GROUNDWATER FLOW IN FRACTURED AQUIFERS IN EMIGRATION CANYON SYNCLINE, UTAH

Analysis of a groundwater flow system within deformed bedrock along the Emigration Canyon syncline, Utah, illustrates interactions between lithology, structural geometry, topography, and fluid flow. Bedrock comprises a thick sequence of Pennsylvanian to Early Cretaceous limestone, dolostone, sandstone, and shale, which was folded and thrust-faulted during the Cretaceous Sevier orogeny, and then uplifted and normal-faulted during Neogene Basin and Range extension. Locations of springs are correlated with intervals of more fractured limestone and porous sandstone that form compartmentalized aquifers, separated by confining intervals of shale and mudstone. Bedding is steeply dipping along the limbs of the syncline, and is cut by cleavage and fracture systems that developed during internal deformation. Results of pump tests indicate that fractured bedrock aquifers have complex, heterogeneous permeability and varying storage properties, related to differences in primary grain-scale porosity and secondary fracture lengths, spacings, and apertures. The ability of fractured bedrock aquifers with low primary porosity to store water is relatively low, resulting in large seasonal fluctuations in water levels and relatively rapid lowering of water levels during pumping. Additionally, aquifers are limited in areal extent due to steeply dipping bedding, such that cones of depression may encounter aquifer boundaries, resulting in increased rates of drawdown, which is also enhanced by closure of apertures during pumping. After prolonged or high rates of pumping, aquifer storage may decrease as fractures permanently close. The combination of complex permeability structure and low storage make estimates of safe yield and determination of source protection areas difficult. Thus, development of fractured bedrock aquifers should include detailed hydrogeologic mapping, and long term monitoring of water levels and chemistry from production wells. One strategy to increase intersection of well bores with fractured intervals in steeply dipping beds is to drill inclined wells; such an inclined well drilled in 2004 has provided a consistent water source for the upper Emigration Canyon area.