Current understanding of fault-zone permeability at regional scales is limited. Particularly,permeability measurements across faults are scarce. Yet, there are clear indications that faults have large hydrogeologic impacts on various geologic processes at this scale, as evidenced by variable hydraulic head gradients, geochemical and geothermal anomalies across faults. There remain questions that are scientifically fundamental and practically significant regarding largescale permeability of fault zones and their impact on aquifers. For example, how geologic and hydrologic information can be best integrated to effectively constrain regional-scale fault permeability? While existing knowledge on fault permeability at small scales is indispensable scientifically, ultimately, it is the regional-scale permeability that is needed in conceptual and numerical models for solving problems related to water resource management and transport of fluid, solute, and energy in the Earth's crust. The scientific objective of this research is to characterize fault-zone permeability at the wellfield to regional scale with constraints from field-based structural observations, hydrogeologic testing, and interpretation from conceptual and numerical modeling. The basis hypotheses are that distinctive hydrogeologic responses around a fault could be observed from cross-fault aquifer tests and that fault-zone permeability features could be established from a comprehensive understanding of the outcrop structural characteristics, borehole data of host rocks, and hydrogeologic tests conducted at multiple scales. Using the Elkhorn thrust fault in Colorado as a field site, the research plan consists of the following: (1) to characterize the geology and internal structure of the fault zone to better understand the influence of juxtaposition of different lithologies on fault-zone hydrogeologic properties, (2) to drill and core new boreholes through the fault zone to obtain direct field observations for fault-zone permeability, (3) to conduct crossfault pumping tests in the new and preexisting holes to provide the much need direct measurements on fault-zone permeability, and (4) to develop a fault-zone permeability model, through numerical modeling that integrates geologic, hydrogeologic, and geophysical field data. Three aspects of the intellectual merit are recognized. First, the study takes a new direction in characterizing fault-zone permeability by closely linking geologic observations and hydrogeologic testing data. Second, this proposed research offers the opportunity to gain hydrogeologic insight into a particular type of fault that is common but highly understudied - a thrust fault with a crystalline hanging wall and sedimentary footwall at regional basin scales. Finally, the cross-fault permeability tests coupled with geologic characterization planned in this research will be a significant advance in characterizing fault-related permeability by providing much-needed field measurement data. The major aspect of the broader impact of this proposed research is on enhancing undergraduate students experience in hydrogeology, in addition to training graduate students and contributing to better groundwater resource management in the western US. This research will benefit a large group of undergraduate students. Undergraduate students will be involved in all stages of this project. Specific approaches to implement this commitment include: (1) Recruit top undergraduates to conduct honor theses, (2) Field exercise related to an upper-division hydrogeology class. Groundwater level mapping and aquifer tests can be incorporated into class related activities, and (3) Drilling new boreholes will be coordinated so that students can observe the operation. Finally, the PIs plan to co-teach a two-credit two-week summer undergraduate field hydrogeology class. The research will provide an excellent venue for many different projects for this field class and help to realization of this plan.
