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I ica[TeChXi W Page 2 G g g 16 Groundwater Momtonng Report Project No 723 2 February 28,2005 ' calculations show that groundwater flow is N63°E at 0 0070 ft/ft This is consistent with the site's historical directions that have been predominantly to the northeast Field evidence recorded during borehole installation indicates that contamination is present on the adjacent property across N El Dorado Street — an active Shell service station GTI ' recommends that the SJC PHS/EHD files be reviewed for the adjacent site to obtain well construction information and historical contaminant data The data suggest that a co-mingled plume situation may exist and the file review will help clarify the site conceptual model Because the lateral extent of the plume down gradient of the MW-81108/2081308 has not been defined, the SJC PHS/EHD will probably require additional site definition ' We recommend that interim remediation be considered before this plume migrates any further and the cost for cleanup continues to increase Interim remedial action can be initiated by installing a high vacuum dual phase extraction unit to address the on site contamination Extraction should be performed on the existing wells EW-1, VEW-1 & VEW-2 and a new extraction well set at a deeper depth The treatment system should include ' air stripping and carbon filtration of extracted groundwater to be discharged to City of Stockton public treatment works The placement of these components at the site will be problematic due to limited space 1.0 GROUNDWATER MONITORING On November 20 - 26, 2003, three new continuous multi-chambered tubing (CMTrM) wells were installed to further define the vertical and lateral extent of the soil and groundwater ' plumes (see Figure 13 for well locations and Table 5 for well construction details) The boreholes for the new wells' were installed to the north, east, and southwest of the site across E Castle,N El Dorado and W Adams streets, respectively Unfortunately, the CMTTM wells were so tightly coiled for shipping that it was difficult to install the wells without the tubing corkscrewing in the borehole, even when the casing was 1 stretched out on the ground in the sun to warm and relax the coils The result of this twist in the tubing is that depth to water measurements could not be made because the instrument probe becomes lodged in the small chambers of the tubing Although depth to water measurements can not be performed in the individual tubing chamber, water samples are obtained by utilizing smaller diameter tubing with a Waterra pumping system 1 Due to this tubing twist constraint the water gradient was determined using wells MW-1 through MW-6 as has been done in the past 1 i 1 1