THE IMPORTANCE OF FAULTS ZONES IN CONTROLLING REGIONAL TO CONTINENTAL SCALE HYDROGEOLOGY AND PERMEABILITY

At the regional to continental scale there is little knowledge about how the fault barrier-conduit paradigm of groundwater flow affects hydrogeology in the earth’s upper crust (<1km), although fault zones have been intensively studied at many local sites. To address this question, we use a combination of geospatial data synthesis and parametric numerical modeling approaches. A spatial database, created from a broad review of fault zone observations and conceptual inferences in data rich regions, such as N. America, Europe, and Japan, is linked to permeability and groundwater flow behaviour data. The conceptual models of fault zones are grouped into dominant behaviour (e.g. conduit, conduit/barrier, barrier, or not significant) and statistically linked to geological attributes (e.g. lithology, structural setting, etc.) and types of observations of fault zone permeability structure on flow behaviour. The linkage of fault zone hydrogeology from site scale to large scale is done through parametric numerical models, to determine appropriate upscaling properties of fault zone behavior on fluid flow. At the local scale, the numerical model is based on observed geology and fault geometries from multiple sites representative of different simplified geologic environments. The model is then scaled to progressively larger regions, guided by attribute characteristics from the spatial database, and results are compared to available analytical methods.

Our initial results confirm that the relative dominance of barrier or conduit fault behaviour depends partly on lithology, and this can be numerically represented with bulk permeability scaling factors for large rock volumes which contain fault zones. The orientation of normal faults in relation to regional flow enhances the barrier effect and it can be represented with horizontal permeability anisotropy ratios at large scales, while the fault conduit behaviour is estimated with vertical permeability anisotropy and it depends more strongly on lithology. At the continental scale the net result of the faults is a potential reduction in horizontal permeability. The developed methodology improves modeling of structural effects on hydrogeology of large regions based on smaller scale fault zone data.