The number of natural attenuation field assessments has expanded over the past few years. Of note are the natural attenuation field characterization studies completed by AFCEE for fuel hydrocarbons and chlorinated solvent sites (Wiedemeier et al. 1995 and 1997). These studies provide the most comprehensive database of geochemical characterization collected in support of natural attenuation currently available. Other studies include Barker et al. (1987), Klecka et al. (1990), Caldwell et al. (1992), Borden et al. (1994), McAllister and Chiang (1994), Stauffer et al. (1994), Rifai and Bedient (1994), Davis et al. (1994), Daniel (1995), Wilson et al. (1995), and Wiedemeier and Chapelle (2000).

The numerous studies listed above as well as additional case studies presented at the Battelle Conference series on In Situ and On Site Bioremediation (Battelle, 1991, 1993, 1995, and 1997) all demonstrate a number of common themes about Natural Attenuation:

1. Natural attenuation processes have been observed and documented at numerous sites;
   
   
2. Natural attenuation controls the extent of migration of contaminants away from the source and limits the extent of the dissolved plume;
   
   
3. Intrinsic bioremediation of fuel hydrocarbons in groundwater occurs universally and many of these plumes are stable or shrinking, while only some sites contaminated with chlorinated solvents exhibit intrinsic bioremediation;
   
   
4. Geochemical characterization of the groundwater provides ample data for quantifying the natural attenuation processes that may be important at a given site;
   
   
5. Biodegradation is one of the most important attenuation mechanisms that contributes to declines in contaminant concentration and loss of mass in almost all fuel hydrocarbon plumes and in some chlorinated solvent plumes;
   
   
6. Aerobic biodegradation of fuel hydrocarbons is evidenced by depletion of dissolved oxygen inside the plume;
   
   
7. Anaerobic biodegradation of fuel hydrocarbons is evidenced by depletion of nitrate and sulfate and production of dissolved iron (Fe[II]) and methane inside the plume;
   
   
8. Anaerobic biodegradation processes, while slower than aerobic biodegradation, are a significant component of natural attenuation due to the abundance of anaerobic electron acceptors (NO₃^-1, Fe(III), Mn, SO₄^-2 and CO₂) relative to dissolved oxygen (see data from the Air Force sites in Appendix A).
   
   
9. While some chlorinated solvents (e.g., DCA) will degrade aerobically, and most will biodegrade during cometabolism, halorespiration is the most important biodegradation reaction involving the common chlorinated solvents. In this reaction electron donors such as BTEX and methanol are fermented to form the direct electron donor, dissolved hydrogen. Halorespirators then use the hydrogen to dechlorinate the chlorinated solvents.
   
   
10. Halorespiration is indicated by: (i) dissolved hydrogen concentrations in certain ranges; (ii) low reduction/oxidation (redox) potential environments from which electron acceptors such as sulfate have been removed; (iii) the presence of methane, which indicates the presence of fermentation reactions and hydrogen; and (iv) the presence of biologically produced daughter products that are not typically released at given sites, such as cis-1,2-dichoroethene (DCE) and VC.