Evaluating Potential for Abiotic Degradation
Although not always fully considered, abiotic degradation can be a substantial or even the primary process for chlorinated hydrocarbon at sites undergoing or transitioning to monitored natural attenuation (MNA). A variety of iron-bearing minerals including iron sulfides (mackinawite and pyrite), iron oxides (magnetite), green rust and iron-bearing clays are capable of complete or near complete degradation of PCE, TCE, and carbon tetrachloride (He et al. 2009). Some iron-bearing minerals also catalyze the degradation of chlorinated ethanes and the lesser chlorinated ethenes, cis-dichloroethene (DCE) and vinyl chloride. While the quantities and types will vary, these reactive iron minerals are frequently identified in subsurface environments under iron reducing and sulfate reducing conditions.
Brown et al. (2007) recommend four avenues for evaluating the role of abiotic processes in contaminant attenuation. First, examining contaminant concentrations along the flow path – decreasing parent compound concentrations with no evidence of accumulation of chlorinated transformation products like cis-DCE and vinyl chloride suggest abiotic degradation. Performing compound specific isotope analysis (CSIA) or monitoring for products unique to abiotic reactions such as acetylene can also provide a strong line of evidence. Microcosm studies with native sediment and killed controls can also be performed. Finally, Brown et al. (2007) suggest performing mineralogical analyses on aquifer sediment to characterize reactive minerals such as magnetite or iron monosulfides.
Additional Analyses to Consider
Dissolved Ferrous Iron (Fe2+): Ferrous iron adsorbed to the surface of minerals including magnetite can increase contaminant degradation rates. Higher dissolved iron concentrations would indicate the potential for such sorption at the mineral surface.
Acid Volatile Sulfide (AVS) : AVS is commonly used to estimate FeS.
Brown, R. A., J. T. Wilson and M. Ferrey (2007). “Monitored natural attenuation forum: The case for abiotic MNA.” Remediation Journal 17(2): 127-137.
Ferrey, M. L., R. T. Wilkin, R. G. Ford and J. T. Wilson (2004). “Nonbiological Removal of cis-Dichloroethylene and 1,1-Dichloroethylene in Aquifer Sediment Containing Magnetite.” Environmental Science & Technology 38(6): 1746-1752.
He, Y., C. Su, J. T. Wilson, R. T. Wilkin, C. Adair, T. Lee, P. Bradley and M. Ferrey (2009). Identification and characterization of methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water, US EPA.
Liu, Y., S. A. Majetich, R. D. Tilton, D. S. Sholl and G. V. Lowry (2005). “TCE Dechlorination Rates, Pathways, and Efficiency of Nanoscale Iron Particles with Different Properties.” Environmental Science & Technology 39(5): 1338-1345.
Song, H. and E. R. Carraway (2005). “Reduction of Chlorinated Ethanes by Nanosized Zero-Valent Iron: Kinetics, Pathways, and Effects of Reaction Conditions.” Environmental Science & Technology 39(16): 6237-6245.