Monitored Natural Attenuation (MNA) of Chlorinated Solvents - Revision history

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 ==Footprints==
-Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Fig. 5).
+Most plumes have some contribution of anaerobic sequential reductive dechlorination. As a result, the primary contaminant and the transformation products of reductive dechlorination are present in the groundwater. The highest concentrations of the primary contaminant will be near the source of contamination, and the flow of groundwater carries the transformation products further downgradient from the source (Figure 5).
 Many plumes of chlorinated solvents also have a contribution of abiotic degradation. As a result, the intermediate degradation products (such as DCE) do not accumulate to stoichiometric concentrations. There is an appearance that degradation of the cDCE has stalled, when in fact it is actively degrading, but not to vinyl chloride (Fig. 5). A quantitative framework<ref name="Lebron2015"/> is now available that allows an evaluation of the contribution of abiotic degradation on magnetite based on the magnetic susceptibility of the sediment, and the contribution of abiotic degradation on pyrite based on the extent of sulfate reduction and the geochemistry of the groundwater.

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 Under anaerobic conditions, the chlorinated solvents act as an electron acceptor. In such cases, electron donors may be in the form of naturally occurring, bioavailable organic carbon, or possibly from comingled plumes of petroleum hydrocarbons. The chlorinated solvents function in bacterial metabolism in the same fashion as oxygen functions in human metabolism. The chlorinated solvents are essentially something for the bacteria to breath in the absence of other electron acceptors such as oxygen, nitrate, or sulfate.
-[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethane.]]
+[[File:Wilson 3 Fig1.png|thumbnail|450 px|right|Figure 1. Degradation chlorinated alkenes to ethene.]]
 In anaerobic groundwater, when conditions are favorable, chlorinated alkenes can undergo a sequential reductive dehalogenation where a chlorine atom is replaced with a hydrogen atom. Degradation proceeds from PCE to TCE, then to DCE, then to VC and finally to ethene (Fig. 1). The minimal geochemical conditions<ref>Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical protocol for evaluating natural attenuation of chlorinated solvents in ground water. EPA-600-R-98-128. [[Media:Wiedemeier-1998-Technical_Protocol_for_Evaluating_Natuaral_Attenuation.pdf|Report pdf]]</ref> that must be taken into account include pH, oxidation-reduction potential (ORP), dissolved oxygen (DO) concentration, total organic carbon (TOC) and competing electron acceptors including oxygen, nitrate, sulfate and ferric iron.
-PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethane are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY.  pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.
+PCE and TCE can be used as an electron acceptor by a wide variety of bacteria<ref>Nyer, E.K., Payne, F., Sutherson, S., 2003. Discussion of environment vs. bacteria or let's play,‘name that bacteria’. Groundwater Monitoring & Remediation, 23(2), 32-48. [http://dx.doi.org/10.1111/j.1745-6592.2003.tb00665.x doi: 10.1111/j.1745-6592.2003.tb00665.x]</ref>. The bacteria can degrade PCE or TCE as far as DCE. The only organisms that can degrade DCE to VC and then degrade VC to the harmless end product ethene are stains of ''Dehalococcoides mccartyi''<ref>Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, Leeson, A., Ward, C.H. Springer, New York, NY.  pgs. 39-88. ISBN: 978-1-4614-4114-4 ISBN 978-1-4614-4115-1. [http://dx.doi.org/10.1007/978-1-4614-4115-1 doi: 10.1007/978-1-4614-4115-1]</ref>.
 The degradation of chlorinated alkanes in anaerobic groundwater is more complicated (Fig. 2).  Chlorinated alkanes can undergo a sequential reductive dehalogenation. In addition, they can undergo the loss of a hydrogen and a chlorine atom to form an alkene (a dehydrochlorination) or the loss of two chlorine atoms to form an alkene (a dichloroelimination).

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