Additional Groundwater investigation Report
<br /> CSUS Multi-Campus Regional Center
<br /> August 19,2003
<br /> Page 9
<br /> Cross-sections (Figure 4, Attachment A) completed across the site are shown in Figures 5, 6, 7, and 8,
<br /> Attachment A. From the surface to approximately 50 feet bgs, soils at the site appear to be predominantly
<br /> ` interbedded clays and silts with interspersed beds of sand. Silty and sandy horizons form lenticular
<br /> features and continuous,undulatory interbeds of variable thickness. Between approximately 50 to 75 feet
<br /> bgs, the soil profile appears to transition from that dominated by fine-grained soil types to one
<br /> characterized by predominately coarse-grained soil types, At depths greater than approximately 75 feet
<br /> bgs, the soil profile appears to be characterizedby sandy horizons interbedded with silt layers of variable
<br /> grain size composition and interspersed with undulatory clay zones. A relatively continuous, coarse to
<br /> gravelly sand horizon of variable thickness was encountered between approximately 115 to 134 feet bgs
<br /> in the location of CPT-7 and CPT-8. Groundwater was encountered at approximately40 feet bgs within
<br /> the sandy horizon and rose in the borings, suggesting the aquifer is under semi-confined conditions
<br /> because of the overlying clays.
<br /> The clay stratum underlying the site varies in composition fromdominantly stiff to very stiff, fine-grained
<br /> clay to silty/sandy clay. The silty horizons range in composition from clayey silt to sandy silt; a relatively
<br /> continuous silty horizon with clay interbeds is present immediately below the former excavation area
<br /> (Figures 5, 6, 7, 8, and 10, Attachment A). Relatively continuous sand horizons of variable thickness
<br /> occur throughout the subsurface and range in composition from clayey sands to medium/coarse-grained
<br /> sands.
<br /> Although the upper soil horizon appears to be dominated by relatively continuous, fine-grained clays, the
<br /> lower soil horizon appears to lack a clay unit that would effectively impede vertical contaminant
<br /> migration. The lower horizon clay units appear to form relatively continuous layers across the site;
<br /> however, these layers lack lateral and vertical uniformity and contain a significant"silt to sand-sized
<br /> component that probably provides a moderate transmissivity to those units.
<br /> E Laboratory analytical results indicate that, in the area proximal to the former excavation, groundwater to
<br /> F depths of at least 128 feet bgs has been impacted by petroleum hydrocarbons. Petroleum hydrocarbon
<br /> constituents were not detected at or above laboratory detection limits in the groundwater samples
<br /> collected from boring CPT-4, with the exception of TBA and 1,2-DCA%1 very low concentrations. TPH-
<br /> G, TPH-D, TBA, and 1,2-DCA were detected in the groundwater samples from boring CPT-5. 1,2-DCA
<br /> was detected at a very low concentration in a groundwater sample collected from boring CPT-6. r.6
<br /> Petroleum hydrocarbon constituents were not detected at or above laboratory detection limits in the
<br /> groundwater samples collected from CPT-7, with the exception of TPH-D and 1,2-DCA in the
<br /> groundwater sample collected from 86-90 feet bgs. BTEX, TPH-G, TPH-D, and 1,2-DCA were detected
<br /> in the groundwater samples from boring CPT-8. '1,2-DCA and TPH-D were detected in the groundwater
<br /> samples collected from boring GP-10; however, the sample chromatogram did not match the standard
<br />�h
<br /> diesel chromatogram. A7 +ro
<br /> Based on the laboratory analytical results of the groundwater samples, the extent of groundwater
<br /> contamination is adequately investigated up-gradient (CPT4) and cross-gradient (MW-1, CPT-6, and
<br /> CPT-7) at the site. In addition, all petroleum hydrocarbon constituents other than TPH-D and 1,2-DCA
<br /> (including BTEX and MTBE) are very limited in extent, laterally and vertically, in the down-gradient
<br /> direction (CPT-5 and GP-10). Many gasoline constituents are present at elevated concentrations in the
<br /> source area (CPT-8) and while attenuating rapidly with depth in that location, are present down to the .
<br /> total depth of the investigation (128 feet bgs). The results indicate that BTEX constituents and gasoline
<br /> oxygenates/additives are not particularly mobile, especially laterally, at the site. Diesel range petroleum
<br /> hydrocarbons, while relatively ubiquitous at the site, appear to be the result of weathered gasoline and/or
<br /> diesel and due to the lack of BTEX and MTBE, may not be particularly important of its own accord. The
<br /> presence of 1,2-DCA in the source area and down-gradient in concentrations elevated well above the
<br /> Is
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