ARCHIVED REPORTS XR0007820 - Laserfiche WebLink

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I ' The following is a summary of milestone events at the site ,u ' In September 1991,six soil borings were drilled at the site = t' ' 1 4 On 9 and 10'February 1994, groundwater monitoring wells MW-1,MW-2 and MW-3 were installed at the site w In November 1994, ground water monitoring well MW-4 was installed at the site >; `r' " Iv •' On 13 June 1995,'three soil probe borings were advanced at the site On 14 and 15 June F_ ' 1995, soil vapor samples were obtained from the site • On 25 and 26 September 1995,four soil bonngs were drilled at the site,borings VE-1, VE-2, ' VE-3 and VE-4 were completed as dual-screened soil vapor extraction wells • On 24 April 1997, groundwater monitoring well MW-5 was installed at the site ' On 22 May 1998, four soil probe bonngs (ASB4 to ASB7)were advanced at the site s • On 04 November 1998, ground water momtonng well MW-6 was installed on-site ' On 10, 11 and 12 January 2000 ground water monitoring wells MW-7 and MW-8 were installed on-site, after a failed sanitary seal (grout) had penetrated the transition seal (bentonite) and infiltrated the sand filter pack ' To date, a total of 1'8 ground water monitoring events have occurred at the site I '• Assessment activities at the site to date indicate that • The vadose zone at the site is composed of fine grained sediments Below the water table, the hydrologic structure is composed primarily of fine grained sediments with discontinuous sand units • r The majority of the unsaturated, hydrocarbon-impacted soil resides below former UST TK1-90, to a depth of 30 feet below surface grade (bsg) Saturated hydrocarbon-impacted soil) extends to a depth of 80 feet bsg, however, below 40 feet bsg, hydrocarbon ' concentrations generally decrease with depth The mass of hydrocarbons in the unsaturated soil is greater than the saturated soil (Figures 3a and 3b) ' The highest concentration of,hydrocarbon-impacted,,ground water has remained below former UST ITK1`,90 The maximum benzene concentration was 41,000 micrograms per liter (µg11), detected in a water sample from vapor extraction well VW-1, located at the former ' UST area q ' • TPH-g and BTEX concentrations have steadily declined in the ground water samples ; collected from the deep screened well MW-6 (screened 70 to 80 feet bsg), located at the ' former UST TK1-90 area, the benzene concentration detected on 28 January 2002 was 2 1 µg11 compared to the initial value of 7,000 µg/1 detected on 11»December 1998 ' The lateral extenbof hydrocarbon-impacted ground water has been assessed down-gradient and cross-gr'adient'o'fthe former UST area (Figure 2) s a I I , J � I