Abiotic and Biotic Pathways in Chlorinated Solvent Natural Attenuation

Abiotic degradative pathways are often overlooked when evaluating natural attenuation at chlorinated solvent sites. Yet, at many sites, significant degradation of parent compounds such as 1,1,1-TCA, PCE and TCE is observed without the corresponding accumulation of daughter products, a sure indication of abiotic reactions. The problems in integrating abiotic processes into MNA are how to prove the existence of the abiotic pathways and how to apportion the degradation between biotic and abiotic pathways. Abiotic processes can be demonstrated and potentially quantified in four ways: plotting plume degradation patterns for the various chlorinated species, conducting mineralogical analyses, monitoring for unique reaction products, and finally, modifying and expanding microcosm study protocols to specifically examine abiotic reactions. INTRODUCTION The natural attenuation of chlorinated volatile organic compounds (CVOCs) solvents has been studied since the early 1980s (Bouwer 1981). The commonly accepted, dominant process in natural attenuation has been the biologically-mediated sequential reductive dechlorination as pictured in Figure 1 (Tiedje, 1992; Vogel and McCarthy, 1985). This process has become the de facto basis of Monitored Natural Attenuation (MNA). Early in the development of the understanding of natural attenuation, Vogel, Criddle, and McCarty (1987) identified a variety of abiotic and biotic processes that could degrade chlorinated aliphatics These degradation processes included hydrogenolysis, dihalo-elimination (loss of two adjacent chlorines forming a C-C bond), and coupling (loss of chlorines on two separate molecules forming a C-C bond, joining the two molecules). These three processes are two electron reductions with the electrons supplied by microbial processes or by chemical reductants. They also identified other, strictly, abiotic processes such as dehydrohalogenation and hydrolysis, neither of which involves a transfer of electrons. Dehydrohalogenation is the loss of a chlorine and an adjacent hydrogen ion, forming a C-C bond. These two non-reductive, abiotic processes are illustrated by the conversion of 1,1,1-trichloroethane (1,1,1-TCA) into 1,1-dichloroethene (1,1-DCE) through dehydrohalogenation, and acetic acid through hydrolysis. While acknowledging that the reduction of chlorinated aliphatics through a two-electron transfer could occur chemically, Vogel concluded that such reactions were primarily microbiallyinduced since chemical reduction was thought to be too slow (Vogel, 1987). The assumption that chemically-based reductive processes are slow compared to biological processes has led to a predominantly biotic focus in natural attenuation. Abiotic degradative pathways are typically overlooked when evaluating natural attenuation at chlorinated solvent sites. Beginning in the mid-1990s and continuing even as late as 2003, the recommended monitoring focus in MNA for CVOCs has been on the production of daughter products and ultimately ethene or ethane as “proof” of natural attenuation (Wiedemeier et al., 1996; NRC, 2003). This focus presupposes a biological pathway. There has been, however, a strong undercurrent, interest in reductive abiotic processes. In 1993, Vogel acknowledged that dechlorination could occur “without microbes” (Vogel, 1993). The growing interest in abiotic reduction has been fueled by two technical developments. One has been the development of zero valent iron (ZVI) technology. The other has been the study of the effect of anaerobic biological processes on minerals. At some sites, significant degradation of parent compounds such as 1,1,1-TCA, PCE and TCE is observed to occur without the corresponding accumulation of daughter products. While highly active, co-existing biodegradation of the daughter products may sometimes be responsible, a significant portion of the degradative removal of the parent compounds may actually be due to abiotic degradation. The occurrence of CVOC degradation without the production of commonly assumed daughter products has sparked an interest in abiotic processes. A central discovery in abiotic processes for MNA has been the role of bound ferrous iron. Researchers have specifically investigated the reductive reactivity of reduced iron minerals such as pyrite and demonstrated that a suspension of pyrite was able to dechlorinate carbon tetrachloride (Kriegman-King, 1994) and reduce dinitrotoluene (Jiayang, 1996). Ferrous iron precipitates, formed by the corrosion (reaction) of ZVI, were also found