Natural attenuation of trichloroethene (TCE) was evaluated for a groundwater plume at the Idaho National Engineering and Environmental Laboratory. Significant evidence demonstrated that reductive dechlorination is occurring, but is limited to a small area around the contamination source. In spite of this, the plume is relatively stable. Three first-order rate estimation methods were used to help understand transport processes affecting TCE in the large, aerobic portion of the plume. Two of the methods gave attenuation half-life estimates for TCE of approximately 8 years; however, these methods do not adequately distinguish between degradation and dispersion. The third method showed TCE attenuation relative to the co-contaminants, tritium and tetrachloroethene (PCE), and used these "tracers" to distinguish between dispersion and degradation. The estimated aerobic degradation half-life for TCE was between 13 and 21 years. Aerobic cometabolism of TCE has been identified as a potential mechanism for the apparent degradation. The importance of distinguishing between dispersion and degradation was shown using an analytical model. The model demonstrated that, in general, the rate of contaminant concentration decrease due to dispersion is not constant with time after the source is removed. This has important implications for predicting the long-term effectiveness of natural attenuation for groundwater restoration.

Environmental Laboratory, involves a trichloroethene (TCE) residual source area in a deep, fractured basalt aquifer that has been undergoing enhanced bioremediation since January 1999.Complete dechlorination from TCE to ethene was documented within nine months of operation, and sodium lactate injections were shown to enhance TCE mass transfer from the residual source.Since that time, optimization of injection strategies has maintained efficient dechlorination while demonstrating accelerated cleanup at a lower cost by changing to a whey powder amendment that solubilizes DNAPL.

A field pilot test in which hydraulic fracturing was used to emplace granular remediation amendment (a mixture of zero‐valent iron [ZVI] and organic carbon) into fine‐grained sandstone to remediate dissolved trichloroethene (TCE)‐contaminated groundwater was performed at a former intercontinental ballistic missile site in Colorado. Hydraulic fracturing was used to enhance the permeability of the aquifer with concurrent emplacement of amendment that facilitates TCE degradation. Geophysical monitoring and inverse modeling show that the network of amendment‐filled fractures extends throughout the aquifer volume targeted in the pilot test zone. Two years of subsequent groundwater monitoring demonstrate that amendment addition resulted in development of geochemical conditions favorable to both abiotic and biological TCE degradation, that TCE concentrations were substantially reduced (i.e., greater than 90 percent reduction in TCE mass), and that the primary degradation processes are likely abiotic. The pilot‐test data aided in re‐evaluating the conceptual site model and in designing the full‐scale remedy to address a larger portion of the TCE‐contaminated groundwater plume. © 2012 Wiley Periodicals, Inc.

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