ARCHIVED REPORTS_XR0011155 - Laserfiche WebLink

Home Browse Search

AMBRIA The groundwater chemical analytical data are summarized on Table 3 and TPHg, benzene, and 1,2-DCA concentrations for the discrete groundwater samples are presented on Figure 2 The most current 1,2-DCA concentrations for all site wells and soil borings are presented on Figure 3 For soil borings, historical TPHg and benzene groundwater results are presented on Figure 4 The certified analytical reports are included in Appendix D DISCUSSION The purpose of the investigation was to delineate the northern extent of the Shell plume Due to the limited available drilling locations north of the site, the parking lot of the property located at 2491 East Fremont Street (the offsite property) was selected as the location to drill our soil borings Two soil borings and two monitoring wells were proposed for this investigation The wells were to be installed to provide repeatable monitoring data to the north of the Shell site Vadose zone soil samples collected from borings advanced during this investigation contained TPHg as high as 19,000 ppm The fact that TPHg is present in unsaturated soil beneath this property suggests that there is a source of petroleum hydrocarbons beneath this site and that the petroleum hydrocarbons detected beneath this site do not originate from the Shell site Groundwater samples collected from the borings advanced during this investigation contained TPHg as high as 19,000 ppb,benzene as high as 1,300 ppb, and 1,2-DCA concentrations as high as 57 ppb We prepared two isoconcentration contour neaps to depict the distribution of TPHg and 1,2-DCA in groundwater beneath the site vicinity The TPHg and 1,2-DCA data collected from the 80 to 88-foot depth was chosen because analytical data was available for most of the borings at this depth, and the deep wells are also screened at this depth Choosing a different depth would result in far fewer data points for creating the isoconcentration contours The isoconcentration contour maps are to be considered conceptual since the data used to create the contours is from different dates — in some cases several years apart We recognize that it is not typical to use data from different dates together to portray the location of a plume, but this is the only data we have to work with We also notice that, at least for 1,2-DCA,the data from the wells appears to be fairly stable over the years, which provides some validity to this analysis We used the first analytical data set collected after well installation for each well Figure 3 depicts the TPHg data and shows two separate plumes One plume is beneath the Shell site and the other is beneath the offsite property Figure 4 depicts the 1,2-DCA data and shows • 0797 g