CHARACTERIZING FAULT PERMEABILITY USING AQUIFER TESTS AND NUMERICAL MODELING

Faults play important roles in various hydrogeological processes. Permeability of faults, however, remains difficult to estimate. This study aims at better understanding the hydrologic behavior of the Elkhorn Fault, a low-angle reverse fault in South Park Basin, Colorado. Aquifer tests and numerical modeling were utilized to characterize the permeability of the fault zone and assess the role of the fault in regional groundwater flow. South Park Basin consists of Precambrian crystalline bedrock, Cretaceous sedimentary rocks, and Tertiary volcanic and sedimentary rocks. The east-dipping Elkhorn Fault marks the eastern boundary of the basin with Precambrian crystalline bedrock thrust over younger sedimentary rocks. Water levels in 32 wells in crystalline and sedimentary aquifers on both sides of the fault were recorded monthly by the USGS from 1997 to 2001. Analyses of the water level data suggest that groundwater flow generally follows the topography. Temporal fluctuations of water level indicate that the aquifers on two sides of the fault behave differently in responding to external hydrologic stresses such as precipitation. Slug tests were performed in four piezometers along a 400-foot transect across the trace of the Elkhorn Fault. Two piezometers in partially fractured crystalline rock in the hanging wall produced hydraulic conductivity in the range of 1 to 50 feet/day. Two others in the sedimentary rocks in the foot wall produced a lower hydraulic conductivity range of 10^-3 to 10^-1 feet/day. Groundwater modeling was carried out to estimate the fault permeability and test the influence of fault properties on groundwater flow in the region, particularly across the fault. Modeling results suggest that the fault is a low-permeability feature with respect to the aquifers it separates. Groundwater flow in the study area is primarily topography driven near the land surface and flow rates decrease with depth. The fault is likely a barrier to groundwater flow, impeding interaction between the crystalline aquifer in the hanging wall and the sedimentary rocks in the foot wall.