Robert J. Pirkle
Microseeps, Inc.

The primary objective of the natural attenuation investigation is to determine whether natural processes will be capable of attaining site-specific remediation objectives in a time period that is reasonable when compared to other alternatives and if it can meet all appropriate State and Federal remediation objectives for the site.

The first step in the process is to evaluate existing site data and develop a conceptual model and to determine if receptor pathways have already been completed. The preliminary conceptual model will help to identify any shortcomings in the data and will facilitate placement of additional data collection points in the most advantageous and cost-effective manner possible. In order to determine the potential of the natural attenuation process, one must determine if the plume is currently stable or migrating and the future extent of the plume based on (1) contaminant properties, including volatility, sorptive properties, and biodegradability; (2) aquifer properties, including hydraulic gradient, hydraulic conductivity, porosity and concentration of organic matter in the sediment and the groundwater; and (3) the location of the plume and contaminant source relative to potential receptor exposure points. If after completing the preliminary conceptual model, it appears that natural attenuation will be a significant factor in contaminant removal and a viable remedial alternative, detailed site characterization activities that will allow evaluation of this remedial option and the refinement of the conceptual model should be performed. The collection of data to evaluate natural attenuation can be integrated into a comprehensive remedial strategy and may help reduce the cost and duration of engineered remedial measures or eliminate the need for them.

During the last two decades, numerous laboratory and field studies have demonstrated that subsurface microorganisms can degrade a variety of chlorinated solvents. Whereas fuel hydrocarbons are degraded by their use as a primary substrate (are oxidized / act as an electron donor) , chlorinated aliphatic hydrocarbons may undergo biodegradation under two different naturally occurring circumstances: (1) they may undergo reductive dechlorination by acting as an electron acceptor; or (2) certain of them may be oxidized by acting as an electron donor. At a given site either or both of these mechanisms may be operative, although at many sites the use of chlorinated aliphatic hydrocarbons as electron acceptors appears to be most important under natural conditions. In this case, biodegradation of chlorinated aliphatic hydrocarbons will be an electron-donor-limited process, i.e.

H₂ + PCE à TCE + Cl + H (1)

where in this case hydrogen is the electron donor and is the limiting factor in this reaction. Conversely, biodegradation of fuel hydrocarbons is an electron-acceptor-limited process, i.e.

BTEX + O₂ à C O₂ + H₂ O (2)

where oxygen is the electron acceptor and is the limiting factor in this reaction.

In an uncontaminated aquifer, native organic carbon is used as an electron donor, and dissolved oxygen (DO) is used first as the prime electron acceptor. Where anthropogenic carbon, such as fuel hydrocarbons, is present, it also will be used as an electron donor. After the DO is consumed, anaerobic microorganisms typically use additional electron acceptors in the following order of preference: nitrate, ferric iron, sulfate, and finally carbon dioxide. Evaluation of the distribution of these electron acceptors can provide evidence of where and how chlorinated aliphatic hydrocarbon biodegradation is occurring. In addition, because chlorinated aliphatic hydrocarbons may be used as electron acceptors or electron donors, in competition with other acceptors or donors, isopleth maps showing the distribution of these compounds and their daughter products can provide evidence of the mechanisms of biodegradation working at the site. As with BTEX, the driving force behind oxidation-reduction reactions resulting in chlorinated aliphatic hydrocarbon degradation is electron transfer. Although thermodynamically favorable, most of the reactions involved in chlorinated aliphatic hydrocarbon reduction and oxidation do not proceed as non-biological processes. Microorganisms are capable of carrying out the reactions, but they facilitate only those oxidation-reduction reactions from which they gain energy.

The most important process for the natural biodegradation of the more highly chlorinated solvents is reductive dechlorination as shown in (1) above. During this process, the chlorinated hydrocarbon is used as an electron acceptor, not as a source of carbon, and a chlorine atom is removed and replaced with a hydrogen atom. In general, reductive dechlorination occurs by sequential dechlorination from PCE to TCE to DCE to VC to ethene. Depending upon environmental conditions, this sequence may be interrupted, with other processes then acting upon the products. Reductive dechlorination of chlorinated solvents is associated with the accumulation of daughter products and an increase in the concentration of chloride ions. Reductive dechlorination affects each of the chlorinated ethenes differently. Of these compounds, PCE is the most susceptible to reductive dechlorination because it is the most oxidized. Conversely, VC is the least susceptible to reductive dechlorination because it is the least oxidized of these compounds. As a result, the rate of reductive dechlorination decreases as the degree of chlorination decreases. Reductive dechlorination has been demonstrated under nitrate and ferric iron reducing conditions, but the most rapid biodegradation rates, affecting the widest range of chlorinated hydrocarbons, occur under sulfate reducing and methanogenic conditions. Because chlorinated aliphatic hydrocarbons are used as electron acceptors during reductive dechlorination, there must be an appropriate source of carbon for microbial growth in order for this process to occur. Potential carbon sources include natural organic matter, fuel hydrocarbons, or other anthropogenic organic compounds such as those found in landfill leachates.

