Laserfiche WebLink
OUR <br /> RWQCB Comment 3: <br /> LFR has requested the cessation of sampling for hydrogen in the Phase V performance <br /> monitoring. The request is based on recent literature that suggests competing processes using <br /> hydrogen are inherent at a wide range of concentrations, and no longer useful as a tool for system <br /> performance. <br /> Staff believes it is premature to eliminate hydrogen analysis. The presence of hydrogen is part of <br /> the total picture for evaluating that ERD has driven the all contaminants and daughter to <br /> innocuous end products. <br /> Response 3: <br /> The current monitoring program was based on protocols developed for monitored natural <br /> attenuation (MNA; U.S. Environmental Protection Agency 1998). In hindsight, hydrogen is a <br /> useful indicator of dominant terminal electron accepting process in "natural environments"; <br /> however, in "substrate-enhanced environments," such as ERD barriers, hydrogen is not as useful <br /> because a wide range of hydrogen concentrations can be present. In ERD barriers, the added <br /> organic substrate is first fermented to molecular hydrogen and low-molecular-weight fatty acids. <br /> These short-chain molecules in turn provide carbon and energy to the microorganisms, which <br /> facilitate reductive dechlorination. In the reductive dechlorination process, microorganisms help <br /> use up the hydrogen by sequentially replacing chlorine atoms with hydrogen. In addition, other <br /> novel degradation mechanisms/factors can affect both VOC and hydrogen concentrations, such as <br /> anaerobic oxidation, the role of electron shuttles, and aerobic cometabolism. These mechanisms <br /> are the subject of ongoing research and are not well understood (e.g., ESTCP Workshop on In <br /> Situ Biogeochemical Transformation of Chlorinated Solvents, February 2008). <br /> In summary, a large range of hydrogen concentrations can be present in "substrate-enhanced <br /> environments" because hydrogen is continuously both produced and consumed. For "substrate- <br /> enhanced environments," direct redox conduction indicators such as DO, ORP, nitrate, <br /> manganese, sulfate, iron (II), and methane are significantly more useful than hydrogen because <br /> they serve as direct indicators of dominant terminal electron accepting process and geochemical <br /> conditions favorable for bioremediation. A recent "Protocol for Bioremediation of Chlorinated <br /> Solvents Using Edible Oil" (U.S. Air Force 2007) recommends only one round of hydrogen <br /> sampling as an optional diagnostic tool. <br /> As documented in the Injection Completion Report, the results of groundwater sampling performed <br /> during and approximately one month after the completion of the injections in support of the Phase <br /> V IRA implementation show the following positive indicators of effectiveness in the ERD barrier <br /> area: increase of mass of PCE daughter products, decrease of both DO and nitrate concentrations, <br /> increase of both dissolved manganese and iron concentrations, and increase of TOC concentration. <br /> The current monitoring program, without dissolved hydrogen, is comprehensive and consistent <br /> with the state-of-the-art in ERD implementation and monitoring protocols. <br /> lir-Response(o RWQCB-Linw1n-Ju108-06750.dm:LMT 4 <br />