Laserfiche WebLink
r- <br /> Mr Ed Ralston <br /> April 16, 2001 <br /> • Page 5 <br /> Shallow groundwater monitoring wells MW-1 through MW-4 were also sampled as part of the <br /> normal quarterly groundwater monitoring event MtBE was detected in samples collected from <br /> MW-3 and MW-4 at 18 and 11 ppb, respectively MW-3 also contained TPH-g at 67 ppb <br /> Shallow impacts are present in the southern part of the site and are primarily MtBE Detection of <br /> TPH-g in MW-3 is the first occurrence in the shallow zone wells (January 2001 results) <br /> Historical shallow zone monitoring results are presented in Attachment C <br /> Deep groundwater impacts in wells MW-5 through MW-7 show attenuation from the <br /> concentrations detected in the groundwater grab sample collected from GP-2 (54 feet bgs) during <br /> July 2000 assessment activities Additional monitoring is necessary to confirm the results of the <br /> January 2001 sampling of the deep wells <br /> CONCLUSIONS <br /> Site assessment activities identified the following subsurface conditions <br /> • Site soils consist of interbedded sands, silty sands, clayey silts and sandy silts to the total <br /> depth explored <br /> • The vertical extent of petroleum hydrocarbon impact to soil is generally defined to a <br /> . depth of 80 feet bgs Highest detected concentrations of TPH-g and benzene are present <br /> beneath the former dispenser islands at approximately 60 feet bgs and concentrations <br /> attenuate with depth The 60 feet bgs impacts attenuate to the east and to the west <br /> • Deep groundwater at 80 feet bgs is impacted by dissolved petroleum hydrocarbons Low <br /> concentrations of MtBE were detected during the initial sampling Additional sampling <br /> and analysis are necessary to verify the results of the initial sampling and to evaluate if <br /> groundwater impacts previously identified at 54 feet bgs were introduced to deeper <br /> intervals during drilling and well installation activities <br /> RECOMMENDATIONS <br /> On behalf of Tosco, IT recommends conducting an additional monitoring event to assess deep <br /> water quality Based upon the results of the next sampling, IT may propose, on behalf of Tosco, <br /> additional on-site deep monitoring wells Additional delineation may be warranted to further <br /> characterize the lateral extent of deeper soil and groundwater impacts encountered between 50 <br /> and 80 feet bgs The present deep well configuration does not allow for evaluation of <br /> groundwater flow in the 80 feet bgs depth interval <br /> N lrancfp I Trod ectsTroj ects1911\01 I M A101021911011 rpt2 doc <br />