Recent evidence suggests that reductive dechlorination is dependent on the supply of molecular hydrogen (H₂) which, in nature, is produced as a result of the anaerobic microbial oxidation of a primary substrate such as lactate, acetate, propionate, butyrate, ethanol, BTEX, benzoate, or other such compounds. Bacteria that facilitate dechlorination compete with sulfate-reducers and methanogens for the H₂ produced in such a system. When degradation of the original substrate/electron donor rapidly yields high concentrations of H₂ , the sulfate-reducers and methanogens appear to be favored over the dechlorinators. Conversely, when substrate degradation produces a steady supply of H₂ at low (1 &emdash; 10 nM) concentrations, the dechlorinators are favored. Thus, while it is generally understood that the type of electron acceptor plays a significant role in reductive dechlorination(i.e. sulfate reduction or methanogenesis vs nitrate or iron(III) reduction), the type of substrate or electron donor can also play a role in how thoroughly a natural system is able to dechlorinate solvents.

Thus, groundwater sampling must be conducted to determine the concentrations and distribution of contaminants, daughter products, and groundwater geochemical parameters including but not limited to those discussed above. A list of all of the parameters, including analytical protocol, necessary to delineate dissolved contamination and to document natural attenuation, including the effects of sorption and biodegradation, is given in Table 2.1 which has been taken directly from the Protocol.

In order to accurately evaluate contaminant fate and transport, estimates or measurement of aquifer parameters are necessary. Parameters which are important include: hydraulic conductivity; hydraulic gradient; and other processes causing apparent reductions in contaminant mass including dilution, sorption, and hydrodynamic dispersion. In order to determine the mass of contaminant removed from the system, it is necessary to correct observed concentrations for the effects of these processes. This is done by normalizing the measured concentrations of each of the contaminants to the concentration of a tracer that is biologically recalcitrant. Because chloride is produced during reductive dechlorination of chlorinated solvents, this analyte can be used as a tracer.

All of the above discussed site investigation data should first be used to refine the conceptual model and quantify groundwater flow, sorption, dilution, and biodegradation. The results of these calculations are used to scientifically document the occurrence and rates of natural attenuation and to help simulate natural attenuation over time. It is the responsibility of the proponent to "make the case" for natural attenuation. This being the case, all available data must be integrated in such a way that the evidence is sufficient to support the conclusion that natural attenuation is occurring. Conceptual model refinement involves integrating newly gathered site characterization data to refine the preliminary conceptual model that was developed on the basis of previously collected site specific data. This process may consist of preparation of geologic logs, hydrogeologic sections, potentiometric surface/water table maps, contaminant and daughter product contour maps, and electron acceptor and metabolic by-product contour maps. In addition, several calculations must be made prior to implementation of the solute fate and transport model, including sorption and retardation calculations, NAPL/water partitioning calculations, ground-water flow velocity calculations, and biodegradation rate-constant calculations. Each of the calculations is discussed and the specifics of each calculation are presented in the Protocol.

Simulating natural attenuation allows prediction of the migration and attenuation of the contaminant plume through time. Natural attenuation modeling is a tool that allows site specific data to be used to predict the fate and transport of solutes under governing physical, chemical and biological processes. Hence the results of the modeling effort are not in themselves sufficient proof that natural attenuation is occurring at a given site. The results of the modeling effort are only as good as the original data input into the model; therefore, an investment in thorough site characterization will improve the validity of the modeling results. Several well documented and widely accepted solute fate and transport models, such as BioScreen, are available for simulating the fate and transport of contaminants under the influence of advection, dispersion, sorption and biodegradation.

Supporting the natural attenuation option generally will involve performing a receptor exposure pathways analysis. This analysis includes identifying potential human and ecological receptors and points of exposure under current and future land and ground-water use scenarios. The results of solute fate and transport modeling are central to the exposure pathways analysis. From this information, the potential for impacts on human health and the environment from contamination present at the site can be assessed.

Additional source removal, treatment, or containment measures, beyond those previously implemented, may be necessary for MNA to be a viable remedial option or to decrease the time needed for natural processes to attain site specific remedial objectives. The solute fate and transport model can also be used to forecast the benefits of source control by predicting the time required to achieve the cleanup objectives under various scenarios.

A long-term monitoring plan must be prepared and used to monitor the plume over time and to verify that natural attenuation is occurring at rates sufficient to attain site-specific remediation objectives within the time frame predicted at the time of remedy selection. In addition, the long-term monitoring-plan should be designed to evaluate long term behavior of the plume, verify that exposure to contaminants does not occur, verify that natural attenuation breakdown products do not pose additional risks, determine actual rather than predicted attenuation rates for refining predictions of remediation time frame, and to document when site specific remediation objectives have been attained.

The results of natural attenuation studies should be presented in the remedy selection document appropriate for the site, such as CERCLA Feasibility Study or RCRA Corrective Measures Study. This will provide scientific documentation that allows an objective evaluation of whether MNA is the most appropriate remedial option for a given site.