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Work Plan for Additional Subsurface Investigation
<br /> Former Helena Chemical Facility
<br /> July 13,2020
<br /> Page 4
<br /> 4.1 STEP-OUT BORINGS AND SHALLOW GROUNDWATER MONITORING WELLS
<br /> ...step-out borings should be advanced until the lateral and vertical boundaries with non-detect levels of
<br /> nitrate and 1,2,3-trichlororopropane(1,2,3-TCP) emanatingfrom the Site are identified.
<br /> Condor proposes to install three shallow groundwater monitoring wells in the locations shown on Figure 6
<br /> (Appendix A);the rationale for each well is presented below:
<br /> 1. Proposed well MW-6:to be installed near the location of highest detected 1,2,3-TCP concentration
<br /> in shallow "grab" groundwater samples (B39 and B42, Table B-3, Appendix B) to confirm
<br /> concentrations of 1,2,3-TCP;
<br /> 2. Proposed well MW-7: to be installed in the northeastern portion of the Site to delineate the lateral
<br /> extent of 1,2,3-TCP and nitrate-N in that direction; and,
<br /> 3. Proposed well MW-8: to be installed off-site, northeast of the location of the highest detected
<br /> 1,2,3-TCP (B39 and B42, Table B-3,Appendix B) and nitrate-N(MW-2, Table B-3,Appendix B)
<br /> in shallow groundwater. Unfortunately, no monitoring well can be placed directly north of the
<br /> 1,2,3-TCP "hot spot" that could define the lateral extent to the north due to the presence of an
<br /> off-site warehouse building. Boring B49, drilled immediately south of the off-site warehouse,
<br /> contained 1,2,3-TCP as high as 0.49 gg/L and nitrate-N as high as 220 mg/L(Table B-3,Appendix
<br /> B).
<br /> Out of an abundance of caution,Condor does not recommend drilling into deeper water-bearing zones until
<br /> the lateral extent of impact by 1,2,3-TCP is known because 1,2,3-TCP is denser than water and may readily
<br /> migrate downward through vertical conduits introduced during drilling.
<br /> 4.2 RECEPTOR SURVEY
<br /> ...a receptor survey should be conducted to identify potential receptors within a 1,000foot radius of the
<br /> Site.
<br /> Condor previously prepared a sensitive receptor survey for the Site, which was presented in the Revised
<br /> Work Plan for Additional Site Investigation, Former Helena Chemical Facility, 2245 West Charter Way,
<br /> Stockton, California, dated February 27, 2018. The sensitive receptor survey portion of that report is
<br /> included in Appendix C.
<br /> 4.3 SUPPLEMENTAL SOIL AND GROUNDWATER SAMPLE ANALYSIS
<br /> ...soil and groundwater at the Site needs to be analyzed for 1,2-DCP and 1,3-DCP, at a minimum, in the
<br /> area where the highest 1,2,3-TCP has been detected.
<br /> Condor will analyze soil and groundwater samples collected from proposed well location MW-6 for the
<br /> presence of 1,2-DCP and 1,3-DCP.If initial analytical results indicate the presence of these chemicals,then
<br /> the remaining monitoring wells will be tested for the presence of these chemicals.
<br /> 4.4 INVESTIGATION OF ON-SITE STORMWATER BASIN
<br /> ...at least one boring be advanced within the stormwater detection basin to assess and confirm that
<br /> 1,2,3-TCP has not migrated to groundwater and created a 1,2,3-TCP Plume in this portion of the Site.
<br /> One stormwater sample collected in May 2016 contained 1,2,3-TCP at a concentration of 0.00026 ug/L,
<br /> which was below the Maximum Concentration Limit (MCL) of 0.005 ug/L. Since a certain amount of
<br /> attenuation would be expected as surface water infiltrate through soil, there is no reason to think
<br /> ��r CONDOR
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