soil sample PB-12-1 at 5 feet showed that contaminants were not detected The PHS-EHD concern of elevated dissolved contaminants near MW-9 from a nearby shallow contaminant source does not appear likely given the vadose soil and capillary soil data since elevated concentrations would be expected in the vadose soil column Some soil contaminant data near the former tank pit (PB-13-2 at 11 5 feet) could be suggestive of a residual source slowly desorbing into groundwater near the former tank pit Soil • samples at the 24- to 26-foot depth showed that contaminants were not detected, although dissolved contaminants were present Previous work had mapped the dissolved plume along an axis, which parallels the trend of the former tank pit, along wells MW-3, MW-9 and MW-2 Quarterly monitoring has shown that the dissolved plume is present along this trend, with wells MW-8 and MW-5 consistently clean Contaminants observed at MW-6, in an upgradient position have consistently been lower than at MW-3, and in our opinion suggest an off-site source The sample data reported herein support this overall plume location (see Figures 4 and 5) Soil capillary fringe samples show residual contaminants, especially just downgradient of the former pit and at MW-9 The presence of sandy beds at depths of 8-10 feet apparently enhanced the dissolved contaminants migration north toward MW-9 However, the silty and clayey strata at depths of 3-8 feet appear to have absorbed UST leakage, creating a residual source at and just north of the former tank pit Wright interprets a similar occurrence at MW-9 where clayey sand strata appear to have caused contanunants to absorb in the vicinity of MW-9 This observation in earlier work, together with contaminant presence was the primary reason that MW-9 was placed at this location Page 7