FRACTURE CONTROL OF REGIONAL GROUNDWATER-FLOW IN A CARBONATE AQUIFER IN A SEMIARID REGION

We integrate fracture mapping and numerical modeling to assess the rol e of fractures in regional ground-water flow Although the importance o f fractures in ground-water flow and solute transport is accepted gene rally, few studies have addressed quantitatively the regional hydrogeo logical implications of fractures. The held-study area in west Texas a nd southeastern New Mexico consists primarily of subhorizontal Permian carbonate rocks cut by extensional faults and fractures. Air-photo an alysis and held mapping reveal a broad fracture zone extending from th e Sacramento Mountains of New Mexico to the Salt Basin near Dell City, Texas. Most fractures are subparallel to major normal faults. The mos t intense fracturing coincides with a prominent trough in the potentio metric surface and an apparent ''plume'' of relatively fresh ground wa ter. Flow models, corroborated by geochemical data, indicate that frac turing has created a high-permeability zone that funnels recharge from the Sacramento Mountains at least 80 km south-eastward to its dischar ge zone. A steady-state finite-element now model uses fracture data to predict the spatial transmissivity distribution, Given the probable r ange of recharge, discharge, and other hydrologic parameters, fracture s are the most important factor affecting the potentiometric surface c onfiguration. Our study implies that: (1) fractures can control ground -water pow over large (> 1000 km(2)) areas; (2) effective recharge are as and regional ground-water chemistry trends are strongly influenced by fractures; and (3) a priori inferences about aquifer properties and regional now are possible by means of fracture studies. This study de monstrates that the timing and nature of fracturing can affect regiona l subsurface fluid flow as web as related processes such as hydrotherm al mineralization, diagenesis, and hydrocarbon transport and entrapmen t.