Seminar

Chlorinated solvent source zones and plumes in fractured sedimentary rock have been investigated intensely at
several industrial sites in Canada, USA and elsewhere. A comprehensive methodology, referred to as the
Discrete Fracture Network (DFN) Field Approach has been developed providing a toolkit for characterizing
these sites. The sites show many common features providing the basis for a general conceptual model for
source zone and plume behavior. Contamination was initiated by DNAPL releases decades ago; however the
source zones have evolved or “aged” over decades due to dissolution and diffusion processes, such that nearly
all mass now occurs as dissolved and sorbed phase in the matrix, causing reduction in source strength such that
mass input to the plume may be controlled more by back diffusion. While nearly all groundwater flow and
downgradient transport occurs in the interconnected fracture network, diffusion in the plume also causes mass
transfer from groundwater flowing in fractures to the rock matrix, causing strong retardation of rates of plume
front migration and strong attenuation of mass discharge within the plume. In many cases, the combined
effects of declining source strength and diffusion causes the plume to be essentially stationary at present time
or possibly even retreating if even slow rates of contaminant degradation are also occurring. This presentation
will provide an overview of the DFN Approach, insights from its application at several sites, and implications
for understanding long term plume behavior, risk assessment and remedial efficacy.