MP4 Organic, Metallic, Organometallic Pollutants and Method
Monday, 14 November 2005: 8:00 AM - 6:30 PM in Exhibit Hall

(BRO-1117-846069) The Role of Abiotic Degradation Processes in Natural Attenuation of Chlorinated Solvents.

Leahy, M¹, Hines, R², Brown, R³, ¹ Environmental Resources Management, Inc, Hartford, CT, USA² Environmental Resources Management, Inc, Mobile, Alabama³ Environmental Resources Management, Inc, Ewing, NJ

ABSTRACT- Natural attenuation occurs through both biotic and abiotic degradation. While this concept is included in discussions of natural attenuation, the bulk of the proof often supplied supports biotic mechanisms only, with few exceptions. The one exception is the abiotic degradation of 1,1,1-trichloroethane (1,1,1-TCA) to form 1,1-dichloroethene (1,1-DCE) and acetic acid, respectively. The seminal work by Vogel and McCarty confirming this pathway, as well as the ease of detection of the abiotic product 1,1-DCE, has made inclusion of this pathway commonplace. However, other abiotic mechanisms, especially reductive reactions, can be equally important. More recently, the work of Ferry and Wilson has shown that reduced mineralogy can also engender the attenuation of chlorinated ethenes. Naturally-occurring reductants such as ferrous iron can degrade a variety of chlorinated solvents such as 1,1,1-TCA, 1,1-DCE, carbon tetrachloride; tetrachloroethene, and trichloroethene. At sites with naturally occurring reduced iron (i.e,.magnetite) or at sites with iron-rich mineralogy and strong reducing conditions, ferrous iron is present and can result in the degradation of chlorinated solvents without the corresponding production of common biological daughter products such as 1,1-DCA from 1,1,1-TCA or 1,2-DCE and vinyl chloride from PCE and TCE. Typically abiotic pathways are overlooked for chlorinated solvent sites. Most degradation of TCE and PCE is assumed to be biologically based. Also, reduced iron mineralogy is not a common natural attenuation parameter evaluated at most sites. Yet there are many sites where significant degradation of the original solvents is observed without the corresponding accumulation of the appropriate daughter products. This is usually interpreted as a highly active co-existing biodegradation of the daughter products. It may however be due to iron-based abiotic degradation. The problem is how to apportion the degradation between biotic and abiotic pathways. This paper presents data from sites at which abiotic degradation appears to be a major attenuation mechanism, and proposes data collection and laboratory microcosms to support this argument. This paper will provide a methodology to prove the existence of abiotic pathways and to provide some quantification of the relative rates.

Key words: natural attenuation, abiotic, solvent degradation, reduced iron