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Lisa Medina [EH] <br /> From: Fortner,Thomas <Thomas.Fortner@WestonSolutions.com> <br /> Sent: Friday, October 18, 2013 9:03 AM <br /> To: Lisa Medina [EH]; bertocchiniv@sjgov.org <br /> Cc: Kerry Wasson [GOVB];Gabriel Karam [FMCP] <br /> Subject: Human Services Agency Low Risk Closure Request <br /> Attachments: Proposed CPT Locations.pdf <br /> Hi Lisa, <br /> I was hoping to have you review our approach to address the EHD's response to the low risk closure request and the <br /> plan to provide sufficient information needed for the evaluation for a low threat closure. I had prepared the following <br /> proposed work to the County Facilities Management and would like to have the EHD's input prior to preparing a formal <br /> work plan: <br /> 1) Demonstrate the down-gradient extent of impacted groundwater from MW-2, the most impacted monitoring <br /> well. -Two groundwater monitoring wells(MW-6 and MW-7)are located in the general down-gradient direction of the <br /> former USTs.The EHD is concerned that the plume may extend between these two wells.WESTON proposes a two-day <br /> sampling event to collect"grab"groundwater samples at locations along the north side of Sutter Street and from the <br /> city owned lot behind the Gan Chy Chinese Restaurant.The sampling can be conducted using a cone penetration testing <br /> (CPT) rig that electronically logs the lithology by driving steel rods into the ground and can also collect groundwater <br /> samples.The CPT truck drives steel rods into the ground using the weight of the truck.The operation of the CPT is not <br /> disruptive and does not create soil cuttings which would require disposal. Proposed VT locations are shown on the <br /> attached figure. nea% 40•-6s + 3b, "') 612 66.'— t w 4 ^µ 4 a, ( ; ,1J.s .k /.r <br /> �2) Provide an estimate of the residual dissolved and sorbed contaminant masses. -This is also referred to as a <br /> secondary source of contamination with the primary source being the USTs.WESTON can make this calculation based on <br /> existing data. D,- 2.,d <br /> v% 3) Provide an estimate of the time required for dissolved contaminant concentrations to achieve background <br /> conditions of groundwater.—WESTON proposes to develop and utilize a fate and transport model based on USEPA's <br /> screening tool BIOSCREEN-AT to predict: <br /> a. How far will the dissolved benzene plume will extend if no engineered controls or further source zone reduction <br /> measures are implemented; and <br /> b. How long will the plume persist under groundwater flow conditions — '/,'(is = �,.Ke 00e 7 <br /> Details of the BIOSCREEN-AT model are presented at the following modeling repositories: e ea tv- COW P-"(7 °K- <br /> • http://www.epa.gov/ada/csmos/models/bioscrn.html <br /> • http://www.sspa.com/software/bioscreen-at.htmi <br /> The framework of the model will be based on the updated CSM to incorporate refinements in the benzene plume <br /> extents based on the proposed CPT investigation. <br /> /�4) Provide a demonstration the secondarysource in the area of MW-2 has been remediated to the extent <br /> practicable.The secondary source of contamination is largely beneath the HSA building. In 1989, prior to construction of <br /> the building,the USTs and 2000 cubic yards of impacted soil were removed to the depth of groundwater. Due to great <br /> fluctuations of the groundwater levels in 1977 and 1999,secondary contamination has been carried down with the <br /> groundwater levels to the depth of 50 to 60 feet and cannot be removed from beneath the building. <br /> 1 <br />