ARCHIVED REPORTS_XR0007038 - Laserfiche WebLink

Home Browse Search

I ERR. • Mr James L Barton ' July 27, 2005 Page 4 ' half of 1985 corresponding with the property changing from a gasoline station to an office building ENSR believes that the sources of petroleum hydrocarbons beneath the site have been removed To demonstrate groundwater plume reduction since remediation began ENSR has included ' charts that show the average concentration and contaminant mass since December 22, 1998 Remediation began at the site in October of 1999 The charts are included as Attachment F ' The average concentration within the plume was determined using the volume function in Surfer® by Golden Software, Inc The mass of contaminant within the plume was found by multiplying the plume volume by an assumed average porosity of 0 3, times the average concentration The average porosity is based on the porosity of sand which is the predominant soil beneath the site Recent and historic soil and groundwater sampling events indicate that the majority of residual and dissolved concentrations of petroleum hydrocarbons are located in the B zone (50 to 70 feet bgs) Therefore, the plume volume was determined by multiplying the planar area by 20 feet The charts show that since December 22, 1998 • o the average concentration of TPI-Ig has decreased from 7,700 NgIL to 1,792 pg/L, o the average concentration of benzene has decreased from 1,249 NgIL to 351 pg/L, © the mass of TPHg has decreased from 276 kilograms (kg)to 59 kg, and, C the mass of benzene has decreased from 43 kg to 10 kg Potential Exposure Pathways ENSR believes that the existing groundwater monitoring well network defines the limits of the SCM The TPI-Ig and benzene plumes appear to be decreasing in size, are stable, and are well defined within the groundwater monitoring well network Groundwater is not considered to be ' an exposure pathway because no drinking water wells exist within the well network The nearest surface water body is the Calaveras River which is located over 4,000 feet to the south-southeast BGsed on the distance from the site, surface water is not cons1UerWU to be a potential exposure pathway The majority of dissolved and residual concentrations of petroleum hydrocarbons are believed to be located approximately 50 to 70 feet bgs and the site and surrounding areas are predominately paved Therefore, soil and biota are not considered to be exposure pathways I ' Celebrating 35 Years of Excellence in Environmental Services