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Appendix C:Initial S"Checklist Farmer Marley Cooling Tower Company,Stockton,CA <br /> zone of the most significant arsenic-impected soil. Two wens will be located In the area of the former <br /> retort pit with well screen lengths extending approximately 150 feet. Three wells will be located east of <br /> the former retort pit, in an area formerly used for solution storage in aboveground storage tanks. The <br /> screen length of the three wells will beapproximately85 feet. Drilling angles and horizontal directional <br /> drilling technology will be employed. This will enable better distribution of flushing water without <br /> disturbing the existing retort pit cap or the near surface arsenicicopper contamination underneath. <br /> 2. Flush all wells with reductant solution to mobilize and treat the chromium from the vadose zone to <br /> groundwater.The effluent water from the treatment plant will be piped to a holding tank for use as the <br /> reductant solution. From the holding tank the effluent water will be piped in parallel by manifold directly <br /> to the horizontal wells at specified rates. The two deep horizontal wells will deliver the reductant <br /> solution in the low part of the vadose zone and groundwater beneath the retort pit area.An initial dosing <br /> of reductant solution will be utilized in the two deep wells and will be determined when specific volumes <br /> and concentrations are fully evaluated. If necessary, periodic redosing would be performed. <br /> 3. Routine operations and monitoring inspections will be performed weekly to ensure that the system Is <br /> functioning correctly. Personnel will inspect all aboveground piping, fittings, and all other associated <br /> equipment and make repairs as needed. <br /> 4. Create 10 in-situ linear reactive treatment barrier zones strategically placed in accessible locations <br /> across the plume to address movement of impacted groundwater via the observed plume migration <br /> trends and natural groundwater flow. This concept allows enhancement of remedial actions despite the <br /> access limitations posed by the existing development of areas overlying the plume. Reductant solution <br /> will be injected using rotary sonic rig drilling technology to create each reactive barrier in a similar <br /> manner and spacing as the Phase II pilot study. Casings are advanced to the bottom of the specified <br /> interval (beginning with the shallowest)and then raised 8 feet to expose the interval of the borehole for <br /> Injection. The reductantsolution is transferred through hoses from mixing tanks into pumps and through <br /> the drill casing into the subsurface. Shallow injections will be advanced across multiple contiguous eight <br /> foot vertical intervals from approximately 55 to 85 feet to treat the shallow groundwater. Deep injections <br /> will be advanced from approximately 65 to 115 feet to treat both the shallow zone and 100-foot sand <br /> Zone. <br /> 5. For the most severely impacted areas such as the core of the plume with concentrations in excess of <br /> 5,000 micrograms of chromium per liter (ug/L), direct treatment in a grid injection approach to eliminate <br /> this principle source of contamination. The cleanup level for chromium is 0.050 milligrams per liter <br /> (IT191 • <br /> 6. Characterize the site lithologic heterogeneity to ensure optimum usage of treatment chemicals, areas <br /> targeted for reductant injection following the same approach used during the pilot study to evaluate the <br /> lateral and vertical distribution of chromium in high and low permeable zones. Soil borings will be <br /> advanced using a Cone Penetrometer Testing rig which pushes rods into the subsurface. Parameters <br /> will be measured to quantify soil characteristics and hydrologic zones. Depth-discrete groundwater <br /> samples will be collected using a hydropunch, or equivalent method. <br /> 7. Modify the compliance and monitoring program to incorporate other analytes of interest(e.g. arsenic) <br /> and to monitor the performance of the remediation. If monitoring suggests elevated concentrations of <br /> arsenic greater than 500 ug7L persist beyond a one (l)-year period, contingency measures will be <br /> implemented including continued or renewed extraction of groundwater to capture the soluble arsenic. <br /> The federal Maximum Contaminant Level (MCL)for arsenic is 10 ug/L in drinking water. Current arsenic <br /> concentrations are below the MGL of 10 ug/L. Evaluation of groundwater sampling analytical results will <br /> be performed after each quarterly groundwater sampling event to monitor the distribution of reductant <br /> solution within the aquifer. Upon completion of the in-situ injection treatment, evaluation of groundwater <br /> sampling results will assist in determining whether any additional in-situ treatments are necessary to <br /> treat remaining hot spots. Due to the heterogeneity of the site lithology, it is possible that supplemental <br /> injections may be necessary in areas of the site where hot spot concentrations remain. Lithoiogic zones <br /> comprising finer grained material (slits and clays) that are less permeable may retain chromium and <br /> require additional dosing to effectively reduce concentrations. If after evaluating the available data, <br /> I additional injection locations are necessary, the proposed Injections, depth of injections, and specific <br /> solution blending details (solution strength) will be presented for regulatory approval prior to additional <br /> Injection implementation, <br /> Page 2 o 25 <br />