Fracture control of regional ground-water flow in a carbonate aquifer in a semi-arid region

摘要

We integrate fracture mapping and numerical modeling to assess the role of fractures in regional round-water flow. Although the importance of fractures in ground-water flow and solute transport is accepted generally, few studies have addressed quantitatively the regional hydrogeological implications of fractures. The field-study area in west Texas and southeastern New Mexico consists primarily of subhorizontal Permian carbonate rocks cut by extensional faults and fractures. Air-photo analysis and field mapping reveal a broad fracture zone extending from the Sacramento Mountains of New Mexico to the Salt Basin near Dell City, Texas. Most fractures are subparallel to major normal faults. The most intense fracturing coincides with a prominent trough in the potentiometric surface and an apparent "plume" of relatively fresh ground water. Flow models, corroborated by geochemical data, indicate that fracturing has created a high-permeability zone that funnels recharge from the Sacramento Mountains at least 80 km southeastward to its discharge zone. A steady-state finite-element flow model uses fracture data to predict the spatial transmissivity distribution. Given the probable range of recharge, discharge, and other hydrologic parameters, fractures are the most important factor affecting the potentiometric surface configuration. Our study implies that: (1) fractures can control ground-water flow over large (>1000 km2) areas; (2) effective recharge areas and regional ground-water chemistry trends are strongly influenced by fractures; and (3) a priori inferences about aquifer properties and regional flow are possible by means of fracture studies. This study demonstrates that the timing and nature of fracturing can affect regional subsurface fluid flow, as well as related processes such as hydrothermal mineralization, diagenesis, and hydrocarbon transport and entrapment.