Multiple distinct but converging lines of evidence, in various forms have been used in recent years to demonstrate natural attenuation (see for example AFCEE 1995, 1997, 1998, 1999, USEPA 1998a, and ASTM, 1998). Any natural attenuation evaluation should use multiple and converging lines of evidence to evaluate natural attenuation. The more complicated the site the greater the weight of evidence typically must be. The most common lines of evidence used to demonstrate natural attenuation of organic compounds dissolved in groundwater include:

1. Historical trends in contaminant data showing plume stabilization and/or loss of contaminant mass over time.
2. Analytical data showing that geochemical conditions are suitable for biodegradation and that active biodegradation has occurred as indicated by the consumption of electron acceptors and/or the production of metabolic byproducts. This chemical and analytical data can include evidence of;
• depletion of electron acceptors and donors
• increasing metabolic byproduct concentrations
• decreasing parent compound concentrations
• increasing daughter compound concentrations
3. Microbiological data that support the occurrence of biodegradation - This line of evidence, microbiological laboratory or field data, can be used to show that indigenous biota are capable of degrading site contaminants. The microcosm study is the most common technique used for this purpose. Microcosm studies should only be undertaken when they are absolutely necessary to obtain biodegradation rate estimates that could not be obtained using the other lines of evidence or when the specific mechanism of degradation is not known.

• Summary of the above three lines of evidence

Ideally, the first or first two lines of evidence should be used to document natural attenuation (AFCEE, 1995, 1997, and 1999). Because of the extensive amount of microbiological research already conducted, microcosm studies typically are not necessary. The American Society for Testing and Materials (ASTM) refers to these lines of evidence as primary, secondary, and optional when dealing with petroleum hydrocarbons. Several types of soil/sediment and groundwater analytical data are used to obtain the first two lines of evidence, as discussed below. In addition to these lines of evidence, analytical or numerical solute transport models can be used to examine the processes influencing the transport of organic contaminants in groundwater and to estimate the influence of natural attenuation mechanisms on solute fate and transport. Solute transport models can also be valuable presentation tools.