ARCHIVED REPORTS XR0010083 - Laserfiche WebLink

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Discussion Soil Chemical Data The site assessment revealed that low concentrations of highly degraded contaminants were present in the capillary fringe at PB-10 near MW-9, and at PB-13 near MW-3 Other capillary fringe samples showed that contarmnants were not detected Intermediate depth soil samples also showed that contaminant were not detected with the exception of PB-14-4 at 22 feet where 0 014 parts per billion (ppb) of Xylene were detected Contaminants were not detected in the deepest soil samples at PB-12-5 at 26 feet and PB-13-5 at 26 feet MTBE and oxygenates were not detected in any soil sample Shallow Groundwater Chemical Data Four reconnaissance groundwater samples were collected, and all contained various concentrations of TPHG ad BTEX contaminants (see Table 2) The highest concentrations observed were at PB-11 (31,000 ppb TPHG and 170 ppb Benzene) and at PB-13 (21,000 ppb TPHG and 160 ppb Benzene MTBE and oxygenates were not detected in any groundwater sample Data Inte1l2retation The most elevated concentrations of both soil and dissolved contaminants occur near the irrigation pipeline These concentrations appear to increase moving south toward the former tank pit (original source) In our opinion on the basis of the sod data, there does not appear to be a deep contaminant penetration, and contaminants were not detected in the underlying confining strata One unverified 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 contarrunants were not detected The PHS EHD concern of elevated dissolved contarrunants near MW-9 from a nearby shallow contaminant source does not appear likely CT given the vadose soil and capillary soil data since elevated concentrations would be expected in the vadose soil column Some sod 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 Relatively elevated dissolved concentrations near MW-3 and MW-9 and occur where historic groundwater monitoring has previously documented the highest contaminant plume concentrations However the persistence of these higher dissolved concentrations at MW-9 have been previously interpreted as a result of plume stabilization It is possible that the nearby irrigation line (when full) may have been influencing the contaminant movement given the temporary appearance of groundwater mounds near MW-9 When the pipeline is full, there could be water leakage causing a local groundwater mound where contanunants are locally desorbed and mobilized near MW-9 (causing periodic concentration rises) However there is insufficient data to confirm this surmise at this time Discussions between Wright and PHS EHD on April 25 and May 18, 2000 resulted m changing the approach to the work given their concern for possible additional contaminant sources Workplan revision approaches discussed were to, perform additional deeper assessment in the former tank pit area, searching the "welding building" for possible tank sources, performing surface geophysical surveys for anomalies and relocating utilities so previously scheduled boreholes may be safely advanced with the utility owner permission Page 4