to react with chlorinated solvents (Matheson, 1994). Recent research has shown that chemically-precipitated ferrous iron will also act as an active reductant for chlorinated volatile organic compounds (CVOCs) (Brown, 2005b). Iron-based reductive chemistry has also been demonstrated in the field by the reactions of naturally occurring, ferrous-containing minerals with chlorinated solvents. In 2002, a plume of cis-1,2dichloroethene (cis-DCE) was shown to be abiotically degraded by magnetite, a mixed ferrous and ferric oxide, at rates comparable to biological processes (Ferrey, 2002). Ferrous iron plays the role in abiotic degradation that microbes play in reductive dechlorination. A second important discovery that has been key to the understanding of abiotic attenuation is the discovery that ferrous iron reacts with chlorinated solvents by mechanisms similar to those observed for ZVI. Chloroacetylenes were observed as products in the reaction of TCE with reduced iron-containing sediments (Szecsody, 2004). This discovery suggests that the processes catalyzed by ferrous iron may be as diverse as those being elucidated for ZVI. The mechanisms of the reaction of ZVI with chlorinated solvents are quite complex and generate multiple products. Orth and Gillham (1996) in a study of the reaction of TCE with ZVI found that ethene and ethane (in the ratio 2:1) accounted for over 80 percent of the original equivalent TCE mass. The typical daughter products formed biologically, such as cis-DCE and VC, accounted for only 3 percent of the original TCE mass. Additional by-products were found including hydrocarbons (C1 to C4) such as methane, propene, propane, l-butene, and butane. At sites with naturally occurring reduced iron (i.e., magnetite) or at sites with ironrich mineralogy and strong reducing conditions, ferrous iron minerals are present and can degrade chlorinated solvents without the corresponding production of common biological daughter products such as 1,1-DCA from 1,1,1-TCA or cis-DCE and vinyl chloride from PCE and TCE. Yet, reduced iron mineralogy is not a common natural attenuation parameter evaluated at most sites. Abiotic attenuation is not yet considered an integral part of MNA. The problems in incorporating abiotic attenuation into MNA are how to prove the existence of the abiotic pathways and how to apportion (if feasible) the degradation between biotic and abiotic pathways. ABIOTIC “FOOTPRINTS” The NRC book, Natural Attenuation For Groundwater Remediation, suggests that many attenuation processes cannot be directly observed, but that they leave “footprints” that can be used as evidence of the process: “Mechanisms that cause contaminants to degrade or transform in the subsurface cannot be observed directly, but they leave footprints that can be detected in groundwater samples.”(NRC 2003) The footprints that the book offers as evidence of MNA include consumption of electron acceptors, presence of daughter products, and detection of metabolites, all of which are biologically derived. Are there equivalent footprints for abiotic processes? Abiotic processes present four footprints: plume degradation patterns, mineralogical characterization, characteristic products, and confirmatory microcosm studies. Plume Degradation Patterns. One of the accepted markers for abiotic attenuation is the loss of the parent compounds without the sequential production of mono-dechlorinated products. If one plots the molar concentrations of the chlorinated compounds with distance from the source area, one can differentiate between abiotic and biotic conditions. Biotic conditions are evidenced by a divergence of the contaminant concentrations with distance and often show a slower degradation or an accumulation of lesser chlorinated compounds. Abiotic conditions are evidenced by a parallel decline in molar concentrations with distance. Consider the following five sites 1. Site A: The source area contained significant levels of petroleum hydrocarbons that “fueled” the biodegradation of 1,1,1-TCA to 1,1-DCA. The dissolved plume extends south from the source approximately 300 meters (m) where it abruptly attenuates to CVOC concentrations <0.01 mg/L. The dissolved plume outside the source area primarily occurs near the alluvium/bedrock interface at 30 to 60 m below ground surface (bgs). In descending order, the stratigraphic sequence includes fill, organic marine clay, silty sand alluvium, and fractured Franciscan Formation bedrock, an iron containing serpentine rock. Abiotic degradation is most likely the primary attenuation process outside of the source area.