ARCHIVED REPORTS_XR0008884 - Laserfiche WebLink

Home Browse Search

Fourteen groundwater sampling events were conducted between 1988 and 1996 Samples were collected from monitoring wells MW-1, MW-2, and MW-3 starting in October of 1988 Samples were analyzed for TPH as gasoline, TPH as diesel, BTEX, chlorobenzene, 1,2- dichlorobenzene, 1,3-dichlorobenzene, and 1,4-dichlorobenzene for the first sampling event Subsequent sampling events included analyses for TPH as diesel and BTEX only MW-1, MW-2, and MW-3 were completed to depths of 63 feet, 64 5 feet, and 65 5 feet, respectively A drop in the regional groundwater elevation required the addition of deeper monitoring wells Monitoring well MW-4 was completed in October 1992 to a depth of 95 feet, and monitoring wells MW-5 and MW-6 were completed in December 1993 to depths of 89 feet and 88 feet, respectively Since August 1995, TPH as diesel was the only required analyte for groundwater analysis Tables 1 to 4 summanze historical soil and groundwater analytical, bioassay, and geotechnical data collected at the site between 1986 and 1996 Figure 2 summarizes historical data collection locations at the site 4.0 Site Geology The subsurface at the site consists primarily of silts and clays, with discontinuous interbedded silty sands. One laterally continuous sand unit (sand and silty sand) was encountered between approximately 45 feet and 55 feet bgs in all site borings To the east of the former excavation, a clean sand unit was encountered in soil boring MW-2 at a depth of 60 feet bgs This unit does not appear to be laterally continuous, as no indication of it was found in other site borings Another sand unit (sand and silty sand) that may be laterally continuous was encountered at a depth of 75 feet bgs in boxings MW-4, MW-5 and MW-6 Figures 4, 5, and 6 provide cross sections of the site Table 3 provides the geotechnical data for four representative soil samples collected between 14.5 feet and 55 feet bgs at the site 5.a Site Hydrogreology ' The direction of groundwater flow has been observed to vary greatly, possibly due to seasonal variations and the effects of extensive pumping within the shallow regional aquifer. The calculated groundwater gradients are consistently low (0 001 ft/ft to 0 002 ft/ft) Figures 7 to 9 illustrate the potentiometnc surface at the site in February, May, and August of 1996 The groundwater elevations measured and the variability in the flow direction aze exemplified in • these maps The vanabdity observed in 1996 was also observed in groundwater monitoring data collected since 1993 M7/06-13-97/VDBF/97-0009 BK 3