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Nestle USA, Inc.—Ripon, CA January 28, 2011 <br /> 2011 Revised Feasibility Study <br /> years. The cost for implementing pump-and-treat is higher than most other <br /> technologies being considered due to its high O&M cost. Disposal options for <br /> extracted and treated water are a challenge in Ripon. Limitations include the <br /> current City WWTP capacity and high TDS concentrations adding costs for TDS <br /> removal for any other beneficial usex'. Therefore, this technology although <br /> potentially suitable as an interim containment option, should be carefully <br /> evaluated as a long-term remedy. <br /> A network of groundwater extraction and injection wells was also evaluated as a <br /> treatment barrier for COC-affected groundwater near the WWTP. This approach <br /> involved groundwater extraction, aboveground treatment, and re-injection of <br /> treated groundwater at an adjacent location. Alternating extraction and injection <br /> wells were assumed, so that all groundwater passing through the barrier would <br /> be captured and treated prior to re-injection. As discussed in Attachment 13.3, <br /> this approach would not be protective of groundwater receptors downgradient of <br /> the recirculation barrier, and therefore no short-term risk reduction would be <br /> realized. Given the very high cost of this approach, and marginal benefit in terms <br /> of overall risk reduction, this technology was not considered a feasible alternative <br /> for treating groundwater affected by historical discharges at the WWTP. <br /> 9.2.2 Impermeable Barriers <br /> Impermeable barriers (or low permeability barriers) provide containment by <br /> limiting groundwater movement away from, to, or through zones of chemically- <br /> affected saturated soils. This approach is intended to isolate a source zone from <br /> the groundwater flow system, thereby limiting the mass discharge from the <br /> source. Because mass discharge of COCs is limited, the source zone or high <br /> concentration dissolved plume could persist for a longer timeframe compared to <br /> other containment options such as groundwater extraction and permeable <br /> reactive barriers, if the CDCs comprising the source zone are not degrading <br /> naturally. <br /> Impermeable barriers typically are designed as permanent subsurface structures <br /> intended to function for long periods of time (decades to centuries) depending on <br /> the anticipated persistence of the source zone chemical constituents in the <br /> subsurface. The most common type of impermeable barriers are slurry walls, <br /> which are often constructed by excavating a trench and backfilling the trench with <br /> site soils augmented with low permeability material, such as bentonite clay. <br /> These types of installations are easily installed to depths of 30 feet bgs or less, <br /> however, it is possible to reach depths greater than 50 feet bgs. Other <br /> technologies have been developed for the construction of deeper containment <br /> systems, including the Vibrated Beam method (a grout injection method) and the <br /> Cutter Soil Mixing method (a deep soil mixing machine). Steel sheet pile walls <br /> are also used to construct impermeable barriers, where the joints between <br /> separate sheet piles are sealed with grout. These installations are limited to <br /> shallower depths and smaller areas, due to cost considerations. <br /> 27 <br />