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A%UO U <br /> very similar, with a rather flat hydraulic gradient of <br /> 0 . 002 ft/ft (a drop of about 1/4 inch per linear foot) . <br /> Figures 4 and 5 describe the most recent calculations of <br /> the distribution of soil and groundwater contaminants , <br /> which are at their highest concentrations in the northwest <br /> quadrant of the site. The extreme borders of the <br /> contaminant plume(s ) have been defined by the placement of <br /> borings advanced during October , 1993 and trenches <br /> excavated during February of 1994 . <br /> During October of 1993 , levels of Total Petroleum Hydro- <br /> carbons as Gasoline (TPH-G) in groundwater along the <br /> eastern boundary of the site were 1 part(s) per million <br /> (ppm) at SBR-5 , 0 . 6 ppm at SBR-8 , and 1 ppm at SBR -3 . <br /> TPH-G in groundwater at the northeast border of the site <br /> (i . e . at SBR-4) was only 0 . 5 ppm. <br /> Soil samples acquired from the capillary fringe at the <br /> trench excavated along a portion of the western boundary of <br /> the site during February, 1994 were free of detectable <br /> concentrations of gasoline, diesel fuel , benzene, toluene, <br /> ethylbenzene, and total xylenes . The oil and grease that <br /> was detected in one of the three samples obtained from the <br /> boundary trench is believed to represent asphalt <br /> interference . The soil sample obtained from the trench <br /> advanced near the northwestern edge of the on site building <br /> was free of detectable concentrations of all target <br /> analytes , indicating that we were successful in removing <br /> all significantly contaminated soil from the "point source" <br /> revealed during preparation of the isocentration contour <br /> map prepared while developing data acquired during <br /> placement of the October, 1993 soil borings. <br /> The isoconcentration contours for TPH-G for both soil and <br /> groundwater reveal an overlapping pattern that provides <br /> reasonably definitive plume boundaries and indicates that <br /> the areal distribution of contaminants is a function of <br /> groundwater movement . Depth wise , significantly <br /> contaminated soil is confined to the portion of the profile <br /> that is , or has been, in contact with the LNAPLs present <br /> near the top of the water table. Like most organic <br /> contaminants , petroleum hydrocarbons may be present in the <br /> subsurface in one, or more of four states : As free <br /> hydrocarbons floating on the surface of the water, as <br /> hydrocarbons dissolved in water, as vapors occupying the <br /> pore spaces above the saturated zone, or as liquid sorbed <br /> onto organic matter or clay particles in the saturated zone <br /> and the zone immediately above the water table. <br /> Page 7 <br />