ARCHIVED REPORTS XR0005957 - Laserfiche WebLink

Home Browse Search

. 4.1.3 Groundwater Flow Direction Eleven hydraulic gradient maps have been constructed since this investigation began in 1994. Figure 5 summarizes the groundwater flow direction determined from these maps. Initially, the flow direction was consistently eastward and the gradient appeared to be relatively planar. The gradient and flow direction became more complex following the,installation of additional monitor wells and when active remediation commenced at the former Chevron station site. Recently, the flow direction has been to the northwest or north. 4.2 Extent of Soil Contamination As discussed in section 3.2.2, the extent of soil contamination was delineated during the second phase of investigation, when five Geoprobe& borings were drilled around the perimeter of the UST cavity. The impacted area is rather limited in extent, and does not extend under the building or to either of the streets that border the site (Figure 3). It is elongate in a north-south direction, parallel to the long axis of the underground tanks, and measures approximately 55 feet long by 35 feet wide. Hydrocarbons are primarily restricted to the interval between 15 feet and 25 feet (Figure 6). Therefore, the estimated volume of soil that was impacted is 55 ft x 35 ft x 10 ft= 19,250 cu ft = 713 cubic yards. Some of this impacted soil was removed during the tank removal, but it is estimated that 500 cubic yards of slightly contaminated soil remain in place around the margins of the UST cavity. Nearly all of this soil is in the lower part of the upper clay bed, although hydrocarbons are also present within the saturated pore space of the sandy middle unit at a depth of 25 feet. 4.3 Extent and Volume of Contaminated Groundwater As discussed in section 3.3, recent samples from five of the seven monitor wells have not contained detectable hydrocarbons. The data indicate that the limit of groundwater contamination lies south of KF-1 and west of KF-2 and KF-4 (Figure 7). The area of impact beneath the Kwikee site forms a right triangle measuring 80 ft x 85 ft, or approximately 3400 square feet in size. It appears that 750 ppb is a reasonable visual estimate of the mean concentration within this area. Using this figure and conservatively estimating that the impacted thickness of the dissolved-phase plume is 3 feet yields an estimate of the volume of gasoline in the aquifer: (750 x 10-9) x(3,400 f12)x 3 ft x 25%porosity x 7.48 gallonslft3 = 0.14 gallons of gasoline The TPH-g isocontours in Figure 7 have been extended westward onto the former'Chevron site, using the May 2001 laboratory data collected by Gentler-Ryan and SECOR. The map shows quite clearly that groundwater contamination beneath Kwikee Foods is part of a larger plume that is situated beneath the eastern portion of the former Chevron site. Overall, the plume is elongate in a west-northwest/east-southeast direction, but the central core of the plume appears to have a strong north-south orientation. The orientation of the plume matches well with the groundwater flow information (Figure 5), which has ranged from southeastward to northeastward during much of the past seven years. This map provides rather conclusive evidence that the Chevron site is the principal source of contamination beneath the Kwikee Foods property. 6 _ '