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' GRANITE CONSTRUCTION FACILITY <br /> WKA No 2088 08 <br /> August 18, 1999 <br /> Page 12 <br /> Dissolved Groundwater Contamination <br /> Dissolved groundwater contamination was assumed to be limited to the area in the immediate <br /> vicinity of well MW3 and extending half way to wells MWI, MW2, MW4, and MW5 <br /> (Plate No 5) <br /> For our calculations, we assumed that the samples were representative of the top twenty four feet <br /> of the groundwater column and that the native sandy silt/clayey silt material has a relative <br /> porosity of approximately 40% Based on these assumptions, the estimated volume of dissolved <br /> petroleum hydrocarbons present in the groundwater beneath the site was calculated to be <br />' approximately 18 gallons Results of laboratory analysis used, along with statistical calculations <br /> are provided on Tables 1 through 5 and 7 through 10 Estimates of mass volume calculation for <br /> soil and groundwater are presented in Appendix C <br /> Conclusions and Recommendations <br /> It is our opinion that the i " <br /> p site has met the criteria for classification as a low risk groundwater <br /> 1 case" as defined by the CVRWQCB,based on the following <br /> e The source of petroleum hydrocarbons (underground storage tanks and dispenser <br /> islands, and contaminated soil has been removed <br /> e The subsurface soil and groundwater beneath the site has been adequately <br />' characterized We estimate that approximately 27 gallons of petroleum <br /> hydrocarbons (0 09 gallons of benzene) remain in subsurface soils and <br /> approximately 18 gallons of petroleum hydrocarbons (0 18 gallons of benzene <br /> 1remain in groundwater in the vicinity of the former USTs <br /> e Petroleum hydrocarbons have decreased significantly in groundwater samples <br /> collected from monitoring wells MWI, MW2 and MW3 located in the vicinity of <br />' the underground storage tanks <br /> e Groundwater depth and flow direction has remained relatively unchanged since at <br /> least 1994 <br /> ie No detectable concentrations of petroleum hydrocarbons have been reported in <br /> monitoring wells (MW4 and MW8) located between known contamination and <br />' the onsite industrial supply wells with the exception of 0 49 µg/1 detected in <br /> MW4 on January 1, 1995 <br /> `�W <br /> WALLACE•KUHL <br /> Recycledpa(x &ASSOCIATES INC <br />