ARCHIVED REPORTS XR0004909 - Laserfiche WebLink

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MASS ESTIMATES Soil: We considered preparing soil isoconcentration contour maps to allow us to calculate mass estimates in soil, but this did not seem appropriate given the following considerations Soli analytical data collected at this site has been collected over a 15-year period Most of the shallow soil data was collected during 1988 to 1993 Using the old data to estimate the mass of hydrocarbons remaining in the soil is not practical as remediation has been performed at the site and natural biodegradation has reduced the mass of residual hydrocarbons remaining in the soil The old data would overestimate the hydrocarbon mass remaining in soil We evaluated the newer data (CPT borings) to see if it could be used for mass estimates We found that there was insufficient data per sample interval to create representative isoconcentration contours In addition, the majority of the soil data from the CPT boring was below the detection limits There are petroleum hydrocarbons present in the soil beneath this site, but the majority of the petroleum hydrocarbons are in saturated soils (below 25 fbg) The available soil data suggest there is little impacted vadose soil Soil data collected to date indicates there is little, if any, MTBE impacted soil beneath the site There has been only one detection of MTBE in soil beneath the site (0 0334 ppm in CPT-1 at 60 fbg) Groundwater The mass of TPHg, benzene, and MTBE in the deeper-screened zones was not estimated because the deep-screened wells are located in a line which does not allow for the triangulation needed to construct isoconcentration contours Once additional deep-screened wells are installed (as proposed), mass estimates in the deep-screened zones can be calculated The mass of TPHg, benzene, and MTBE remaining in shallow groundwater was estimated using the following equation Mass(lbs)=Aquifer Volume(gal)x Porosity(%)x Avg Concentrat►on(pglL)x 109 kg/pg x 3 785(Ugal)x 2 205(lbslkg) Cambria used the groundwater analytical data from the August 2004 groundwater sampling event for these calculations Isoconcentration contour maps are presented as Figures Al, A2, and A3 A depth interval of 20 fbg (the typical screened interval for monitoring wells at the site) was assumed for these calculations The porosity was assumed to be 40 percent, based on typical porosity values for soil types published in Groundwater, by Freeze and Cherry, 1979 Since the soil types found beneath the site primarily consist of sand, silt, and clay, the midpoint of where