ARCHIVED REPORTS XR0006986 - Laserfiche WebLink

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' approximately 31 tact bgs in IIP-2, N TBL was not detected at or above the laboratory reported detection limit in any of the groundwater samples collected from the deeper strata in I IP-1, HP-2, and IIP-3 Field observations indicated that the site soil present to the depth investigated generally consists of peat, very fine to fine-grained sand clayey sand silty sand, silty clay, and sandy clay The stratigraphy Wow the peat at the site, as observed in the three deep continuously cored borings (DP-3, DP-7, and DP-8) is tine-grained and ' complex with a relatively high degree of heterogeneity and anisotropy Sand, silt, and clay content alternate and vary frequently with an increase in depth The stratigraphy indicated in DP-7 correlates well with the stratigraphy in DP-8 T he stratigraphy indicated in DP-3, however, does not correlate as closely with that of DP- ' 7 and DP-8 I-his indicates that the various sail strata at the site are not laterally extensive but instead vary in composition within a relatively short distance I he presence of a laterally and vertically extensive clay hon7on in the site subsurface (that would serve to halt vertical migration of the contaminants), therefore, was not ' observed nor indicated Although a confining unit (laterally and vertically extensive with high clay content) was not observed at the site, ' virtually all of the subsurface is composed of a relatively tight, fine-grained matrix that should serve to inhibit the vertical migration of contaminants The laboratory analytical results of the soil and groundwater samples indicated that only trace concentrations of MTBL were present in groundwater from the deeper soil strata at the ' locations investigated The results of the soil and groundwater investigation indicated that the MTBE plume generally extends in ' relatively low concentrations in shallow groundwater beyond the drainage ditch and into the agricultural field to the east of the site Relatively high MTBE concentrations are present in shallow groundwater to the west of the drainage ditch, southeast of site monitor well MW-2 The concentration of MTBE present at the location of DP- 5 is approximately three orders of magnitude greater than the concentration of MTBE at the location of DP-9 lieThis relatively rapid attenuation of MTBL concentrations suggests that the MTBE plume is being truncated by the hydraulic influence exerted by the island drainage channel pumping that occurs at the south portion of the site The sensitive receptor survey did not indicate any readily identifiable sensitive receptors within a radius of ' 2,000-feet around the site (other than the surface water bodies adjacent to the site) Based on the results of the soil and groundwater investigation, Condor recommended that an engineered ' approach to remediation of the MTBE plume not be initiated Alternatively, Condor recommended further investigation and monitoriing of the apparent leading edge of the MTBE plume rather than conducting remedial operations beyond what is presently occurring at the site Additional extraction and treatment of MTBE at the ' site would be a very expensive and time-consuming approach with a low chance of technical success or results Condor specifically recommended continuing the current quarterly surface and groundwater monitoring program ' at the site Condor also recommended additional groundwater investigation m the area south, southeast, and southwest of MW-2, along Light Mile Road, to verify containment of the south extent of the MTBE plume by the island drainage channel pumping in addition, the installation, development, surveying and sampling of ' additional monitor wells was recommended to be conducted in a subsequent phase to verify the results of the past and proposed soil and groundwater investigation and to monitor the extent of the MTBE plume The additional monitor wells would include at least one monitor well located at the anticipated east lateral extent of the MTBE ' plume which would be screened in the deeper strata In addition, Condor further recommended that the groundwater sampling of site monitor well MW-I be ' discontinued or reduced to an annual frequency This recommendation was based on the results of several successive quarterly monitoring events that indicate that petroleum hydrocarbons are not present at or above the laboratory reported detection limits at that location