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Nestle USA, Inc.—Ripon, CA January 28, 2011 <br /> 2011 Revised Feasibility Study <br /> because their permeable nature does not substantially disrupt the groundwater <br /> flow system (if functioning correctly). A PRB can be thought of as a highly <br /> controlled, relatively narrow in-situ reaction zone. The reactivity and hydraulic <br /> performance of the PRB must outlast the timeframe that the source zone <br /> threatens groundwater quality. In general, PRBs are permanent containment <br /> installations intended to function for long time periods (decades). Although PRBs <br /> destroy mass in-situ, they do not accelerate source zone cleanup because COC <br /> mass is delivered to the PRB by groundwater flow, which is a relatively slow <br /> process. <br /> PRBs composed of zero-valent iron (ZVI) have been proven to be effective for <br /> removing TCE and its chlorinated breakdown products from groundwater"'. The <br /> degradation reaction occurs by two separate abiotic mechanisms: <br /> ZVI (Fe°) reduces chlorinated VOCs by transferring electrons from the metal to <br /> the target compound, as shown for PCE: <br /> PCE + 4Fe° +4H2O—> ethene + 4Fe2+ + 4C1-+40H- <br /> Where: <br /> PCE = tetrachloroethene <br /> FeO= ZVI <br /> H2O = water <br /> Fe 2+= ferrous iron ion <br /> Cl-= chloride ion <br /> OH-= hydroxide ion <br /> The general reaction above summarizes different treatment pathways which <br /> occur concurrently (primarily electron removal [R-elimination] and reductive <br /> dechlorination) that reduce PCE"1"°X1"',X1'vIn addition, ZVI corrodes under anaerobic <br /> conditions to produce ferrous iron, hydroxide (which can cause a pH increase), <br /> and hydrogen gas (which can stimulate the biological reduction of PCE): <br /> FeO + 21­12O —> Fe 2+ + 2OH- + H2 <br /> Where: <br /> H2= hydrogen gas <br /> PRBs can also be constructed with reactive media such as mulch and other <br /> organic substrates to promote the passive biological degradation of dissolved <br /> TCE in groundwater moving across the PRB alignmentX" The primary reaction <br /> is the biological reductive dechlorination, which is already occurring in site <br /> groundwater where cis-1,2-DCE and VC are present. In general, the cost for <br /> these biowalls can be as low as a quarter of the cost of the ZVI-PRB option, but <br /> 29 <br />