Remediation attenuation

Schirmer M, JF Barker (1998) A study of long-term MTBE attenuation in the Borden aquifer, Ontario, Canada. Ground Water Monitoring and Remediation. 113-122. 

Other U.S. EPA reports explain what MNA means when the term is used to describe a potential strategy to remediate a contaminated site. They also describe the various physical, chemical, and biological processes of natural attenuation that may occur at a site. Other informational materials are in preparation and will provide more specific details and scientific depth for the evaluation of MNA as a remedy at specific sites. 

U.S. EPA defines MNA as the reliance on natural processes, within the context of a carefully controlled and monitored site cleanup approach, to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, stabilization, and transformation. These processes reduce site risk by transforming contaminants to less toxic forms, reducing contaminant concentrations, and reducing contaminant mobility and bioavailability. Other terms for natural attenuation in the literature include intrinsic remediation, intrinsic bio-remediation, passive bioremediation natural recovery, and natural assimilation. 30... 

Rittmann, B.E., Monitored natural attenuation of MTBE, in MTBE Remediation Handbook, Moyer, E.E. and Kostecki, P.T., Eds, Amherst Scientific Publishers, 2003. 

Leading new technologies include the landfill as a bioreactor,1 and the use of natural attenuation processes to enhance effectiveness of remediation at reduced cost by naturally renewable and continuing processes.2-3... 

Fig. 32.2. Remediation of the aquifer shown in Figure 32.1, as the simulation continues. After water contaminated with Pb++ displaces half of the aquifer s pore volume, clean water is flushed through the aquifer until a total of 30 pore volumes have been replaced. Flushing attenuates Pb++ concentration in the groundwater (top), so that it gradually approaches drinking water standards (MCL, or Maximum Contamination Level), and slowly displaces most of the sorbed metal from the Fe(OH)3 surface, primarily from the weak surface sites.

Brady, P. V., M. V. Brady and D. J. Borns, 1998, Natural Attenuation, CERCLA, RBCA s, and the Future of Environmental Remediation. CRC Press, Boca Raton. 

Smith, G., Adams, T. V., and Jurka, V., 1998, Closing a DNAPL Site through Source Removal and Natural Attenuation In Physical, Chemical, and Thermal Technologies — Remediation of Chlorinated and Recalcitrant Compounds (edited by G. B. Wickramanayake and R. E. Hinchee), Battelle Press, Columbus, OH, pp. 97-102. 

Confirmation of certain conditions usually needs to be demonstrated for natural attenuation to be deemed acceptable as a remedial option. This demonstration may involve showing the contaminant plume to be stable and contained, shrinking, or to be intrinsically remediated at the same rate as its spreading (Figure 10.11). When the plume is essentially being remediated at the same rate that it is spreading, dynamic equilibrium conditions exist. 

Ideally, the site characterization study has defined the vertical and horizontal extent of the contamination. Contoured site maps showing the (three-dimensional) distribution of the contaminants allow identification of areas that require extensive restoration, or may be allowed to be monitored to closure under natural attenuation. Knowledge of how much contamination exists and its location is the important first step in the remediation process. Evaluation of these data will permit consideration of the various remediation remedies available. Where the contaminant is contained within the shallow (<6 m) unsaturated zone and is recalcitrant (not readily biodegradable), excavation for off-site treatment or disposal may be the most expeditious procedure. Alternatively, depending on the contaminant, a variety of in situ procedures, including bioremediation, air sparging, soil vapor extraction, and fixation, may be applicable. 

Natural attenuation should not be perceived as a permanent remedy or as a means to achieve certain cleanup levels, but rather as (1) an interim measure until future technologies are developed, (2) a managerial tool for reducing site risks, and (3) a bridge from active engineering (i.e., pump-and-treat, vapor extraction, etc.) to no further action. 

There exist several methods or lines of evidence to demonstrate whether or not natural attenuation is occurring. Because sites can vary dramatically in their complexity and amount of effort required, the level of documentation that can be reasonably obtained will vary. What is important is that because many impacted sites will be or are difficult to clean up completely to the satisfaction of all parties, the use of natural attenuation will likely be at minimum considered at some stage of the project or remedial action. It is thus prudent to generate evidence and documentation to assess the suitability of natural attenuation as part of the site subsurface characterization process. 

Primary Parameters for Evaluation of Natural Attenuation as a Remedial Strategy... 

It was decided that natural attenuation with long-term monitoring was appropriate for the site based on the low risk the affected soil presented. An explanation of significant differences (ESD) was prepared by the regulators to revise the remedy selected in the ROD. In the ESD, active remediation of the upper and lower smear zone was rejected by the regulators for several reasons ... 

To reach the ultimate objective of no further action, a remedial approach of natural attenuation was proposed. This approach, summarized in Table 13.5, is based upon the following criteria ... 

Natural attenuation was favorably received and approved as an appropriate remedial alternative for these two sites. Periodic gauging and monitoring of key parameters will continue for a minimum period of 2 to 3 years to demonstrate continued stability and/or decline in dissolved BTEX prior to site closure. 

Barker, J. F. and Wilson, J. T., 1997, Natural Biological Attenuation of Aromatic Hydrocarbons under Anaerobic Conditions In Subsurface Remediation (edited by C. H. Ward, J. A. Cherry, and M. R. Scalf), Ann Arbor Press, Chelsea, MI, pp. 289-300. 

McAllister, P. M. and Chiang, C. Y., 1994, A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water Ground Water Monitoring and Remediation, Spring, Vol. 14, No. 2, pp. 161-173. 

Ollila, R W., 1996, Evaluating Natural Attenuation with Spreadsheet Analytical Fate and Transport Models Ground Water Monitoring Remediation, Fall, Vol. 16, No. 4, pp. 69-75. 

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