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..M. <br /> break down through natr:ral degradation processes over time. <br /> Given the small volume of soil and the fact that it is capped <br /> by a relatively impermeable asphalt layer, no additional <br /> mitigative measures are recommended with regard to existing <br /> soil conditions. <br /> 4.2 Groundwater Contamination <br /> Laboratory results of groundwater samples collected from site <br /> monitoring wells detected xylene at levels ranging from <br /> om 2.2ne wto <br /> 240 ppb, well below the accepted MCL of 1750 pp <br /> b. as <br /> not entein M in any 0 the watch exceedsbut <br /> the MCLbofZene was <br /> 680 ppb <br /> present in Mn13 at 160Q ppb, <br /> for this constituent. <br /> Groundwater sampling has shown that the hydrocarbons <br /> associated with the former xylene tank may heave impacted <br /> groundwater beneath the site. However, ethylbe;nzene is the <br /> only hydrocarbon constituent detected that exceeds the MCL fo <br /> drinking water. This may reflect the processes of dispersion <br /> and natural biodegradation which are observed at most <br /> underground tank sites involving high boiling point <br /> hydrocarbons (Hadley and Armstrong, 1991). <br /> WaterWork Corporation recommends monitoring and sampling of <br /> the site wells to confirm the groundwater gradient and the <br /> distribution of dissolved contaminants. Frequently, <br /> monitoring wells record their highest levels in the first <br /> sampling event, due to drilling effects. Depending upon these <br /> results, the need for remedial measures, additional wells, or <br /> i on-going monitoring can be assessed. <br /> 5.0 REFERENCES <br /> Califcrnia Department of Water Resources, December 1981, Water <br /> Well Standards: State of Carifornia Bulletin 74-8.1, 92p. <br /> Hadley, P.W. and Armstrong, R. , 1991, Where's the Benzene <br /> - Examining California Groundwater Quality <br /> Surveys, Ground Water, v. 29, no 1, p.35. <br /> Page, .Z.W. , 1986, Geology of the Fresh Groundwater Basin of <br /> the Central Valley California with Restore Maps and Section: <br /> U.S. Geological Survey Professional Paper, 1401-0, 54p. <br /> 6 <br /> LCROSl0-p <br /> Yd11YS1IfJl�41 <br />