EPA Recommends Against Using Models for Sediment Movement Predictions
The U.S. EPA has recommended ending the use of numerical models and fluorescent materials to predict future conditions at contaminated sediment sites for purposes of decision making.
The recommendation was one of 11 listed in a January 2017 directive from the EPA’s Office of Land and Emergency Management (OLEM) sent to the directors at EPA Regions 1 through 10. It is based on lessons learned, while clarifying 2002 and 2005 guidance documents for remediating contaminated sediment sites.
Regarding models, the document states that predictions are sometimes presented with a degree of certainty that fail to account for the inherent unknown accuracy of those predictions. “Since the accuracy and uncertainty of future projections are generally not known, any use of models should be made with a degree of caution, if at all,” the document states.
It explains that at many large sediment sites, computational models have been developed to simulate the complex interconnected processes of water flows, sediment transport, and contaminant fate and bioaccumulation by organisms. Regardless of their sophistication and level of complexity, models are simplifications of the complex environmental processes and the time and spatial scales over which the processes occur. Model uncertainty, (i.e. how well it can describe existing water sediment and contaminant transport conditions, and especially how well a model’s assigned parameter values can successfully predict future conditions) may limit, if not preclude, its successful application. Such potential limitations must be considered by regions when making remedy selection decisions.
The document quotes a 2007 National Research Council report that said “models will always be constrained by computational limitations, assumptions and knowledge gaps. They can best be viewed as tools to help inform decisions rather than as machines to generate truth or make decisions. Scientific advances will never make it possible to build a perfect model that accounts for every aspect of reality, or to prove that a given model is correct in all aspects for a particular regulatory application.”
The recommendation includes the use of fluorescent sediments (dyes) placed in the system to evaluate sediment movement, which will also be used as supporting evidence but not the basis for final decisions.
On September 18, the Environmental Law Education Center presented a seminar on “Environmental Contamination Cleanup: CERCLA + MTCA + Sediments” in Seattle.
Jim Woolford, director of the EPA office of Site Remediation and Technology Innovation and of OLEM, presented an explanation of the 11 recommendations under the major themes site characterization and monitoring; remedial action objectives and remedial goals; decisions and actions; and actions to integrate CERCLA (Comprehensive Environmental Response, Compensation, and Liability Act, or Superfund) and CWA (Clean Water Act) at Superfund sediment sites. He indicated that the EPA would be open to new ideas for accomplishing the goals.
The OLEM publication can be viewed at: https://semspub.epa.gov/work/HQ/196834.pdfEdit Module