Ascertaining the effect of reductive dehalogenation on chlorinated hydrocarbon plume lengths in groundwater: Analyses of multisite data

Chlorinated hydrocarbon groundwater plume data from a multisite study were evaluated by a variety of statistical techniques (correlation, analysis of covariance, principal components) to quantify the effects of biotransformations (reductive dehalogenation) on plume length. After accounting for the effects of groundwater velocity, source strength, and biases in the data collection process, chlorinated hydrocarbon plume lengths at sites where reductive dehalogenation was occurring were found to be significantly shorter on average, by roughly a factor of two, than those where it was not. Moreover, principal component analyses indicated significant differences in the behavior of chlorinated hydrocarbon plumes between sites with and without evidence of reductive dehalogenation, respectively. The advantage in examining plume behavior from this population-oriented perspective is that overall trends in plume behavior can be evaluated despite site-specific influences such as heterogeneities and unique release histories. Ultimately, it is these average trends that would be of the most interest to policymakers because they represent the ranges of conditions that will be encountered. This is especially important in the case of chlorinated hydrocarbons because they will biotransform at rates significant for appreciable natural attenuation only in certain instances.