Laserfiche WebLink
Sent 8y: RWQC8 SACRAMENTO; 916 255 3015; Apr-6.99 8:34; Page 517 <br /> - 3 - MAR 2 Z G99 <br /> There are no wells present or soil borings drilled at a distance of 150 feet from the site which <br /> "confirm"the presence of"high"concentsadorts of petroleum constituents in soil or <br /> groundwater. Further, soil and groundwater analytical data for down-gradient monitor wells IM <br /> than 100 fit from the site show only low to"non.detect"concentrations of constituents of <br /> concern. <br /> Inability ill without an OAMM of the maw of bydrocarbousin a i1 an <br /> to . <br /> Phone stability,particularly plumes associated with releases over ten years old, is independent of <br /> the mass of residual petroleum Hydrocarbons in soil or dissolved phase hydrocarbons in <br /> groundwater. As demonstrated by water quality data obtained is February of 1999 and earlier, <br /> the plume has been stable for at least the.past nine years and the residual dissolved please mass <br /> hasbeen�diminishing through natural attenuation,including intrinsic t <br /> Groundwater samples at the site have been analyzed for MTBE and other oxygenates using EPA <br /> Method 8260 on a quarterly basis since March of 1998(five sampling events through February <br /> of 1999). NME has been detected once in each of six monitor wells,and twice in one of the <br /> 11 site wells. Five of the eight`%its",with concentrations ranging from 2.3 to 10 ppb,were <br /> reported in February of 1999,and occurred in wells within the existing,defined hydrocarbon <br /> plume as well as in up-gradient,cross-gradient and down-gradient wells The widespread <br /> distribution and low concentrations of these detections indicate that the occurrences of MTBE at <br /> this site are unrelated to the pre.1999 release,but rather are representative of backSmund water ? T Z <br /> quality. T <br /> Vertical_dtfinif <br /> M of oetrolceem hy_drxns, <br /> In hydrogeologic settings such as is present at the subject site(predominantly silty and clayey <br /> soil with subordinate fine sand),the vertical extent of petroleum hydrocarbon affected soil and <br /> groundwater in the immediate area of a release readily diminishes with depth below the water <br /> table. At this site the vortical extent has been complicated slightly by the fluctuation of the water <br /> table over an estimated range of 25 to 30 feet since the time of the release. Nonethcl ss.the <br /> lowest apparent stand of the water table(about 50 feet below grads)does serve as the datum <br /> below which concentrations of pctrolcutri hydrocarbon constituents in soil and groundwater will <br /> rapidly diminish(usually by two to three orders of magnitude over relatively short vertical <br /> distances). Based on data developed from wont performed in 1992,the vertical extent of <br /> affected soil was Sufficiently characterized to conclude that petroleum hydrocarbon impacts to <br /> soil and groundwater below the depth of about 55 feet were relatively minor and likely not <br /> detectable at a depth of 75 to 85 feet. <br /> California Envirennrenfal Protection Agency <br /> Reayakd POW <br />