ARCHIVED REPORTS XR0002741 - Laserfiche WebLink

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Atlantic Richfield Company 4 ARCO 4932, 16 E Harding Way,Stockton CA June 7,2002 Also in 2001, SECOR performed a utility survey to evaluate whether nearby off-site utilities or other underground improvements may act as potential conduits for migration of petroleum hydrocarbons Since depth to water at the site is approximately 27 feet bgs and no underground improvements or utilities were identified near this depth, it is ' unlikely that any subsurface utilities or other unknown underground improvements are preferential migration pathways for petroleum hydrocarbon-impacted groundwater SECOR concluded that higher potential exists for vapor-phase contaminant migration ' through these areas (SECOR,Additional Site Assessment Report, dated November 9, 2001) ' As a result of the discovery of liquid phase hydrocarbons (LPH) in wells MW-4 and SVE-1 during the third quarter 2001 sampling event on July 20, 2001, weekly interim groundwater remediation was initiated on October 29, 2001, which consisted of the ' removal of LPH and petroleum hydrocarbon-impacted groundwater from the wells with a bailer Interim GWE continued for approximately six weeks until measurable LPH was no longer present Between October 29, 2001 and December 17, 2001, approximately 10 to 15 gallons of LPH had been removed from the wells As a result of the continued presence of elevated concentrations of petroleum hydrocarbons and ' MtBE in on-site wells, interim GWE was reinitiated on March 18, 2002 utilizing a vacuum truck Interim GWE by vacuum truck is currently conducted on a bimonthly basis utilizing selected wells containing LPH or elevated concentrations of petroleum hydrocarbons and MtBE (MW-1, MW-4, MW-6, SVE-1, EW-1) Between March 18 and May 13, 2002, approximately 7,500 gallons of LPH and petroleum hydrocarbon- impacted groundwater had been removed from beneath the site i ASSESSMENT ACTIVITIES AND FINDINGS ' On March 14 and 15, 2002, West Hazmat Drilling (West Hazmat) of Rancho Cordova, California installed one deep groundwater monitoring well (DMW-1) and one GWE well ' (EW-1) at the site (Figures 2 and 3) Wells DMW-1 and EW-1 were completed at 81 feet bgs and 44 feet bgs, respectively Well DMW-1 was screened from 75 feet to 80 feet bgs Well EW-1 was installed with a stainless steel wire-wrap screen, extending from 21 feet to ' 41 feet bgs Well construction details are included in Table 1 Subsurface Conditions In each of the borings for wells DMW-1 and EW-1, soil consisted predominantly of sandy silt from ground surface to approximately 25 feet to 30 feet bgs, with occasional ' interbedded layers of silty sand Below approximately 25 feet to 30 feet bgs, soils within borings for DMW-1 and EW-I were comprised of interbedded layers of sandy silt, clayey silt, sand, and silty sand to the total depths explored (81 feet and 50 feet bgs, ' respectively) Boring logs showing soil stratigraphy are included in Attachment B . Groundwater was first encountered in wells DMW-1 and EW-1 at 30 5 feet and 28 S feet bgs, respectively Stabilized groundwater levels in wells DMW-1 and EW-1 were i 1ARCO\49321Reports\Asmt&AgfrTstRpt doc h