Contaminant Plume and Flow System Characteristics in Fractured Sedimentary Rocks
The general nature of contaminant plumes in groundwater differs greatly between the major types of hydrogeologic systems. This presentation concerns 'aged' chlorinated solvent plumes in fractured sedimentary rock emanating from DNAPL source zones serving as long-term natural gradient tracer experiments of flow system and contaminant transport conditions. Generalizations can be made from intensive field studies using high-resolution methods at a dozen field sites in a variety of sedimentary rock types including sandstone, mudstone, shale, dolostone and limestone. Hydraulically active fractures organized parallel to bedding planes and joint sets nearly orthogonal to the bedding are common but display variable density and connectivity between distinct hydrogeologic units. The Freeze/ Cherry GROUNDWATER book drew attention to the influence of heterogeneity and hydrologic layers directing flow paths by drawing attention to the work by Freeze and Witherspoon (1967). Our understanding of heterogeneity and hydrologic units directs flow paths through layers with distinct residence times and hydrogeochemical properties, important to identifying discharge quantity and quality. The fracture network influences the bulk hydraulic properties of the flow system and groundwater flow paths that serve as the solute transport pathways and surface area for diffusion, also emphasized in this textbook with the timely recognition of the work by Foster (1975) showing the importance of diffusion on plume front advance rates in fractured porous geologic media. My research program on quantifying processes important to plume behavior in sedimentary rocks is founded on these two important concepts. New and traditional methods are used to provide concentration and hydraulic properties in spatial and temporal detail to inform, calibrate and validate discrete-fracture network numerical models for flow and transport. Strong matrix diffusion effects in both in the source and plume create nearly stationary plume conditions after a few decades even without degradation, much different from plumes in sand or gravel aquifers where substantial degradation is required. Thus, distinct criteria are needed for assessing natural attenuation and risks to receptors for these chlorinated solvent plumes in sedimentary rocks.
College of Physical and Engineering Science, G360, University of Guelph, Guelph, ON, Canada
