The NRC panel on intrinsic remediation in its report of March 1, 2000 stated a number of concerns about the MNAtoolbox that we address below. There are also some nice comments about the toolbox being easy to use, its development being an overwhelming task, and its documentation being an important first step. While we appreciate the complements, many of their concerns seem to be based on misconceptions about the use of MNAtoolbox. We have tried to identify each of these concerns and address them in a forthright manner below. If anyone would like to discuss them with us further - please email me, Pat Brady, at pvbrady@sandia.gov
NRC Point 1 - Pg. 12 "Although the DOE protocol proposes a method for assessing natural attenuation of inorganic contaminants, such processes are exceedingly complex, and the DOE protocol does not address this complexity".
Response The complexity of inorganic attenuation is alluded to repeatedly in the toolbox documentation and a multitude of scientific documentation is provided to identify the various sources of uncertainty. Note though that the toolbox in general, and the scorecard in particular, were developed to provide site managers a rapid means for identifying the primary processes affecting attenuation of specific contaminants. As noted in the toolbox documentation, it is not meant to be used for transport calculations, risk assessments, or final site decisions, but rather to rapidly tell site managers whether MNA bears further examination. The toolbox was specifically designed to simplify processes to the greatest extent possible. That is the intended object of a screening tool. A much higher level of evidence is required if an actual decision to implement MNA were made. This complexity was not addressed because it was not the ultimate object of the toolbox.
NRC Point 2 - Pg. 15 "the toolbox should not be used…" because "…scoring systems are generally too simple to represent the complex processes involved and are often used erroneously in judging the suitability of a site for natural attenuation".
Response To begin with, we agree that scorecards should only be used for their stated purpose. Note though that the DOE scorecard is designed to guide further site characterization needed for natural attenuation if the circumstances favor it, not to make final site decisions. To this end, we believe the use of the toolbox is useful and consistent with EPA guidance. More importantly, it should be noted that all sites are complex and this complexity is rarely confined to geochemical factors. Full quantification of site complexity, be it associated with contaminant geochemistry and/or subsurface hydrology, is impossible to attain, and moreover, is seldom needed to make rational decisions about cleanup. Indeed, the existence of this uncertainty is the justification for long-term monitoring and contingency planning - DOE has recently issued protocols for use with MNA that cover both of these - see http://www2.em.doe.gov/framework/. Apparently these were not reviewed by the NRC panel. What is clearly needed in the meantime is a science-based method for identifying the likely sources of uncertainty before a natural attenuation argument is subsequently developed. The toolbox does just that.
NRC Point 3 - Pg. 195 "The natural attenuation factor (NAF)… is difficult to gasp", and "additional discussion of the NAF … in terms of classical retardation or attenuation concepts would be helpful". "It steps beyond the bounds of scientific understanding and vastly oversimplifies various processes". NRC then cites the example of irreversible sorption, notes that it is poorly understood and that much more research is needed before it can be quantified.
Response To begin with, there are no "classical retardation or attenuation concepts" that deal specifically with irreversibly sorbed contaminants - This is an area of ongoing research, both in our lab and in others. In the absence of a "classical approach" for assessing the potential of MNA, the NAF was developed. The NAF is merely the simplest way to sum the four primary attenuation processes - degradation, sorption, irreversible uptake, and dilution. The summing of the four and their normalization to a value between 0 and 100 is purely numerical - but the scientific bases for the constituent terms are reasonably well known, and exhaustively documented.
The toolbox documentation notes that irreversible sorption is poorly understood and cannot be predicted using existing approaches. Note though that even simple processes that are reasonably well understood – such as reversible sorption, or even the transport of non-reactive tracers - cannot be predicted with existing approaches. Yet, this does not prevent remediation from being considered. We agree that further research might work out the fine scale features of irreversible sorption. We reject the implication that site cleanup should wait on these results.
NRC Point 4 - Pg. 195 Because of DOE site complexity and inadequate recognition of this complexity, the "use of NAF calculations is not justified from a scientific perspective".
Response At even the best studied sites there are typically order-of-magnitude, or greater, uncertainties in subsurface contaminant behavior. In other words, site complexity will always be with us. Its presence should not prevent us from screening sites though. Requiring quantification of the more than the 3 or 4 primary processes beforehand is a disingenuous expectation for a site-screening tool.
NRC Point 5 - Pg. 201 and Pg. 14 NRC calls for further review and testing of the toolbox and for peer review.
Response – To begin with, the toolbox was developed after extensive
consultation with scientists at several national labs and universities.
Moreover, the toolbox documentation relied wherever possible on peer-reviewed
scientific literature. There are over 140 supporting references and the
vast majority come from the peer reviewed literature.
Conclusion
The toolbox was developed to address a knowledge gap formed by the absence
of clear guidance that rapidly points to probable attenuation mechanisms.
We maintain that that gap still exists, that MNAtoolbox successfully fills
that gap, and that it is useful when applied in conjunction with EPA and
DOE guidelines. In the near future we hope to calibrate the scorecard with
historical case analysis data for plumes containing BTEX, MTBE, U, chlorinated
solvents, Cs, and Sr and expect the usefulness of MNAtoolbox to only increase.