SECTION F

Processes that Control the Natural Attenuation of Chlorinated Solvents

In the early 1980's, when the Toxic Substances Hydrology Program of U.S. Geological Survey began, chlorinated solvents like trichloroethene (TCE) were considered to be non-biodegradable. In fact, it wasn't until 1985 that a mechanism for microbial degradation of TCE was first documented. Since that time, numerous biodegradation mechanisms including reductive dechlorination, direct oxidation, and various cometabolic mechanisms have been identified. These processes act in conjunction with non-biologic processes such as adsorption, dilution, and dispersion, to limit the migration of chlorinated solvents in ground-water systems. Perhaps the best-documented example of this behavior was observed at the Toxic Substances Hydrology site at Picatinny Arsenal, New Jersey. Data collected from this site shows that a large plume of solvent-contaminated ground water had come to a steady-state condition, and that this behavior reflected the combination of biological (reductive dechlorination), geochemical (sorption), and hydrologic (dilution) processes. Because these have been so well documented, Monitored Natural Attenuation (the use of these natural processes to reach remediation goals) has become an accepted remedial strategy.

Because of the many different processes that contribute to the natural attenuation of chlorinated solvents, assessing their efficiency can be a complex task. The papers in this section reflect this complexity. Depending on the characteristics of different sites, sorption may be a controlling process (Imbrigiotta), interactions with wetlands may be significant (Lorah-Devereux), temporal changes in redox conditions may be important (Mcguire and others), uptake by trees may occur (Vroblesky and others), biodegradation may be reflected in the diversity of microbial biomass (Haack and Reynolds), or previously unknown biodegradation processes might be observed (Bradley and Chapelle).

The scope of papers presented in this section reflects the diversity of processes that affect the natural attenuation of chlorinated solvents. Considering that just 15 years ago TCE was considered to be non-biodegradable, this shear variety is astounding. However, given the impressive advances of the last 15 years, it is reasonable to presume that much remains to be learned about the behavior of these compounds. This knowledge, in turn, will help make assessments of natural attenuation that are much more reliable and technically feasible.

For additional information contact:

Francis H. Chapelle,
USGS, Columbia, South Carolina
(email: )