Laserfiche WebLink
• • <br /> page 2, 640 EI Dorado <br /> completing the vertical assessment in the former UST pit area at this time, Ground Zero <br /> proposes to investigate to greater depths during the installation of the currently proposed <br /> monitoring wells. SJC/EHD finds this line of reasoning by Ground Zero against further <br /> vertical assessment in the former UST area at this time to be more acceptable, therefore <br /> SJC/EHD will postpone requiring the vertical assessment in the former UST area during <br /> this phase of work. However, to address SJC/EHD concerns regarding migration of the <br /> contaminant plume at greater depth, the following is required: <br /> 1. The currently proposed monitoring well borings are to be advanced to 50 feet <br /> below surface grade (bsg), as proposed by Ground Zero, and continuous <br /> cores are to be collected from 20 feet bsg to total depth. <br /> 2. The pilot borings will extend past 50 feet bsg if necessary to avoid bottoming <br /> in a sand lens with an unknown lower extent. The borings should bottom in a <br /> significant fine-grained unit. <br /> 3. The third well proposed for the corner of EI Dorado Street and Park Street is <br /> installed following the same procedures as the currently proposed wells. <br /> 4. If more than two sand units are encountered in the saturated zone during <br /> advancement of the pilot borings for the monitoring wells, additional grab <br /> groundwater samples should be obtained for laboratory analysis. <br /> SJC/EHD believes these conditions are necessary for the following reasons: <br /> 1. While hydrocarbons do not appear to be a'significant issue, other than as free <br /> product, the VOC's are of concern and require careful characterization of <br /> potential contaminant migration pathways, particularly the sand units. <br /> 2. SJC/EHD's and Ground Zero's interpretations of the sand units are not quite <br /> the same, although both are supported by the currently available data. The <br /> differences can be significant for properly characterizing the extent and <br /> migration pathways of the dissolved solvents. <br /> 3. If Ground Zero's interpretation is correct, then there may be a southerly slope <br /> to the 25-30 foot sand unit, which would influence migration of the heavier <br /> solvents if released as free phase. If SJC/EHD's interpretation is correct, the <br /> sand unit in MW-1 would not be the same unit as in MW-3 and other potential <br /> migration pathways must be considered. <br /> 4. The locations of the currently proposed two wells will cause a fairly linear <br /> arrangement of monitoring wells, which contributes to error in determining <br /> groundwater flow direction and structural attitude of hydrologic units. <br /> 5. The third originally proposed well may contribute to understanding the origin <br /> of the solvents by providing delineation of the plume toward the northwest or <br /> potentially demonstrating the possibility of an offsite source. <br /> Ground Zero states that they want to attempt removal of the free product that has been <br /> present for the past year in the source area well, MW-3, before attempting to drill in this <br /> area. However, Ground Zero did not discuss a proposal for how they wanted to remove <br /> the free product. Ground Zero included in the work plan a charted schedule of proposed <br /> work for this site. One task listed on this chart is the installation of a "passive or active <br /> skimmer for continuous product removal from MW-3". SJC/EHD presumes that this, <br /> therefore, is the method Ground Zero intends to use to remove the free product. <br />