Although not a line of evidence, analytical or numerical solute fate and transport models can prove valuable when evaluating natural attenuation. For example, models can be used to evaluate the relative importance of natural attenuation mechanisms if sufficient historical site data are available. The dominant transport mechanisms affecting the transport of dissolved organic contaminants in many subsurface systems are advection, dispersion, sorption, and degradation (biotic or abiotic). Thus, observed changes in contaminant concentrations downgradient from the NAPL source area will represent the sum of the influences exerted by these processes. Solute transport models that incorporate these mechanisms can be a valuable tool for estimating the relative importance of the various attenuation mechanisms. In addition, they can be used to show graphically that without the influence of degradation and mass removal, many contaminants would migrate some distance further than observed. Visit the models page to view some models useful for evaluating natural attenuation. The page also contains information about free public-domain software and documentation on the following MNA screening tools:

• Bioscreen Natural Attenuation Decision Support System - a screening model that simulates remediation through natural attenuation of dissolved hydrocarbons. The software is programmed in a Microsoft Excel spreadsheet environment.
• Biochlor Natural Attenuation Modeling Software for Chlorinated Solvents - a screening tool that models reductive dechlorination of chlorinated ethenes and ethanes.
• AFCEE-developed Biodegradation Screening- Analytical Parameters and Weighting for Preliminary Screening for Anaerobic Biodegradation Process.