The role of fractures in regional groundwater flow : field evidence and model results from the basin-and-range of Texas and New Mexico
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This study integrates fracture mapping and groundwater flow modeling to assess the role of fractures in regional groundwater flow. This is an important topic because fractures play a prominent role in groundwater flow in many aquifers. Furthermore, few studies have addressed quantitatively the regional hydrogeological implications of fractures. The study area is located in west Texas and southern New Mexico, between the Salt Basin and the Tularosa Valley. The region is largely undeformed, but the Permian carbonate bedrock is cut by many extensional faults and fractures. Air-photo analysis and field mapping reveal a broad fracture zone extending from the Sacramento Mountains to the Salt Basin near Dell City, Texas. Most fractures roughly parallel major normal faults and are oriented approximately N20W. The most intense fracturing coincides with a prominent trough in the potentiometric surface and an apparent "plume" of relatively fresh groundwater. Flow simulation and chemical modeling suggest that fracturing has created a high permeability zone that funnels recharge from the Sacramento Mountains at least 80 km southeastward to discharge points in the Salt Basin and the Dell City irrigation district. To estimate the regional transmissivity and to test the role of fractures in regional flow, a steady-state finite-element flow model was constructed in which fracture data are used to constrain a spatially distributed transmissivity. Given the probable range of recharge, discharge and other hydrologic parameters, fractures are the most important single constraint on the configuration of the potentiometric surface. Major results include: (1) fracturing can control groundwater flow over large (>1000 km²) areas, (2) effective recharge areas and regional groundwater chemistry trends are strongly influenced by fractures, and (3) through fracture studies, a priori inferences about aquifer properties and regional flow are possible. Finally, this study demonstrates one mechanism by which the timing and nature of tectonic events can affect regional subsurface fluid flow and, perhaps more importantly, related processes such as hydrothermal mineralization, diagenesis, and hydrocarbon transport and entrapment.