The occurrence of fractures in a consolidated rock mass can result in rapid and widespread migration of DNAPL. To be effective, remediation methods require knowledge of the extent of travel of the contaminant from the point of release and of the volume of DNAPL retained in the fractures. Whereas the retention of DNAPL residual in porous media and its subsequent remediation have been extensively discussed in the literature, the capacity of fractured rock to retain DNAPL has yet to be established. Limestone samples (0.3mx0.3mx0.1m) were fractured in the laboratory along existing planes of weakness and the fracture flooded with perchloroethylene (PCE) following saturation with water. A retention curve is presented as a function of capillary number for the example case of a rough-walled fracture exposed to PCE where hydraulic gradients of up to 1.0 could not displace all DNAPL from the fracture.
Dense, nonaqueous phase liquids (DNAPLs) such as chlorinated solvents, PCB oils, and creosote are commonly encountered groundwater contaminants throughout industrial areas of North America. DNAPLs are denser-than-water liquids which are generally considered to be immiscible with water; however, these chemicals do have finite aqueous solubilities which, although very low, are typically well above allowable drinking water levels. Of the thousands of sites throughout North America contaminated by these hazardous liquids, hundreds are underlain by fractured rock. Exposed rock masses typically contain fractures on every dimensional scale (Brown and Scholz, 1985a). The presence of fractures, from microcracks to joints and faults, significantly affects both the physical characteristics (Walsh, 1965 a,b) and the transport properties (Neuzil and Tracy, 1981; Raven and Gale, 1985) of a rock mass. In the case of a fractured rock mass, interconnected fractures form the dominant flow paths (Raven and Gale, 1985). Examination of the transport capacity of such a fracture system is critical to determining both contaminant migration and remediation potential. However, contaminant behavior in a single fracture must be understood before reasonable conclusions can be made for networks and systems. Although residual formation and mobilization in porous media have been extensively discussed in the literature, the retention capacity of natural rock fractures has yet to be established. Effects of variations in parameters such as hydraulic aperture, hydraulic flood gradient, surface roughness and fluid properties on the retention of organic contaminants in a fractured rock environment are currently poorly understood. This paper presents results of a laboratory-scale investigation of the retention capacity of fractured limestone. Comparisons with porous media behavior are discussed in the context of capillary number plots.
Fractures present in clay and rock can represent primary migration pathways for DNAPLs present in the subsurface. Figure 1 illustrates a typical DNAPL contamination scenario in an aquifer underlain by a zone of fractured rock. Not only do the fractures permit transmission of the pure phase liquid through an otherwise essentially impermeable matrix, but they also facilitate migration of dissolved phase. Their work centered on rough-walled fractures, those defined as having a spatially variable aperture distribution.
