Fate and Transport Model Reporting Requirements
When a site has low levels of contamination, it may be feasible to allow the regulated substances to naturally attenuate as opposed to an active remediation method. In order to approve monitored natural attenuation, the responsible parties must demonstrate that the site is likely to reach clean-up standards within a reasonable time. One of the requirements for this demonstration is a fate and transport model.
The model may be either analytical or numerical and should include a report with sufficient detail that the reviewer would be able to duplicate the model. In order to assure that the model has been through an acceptable peer review, the model must also be associated with the EPA, either approved in an EPA document, written by the EPA, or some other disclosed association.
A fate and transport model report should include the following:
- Description of the model, how it approaches the problem, and its limitations, showing that the model is appropriate for use at the site.
- Description of the model association with the EPA.
- Summary of current site contamination.
- Most recent potentiometric surface map for the site.
- Model input values in tabular form including:
- Field measurements
- References or explanation of input values
- Examples of all calculations.
- Model calibration procedures.
- Sensitive parameters for model, any sensitivity analysis showing range of acceptable model adjustments, and rational for selecting input values for sensitive parameters.
- Model output showing range of results.
- If simple calculation model used, output data should be presented in tabular form.
- If computer model used, output data should be presented in both tabular form and a printout of the output pages should be provided.
- A series of maps showing reduction of regulated substances over time.
- Maps must show reference points (monitoring wells, targets, and significant landmarks).
- Maximum 5-year span between map.
- Other figures, tables, or discussions may be required depending on the complexity of the model and/or the site.
- Model results
- Statement of how many years for site to come into compliance with applicable risk reduction standards.
Common Modeling Errors:
- Units are inconsistent (For example, using standard and metric units without converting).
- Insufficient field data for calibration.
- Insufficient boundary size and/or conditions.
- Inaccurate hydrologic assumptions.
- Incorrect sign for pumping or recharge.
- Typos or general mistakes in input values.
- Using input data that doesn’t match the site.
- Excluding data from wells with the highest contamination.
- Improper selection and use of source and target wells.
- Target wells clustered in only small portion of the model.
- Incorrect assumptions regarding the effect of soil/source removal on source area groundwater contamination (Example, assume 50% contamination loss in source well due to removal of overlying soil).
- Forcing data to fit using maximum or minimum ranges of input values.
- Acceptance of model output without logical assessment.
The most popular soil leach model is the US EPA’s Soil Screening Guidance: User’s Guide, equations 10, 11, and 12 (http://www.epa.gov/superfund/resources/soil/ssg496.pdf). Other popular soil leach models include: LEACH, SAM, SEVIEW, SESOIL, VDSAT, VLEACH, VS2DT.
Popular groundwater contaminant transport models include: AT123D, BIOBALANCE, BIOCHLOR, BIOPLUME, BIOSCREEN, Disperse, Domenico, FATE5, MOC, MULTIMED, MT3D, Random Walk, SOLUTE, Summers, VADSAT, Chemflo.
The EPA has additional information on many of these models on their Center for Subsurface Modeling Support web page (http://www.epa.gov/ada/csmos/index.html)
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