Characteristics of Fractured Rock Hydrogeology that Impact on Contaminated Site Remediation

Abstract: All rock masses are fractured to some extent. When the rock matrix has low to moderate permeability, but significant porosity, fractures form the main pathways for fluid flow and their geometry have to be well characterized when designing contaminant recovery programs. At the contaminated site scale, fractured rock masses are characterized by fractures that are grouped into sets that have preferred orientations which contribute a strong permeability anisotropy and heterogeneity to such rock masses. While the permeability of the individual fracture planes can be quite high, they have to be interconnected one with the other to provide a pathway for flow through the rock mass unless the matrix is permeable. At the scale of the single fracture, flow and transport rates in most fracture systems are controlled by the geometry of the fracture pore space. Rough extensional fractures, in contrast to the more tabular pore space of shear fractures, exhibit a high trapping capacity for NAPLs and highly variable capillary pressures. Results from a field study of arsenic contamination at a 50-year-old gold mine, and detailed laboratory experiments on discrete fractures in which the fracture pore space was mapped in detail, demonstrate the importance of these large- and small-scale fracture characteristics on the design of contaminant recovery operations in fractured rock masses