Laserfiche WebLink
1wit) <br /> FRI 205513 OSRIO Tosco 76 Service ! a Q�tober 27 09 <br /> In April 1997, ERI performed an investigation to evaluate the source and extent of methyl tertiary butyl <br /> ether(MTBE) (ERI, June 3, 1997) The investigation included hand-auguring three soil borings <br /> adjacent to catch basins, drilling four soil borings, and constructing dual-completion groundwater <br /> monitoring wells (MW4SID through MW7S/D) at discrete depths in these borings Work also included <br /> associated well development and sampling, and analyses of soil and groundwater samples The data <br /> obtained during the investigation indicated the following <br /> • Residual hydrocarbons were not detected at or above the laboratory method detection limits in soil <br /> samples collected from adjacent to three on-site catch basins located near the pump islands, except <br /> for maximum concentrations of 3 3 parts per million (ppm) total purgeable petroleum hydrocarbons <br /> as gasoline (TPPHg), 8 4 ppm total extractable petroleum hydrocarbons as diesel (TEPHd), and <br /> 12 ppm MTBE <br /> • Residual hydrocarbons in soil appear to have been delineated beneath the site <br /> • Groundwater was encountered at approximately 9 to 14 feet bgs During May 1997, static <br /> groundwater elevations in the groundwater monitoring wells ranged from approximately 6 to 9 5 ft <br /> bgs, and the groundwater gradient was towards the southwest at a calculated hydraulic gradient of <br /> 0 002 Groundwater elevations were essentially the same in the shallow and deep wells, indicating <br /> hydraulic communication between the two depths <br /> • The lateral and vertical extent of TPPHg and benzene, toluene, ethylbenzene, and total xylenes <br /> (BTEX) appear to have been delineated beneath the site Concentrations of MTBE were delineated <br /> in each direction, except towards the south and southwest based on the initial sampling of wells <br /> MW4SID through MW7S/D <br /> Based on the results of the investigation, ERI submitted a Work PIan which included 1) obtaining and <br /> reviewing available MTBE data from previous investigations, 2) performing two additional <br /> groundwater monitoring and sampling events and interpreting the data to further evaluate the extent of <br /> dissolved MTBE in groundwater, and select additional well locations (if necessary), 3) drilling three <br /> soil borings adjacent to former catch basins, 4) obtaining appropriate permit(s) and access <br /> agreement(s), 5) drilling off-site soil boring(s), 6) collecting soil samples, 7) constructing groundwater <br /> monitoring well(s), 8) developing and purging the new well(s), 9) collecting groundwater samples, 10) <br /> analyzing selected soil and groundwater samples, and, 11) preparing a report <br /> The County response to the Work Plan requested Tosco complete the following 1) perform a detailed <br /> site assessment to evaluate the lateral and vertical extent of MTBE in groundwater and whether these <br /> compounds have migrated off site, and in addition, analyze groundwater samples for the seven <br /> oxygenated compounds identified in the California Regional Water Quality Control Board-Central <br /> Valley Region (Regional Board) document dated July IS, 1997, 2) perform a conduit study and well <br /> survey to identify underground facilities that may provide potential preferential groundwater flow and <br /> constituent migration, 3) characterize hydrogeology beneath and in the vicinity of the site, and <br /> 4) identify the source(s) of MTBE detected in groundwater beneath the site and the approximate time <br /> frame of MTBE/additives release and history <br /> At the request of Tosco, ERI submitted the Work Plan addendum(dated August 8, 1997) The <br /> addendum addressed the directive for a full dimensional definition and investigation of soil and <br /> 2 <br />