GEOLOGIC-STRUCTURAL CONTROL OF FRACTURE-CONTROLLED GROUND-WATER FLOW AT A VARIETY OF SPATIAL SCALES

The role of structural features in controlling subsurface flow and surface water - ground water interactions is being investigated in a number of areas in the western U.S. through combined geologic, geochemical, and hydrologic studies. Hydraulic head measurements and physical and chemical characteristics of water samples are linked to structural setting at a variety of scales to understand fracture-controlled flow and spatial changes in water quality in surface water-ground water systems associated with mineral deposits. At Battle Mountain, Nevada, changes in surface-water flow due to ground water discharge and recharge occur in locations where large-scale (10’s of km) through-going fracture systems cross a stream drainage. In the nearby Osgood Mountains, ground water - surface water interactions are controlled by fracture systems at spatial scales from meters to kilometers, as confirmed by hydraulic head measurements and geochemical samples. In the Patagonia Mountains, southern Arizona, surface water disappeared into the ground as a stream intersected a major fracture zone and re-emerged several hundred meters downstream, where it mixed with water draining an abandoned mine. Chemical parameters and temperature provided a clear indication of the presence of the re-emergent ground water. A final case study on Mt. Emmons in southwest Colorado examined local flow phenomena within a fault duplex that formed in the Tertiary, but whose fracture sets are still hydraulically conductive. The main strike-slip faults of the duplex extend for several kilometers and are about 1.5 km apart. Local flow regimes are established at the scale of tens of meters within this system, and are observed by chemically dissimilar springs within meters of each other, mixing of dissimilar waters from adjacent stream drainages, etc. The results of these studies are being used to help evaluate the environmental effects of mineral deposits throughout the U.S., and the scales of study required to properly evaluate those systems.