Small Displacement Faults in Sand: What Control Do They Exert on Saturated and Unsaturated Flow and Transport?

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9614385 Wilson We propose to study the influence of small-displacement faults on both saturated and unsaturated water flow and chemical transport in poorly consolidated sandy sediments. Previous work has focused on the influence of faults on saturated fluid flow in rock, rather than sediments. These studies cannot be directly applied to flow in sediments because the mechanics of deformation and resultant structures are different for sediments than for rocks, and cannot be applied to unsaturated flow because the distribution of permeability in a given material is different under unsaturated versus saturated conditions. A complex set of variables controls the influence of faulting on fluid flow. Given the lack of information about the behavior of faults in unconsolidated or poorly consolidated sediments, addressing all of the pertinent variables is beyond the scope of this proposal. We will focus on a small, but very important, aspect of the problems: how deformation influence saturated and unsaturated permeability of small-displacement faults in sand, the most common aquifer material in the southwestern United States. Small-displacement faults are far more common than the large-displacement faults typically included on geologic maps. Our preliminary work suggests that faulting in such material will result in decreased saturated permeability and, for the arid moisture conditions typical of the region, increased unsaturated permeability, both resulting from pore-size reduction caused by deformation. Thus in the saturated groundwater zone faults act as barrier, while in the Vaduz zone they may act as fast paths that lead chemicals directly to the water table. We will test the following hypotheses the course of the study: Fault in sand, especially small-displacement faults, are relatively common features in tectonically active extensional regions like the Rio Grande Rift. At the porous-media scale, fault-zone saturated and unsaturated permeability is altered by deformation and d iagenesis, through grain-size reduction, grain reorientation, and cementation. We will focus our attention on the first two processes. (The third process will be the subject of future research.) The altered fault-zone permeability influences saturated groundwater flow and unsaturated Vaduz zone flow, and the associated movement of chemicals. The degree influence depends on the permeability contrast between fault zones and host sands, spatial frequency of faults. In saturated sandy aquifers these fault zones become barriers to flow because of reductions in saturated permeability. In the vadose zone fault zones can become preferred flow paths under the arid conditions typical of the southwestern US; in wetter conditions they become barriers. We will test these hypotheses using a combination of geologic and hydrologic analyses. The geology of several study sites will be mapped at different scales. They will then determine the permeability and petrographic characteristics of the mapped hydrogeologic units in both deformed and undeformed materials. This field-based data will be used to construct numerical models of fault systems in which the influence of fault properties on flow and transport can be evaluated. We will also make geochemical measurement of cementation that is diagnostic of past flow regimes. By addressing the problem with a interdisciplinary approach, using a variety of techniques at different scales, we hope to determine whether these faults are significant hydrogeologic features.