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. been detected at a maximum depth of approximately 700 feet Ground water is typically encountered at
<br /> depths of 250 to 300 feet below grade Another plume (the northeast plume) that apparently originates
<br /> near the southern boundary of the former industrial area currently extends off the installation at its
<br /> northeast boundary and may ultimately commingle with the main plume The impacted ground water is
<br /> an environmental concern of the State due to its presence in an aquifer potentially used for irrigation,
<br /> livestock watering, and domestic use The site is currently regulated under RCRA
<br /> 2 3 Source Remediation The originally identified source areas have been partially remediated
<br /> Sludge from the distribution ditches was removed and placed within the IWL Then, both the IWL and
<br /> the distnbution ditches were capped with a RCRA-type cap It was felt that by capping the IWL and the
<br /> distribution ditches, the further migration of the contaminants held within the vadose zone would be
<br /> halted However it is possible that lateral migration of contaminants, beyond the limits of the capped
<br /> areas, has occurred within the vadose zone A very substantial vadose zone, often exceeding 300 feet
<br /> thick, exists beneath both the distribution ditches and the IWL Other potential sources in the vicinity of
<br /> the Industrial Area are currently being investigated
<br /> 2 4 Contaminant(s) of Concern The primary COC is the volatile organic compound (VOC) TCE
<br /> Additional CDCs identified in the Post-Closure Permit are Carbon Tetrachloride (CC14), 1,1-
<br /> Dichloroethene (1,1-DCE) 1,2-Dichloroethene (1,2-DCE), 1,1-Dichloroethane (1,1-DCA), methylene
<br /> chloride, 1,1,1-Trichloroethane (1,1,1-TCA), Tetrachloroethene, Chloroform, Chloroethane, 1,2-
<br /> Dichloropropane (1,2-DCP), Benzene, Toluene, Ethylbenzene, Xylene, and Chromium 2,4-
<br /> Dimethylphenol (2,4-DMP) was identified in the initial characterization and the Post Closure Permit but
<br /> was deleted in May 1995 The compound had only been detected in one well since monitoring began
<br /> and the last detection was in 1988
<br /> 2 5 Historical Contaminant Concentrations The highest ground water contaminant
<br /> concentrations associated with the IWL and ditches were recorded as follows (well in which
<br /> concentration occurred and date are given in parentheses) 250 ppb TCE (T-1, T-3, 11186, 8/84,
<br /> respectively), 54 ppb CC14 (B-26, 3/96), 4 2 ppb 1,1-DCE (B-15, 10/86), 340 ppb 1,1,-DCA (N-2C,
<br /> 8/84), 12 ppb methylene chloride (B-13, 10/86), 200 ppb 1,1,1-TCA (N-2C, 3/88), 5 8 ppb chloroform
<br /> (B-5, 9191), 6 78 ppb benzene (B-18, 6/90), 35 ppb toluene (A-7, 4188), 2 6 ppb ethylbenzene (B-5,
<br /> 9/91) and 3 8 ppb xylene (N-2C, 3/88) For many of the COCs, the highest detected concentrations were
<br /> in the ground water directly beneath or near the IWL or distribution ditches during the early years of the
<br /> investigation Concentrations have since diminished, in some areas to undetectable levels, with time and
<br /> with the implementation of the ground water treatment system Only the COCs TCE, CC14, and 1,1-
<br /> DCA currently persist in the plume generated by the IWL and the associated distribution ditches TCE
<br /> is the most widespread and the most concentrated COC, and TCE defines the lateral and vertical extent
<br /> of the plume The CC14 plume is much smaller than the TCE and 1,1-DCA is only currently found in
<br /> 11 rn
<br /> wV, T11at L?-LV
<br /> 2 6 Aquifer System The aquifer system impacted in the main plume area is composed primarily
<br /> of coarse-grained alluvial materials with some lake deposits The unconsolidated materials, which are
<br /> generally hundreds of feet thick, are underlain by a bedrock aquifer consisting of various strata,
<br /> including quartzite, sandstone, and limestone In a portion of the study area near the former IWL, an
<br /> uplifted block of the bedrock aquifer is encountered at much shallower depths and the water table
<br /> actually resides in the bedrock This "bedrock block" has a significant impact on the ground water flow
<br /> 06/07/99
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