Oxygenates Natural Attenuation : Is Dilution the Solution ?

The term “Natural Attenuation” (NA) has been defined as “naturally occurring processes in soil and groundwater environments that act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in those media” (Wiedemeier et al., 1999). This popular definition goes on to mention that the “in-situ” processes of NA include biodegradation, dispersion, dilution, adsorption, volatilization, and chemical or biological stabilization or destruction of contaminants, meaning that natural attenuation is composed of numerous contributing factors of which biodegradation is only one. In practice, unfortunately, the term “natural attenuation” is often used synonymously with such terms as intrinsic bioremediation, self remediation, natural restoration, passive bioremediation, or intrinsic remediation. The negative result of this is that it is increasingly common to interchange “natural attenuation” with “remediation," when in fact they are not synonymous. Natural attenuation occurs to some degree at every site; however, depending on site conditions, there can be definite limits to its effectiveness as an interim or long-term solution because natural attenuation does not necessarily imply that contaminants are removed. Furthermore, the site-specific conditions that often limit the effectiveness of natural attenuation as a contaminant removal/ destruction process are rarely properly evaluated. It is vital that we distinguish between destructive processes and dilution. To do this it is first necessary to establish the types of biological processes that may be induced or monitored at a site. Intrinsic or Engineered? Consider a “Biologically Active Zone," or BAZ, which occurs in close proximity to the contaminant source in the presence of electron donors in the mix of available electron acceptors. Contamination that escapes the BAZ escapes biological reaction and continues to move downgradient. Perhaps this is the reason why many of our chlorinated solvents plumes are so long (miles and kilometers long). For chlorinated solvents dissolved in water, biodegradation typically occurs within a BAZ, and the limiting factor is the availability of electron donors (primary substrates) for which a zone of increased biological activity can be established. In other words, there must be some growth of bacteria in order for biodegradation to occur, and growth requires the overlap of bacteria, electron donors, and acceptors.